readcon_core/

iterators.rs

//=============================================================================
// The Public API - A clean iterator for users of our library
//=============================================================================

use crate::parser::{LineStream, parse_declared_sections};
use crate::{error, types};
use std::path::Path;

/// memchr-backed line cursor for the full parse path (not only frame skip).
///
/// Profile note: `str::Lines` + `Peekable` showed up under
/// `ConFrameIterator::next` on multi-atom multi-frame workloads. One cursor
/// serves `next` / `peek` / `forward_fast` so skip and full parse share the
/// same O(1) newline scan rather than two desynchronized views of the buffer.
pub struct MemchrLines<'a> {
    bytes: &'a [u8],
    pos: usize,
    peeked: Option<&'a str>,
    /// When the buffer was not already a `&str`, header lines are checked
    /// as they are tokenized. Coordinate rows stay on the byte parser.
    check_utf8: bool,
    utf8_bad: bool,
    /// Absolute offsets of every `\n`. Set only for a frame-start scan.
    /// `None` keeps the per-line `memchr` path used by a full parse.
    newlines: Option<Vec<usize>>,
    /// Index of the line that starts at `pos` when `newlines` is in use.
    index_line: usize,
    /// 1-based index of the last line produced by [`Self::read_one`], including
    /// a line rejected as UTF-8. 0 before that.
    line_no: u32,
}

impl<'a> MemchrLines<'a> {
    pub fn new(text: &'a str) -> Self {
        Self::from_parts(text.as_bytes(), false)
    }

    /// Cursor over bytes that are not yet known to be UTF-8.
    ///
    /// ASCII coordinate rows are accepted by the numeric scan. Text lines
    /// are validated when they are materialized, so there is no separate
    /// whole-buffer UTF-8 pass.
    pub fn from_bytes(bytes: &'a [u8]) -> Self {
        Self::from_parts(bytes, true)
    }

    fn from_parts(bytes: &'a [u8], check_utf8: bool) -> Self {
        Self {
            bytes,
            pos: 0,
            peeked: None,
            check_utf8,
            utf8_bad: false,
            newlines: None,
            index_line: 0,
            line_no: 0,
        }
    }

    /// 1-based index of the last line read, including a line that is only peeked.
    #[inline]
    pub fn line_no(&self) -> u32 {
        self.line_no
    }

    #[inline]
    fn read_one(&mut self) -> Option<&'a str> {
        if self.newlines.is_some() {
            return self.read_one_indexed();
        }
        if self.utf8_bad || self.pos >= self.bytes.len() {
            return None;
        }
        let rest = &self.bytes[self.pos..];
        let (line_bytes, advance) = match memchr::memchr(b'\n', rest) {
            Some(i) => (&rest[..i], i + 1),
            None => (rest, rest.len()),
        };
        let trimmed = if line_bytes.last() == Some(&b'\r') {
            &line_bytes[..line_bytes.len() - 1]
        } else {
            line_bytes
        };
        if self.check_utf8 {
            match std::str::from_utf8(trimmed) {
                Ok(s) => {
                    self.accept_line(advance);
                    Some(s)
                }
                Err(_) => {
                    // Stay on the rejected line. Moving past it makes a
                    // whitespace tail look like a clean end of file.
                    // The line number still names this line.
                    self.line_no = self.line_no.saturating_add(1);
                    self.utf8_bad = true;
                    None
                }
            }
        } else {
            self.accept_line(advance);
            // SAFETY: source was `&str`; line is a UTF-8 prefix cut on ASCII `\n`/`\r`.
            Some(unsafe { std::str::from_utf8_unchecked(trimmed) })
        }
    }

    #[inline]
    fn accept_line(&mut self, advance: usize) {
        self.pos += advance;
        self.line_no = self.line_no.saturating_add(1);
    }

    #[inline]
    pub fn next_line(&mut self) -> Option<&'a str> {
        if let Some(p) = self.peeked.take() {
            return Some(p);
        }
        self.read_one()
    }

    #[inline]
    pub fn peek_line(&mut self) -> Option<&'a str> {
        if self.peeked.is_none() {
            self.peeked = self.read_one();
        }
        self.peeked
    }

    /// Drop any peek buffer (required before bulk cursor advances).
    fn clear_peek(&mut self) {
        if let Some(p) = self.peeked.take() {
            // Rewind pos to the start of the peeked line.
            let start = p.as_ptr() as usize - self.bytes.as_ptr() as usize;
            self.pos = start;
            // A peek already counted this line. Rewinding must drop that
            // count so the next read names it once, and must put the
            // newline index back on the line `pos` now starts.
            self.line_no = self.line_no.saturating_sub(1);
            if let Some(nl) = self.newlines.as_deref() {
                self.index_line = line_no_for_pos(nl, self.bytes.len(), start);
            }
        }
    }

    /// One line from the precomputed newline index.
    ///
    /// The cursor must sit on a line start (or at the end). The bytes of
    /// the line are the same slice `memchr` would return, including a line
    /// whose body crosses a chunk boundary: only the `\n` is indexed.
    fn read_one_indexed(&mut self) -> Option<&'a str> {
        if self.utf8_bad || self.pos >= self.bytes.len() {
            return None;
        }
        let len = self.bytes.len();
        let line_no = {
            let nl = self.newlines.as_deref().unwrap();
            if self.pos == line_start_pos(nl, len, self.index_line) {
                self.index_line
            } else {
                line_no_for_pos(nl, len, self.pos)
            }
        };
        let span = indexed_span(self.newlines.as_deref().unwrap(), self.bytes, line_no)?;
        let trimmed = &self.bytes[span.start..span.content_end];
        if self.check_utf8 {
            match std::str::from_utf8(trimmed) {
                Ok(s) => {
                    self.pos = span.next_pos;
                    self.index_line = line_no + 1;
                    self.line_no = self.index_line as u32;
                    Some(s)
                }
                Err(_) => {
                    self.utf8_bad = true;
                    self.index_line = line_no;
                    // The rejected line is 1-based `line_no + 1`. The index
                    // stays on that line so a later read does not skip it.
                    self.line_no = (line_no as u32).saturating_add(1);
                    None
                }
            }
        } else {
            self.pos = span.next_pos;
            self.index_line = line_no + 1;
            self.line_no = self.index_line as u32;
            // SAFETY: source was `&str`; the span is a UTF-8 line cut on ASCII `\n`/`\r`.
            Some(unsafe { std::str::from_utf8_unchecked(trimmed) })
        }
    }

    fn utf8_error(&self) -> Option<error::ParseError> {
        self.utf8_bad.then(invalid_utf8)
    }

    /// Keep `err` unless this cursor already rejected a line as UTF-8.
    fn prefer_utf8(&self, err: error::ParseError) -> error::ParseError {
        if self.utf8_bad { invalid_utf8() } else { err }
    }
}

fn invalid_utf8() -> error::ParseError {
    error::ParseError::ValidationError("invalid UTF-8".into())
}

impl<'a> Iterator for MemchrLines<'a> {
    type Item = &'a str;
    fn next(&mut self) -> Option<&'a str> {
        self.next_line()
    }
}

impl<'a> LineStream<'a> for MemchrLines<'a> {
    #[inline]
    fn next_line(&mut self) -> Option<&'a str> {
        MemchrLines::next_line(self)
    }
    #[inline]
    fn peek_line(&mut self) -> Option<&'a str> {
        MemchrLines::peek_line(self)
    }
    #[inline]
    fn next_coord_row(
        &mut self,
        spec_version: u32,
        default_id: u64,
    ) -> Result<([f64; 3], [bool; 3], u64), crate::error::ParseError> {
        if let Some(p) = self.peeked.take() {
            return crate::parser::parse_coord_row_at(p.as_bytes(), 0, spec_version, default_id)
                .map(|(row, _)| row);
        }
        // The failing row is this line even when the numeric scan returns Err
        // before the cursor moves.
        self.line_no = self.line_no.saturating_add(1);
        let (row, pos) =
            crate::parser::parse_coord_row_at(self.bytes, self.pos, spec_version, default_id)?;
        self.pos = pos;
        Ok(row)
    }
    #[inline]
    fn next_vector_row(
        &mut self,
        spec_version: u32,
    ) -> Result<([f64; 3], [bool; 3], u64), crate::error::ParseError> {
        if let Some(p) = self.peeked.take() {
            return crate::parser::parse_vector_row_at(p.as_bytes(), 0, spec_version)
                .map(|(row, _)| row);
        }
        let (row, pos) = crate::parser::parse_vector_row_at(self.bytes, self.pos, spec_version)?;
        self.pos = pos;
        Ok(row)
    }
    fn remainder_is_whitespace(&mut self) -> bool {
        if self.utf8_bad {
            return false;
        }
        if let Some(line) = self.peek_line()
            && !line.trim().is_empty()
        {
            return false;
        }
        // `peek_line` may have rejected a line. That is a decode error,
        // including when every byte after it is ASCII whitespace.
        if self.utf8_bad {
            return false;
        }
        self.bytes[self.pos..]
            .iter()
            .all(|b| b.is_ascii_whitespace())
    }
    fn blank_line_opens_section(&mut self) -> bool {
        // `peek_line` has already moved `pos` to the line after the peek.
        let Some(peeked) = self.peeked else {
            return false;
        };
        if !peeked.trim().is_empty() {
            return false;
        }
        match probe_section(&self.bytes[self.pos..]) {
            SectionProbe::Opens => true,
            SectionProbe::Closed => false,
            // Leave the rejected bytes on the cursor. `prefer_utf8` then
            // reports the decode error instead of a missing section.
            SectionProbe::InvalidUtf8 => {
                self.utf8_bad = true;
                false
            }
        }
    }

    fn blank_opens_velocity_block(&mut self) -> bool {
        match self.peek_line() {
            Some(line) if line.trim().is_empty() => self.blank_line_opens_section(),
            _ => true,
        }
    }
}

fn split_raw_line(bytes: &[u8]) -> Option<(&[u8], &[u8])> {
    if bytes.is_empty() {
        return None;
    }
    match memchr::memchr(b'\n', bytes) {
        Some(i) => Some((&bytes[..i], &bytes[i + 1..])),
        None => Some((bytes, &[])),
    }
}

fn strip_cr(line: &[u8]) -> &[u8] {
    if line.last() == Some(&b'\r') {
        &line[..line.len() - 1]
    } else {
        line
    }
}

enum SectionProbe {
    Opens,
    Closed,
    InvalidUtf8,
}

/// A blank line opens a section when the next line is a symbol and the
/// line after that is a component label (CON-file.5, CON-header.4).
fn probe_section(after_blank: &[u8]) -> SectionProbe {
    let Some((raw1, after1)) = split_raw_line(after_blank) else {
        return SectionProbe::Closed;
    };
    let line1 = strip_cr(raw1);
    if line1.iter().all(|b| matches!(b, b' ' | b'\t')) {
        return SectionProbe::Closed;
    }
    let Ok(text1) = std::str::from_utf8(line1) else {
        return SectionProbe::InvalidUtf8;
    };
    if text1.trim_start().starts_with('{') {
        return SectionProbe::Closed;
    }
    let Some((raw2, _)) = split_raw_line(after1) else {
        return SectionProbe::Closed;
    };
    let Ok(text2) = std::str::from_utf8(strip_cr(raw2)) else {
        return SectionProbe::InvalidUtf8;
    };
    if crate::parser::line_has_component_label(text2) {
        SectionProbe::Opens
    } else {
        SectionProbe::Closed
    }
}

fn blank_opens_section(after_blank: &[u8]) -> bool {
    matches!(probe_section(after_blank), SectionProbe::Opens)
}

/// An iterator that lazily parses simulation frames from a `.con` or `.convel`
/// file's contents.
///
/// This struct wraps a memchr line cursor over the file buffer and, upon each
/// iteration, attempts to parse a complete `ConFrame`. A blank line after the
/// coordinates is a velocity section only when it begins a complete velocity
/// block. Otherwise that line is line 1 of the next frame, and it may be empty.
///
/// The iterator yields items of type `Result<ConFrame, ParseError>`, allowing for
/// robust error handling for each frame.
pub struct ConFrameIterator<'a> {
    pub(crate) lines: MemchrLines<'a>,
    /// 0-based index of the frame the cursor is about to read.
    /// Successful `next` and `forward_fast` advance it. A failure leaves
    /// it on the frame that failed.
    frame_index: u64,
}

/// Newlines seen and the byte index just past the `n`-th `\n`.
///
/// When fewer than `n` newlines exist, the count is how many were seen
/// and the index is `hay.len()`.
///
/// # Safety
/// The CPU supports AVX2.
#[cfg(target_arch = "x86_64")]
#[target_feature(enable = "avx2,popcnt")]
#[inline]
unsafe fn skip_newlines_avx2(hay: &[u8], n: usize) -> (usize, usize) {
    use std::arch::x86_64::{
        __m256i, _mm256_cmpeq_epi8, _mm256_loadu_si256, _mm256_movemask_epi8, _mm256_set1_epi8,
        _popcnt32,
    };
    if n == 0 {
        return (0, 0);
    }
    let mut need = n;
    let mut offset = 0usize;
    let ptr = hay.as_ptr();
    let len = hay.len();
    let nl = _mm256_set1_epi8(b'\n' as i8);
    while offset + 32 <= len {
        // SAFETY: `offset + 32 <= len`, so the 32-byte load stays inside `hay`.
        let chunk = unsafe { _mm256_loadu_si256(ptr.add(offset) as *const __m256i) };
        let eq = _mm256_cmpeq_epi8(chunk, nl);
        let mask = _mm256_movemask_epi8(eq) as u32;
        let count = _popcnt32(mask as i32) as usize;
        if count >= need {
            let mut bits = mask;
            for _ in 1..need {
                bits &= bits - 1;
            }
            let bit = bits.trailing_zeros() as usize;
            return (n, offset + bit + 1);
        }
        need -= count;
        offset += 32;
    }
    while offset < len {
        if hay[offset] == b'\n' {
            need -= 1;
            if need == 0 {
                return (n, offset + 1);
            }
        }
        offset += 1;
    }
    (n - need, len)
}

impl<'a> ConFrameIterator<'a> {
    /// Creates a new `ConFrameIterator` from a string slice of the entire file.
    ///
    /// # Arguments
    ///
    /// * `file_contents` - A string slice containing the text of one or more `.con` frames.
    pub fn new(file_contents: &'a str) -> Self {
        ConFrameIterator {
            lines: MemchrLines::new(file_contents),
            frame_index: 0,
        }
    }

    /// Like [`Self::new`], but reading from byte `pos` of `file_contents`.
    ///
    /// `pos` must be 0 or follow a `\n`. The iterator keeps no state from
    /// earlier frames, so skipping from `pos` sees what a scan sees there.
    pub(crate) fn starting_at(file_contents: &'a str, pos: usize) -> Self {
        debug_assert!(pos == 0 || file_contents.as_bytes()[pos - 1] == b'\n');
        let mut it = Self::new(file_contents);
        it.lines.pos = pos;
        it
    }

    /// Like [`Self::new`] for a buffer that has not been UTF-8 validated.
    ///
    /// Text lines are checked as they are tokenized. Numeric rows reject a
    /// non-ASCII byte during the fused parse, which is the UTF-8 check for
    /// those lines.
    pub fn from_bytes(bytes: &'a [u8]) -> Self {
        ConFrameIterator {
            lines: MemchrLines::from_bytes(bytes),
            frame_index: 0,
        }
    }

    /// 0-based index of the frame the cursor is on.
    ///
    /// This is the frame a following error belongs to. It is not the
    /// number of frames still ahead.
    pub(crate) fn frame_index(&self) -> u64 {
        self.frame_index
    }

    fn note_skipped_frame(&mut self) -> Option<Result<(), error::ParseError>> {
        self.frame_index = self.frame_index.saturating_add(1);
        Some(Ok(()))
    }

    /// Bulk-skips `n` lines from the shared memchr cursor.
    ///
    /// A final line with no trailing newline still counts when it is the
    /// last line this call needs and its bytes are present. The full
    /// parser accepts that line. Further lines after it are missing.
    ///
    /// The header has already named the line count, so the skip counts
    /// newlines instead of parsing the rows. On AVX2 that count is one
    /// pass over the span.
    #[inline(always)]
    fn advance_lines(&mut self, n: usize) -> Result<(), error::ParseError> {
        self.lines.clear_peek();
        if self.lines.newlines.is_some() {
            return self.advance_lines_indexed(n);
        }
        #[cfg(target_arch = "x86_64")]
        {
            if n > 1 && std::is_x86_feature_detected!("avx2") {
                return self.advance_lines_avx2(n);
            }
        }
        self.advance_lines_memchr(n)
    }

    fn advance_lines_memchr(&mut self, n: usize) -> Result<(), error::ParseError> {
        for i in 0..n {
            let rest = &self.lines.bytes[self.lines.pos..];
            match memchr::memchr(b'\n', rest) {
                Some(pos) => {
                    self.lines.pos += pos + 1;
                    self.lines.line_no = self.lines.line_no.saturating_add(1);
                }
                None => {
                    if rest.is_empty() {
                        return Err(error::ParseError::IncompleteFrame);
                    }
                    self.lines.pos = self.lines.bytes.len();
                    // Count the unterminated line so a later located error
                    // names it. It satisfies this skip only when it is the
                    // last line still required; the full parser accepts it.
                    self.lines.line_no = self.lines.line_no.saturating_add(1);
                    if i + 1 == n {
                        return Ok(());
                    }
                    return Err(error::ParseError::IncompleteFrame);
                }
            }
        }
        Ok(())
    }

    /// [`Self::advance_lines_memchr`] counting newlines in AVX2 chunks.
    ///
    /// The landed position and the incomplete-frame condition match the
    /// per-line search. `n > 1` is the caller's guard: a one-line skip
    /// is already a single `memchr`.
    #[cfg(target_arch = "x86_64")]
    #[inline]
    fn advance_lines_avx2(&mut self, n: usize) -> Result<(), error::ParseError> {
        let rest = &self.lines.bytes[self.lines.pos..];
        let (found, end) = unsafe { skip_newlines_avx2(rest, n) };
        if found == n {
            self.lines.pos += end;
            // The per-line walk counts each skipped line. The vectorized
            // jump has to keep the same 1-based file line.
            self.lines.line_no = self
                .lines
                .line_no
                .saturating_add(u32::try_from(n).unwrap_or(u32::MAX));
            return Ok(());
        }
        if rest.is_empty() {
            return Err(error::ParseError::IncompleteFrame);
        }
        let has_tail = rest.last().copied() != Some(b'\n');
        self.lines.pos = self.lines.bytes.len();
        let counted = found + usize::from(has_tail);
        self.lines.line_no = self
            .lines
            .line_no
            .saturating_add(u32::try_from(counted).unwrap_or(u32::MAX));
        if counted >= n {
            Ok(())
        } else {
            Err(error::ParseError::IncompleteFrame)
        }
    }

    /// [`Self::advance_lines`] for `n` lines expected to end at `end`.
    ///
    /// When `end` follows a `\n` and exactly `n` newlines lie before it,
    /// the per-line search would stop at `end` too, so one vectorized
    /// count replaces it. Any other case takes the per-line search.
    fn advance_lines_to(&mut self, n: usize, end: Option<usize>) -> Result<(), error::ParseError> {
        // The newline index jumps by line count. `end` does not change
        // where that jump lands; it only shortcuts the per-line search.
        if self.lines.newlines.is_some() {
            return self.advance_lines(n);
        }
        self.lines.clear_peek();
        let pos = self.lines.pos;
        let bytes = self.lines.bytes;
        if let Some(end) = end
            && pos < end
            && end <= bytes.len()
            && bytes[end - 1] == b'\n'
            && memchr::memchr_iter(b'\n', &bytes[pos..end]).count() == n
        {
            self.lines.pos = end;
            // The per-line walk would count each of these lines. The
            // vectorized jump has to keep the same 1-based line.
            self.lines.line_no = self
                .lines
                .line_no
                .saturating_add(u32::try_from(n).unwrap_or(u32::MAX));
            return Ok(());
        }
        self.advance_lines(n)
    }

    /// [`Self::advance_lines`] using the stitched newline index.
    ///
    /// A missing line and a final line with no `\n` land on the same byte
    /// as the `memchr` loop. The index is not consulted for a full parse.
    fn advance_lines_indexed(&mut self, n: usize) -> Result<(), error::ParseError> {
        let len = self.lines.bytes.len();
        let step = {
            let nl = self.lines.newlines.as_deref().unwrap();
            let nlines = line_count(nl, len);
            let line_no = if self.lines.pos == line_start_pos(nl, len, self.lines.index_line) {
                self.lines.index_line
            } else {
                line_no_for_pos(nl, len, self.lines.pos)
            };
            if n == 0 {
                IndexStep::Stay(line_no)
            } else if line_no >= nlines {
                IndexStep::Missing(line_no)
            } else {
                match line_no.checked_add(n) {
                    Some(target) if target <= nlines => {
                        let pos = if target == nlines {
                            len
                        } else {
                            line_start_pos(nl, len, target)
                        };
                        IndexStep::Landed(target, pos)
                    }
                    _ => IndexStep::Short(nlines, len),
                }
            }
        };
        match step {
            IndexStep::Stay(line_no) => {
                self.lines.index_line = line_no;
                self.lines.line_no = u32::try_from(line_no).unwrap_or(u32::MAX);
                Ok(())
            }
            IndexStep::Missing(line_no) => {
                self.lines.index_line = line_no;
                self.lines.line_no = u32::try_from(line_no).unwrap_or(u32::MAX);
                Err(error::ParseError::IncompleteFrame)
            }
            IndexStep::Landed(line_no, pos) => {
                self.lines.index_line = line_no;
                self.lines.line_no = u32::try_from(line_no).unwrap_or(u32::MAX);
                self.lines.pos = pos;
                Ok(())
            }
            IndexStep::Short(line_no, pos) => {
                self.lines.index_line = line_no;
                self.lines.line_no = u32::try_from(line_no).unwrap_or(u32::MAX);
                self.lines.pos = pos;
                Err(error::ParseError::IncompleteFrame)
            }
        }
    }

    /// One line from the shared cursor (same as full-parse path).
    fn read_line_str(&mut self) -> Option<&'a str> {
        self.lines.clear_peek();
        self.lines.next_line()
    }

    /// memchr-backed equivalent of [`Self::forward`]. Skips the next
    /// frame without fully parsing its atom data. Shares the same line
    /// cursor as [`Iterator::next`], so skip and full parse interleave safely.
    pub fn forward_fast(&mut self) -> Option<Result<(), error::ParseError>> {
        self.forward_fast_to(None)
    }

    /// [`Self::forward_fast`] for a frame expected to end at byte `end`.
    ///
    /// The result and the new position are the same as without `end`.
    /// Line blocks that end exactly at `end` are counted, not searched.
    #[inline(always)]
    pub(crate) fn forward_fast_to(
        &mut self,
        end: Option<usize>,
    ) -> Option<Result<(), error::ParseError>> {
        self.lines.clear_peek();
        if self.lines.pos >= self.lines.bytes.len() {
            return None;
        }
        // Lines 1..=6 of the header are skipped wholesale.
        if let Err(e) = self.advance_lines(6) {
            return Some(Err(e));
        }
        // Line 7: natm_types.
        let natm_types: usize = match self.read_line_str() {
            Some(line) => match crate::parser::parse_count_line(line, 1) {
                Ok(v) => v[0],
                Err(e) => return Some(Err(e)),
            },
            None => return Some(Err(error::ParseError::IncompleteHeader)),
        };
        if natm_types == 0 {
            // Lines 8 and 9 are whitespace, or absent at EOF. Do not scan
            // for coordinates or a legacy velocity block (CON-count.1).
            return match crate::parser::read_zero_atom_tail(|| self.read_line_str()) {
                Ok(()) => self.note_skipped_frame(),
                Err(e) => Some(Err(e)),
            };
        }
        // Line 8: natms_per_type.
        let natms_per_type: Vec<usize> = match self.read_line_str() {
            Some(line) => match crate::parser::parse_count_line(line, natm_types) {
                Ok(v) => v,
                Err(e) => return Some(Err(e)),
            },
            None => return Some(Err(error::ParseError::IncompleteHeader)),
        };
        if natms_per_type.contains(&0) {
            return Some(Err(error::ParseError::InvalidCount(
                "each per-type count must be >= 1".into(),
            )));
        }
        // Line 9: masses_per_type, consumed.
        if let Err(e) = self.advance_lines(1) {
            return Some(Err(e));
        }
        let total_atoms: usize = natms_per_type.iter().sum();
        let coord_block_lines = total_atoms + natm_types * 2;
        if let Err(e) = self.advance_lines_to(coord_block_lines, end) {
            return Some(Err(e));
        }
        // Optional sections: blank line + same-shape block, repeated.
        self.lines.clear_peek();
        loop {
            let rest = &self.lines.bytes[self.lines.pos..];
            if rest.is_empty() {
                break;
            }
            let next_eol = memchr::memchr(b'\n', rest);
            let line = match next_eol {
                Some(pos) => &rest[..pos],
                None => rest,
            };
            let is_blank = line.iter().all(|b| matches!(b, b' ' | b'\t' | b'\r'));
            if !is_blank {
                break;
            }
            let after = match next_eol {
                Some(pos) => &rest[pos + 1..],
                None => &[][..],
            };
            // A blank line and then only whitespace is the end of the file,
            // not a section (CON-frame.5).
            if after.iter().all(|b| b.is_ascii_whitespace()) {
                self.lines.pos = self.lines.bytes.len();
                break;
            }
            // A blank that does not begin a section is line 1 of the next
            // frame, including an empty user line (CON-frame.7, CON-header.4).
            if !blank_opens_section(after) {
                break;
            }
            // Consume the blank separator and the section block.
            // The separator is a file line. Skipping it without counting
            // shifts every later frame's reported line.
            self.lines.pos += next_eol.map(|p| p + 1).unwrap_or(rest.len());
            // The separator is a file line. The section block is counted by
            // the advance, including the vectorized jump.
            self.lines.line_no = self.lines.line_no.saturating_add(1);
            if let Err(e) = self.advance_lines_to(coord_block_lines, end) {
                return Some(Err(e));
            }
        }
        self.note_skipped_frame()
    }

    /// Skips the next frame without fully parsing its atomic data.
    ///
    /// This is more efficient than `next()` if you only need to advance the
    /// iterator. It reads the frame's header to determine how many lines to skip,
    /// including any velocity section if present.
    ///
    /// # Returns
    ///
    /// * `Some(Ok(()))` on a successful skip.
    /// * `Some(Err(ParseError::...))` if there's an error parsing the header.
    /// * `None` if the iterator is already at the end.
    pub fn forward(&mut self) -> Option<Result<(), error::ParseError>> {
        // Prefer the shared memchr skip path (same cursor as full parse).
        self.forward_fast()
    }

    /// Byte offset of the next frame start in the buffer passed to [`Self::new`].
    pub fn byte_pos(&self) -> usize {
        self.lines.pos
    }

    /// Skip `n` frames without parsing atom data. Returns how many were skipped.
    /// Named `skip_frames` so it does not collide with [`Iterator::skip`].
    pub fn skip_frames(&mut self, n: usize) -> Result<usize, error::ParseError> {
        self.skip_frames_counted(n).map_err(|(_, err)| err)
    }

    /// Like [`Self::skip_frames`]. The error's first field is how many frames
    /// were skipped before the failure, which is the 0-based index of the
    /// failing frame when the iterator started at frame 0.
    pub(crate) fn skip_frames_counted(
        &mut self,
        n: usize,
    ) -> Result<usize, (usize, error::ParseError)> {
        let mut skipped = 0usize;
        for _ in 0..n {
            match self.forward() {
                None => break,
                Some(Err(e)) => return Err((skipped, e)),
                Some(Ok(())) => skipped += 1,
            }
        }
        Ok(skipped)
    }

    /// Next frame plus the exact substring of the buffer passed to [`Self::new`].
    ///
    /// **Corpus ingest contract:** successive successful spans from the same
    /// `file_contents` are contiguous (`end` of frame *i* equals `start` of frame
    /// *i+1*) and, for a buffer that is only multi-frame CON (no prefix garbage),
    /// concatenating all spans reproduces the trajectory text. Campaign stores
    /// (`readcon-db`) must persist these spans as authoritative blobs—do not
    /// re-serialize on the hot ingest path unless the caller supplied in-memory
    /// [`types::ConFrame`] values without source text.
    ///
    /// See also [`crate::index_proj::frame_byte_spans`] and
    /// [`crate::index_proj::spans_cover_buffer`].
    pub fn next_with_raw_span(
        &mut self,
        file_contents: &'a str,
    ) -> Option<Result<(types::ConFrame, &'a str), error::ParseError>> {
        let base = file_contents.as_ptr() as usize;
        let start = {
            let line = self.lines.peek_line()?;
            line.as_ptr() as usize - base
        };
        let frame = match self.next()? {
            Ok(f) => f,
            Err(e) => return Some(Err(e)),
        };
        let end = match self.lines.peek_line() {
            Some(line) => line.as_ptr() as usize - base,
            None => file_contents.len(),
        };
        debug_assert!(end >= start && end <= file_contents.len());
        Some(Ok((frame, &file_contents[start..end])))
    }

    /// [`Iterator::next`] plus the byte offset where the frame ends.
    pub(crate) fn next_with_end(
        &mut self,
    ) -> Option<Result<(types::ConFrame, usize), error::ParseError>> {
        let frame = match self.next()? {
            Ok(f) => f,
            Err(e) => return Some(Err(e)),
        };
        let end = match self.lines.peek_line() {
            Some(line) => line.as_ptr() as usize - self.lines.bytes.as_ptr() as usize,
            None => self.lines.pos,
        };
        Some(Ok((frame, end)))
    }
}

impl<'a> Iterator for ConFrameIterator<'a> {
    /// The type of item yielded by the iterator.
    ///
    /// Each item is a `Result` that contains a successfully parsed `ConFrame` or a
    /// `ParseError` if the frame's data is malformed.
    type Item = Result<types::ConFrame, error::ParseError>;

    /// Advances the iterator and attempts to parse the next frame.
    ///
    /// This method will return `None` only when there are no more lines to consume.
    /// If there are lines but they do not form a complete frame, it will return
    /// `Some(Err(ParseError::...))`. Invalid UTF-8 is a validation error, not
    /// the end of the stream.
    fn next(&mut self) -> Option<Self::Item> {
        // A line rejected earlier in this cursor (for example while probing
        // for a velocity block) must not be reported as a clean EOF.
        if let Some(err) = self.lines.utf8_error() {
            return Some(Err(err));
        }
        // Trailing whitespace after the last complete frame is not a frame
        // (CON-frame.1, D2). An empty line that is followed by more text is
        // line 1 of the next frame (CON-frame.7).
        if self.lines.remainder_is_whitespace() {
            if let Some(err) = self.lines.utf8_error() {
                return Some(Err(err));
            }
            while self.lines.next_line().is_some() {}
            return None;
        }
        if let Some(err) = self.lines.utf8_error() {
            return Some(Err(err));
        }
        // Otherwise, attempt to parse the next frame from the available lines.
        let mut frame = match crate::parser::parse_single_frame_ls(&mut self.lines) {
            Ok(f) => f,
            Err(e) => return Some(Err(self.lines.prefer_utf8(e))),
        };
        // Optional sections mutate AoS; only re-sync section SoA when needed.
        // Plain .con assembly already filled positions/ids/masses (no O(N)
        // post-scan when no velocity/force sections were applied).
        let sections =
            match parse_declared_sections(&mut self.lines, &mut frame.header, &mut frame.atom_data)
            {
                Ok(n) => n,
                // A section line that failed UTF-8 makes `next_line` return
                // `None`. That must not surface as `IncompleteSection`.
                Err(e) => return Some(Err(self.lines.prefer_utf8(e))),
            };
        if sections > 0 {
            // One flat fill per section that was actually parsed. The full
            // sync walks every absent section and writes each row through
            // ArcArray uniqueness checks.
            frame.install_declared_section_soa();
        }
        self.frame_index = self.frame_index.saturating_add(1);
        Some(Ok(frame))
    }
}

#[cfg(test)]
mod aos_soa_agreement_tests {
    use super::*;
    use std::path::PathBuf;

    #[test]
    fn iterator_vel_forces_soa_matches_aos() {
        let p = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
            .join("resources/test/tiny_cuh2_vel_forces.con");
        let text = std::fs::read_to_string(&p).expect("fixture");
        let fr = ConFrameIterator::new(&text)
            .next()
            .expect("frame")
            .expect("parse");
        let n = fr.atom_data.len();
        assert!(n > 0);
        assert_eq!(fr.positions.nrows(), n);
        let has_vel = fr.atom_data.iter().any(|a| a.velocity().is_some());
        let has_frc = fr.atom_data.iter().any(|a| a.force().is_some());
        if has_vel {
            assert_eq!(
                fr.velocities.nrows(),
                n,
                "SoA velocities must match AoS after section parse"
            );
        }
        if has_frc {
            assert_eq!(fr.forces.nrows(), n, "SoA forces must match AoS");
        }
        for (i, a) in fr.atom_data.iter().enumerate() {
            let p = fr.positions.as_f64_row(i);
            assert_eq!([a.x, a.y, a.z], p);
            if let Some(v) = a.velocity() {
                assert_eq!(v, fr.velocities.as_f64_row(i));
            }
            if let Some(f) = a.force() {
                assert_eq!(f, fr.forces.as_f64_row(i));
            }
        }
    }

    /// After SoA-primary parse, section sync must not require rewriting positions
    /// (nrows already equals N); forces SoA still filled from AoS.
    #[test]
    fn sync_skips_pos_when_nrows_matches_keeps_force_soa() {
        let p =
            PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("resources/test/tiny_cuh2_forces.con");
        let text = std::fs::read_to_string(&p).expect("fixture");
        let fr = ConFrameIterator::new(&text)
            .next()
            .expect("frame")
            .expect("parse");
        let n = fr.atom_data.len();
        assert_eq!(fr.positions.nrows(), n);
        assert_eq!(fr.forces.nrows(), n);
        // Snapshot first position SoA row then re-sync; coords must stay bit-identical
        // (no needless rewrite would change nothing but we still require agreement).
        let p0 = fr.positions.as_f64_row(0);
        let mut fr2 = fr.clone();
        fr2.sync_arrays_from_atom_data();
        assert_eq!(fr2.positions.as_f64_row(0), p0);
        assert_eq!(fr2.forces.nrows(), n);
        assert_eq!(
            fr2.forces.as_f64_row(0),
            fr2.atom_data[0].force().expect("force")
        );
    }

    /// Section SoA is filled from one flat buffer. Every value must match the
    /// per-atom tail, including past a small-vector boundary.
    #[test]
    fn declared_sections_soa_matches_every_atom() {
        let n = 400;
        let mut text = String::new();
        text.push_str("soa fill\n");
        text.push_str(
            "{\"con_spec_version\":2,\"sections\":[\"velocities\",\"forces\",\"energies\"]}\n",
        );
        text.push_str("10.0 10.0 10.0\n90.0 90.0 90.0\n0 0\n0 0 0\n1\n");
        text.push_str(&format!("{n}\n63.546\nCu\nCoordinates of Component 1\n"));
        for i in 0..n {
            let x = (i as f64) * 0.125 - 3.5;
            let y = (i as f64) * -0.05 + 1.0;
            let z = (i % 17) as f64 * 0.333;
            text.push_str(&format!("{x:.8} {y:.8} {z:.8} 0 {i}\n"));
        }
        text.push_str("\nCu\nVelocities of Component 1\n");
        for i in 0..n {
            let vx = (i as f64) * 1.0e-4 - 0.02;
            let vy = ((i * 3) % 11) as f64 * -1.0e-3;
            let vz = (i as f64).sin() * 0.01;
            text.push_str(&format!("{vx:.8} {vy:.8} {vz:.8} 0 {i}\n"));
        }
        text.push_str("\nCu\nForces of Component 1\n");
        for i in 0..n {
            let fx = (i as f64) * 0.01;
            let fy = -fx / 2.0;
            let fz = (i % 5) as f64;
            text.push_str(&format!("{fx:.8} {fy:.8} {fz:.8} 0 {i}\n"));
        }
        text.push_str("\nCu\nEnergies of Component 1\n");
        for i in 0..n {
            let e = (i as f64) * 0.02 - 4.0;
            text.push_str(&format!("{e:.8} 0 {i}\n"));
        }
        let frame = ConFrameIterator::new(&text)
            .next()
            .expect("frame")
            .expect("parse");
        assert_eq!(frame.atom_data.len(), n);
        assert_eq!(frame.velocities.nrows(), n);
        assert_eq!(frame.forces.nrows(), n);
        assert_eq!(frame.atom_energies.len(), n);
        assert_eq!(frame.positions.nrows(), n);
        assert!(frame.magmoms.nrows() == 0);
        for i in 0..n {
            let atom = &frame.atom_data[i];
            let pos = frame.positions.as_f64_row(i);
            assert_eq!(pos[0].to_bits(), atom.x.to_bits());
            assert_eq!(pos[1].to_bits(), atom.y.to_bits());
            assert_eq!(pos[2].to_bits(), atom.z.to_bits());
            let vel = atom.velocity().unwrap();
            let soa_v = frame.velocities.as_f64_row(i);
            assert_eq!(soa_v[0].to_bits(), vel[0].to_bits());
            assert_eq!(soa_v[1].to_bits(), vel[1].to_bits());
            assert_eq!(soa_v[2].to_bits(), vel[2].to_bits());
            let frc = atom.force().unwrap();
            let soa_f = frame.forces.as_f64_row(i);
            assert_eq!(soa_f[0].to_bits(), frc[0].to_bits());
            assert_eq!(soa_f[1].to_bits(), frc[1].to_bits());
            assert_eq!(soa_f[2].to_bits(), frc[2].to_bits());
            assert_eq!(
                frame.atom_energies.get_f64(i).to_bits(),
                atom.energy().unwrap().to_bits()
            );
            assert_eq!(frame.atom_ids[i], atom.atom_id);
        }
    }
}

/// Reads all frames from a file.
///
/// The bytes are read into one owned buffer and parsed on the calling
/// thread. A worker pool runs only when [`read_all_frames_with_threads`]
/// is asked for two or more threads.
pub fn read_all_frames(path: &Path) -> Result<Vec<types::ConFrame>, Box<dyn std::error::Error>> {
    read_all_frames_with_threads(path, None)
}

/// Collect every frame from `text`.
///
/// `None`, `Some(0)`, and `Some(1)` parse on the calling thread.
/// `Some(n)` with `n >= 2` uses a Rayon pool of that size when the
/// `parallel` feature is on. Builds without `parallel` always parse
/// sequentially.
pub fn frames_from_text(
    text: &str,
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, error::ParseError> {
    #[cfg(feature = "parallel")]
    {
        if let Some(n) = num_threads.filter(|n| *n >= 2) {
            let parts = parse_frames_parallel_with_threads(text, Some(n));
            let mut frames = Vec::with_capacity(parts.len());
            for r in parts {
                frames.push(r?);
            }
            return Ok(frames);
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
    }
    ConFrameIterator::new(text).collect()
}

/// Like [`read_all_frames`], with an explicit worker count.
///
/// `None`, `Some(0)`, and `Some(1)` parse on the calling thread.
/// `Some(n)` with `n >= 2` pins a Rayon pool when `parallel` is on.
///
/// A parse failure is the [`error::LocatedParseError`] from that parse.
/// Its line is the cursor that failed and its frame is the 0-based index.
/// The path is not opened again.
///
/// A plain file of 64 KiB or more stays mapped. Smaller files and
/// compressed input are read into one buffer.
pub fn read_all_frames_with_threads(
    path: &Path,
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, Box<dyn std::error::Error>> {
    let bytes = crate::compression::read_bytes_owned(path)?;
    Ok(frames_from_bytes_located(bytes.as_slice(), num_threads)?)
}

/// Positions of every frame, one row-major `f64` buffer.
#[derive(Debug)]
///
/// A parse failure is the [`error::LocatedParseError`] from the boundary
/// scan or the span parse that failed. The path is not opened again.
///
/// The boundary walk records each frame start. Each worker then parses
/// only its span and writes that frame's coordinates at
/// `frame * n_atoms * 3`. The returned buffer is the only retained
/// coordinate storage. A file whose frames do not share one atom count
/// is rejected, because the buffer is rectangular.
pub struct PositionBlock {
    pub n_frames: usize,
    pub n_atoms: usize,
    pub values: Vec<f64>,
}

/// Read every frame's coordinates through the column walker.
///
/// `num_threads` of `None`, `Some(0)`, and `Some(1)` stay on the calling
/// thread. `Some(n)` with `n >= 2` uses that many workers when `parallel`
/// is enabled. The buffer is rectangular: frames that do not share one
/// atom count are an error. Python `read_positions` returns this buffer.
pub fn read_positions_with_threads(
    path: &Path,
    num_threads: Option<usize>,
) -> Result<PositionBlock, Box<dyn std::error::Error>> {
    let contents = crate::compression::read_file_contents(path)?;
    let block = match &contents {
        crate::compression::FileContents::Owned(text) => {
            fill_position_block(text.as_bytes(), num_threads)
        }
        crate::compression::FileContents::Mapped(map) => fill_position_block(map, num_threads),
    }?;
    Ok(block)
}

/// Masses and cell that belong with [`read_positions_with_threads`].
///
/// The column walker fills these when it certifies the file. Otherwise they
/// come from the frame parser, which is the same source as before.
pub struct CoordColumns {
    pub block: PositionBlock,
    pub masses: Vec<f64>,
    pub cells: Vec<f64>,
}

/// Positions, masses, and cells for a stock CON or convel file.
///
/// A certified file is one coordinate pass. Anything else uses the frame
/// reader. Python `read_coords` returns these columns.
pub fn read_coord_columns(
    path: &Path,
    num_threads: Option<usize>,
) -> Result<CoordColumns, Box<dyn std::error::Error>> {
    let contents = crate::compression::read_file_contents(path)?;
    let bytes = contents.as_bytes();
    if let Ok(Some(columns)) = certified_columns(bytes, num_threads) {
        return Ok(columns);
    }
    let frames = frames_from_bytes_located(bytes, num_threads)?;
    Ok(columns_from_frames(&frames))
}

fn certified_columns(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Result<Option<CoordColumns>, error::LocatedParseError> {
    let parsed = match crate::columns::positions_from_con_bytes(bytes) {
        Ok(parsed) if parsed.stock => parsed,
        _ => return Ok(None),
    };
    if parsed.n_frames == 0 {
        return Ok(Some(CoordColumns {
            block: PositionBlock {
                n_frames: 0,
                n_atoms: 0,
                values: Vec::new(),
            },
            masses: Vec::new(),
            cells: Vec::new(),
        }));
    }
    let Some(n_atoms) = parsed.rectangular_atoms() else {
        let first = parsed.counts[0];
        let index = parsed
            .counts
            .iter()
            .position(|count| *count != first)
            .unwrap_or(0);
        let line = file_line_at_bytes(
            bytes,
            parsed.spans.get(index).map(|span| span.start).unwrap_or(0),
        );
        return Err(error::LocatedParseError::new(
            error::ParseError::InvalidCount(
                "frame atom count does not match the first frame".into(),
            ),
            line,
            index as u64,
        ));
    };
    let values = if parallel_column_threads(num_threads) {
        parallel_column_values(bytes, &parsed, n_atoms, num_threads)
            .map_err(|err| error::LocatedParseError::new(err, 0, 0))?
    } else {
        parsed.positions
    };
    Ok(Some(CoordColumns {
        block: PositionBlock {
            n_frames: parsed.n_frames,
            n_atoms,
            values,
        },
        masses: parsed.masses,
        cells: parsed.cells,
    }))
}

fn parallel_column_threads(num_threads: Option<usize>) -> bool {
    // `None`, `Some(0)`, and `Some(1)` stay on the calling thread. A pool
    // runs only for an explicit count of two or more.
    #[cfg(feature = "parallel")]
    {
        matches!(num_threads, Some(n) if n >= 2)
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
        false
    }
}

fn parallel_column_values(
    bytes: &[u8],
    parsed: &crate::columns::ConPositions,
    n_atoms: usize,
    num_threads: Option<usize>,
) -> Result<Vec<f64>, error::ParseError> {
    let stride = n_atoms * 3;
    if stride == 0 {
        return Ok(Vec::new());
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = (bytes, num_threads);
        Ok(parsed.positions.clone())
    }
    #[cfg(feature = "parallel")]
    {
        let mut values = vec![0.0_f64; parsed.n_frames * stride];
        let fill_one = |index: usize, dest: &mut [f64]| {
            let span = &parsed.spans[index];
            let slice = crate::columns::positions_from_con_bytes(&bytes[span.start..span.end])?;
            if slice.counts.first().copied() != Some(n_atoms) || slice.positions.len() != stride {
                return Err(error::ParseError::InvalidCount(
                    "frame atom count does not match the first frame".into(),
                ));
            }
            dest.copy_from_slice(&slice.positions);
            Ok(())
        };
        use rayon::prelude::*;
        let mut run = || match values
            .par_chunks_mut(stride)
            .enumerate()
            .find_map_first(|(index, dest)| fill_one(index, dest).err())
        {
            Some(err) => Err(err),
            None => Ok(()),
        };
        match num_threads {
            Some(n) if n >= 2 => {
                let pool = rayon::ThreadPoolBuilder::new()
                    .num_threads(n)
                    .build()
                    .map_err(|err| error::ParseError::ValidationError(err.to_string()))?;
                pool.install(run)?;
            }
            _ => run()?,
        }
        Ok(values)
    }
}

fn columns_from_frames(frames: &[types::ConFrame]) -> CoordColumns {
    let n_frames = frames.len();
    let n_atoms = frames
        .first()
        .map(|frame| frame.positions.nrows())
        .unwrap_or(0);
    let mut values = Vec::with_capacity(n_frames * n_atoms * 3);
    let mut masses = Vec::with_capacity(n_frames * n_atoms);
    let mut cells = Vec::with_capacity(n_frames * 6);
    for frame in frames {
        cells.extend_from_slice(&frame.header.boxl);
        cells.extend_from_slice(&frame.header.angles);
        if let Some(src) = frame.positions.f64_slice() {
            values.extend_from_slice(src);
        } else {
            for row in 0..frame.positions.nrows() {
                values.extend_from_slice(&frame.positions.as_f64_row(row));
            }
        }
        for (type_index, &count) in frame.header.natms_per_type.iter().enumerate() {
            let mass = frame
                .header
                .masses_per_type
                .get(type_index)
                .copied()
                .unwrap_or(0.0);
            masses.extend(std::iter::repeat_n(mass, count));
        }
    }
    CoordColumns {
        block: PositionBlock {
            n_frames,
            n_atoms,
            values,
        },
        masses,
        cells,
    }
}

/// 1-based file line of the line that contains `byte`.
fn file_line_at_bytes(bytes: &[u8], byte: usize) -> u32 {
    let end = byte.min(bytes.len());
    1 + memchr::memchr_iter(b'\n', &bytes[..end]).count() as u32
}

/// File line of a cursor that counted from the start of a share.
///
/// `local` is 1-based inside the share. A cursor that never moved still
/// names the share's first line.
fn share_cursor(local: u32, bytes: &[u8], share_start: usize) -> u32 {
    let base = file_line_at_bytes(bytes, share_start);
    if local == 0 {
        base
    } else {
        base.saturating_add(local.saturating_sub(1))
    }
}

fn frame_starts(bytes: &[u8]) -> Result<Vec<usize>, error::LocatedParseError> {
    let mut scanner = ConFrameIterator::from_bytes(bytes);
    let mut boundaries = Vec::new();
    loop {
        scanner.lines.clear_peek();
        let start = scanner.lines.pos;
        if start >= bytes.len() || bytes[start..].iter().all(u8::is_ascii_whitespace) {
            break;
        }
        boundaries.push(start);
        match scanner.forward_fast() {
            Some(Ok(())) => {}
            Some(Err(err)) => {
                return Err(error::LocatedParseError::new(
                    err,
                    scanner.lines.line_no(),
                    scanner.frame_index(),
                ));
            }
            None => break,
        }
    }
    Ok(boundaries)
}

/// Parse one span. The error keeps this frame's file line.
fn parse_span_located(
    bytes: &[u8],
    start: usize,
    end: usize,
    frame_index: u64,
) -> Result<types::ConFrame, error::LocatedParseError> {
    let mut iter = ConFrameIterator::from_bytes(&bytes[start..end]);
    match iter.next() {
        Some(Ok(frame)) => Ok(frame),
        Some(Err(err)) => Err(error::LocatedParseError::new(
            err,
            share_cursor(iter.lines.line_no(), bytes, start),
            frame_index,
        )),
        None => Err(error::LocatedParseError::new(
            error::ParseError::IncompleteFrame,
            file_line_at_bytes(bytes, start),
            frame_index,
        )),
    }
}

fn copy_positions(frame: &types::ConFrame, dest: &mut [f64]) -> Result<(), error::ParseError> {
    if frame.positions.nrows().saturating_mul(3) != dest.len() {
        return Err(error::ParseError::InvalidCount(
            "frame atom count does not match the first frame".into(),
        ));
    }
    if let Some(src) = frame.positions.f64_slice() {
        dest.copy_from_slice(src);
        return Ok(());
    }
    for (i, slot) in dest.as_chunks_mut::<3>().0.iter_mut().enumerate() {
        slot.copy_from_slice(&frame.positions.as_f64_row(i));
    }
    Ok(())
}

fn fill_position_block(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Result<PositionBlock, error::LocatedParseError> {
    match certified_columns(bytes, num_threads) {
        Ok(Some(columns)) => return Ok(columns.block),
        Ok(None) => {}
        Err(err) => return Err(err),
    }
    let boundaries = frame_starts(bytes)?;
    if boundaries.is_empty() {
        return Ok(PositionBlock {
            n_frames: 0,
            n_atoms: 0,
            values: Vec::new(),
        });
    }
    let end0 = boundaries.get(1).copied().unwrap_or(bytes.len());
    let first = parse_span_located(bytes, boundaries[0], end0, 0)?;
    let n_atoms = first.positions.nrows();
    let n_frames = boundaries.len();
    let stride = n_atoms.saturating_mul(3);
    let mut values = vec![0.0_f64; n_frames.saturating_mul(stride)];
    if stride == 0 {
        return Ok(PositionBlock {
            n_frames,
            n_atoms,
            values,
        });
    }
    fill_position_spans(bytes, &boundaries, n_atoms, num_threads, &mut values)?;
    Ok(PositionBlock {
        n_frames,
        n_atoms,
        values,
    })
}

fn fill_position_spans(
    bytes: &[u8],
    boundaries: &[usize],
    n_atoms: usize,
    num_threads: Option<usize>,
    values: &mut [f64],
) -> Result<(), error::LocatedParseError> {
    let stride = n_atoms * 3;
    let write_one = |i: usize, dest: &mut [f64]| {
        let start = boundaries[i];
        let end = boundaries.get(i + 1).copied().unwrap_or(bytes.len());
        let frame = parse_span_located(bytes, start, end, i as u64)?;
        copy_positions(&frame, dest).map_err(|error| {
            error::LocatedParseError::new(error, file_line_at_bytes(bytes, start), i as u64)
        })
    };
    #[cfg(feature = "parallel")]
    {
        let parallel = !matches!(num_threads, Some(n) if n <= 1);
        if parallel {
            use rayon::prelude::*;
            // The serial walk reports the earliest failing span. A pool
            // that stops on the first finished failure can hand back a
            // later span of the same file. `find_map_first` keeps the
            // earliest slot and still stops spans to its right.
            let mut run = || match values
                .par_chunks_mut(stride)
                .enumerate()
                .find_map_first(|(i, dest)| write_one(i, dest).err())
            {
                Some(err) => Err(err),
                None => Ok(()),
            };
            return match num_threads {
                Some(n) if n >= 2 => {
                    let pool = rayon::ThreadPoolBuilder::new()
                        .num_threads(n)
                        .build()
                        .map_err(|err| {
                            error::LocatedParseError::new(
                                error::ParseError::ValidationError(err.to_string()),
                                0,
                                0,
                            )
                        })?;
                    pool.install(run)
                }
                _ => run(),
            };
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
    }
    for (i, dest) in values.chunks_mut(stride).enumerate() {
        write_one(i, dest)?;
    }
    Ok(())
}

/// [`frames_from_text`] for bytes that may not be UTF-8 yet.
pub fn frames_from_bytes(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, error::ParseError> {
    #[cfg(feature = "parallel")]
    {
        if let Some(n) = num_threads.filter(|n| *n >= 2) {
            let parts = parse_frames_parallel_bytes(bytes, Some(n));
            let mut frames = Vec::with_capacity(parts.len());
            for r in parts {
                frames.push(r?);
            }
            return Ok(frames);
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
    }
    let mut frames = Vec::new();
    for r in ConFrameIterator::from_bytes(bytes) {
        frames.push(r?);
    }
    Ok(frames)
}

/// [`frames_from_bytes`] that keeps the line and frame of the first error.
fn frames_from_bytes_located(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, error::LocatedParseError> {
    #[cfg(feature = "parallel")]
    {
        if let Some(n) = num_threads.filter(|n| *n >= 2) {
            let timed = parse_bytes_timed(bytes, Some(n));
            return collect_located_slots(timed.slots, &timed.fail_lines);
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
    }
    collect_bytes_located(bytes)
}

/// Serial byte parse. The cursor is the iterator's file line.
fn collect_bytes_located(bytes: &[u8]) -> Result<Vec<types::ConFrame>, error::LocatedParseError> {
    let mut frames = Vec::new();
    let mut iter = ConFrameIterator::from_bytes(bytes);
    while let Some(item) = iter.next() {
        match item {
            Ok(frame) => frames.push(frame),
            Err(error) => {
                return Err(error::LocatedParseError::new(
                    error,
                    iter.lines.line_no(),
                    iter.frame_index(),
                ));
            }
        }
    }
    Ok(frames)
}

/// First failed slot, with the file line its worker stored.
#[cfg(feature = "parallel")]
fn collect_located_slots(
    slots: Vec<Result<types::ConFrame, error::ParseError>>,
    fail_lines: &[(usize, u32)],
) -> Result<Vec<types::ConFrame>, error::LocatedParseError> {
    let mut frames = Vec::with_capacity(slots.len());
    for (index, part) in slots.into_iter().enumerate() {
        match part {
            Ok(frame) => frames.push(frame),
            Err(error) => {
                let line = fail_lines
                    .iter()
                    .find(|(slot, _)| *slot == index)
                    .map(|(_, line)| *line)
                    .unwrap_or(0);
                return Err(error::LocatedParseError::new(error, line, index as u64));
            }
        }
    }
    Ok(frames)
}

#[cfg(feature = "parallel")]
fn span_end(boundaries: &[usize], index: usize, len: usize) -> usize {
    if index + 1 < boundaries.len() {
        boundaries[index + 1]
    } else {
        len
    }
}

/// Pointer to slot storage shared by the workers that claim chunks.
///
/// Each claimed index is written by one thread, so the alias is the
/// partition. The pointer stays valid until those workers join.
#[cfg(feature = "parallel")]
#[derive(Copy, Clone)]
struct SlotWritePtr(*mut std::mem::MaybeUninit<Result<types::ConFrame, error::ParseError>>);

#[cfg(feature = "parallel")]
impl SlotWritePtr {
    /// # Safety
    /// `index` is in range and this call is the only write of that slot.
    unsafe fn write_slot(self, index: usize, value: Result<types::ConFrame, error::ParseError>) {
        unsafe {
            (*self.0.add(index)).write(value);
        }
    }
}

// SAFETY: `SlotWritePtr` is copied into the claim loop, and that loop
// writes each slot index from exactly one worker. The allocation
// outlives the join.
#[cfg(feature = "parallel")]
unsafe impl Send for SlotWritePtr {}

// SAFETY: same partition as `Send`. Workers do not read a slot another
// worker is writing.
#[cfg(feature = "parallel")]
unsafe impl Sync for SlotWritePtr {}

/// Minor faults of the calling thread.
///
/// `RUSAGE_THREAD` is Linux. Other hosts report 0; the counter is
/// diagnostic and the parse result does not depend on it.
#[cfg(feature = "parallel")]
fn thread_minflt() -> i64 {
    #[cfg(target_os = "linux")]
    {
        unsafe {
            let mut usage = std::mem::zeroed::<libc::rusage>();
            libc::getrusage(libc::RUSAGE_THREAD, &mut usage);
            usage.ru_minflt
        }
    }
    #[cfg(not(target_os = "linux"))]
    {
        0
    }
}

/// Hardware LLC-miss counter for this process.
///
/// `-1` means the guest refused the event. This KVM guest returns
/// `EPERM` for `perf_event_open`, so the parse explanation uses the
/// per-worker minor faults instead.
#[cfg(feature = "parallel")]
fn sample_llc_misses() -> i64 {
    // `perf_event_open` is Linux. `-1` is the existing "counter refused" value.
    #[cfg(not(target_os = "linux"))]
    {
        return -1;
    }
    #[cfg(target_os = "linux")]
    {
        #[repr(C)]
        struct PerfEventAttr {
            typ: u32,
            size: u32,
            config: u64,
            rest: [u64; 14],
        }
        let attr = PerfEventAttr {
            typ: 3,
            size: std::mem::size_of::<PerfEventAttr>() as u32,
            // PERF_COUNT_HW_CACHE_LL, OP_READ, RESULT_MISS.
            config: 2 | (1 << 16),
            rest: [0; 14],
        };
        let fd = unsafe {
            libc::syscall(
                libc::SYS_perf_event_open,
                &attr as *const PerfEventAttr,
                0i32,
                -1i32,
                -1i32,
                0u64,
            )
        };
        if fd < 0 {
            return -1;
        }
        let fd = fd as libc::c_int;
        let mut value = 0u64;
        let n = unsafe { libc::read(fd, &mut value as *mut u64 as *mut libc::c_void, 8) };
        unsafe {
            libc::close(fd);
        }
        if n == 8 { value as i64 } else { -1 }
    }
}

/// Pages that do not sit on node 0.
///
/// A guest with one memory node has no remote pages. The count is 0
/// in that case. A wider node list sums the pages `numa_maps` records
/// on every node other than 0.
#[cfg(feature = "parallel")]
fn numa_remote_pages() -> i64 {
    let online = std::fs::read_to_string("/sys/devices/system/node/online").unwrap_or_default();
    let online = online.trim();
    let nodes = if let Some((start, end)) = online.split_once('-') {
        let start: i64 = start.parse().unwrap_or(0);
        let end: i64 = end.parse().unwrap_or(0);
        end - start + 1
    } else {
        1
    };
    if nodes <= 1 {
        return 0;
    }
    let text = std::fs::read_to_string("/proc/self/numa_maps").unwrap_or_default();
    let mut remote = 0i64;
    for line in text.lines() {
        for token in line.split_whitespace() {
            if let Some(rest) = token.strip_prefix('N')
                && let Some((node, pages)) = rest.split_once('=')
                && node != "0"
            {
                remote += pages.parse::<i64>().unwrap_or(0);
            }
        }
    }
    remote
}

/// Contiguous frame ranges that cover `boundaries` and split the file
/// into similar byte lengths.
///
/// There are up to four ranges per worker, and never more ranges than
/// frames. Each range holds at least one frame. The cuts stay on frame
/// starts, so a worker parses a whole frame.
#[cfg(feature = "parallel")]
fn byte_balanced_ranges(boundaries: &[usize], len: usize, workers: usize) -> Vec<(usize, usize)> {
    let n = boundaries.len();
    if n == 0 {
        return Vec::new();
    }
    let workers = workers.max(1).min(n);
    let chunks = workers.saturating_mul(4).clamp(workers, n);
    let origin = boundaries[0];
    let total = len.saturating_sub(origin).max(1) as u128;
    let mut ranges = Vec::with_capacity(chunks);
    let mut start = 0usize;
    for chunk in 0..chunks {
        if start >= n {
            break;
        }
        let end = if chunk + 1 == chunks {
            n
        } else {
            let target = origin + ((total * (chunk as u128 + 1)) / chunks as u128) as usize;
            let mut idx = if start + 1 < n {
                boundaries[start + 1..].partition_point(|&b| b < target) + start + 1
            } else {
                n
            };
            let min_end = start + 1;
            let reserved = chunks - chunk - 1;
            let max_end = n.saturating_sub(reserved);
            if max_end >= min_end {
                idx = idx.clamp(min_end, max_end);
            } else {
                idx = min_end.min(n);
            }
            idx
        };
        if end > start {
            ranges.push((start, end));
            start = end;
        }
    }
    if start < n {
        match ranges.last_mut() {
            Some(last) => last.1 = n,
            None => ranges.push((start, n)),
        }
    }
    ranges
}

/// Parse each frame into a slot allocated before any worker runs.
///
/// Workers claim chunk indexes from an atomic counter and write only
/// those slots. The returned vector is that slot storage, in boundary
/// order, which is the serial iterator order. `workers < 2` parses on
/// the calling thread.
#[cfg(feature = "parallel")]
fn parse_into_slots<F>(
    bytes: &[u8],
    len: usize,
    boundaries: &[usize],
    workers: usize,
    parse_span: F,
) -> Vec<Result<types::ConFrame, error::ParseError>>
where
    F: Fn(usize, usize) -> Result<types::ConFrame, error::ParseError> + Sync,
{
    parse_into_slots_timed(bytes, len, boundaries, workers, parse_span).slots
}

/// Slot allocation and the worker parse, timed apart.
#[cfg(feature = "parallel")]
struct SlotParse {
    slots: Vec<Result<types::ConFrame, error::ParseError>>,
    alloc_s: f64,
    work_s: f64,
    sum_s: f64,
    max_s: f64,
    /// One entry per worker range, in worker order. This is CPU time.
    chunk_s: Vec<f64>,
    /// Minor faults on each worker during its range.
    worker_minflt: Vec<i64>,
    /// Frames in each worker's share.
    share_frames: Vec<u64>,
    /// File bytes in each worker's share.
    share_bytes: Vec<u64>,
    /// `(slot, file line)` for each parse that failed. The line is the
    /// cursor of that parse, not a later reread.
    fail_lines: Vec<(usize, u32)>,
}

/// Slot allocation and the worker parse, timed apart.
///
/// `alloc_s` is the slot vector. `work_s` is the parse wall. `sum_s` is
/// the sum of per-chunk parse times and `max_s` is the slowest chunk.
/// `chunk_s` keeps every chunk. `workers < 2` still reserves the slot
/// vector before the calling thread fills it, and that one chunk's time
/// is the sum, the max, and the only entry.
#[cfg(feature = "parallel")]
fn parse_into_slots_timed<F>(
    bytes: &[u8],
    len: usize,
    boundaries: &[usize],
    workers: usize,
    parse_span: F,
) -> SlotParse
where
    F: Fn(usize, usize) -> Result<types::ConFrame, error::ParseError> + Sync,
{
    let n = boundaries.len();
    if n == 0 {
        return SlotParse {
            slots: Vec::new(),
            alloc_s: 0.0,
            work_s: 0.0,
            sum_s: 0.0,
            max_s: 0.0,
            chunk_s: Vec::new(),
            worker_minflt: Vec::new(),
            share_frames: Vec::new(),
            share_bytes: Vec::new(),
            fail_lines: Vec::new(),
        };
    }
    if workers < 2 {
        let t_alloc = std::time::Instant::now();
        let mut slots = Vec::with_capacity(n);
        let alloc_s = t_alloc.elapsed().as_secs_f64();
        let flt0 = thread_minflt();
        let t_work = std::time::Instant::now();
        for i in 0..n {
            slots.push(parse_span(boundaries[i], span_end(boundaries, i, len)));
        }
        let work_s = t_work.elapsed().as_secs_f64();
        let worker_minflt = vec![thread_minflt().saturating_sub(flt0)];
        return SlotParse {
            slots,
            alloc_s,
            work_s,
            sum_s: work_s,
            max_s: work_s,
            chunk_s: vec![work_s],
            worker_minflt,
            share_frames: vec![n as u64],
            share_bytes: vec![len as u64],
            fail_lines: Vec::new(),
        };
    }
    let t_alloc = std::time::Instant::now();
    let mut slots = take_slot_buf(n);
    let alloc_s = t_alloc.elapsed().as_secs_f64();
    // Several byte-balanced ranges per worker. A slow range can be
    // stolen. Each range is still one iterator, not one iterator per
    // frame: a fresh iterator per frame faulted a private heap.
    let ranges = byte_balanced_ranges(boundaries, len, workers);
    let chunk_ns: Vec<std::sync::atomic::AtomicU64> = (0..ranges.len())
        .map(|_| std::sync::atomic::AtomicU64::new(0))
        .collect();
    let chunk_flt: Vec<std::sync::atomic::AtomicI64> = (0..ranges.len())
        .map(|_| std::sync::atomic::AtomicI64::new(0))
        .collect();
    let slots_ptr = SlotWritePtr(slots.as_mut_ptr());
    let fail_lines = std::sync::Mutex::new(Vec::<(usize, u32)>::new());
    let t_work = std::time::Instant::now();
    {
        let parse_range = |range_index: usize, start: usize, end: usize| {
            if start >= end {
                return;
            }
            let flt0 = thread_minflt();
            let t_chunk = std::time::Instant::now();
            let byte_start = boundaries[start];
            let byte_end = span_end(boundaries, end - 1, len);
            let mut iter = ConFrameIterator::from_bytes(&bytes[byte_start..byte_end]);
            for i in start..end {
                let frame = match iter.next() {
                    Some(Ok(frame)) => Ok(frame),
                    Some(Err(error)) => {
                        let line = share_cursor(iter.lines.line_no(), bytes, byte_start);
                        fail_lines.lock().expect("fail lines").push((i, line));
                        Err(error)
                    }
                    None => {
                        let line = share_cursor(iter.lines.line_no(), bytes, byte_start);
                        fail_lines.lock().expect("fail lines").push((i, line));
                        Err(error::ParseError::IncompleteFrame)
                    }
                };
                // SAFETY: this range owns `start..end`, so `i` has one
                // writer. `slots` outlives the join.
                unsafe {
                    slots_ptr.write_slot(i, frame);
                }
            }
            chunk_ns[range_index].store(
                t_chunk.elapsed().as_nanos() as u64,
                std::sync::atomic::Ordering::Relaxed,
            );
            chunk_flt[range_index].store(
                thread_minflt().saturating_sub(flt0),
                std::sync::atomic::Ordering::Relaxed,
            );
        };
        let run_ranges = || {
            use rayon::prelude::*;
            ranges
                .par_iter()
                .enumerate()
                .for_each(|(range_index, &(start, end))| {
                    parse_range(range_index, start, end);
                });
        };
        if rayon::current_thread_index().is_some() {
            run_ranges();
        } else {
            match rayon::ThreadPoolBuilder::new().num_threads(workers).build() {
                Ok(pool) => pool.install(run_ranges),
                Err(_) => {
                    for (range_index, &(start, end)) in ranges.iter().enumerate() {
                        parse_range(range_index, start, end);
                    }
                }
            }
        }
    }
    let work_s = t_work.elapsed().as_secs_f64();
    let mut worker_sum_s = 0.0_f64;
    let mut worker_max_s = 0.0_f64;
    let mut chunk_s = Vec::new();
    let mut worker_minflt = Vec::new();
    let mut share_frames = Vec::new();
    let mut share_bytes = Vec::new();
    for (range_index, &(start, end)) in ranges.iter().enumerate() {
        if start >= end {
            continue;
        }
        let seconds =
            chunk_ns[range_index].load(std::sync::atomic::Ordering::Relaxed) as f64 / 1.0e9;
        chunk_s.push(seconds);
        worker_sum_s += seconds;
        worker_max_s = worker_max_s.max(seconds);
        worker_minflt.push(chunk_flt[range_index].load(std::sync::atomic::Ordering::Relaxed));
        share_frames.push((end - start) as u64);
        let nbytes = span_end(boundaries, end - 1, len).saturating_sub(boundaries[start]);
        share_bytes.push(nbytes as u64);
    }
    // Each index was written once. Reinterpreting the buffer avoids a
    // second serial walk of the frame shells after the workers join.
    let slots = unsafe {
        let mut raw = std::mem::ManuallyDrop::new(slots);
        Vec::from_raw_parts(
            raw.as_mut_ptr() as *mut Result<types::ConFrame, error::ParseError>,
            n,
            raw.capacity(),
        )
    };
    let fail_lines = fail_lines.into_inner().unwrap_or_default();
    SlotParse {
        slots,
        alloc_s,
        work_s,
        sum_s: worker_sum_s,
        max_s: worker_max_s,
        chunk_s,
        worker_minflt,
        share_frames,
        share_bytes,
        fail_lines,
    }
}

#[cfg(feature = "parallel")]
fn parse_byte_span(
    bytes: &[u8],
    start: usize,
    end: usize,
) -> Result<types::ConFrame, error::ParseError> {
    let mut iter = ConFrameIterator::from_bytes(&bytes[start..end]);
    match iter.next() {
        Some(result) => result,
        None => Err(error::ParseError::IncompleteFrame),
    }
}

#[cfg(feature = "parallel")]
fn parse_text_span(
    text: &str,
    start: usize,
    end: usize,
) -> Result<types::ConFrame, error::ParseError> {
    let mut iter = ConFrameIterator::new(&text[start..end]);
    match iter.next() {
        Some(result) => result,
        None => Err(error::ParseError::IncompleteFrame),
    }
}

#[cfg(feature = "parallel")]
struct TimedParse {
    slots: Vec<Result<types::ConFrame, error::ParseError>>,
    /// Still alive so later phases of this read use the same workers.
    /// `None` when the call stayed on the calling thread.
    pool: Option<std::sync::Arc<rayon::ThreadPool>>,
    pool_s: f64,
    /// Zero while `pool` is still alive. The owner times the join.
    pool_join_s: f64,
    scan_s: f64,
    scan_minflt: i64,
    slot_alloc_s: f64,
    worker_parse_s: f64,
    worker_sum_s: f64,
    worker_max_s: f64,
    chunk_s: Vec<f64>,
    worker_minflt: Vec<i64>,
    share_frames: Vec<u64>,
    share_bytes: Vec<u64>,
    /// `(slot, file line)` recorded by the worker that failed.
    fail_lines: Vec<(usize, u32)>,
    llc_misses: i64,
    numa_remote: i64,
    /// [`SCAN_SERIAL`], [`SCAN_INDEXED`], or [`SCAN_EQUAL_SPAN`].
    scan_kind: u8,
}

/// Keep freed parse pages in the heap.
///
/// Eight workers otherwise return those pages to the kernel, and the
/// next file faults them again. One worker already keeps a smaller set.
/// The same policy on every width leaves the parse fault count flat.
#[cfg(feature = "parallel")]
fn keep_parse_pages() {
    // glibc `M_MMAP_MAX` and `M_TRIM_THRESHOLD`. macOS and Windows have
    // no `mallopt`; leaving the pages to the system allocator is enough
    // for the parse to be correct.
    #[cfg(target_os = "linux")]
    unsafe {
        libc::mallopt(-4, 0);
        libc::mallopt(-1, i32::MAX);
    }
}

/// Slot storage kept between parses.
///
/// Eight workers write one `ConFrame` shell per frame into this buffer.
/// A fresh buffer is a new mapping, and those writes are the repeating
/// minor faults. The next parse reuses the same pages.
#[cfg(feature = "parallel")]
fn take_slot_buf(
    n: usize,
) -> Vec<std::mem::MaybeUninit<Result<types::ConFrame, error::ParseError>>> {
    let mut buf = slot_cache()
        .lock()
        .map(|mut slot| std::mem::take(&mut *slot))
        .unwrap_or_default();
    if buf.capacity() < n {
        buf = Vec::with_capacity(n);
    }
    unsafe { buf.set_len(n) };
    buf
}

#[cfg(feature = "parallel")]
fn slot_cache()
-> &'static std::sync::Mutex<Vec<std::mem::MaybeUninit<Result<types::ConFrame, error::ParseError>>>>
{
    static CACHE: std::sync::Mutex<
        Vec<std::mem::MaybeUninit<Result<types::ConFrame, error::ParseError>>>,
    > = std::sync::Mutex::new(Vec::new());
    &CACHE
}

/// One pool per worker count, kept for the life of the process.
///
/// A fresh pool on every file made each worker fault its frames again.
/// Reusing the workers lets the allocator hand back the pages just freed.
#[cfg(feature = "parallel")]
#[allow(clippy::type_complexity)]
fn shared_pool(workers: usize) -> Option<std::sync::Arc<rayon::ThreadPool>> {
    use std::sync::{Arc, Mutex, OnceLock};
    static CACHE: OnceLock<Mutex<Vec<(usize, Arc<rayon::ThreadPool>)>>> = OnceLock::new();
    let cache = CACHE.get_or_init(|| Mutex::new(Vec::new()));
    let mut guard = cache.lock().ok()?;
    if let Some((_, pool)) = guard.iter().find(|(count, _)| *count == workers) {
        return Some(Arc::clone(pool));
    }
    keep_parse_pages();
    let pool = Arc::new(
        rayon::ThreadPoolBuilder::new()
            .num_threads(workers)
            .build()
            .ok()?,
    );
    guard.push((workers, Arc::clone(&pool)));
    Some(pool)
}

/// Minor faults of this process.
///
/// `getrusage` is Unix. Windows reports 0.
#[cfg(feature = "parallel")]
pub fn process_minflt() -> i64 {
    #[cfg(unix)]
    {
        unsafe {
            let mut usage = std::mem::zeroed::<libc::rusage>();
            libc::getrusage(libc::RUSAGE_SELF, &mut usage);
            usage.ru_minflt
        }
    }
    #[cfg(not(unix))]
    {
        0
    }
}

/// Page size from `sysconf`, with the POSIX page as the fallback.
#[cfg(feature = "parallel")]
fn host_page_size() -> usize {
    #[cfg(unix)]
    {
        let raw = unsafe { libc::sysconf(libc::_SC_PAGESIZE) };
        if raw > 0 { raw as usize } else { 4096 }
    }
    #[cfg(not(unix))]
    {
        4096
    }
}

/// Fault one word on each page of `values`.
///
/// The page count is split by the worker count. The step is the host
/// page, so the slice a worker faults is its own. No extra chunk size.
#[cfg(feature = "parallel")]
fn first_touch_f64(values: &mut [f64], workers: usize) {
    use rayon::prelude::*;
    if values.is_empty() || workers < 2 {
        return;
    }
    let page = host_page_size().max(std::mem::size_of::<f64>());
    let step = (page / std::mem::size_of::<f64>()).max(1);
    let pages = values.len().div_ceil(step);
    let share = pages.div_ceil(workers);
    if rayon::current_thread_index().is_some() {
        // One chunk per page. The installed pool hands each page to one
        // worker, so that worker faults its own slice.
        values.par_chunks_mut(step).for_each(|chunk| {
            if let Some(slot) = chunk.first_mut() {
                *slot = 0.0;
            }
        });
        return;
    }
    std::thread::scope(|scope| {
        let mut rest = values;
        for worker in 0..workers {
            if rest.is_empty() {
                break;
            }
            let pages_here = share.min(rest.len().div_ceil(step));
            let take = (pages_here * step).min(rest.len());
            let (head, tail) = rest.split_at_mut(take);
            rest = tail;
            let _ = worker;
            scope.spawn(move || {
                let mut at = 0;
                while at < head.len() {
                    head[at] = 0.0;
                    at += step;
                }
            });
        }
    });
}

/// Move every `Ok` frame on the read's pool.
///
/// A serial walk of the slot vector grows with the frame count. The
/// pool that parsed the frames does this move. One thread still walks
/// the slots when that pool is absent.
#[cfg(feature = "parallel")]
fn finish_results(
    pool: Option<&rayon::ThreadPool>,
    slots: Vec<Result<types::ConFrame, error::ParseError>>,
) -> Result<Vec<types::ConFrame>, error::ParseError> {
    if slots.len() < 2 || pool.is_none() {
        let mut frames = Vec::with_capacity(slots.len());
        for part in slots {
            frames.push(part?);
        }
        return Ok(frames);
    }
    let pool = pool.expect("pool");
    pool.install(|| {
        use rayon::prelude::*;
        if let Some(index) = slots.par_iter().position_first(|item| item.is_err()) {
            return Err(match slots.into_iter().nth(index) {
                Some(Err(err)) => err,
                _ => error::ParseError::IncompleteFrame,
            });
        }
        Ok(slots
            .into_par_iter()
            .map(|item| item.expect("ok"))
            .collect())
    })
}

/// Header walk on the calling thread.
#[cfg(feature = "parallel")]
pub const SCAN_SERIAL: u8 = 0;
/// Newline index in chunks, then a header walk on the calling thread.
///
/// Frames that do not share one byte length take this path. The newline
/// bytes are split across the pool. The header walk is not, because a
/// later frame start is the end of the skip, and that end is not known
/// until the skip runs.
#[cfg(feature = "parallel")]
pub const SCAN_INDEXED: u8 = 1;
/// Equal-length frames. Each worker boundary is checked.
#[cfg(feature = "parallel")]
pub const SCAN_EQUAL_SPAN: u8 = 2;

/// Frame starts when every frame occupies the same number of bytes.
///
/// The first frame sets the span. Each worker boundary is skipped on
/// its own slice. A span that does not tile the
/// buffer, or a skip that does not land on the next span, returns
/// `None` and the caller keeps the serial walk. The starts are
/// `0, span, 2*span, ...` and match [`collect_frame_starts`].
#[cfg(feature = "parallel")]
#[cfg_attr(not(test), allow(dead_code))]
fn equal_span_boundaries(bytes: &[u8], workers: usize) -> Option<Vec<usize>> {
    if workers < 2 || bytes.len() < 2 {
        return None;
    }
    let mut first = ConFrameIterator::from_bytes(bytes);
    match first.forward_fast() {
        Some(Ok(())) => {}
        _ => return None,
    }
    let span = first.lines.pos;
    if span < 2 {
        return None;
    }
    let n = bytes.len() / span;
    if n < 2 {
        return None;
    }
    // Bytes after the last full span are a tail, not another frame.
    // The serial walk ignores a whitespace tail.
    let tiled = n * span;
    if bytes[tiled..].iter().any(|b| !b.is_ascii_whitespace()) {
        return None;
    }
    // Equal byte ranges. A cut that falls inside a frame resyncs to the
    // next frame start, and only that header is checked. The checks stay
    // on this worker: a nested pool wake was larger than the checks.
    let byte_share = tiled.div_ceil(workers);
    for worker in 0..workers {
        let byte_cut = worker.saturating_mul(byte_share).min(tiled);
        if byte_cut >= tiled {
            continue;
        }
        let index = if worker == 0 {
            0
        } else {
            byte_cut.div_ceil(span).min(n - 1)
        };
        let start = index.saturating_mul(span);
        if start >= tiled {
            continue;
        }
        let end = (start + span).min(tiled);
        let mut iter = ConFrameIterator::from_bytes(&bytes[start..end]);
        match iter.forward_fast() {
            Some(Ok(())) if iter.lines.pos == end - start => {}
            _ => return None,
        }
    }
    Some(arithmetic_starts(span, n))
}

/// Starts `0, span, 2*span, ...` from a buffer kept between files.
///
/// Building that buffer from a fresh allocation stalled for milliseconds
/// after a large free. The next file fills the same pages.
#[cfg(feature = "parallel")]
static START_CACHE: std::sync::Mutex<Vec<usize>> = std::sync::Mutex::new(Vec::new());

#[cfg(feature = "parallel")]
#[cfg_attr(not(test), allow(dead_code))]
fn arithmetic_starts(span: usize, n: usize) -> Vec<usize> {
    let mut starts = START_CACHE
        .lock()
        .map(|mut slot| std::mem::take(&mut *slot))
        .unwrap_or_default();
    if starts.len() != n {
        starts.resize(n, 0);
    }
    for (index, slot) in starts.iter_mut().enumerate() {
        *slot = index * span;
    }
    starts
}

#[cfg(feature = "parallel")]
fn recycle_starts(starts: Vec<usize>) {
    if let Ok(mut slot) = START_CACHE.lock()
        && starts.capacity() >= slot.capacity()
    {
        *slot = starts;
    }
}

#[cfg(feature = "parallel")]
fn parse_frames_parallel_bytes(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Vec<Result<types::ConFrame, error::ParseError>> {
    parse_bytes_timed(bytes, num_threads).slots
}

/// Scan, slot allocation, and worker parse of `bytes`.
///
/// `Some(n)` with `n >= 2` uses a pool of that size. Every other count
/// stays on the calling thread. `None` follows [`scan_thread_count`].
/// The pool build is `pool_s`. The pool join is `pool_join_s`.
/// Neither one is inside the scan or the parse.
#[cfg(feature = "parallel")]
fn parse_bytes_timed(bytes: &[u8], num_threads: Option<usize>) -> TimedParse {
    let parse_workers = match num_threads {
        Some(n) if n >= 2 => n,
        _ => 1,
    };
    let run = || {
        let fault0 = process_minflt();
        let t_scan = std::time::Instant::now();
        // Equal-length frames skip the serial header walk. Anything else
        // still walks headers on this thread. Workers then parse the
        // spans in file order.
        let (boundaries, scan_kind) =
            if let Some(bounds) = equal_span_boundaries(bytes, parse_workers) {
                (bounds, SCAN_EQUAL_SPAN)
            } else {
                (
                    collect_frame_starts(ConFrameIterator::from_bytes(bytes), true),
                    SCAN_SERIAL,
                )
            };
        let scan_s = t_scan.elapsed().as_secs_f64();
        let scan_minflt = process_minflt().saturating_sub(fault0);
        let parsed = parse_into_slots_timed(
            bytes,
            bytes.len(),
            &boundaries,
            parse_workers,
            |start, end| parse_byte_span(bytes, start, end),
        );
        recycle_starts(boundaries);
        (parsed, scan_s, scan_kind, scan_minflt)
    };
    let (pool_s, pool, packed) = match num_threads {
        None => (0.0, None, run()),
        Some(n) if n <= 1 => {
            let _ = n;
            keep_parse_pages();
            let fault0 = process_minflt();
            let t_scan = std::time::Instant::now();
            let boundaries = collect_frame_starts(ConFrameIterator::from_bytes(bytes), true);
            let scan_s = t_scan.elapsed().as_secs_f64();
            let scan_minflt = process_minflt().saturating_sub(fault0);
            let parsed =
                parse_into_slots_timed(bytes, bytes.len(), &boundaries, 1, |start, end| {
                    parse_byte_span(bytes, start, end)
                });
            (0.0, None, (parsed, scan_s, SCAN_SERIAL, scan_minflt))
        }
        Some(n) => {
            let t_pool = std::time::Instant::now();
            match shared_pool(n) {
                Some(pool) => {
                    let built = t_pool.elapsed().as_secs_f64();
                    let t_wake = std::time::Instant::now();
                    let (wake, body) = pool.install(|| {
                        let wake = t_wake.elapsed().as_secs_f64();
                        (wake, run())
                    });
                    // The same workers serve later files of this width.
                    // Their freed blocks stay warm for the next parse.
                    (built + wake, Some(pool), body)
                }
                None => (t_pool.elapsed().as_secs_f64(), None, run()),
            }
        }
    };
    let (parsed, scan_s, scan_kind, scan_minflt) = packed;
    TimedParse {
        slots: parsed.slots,
        pool,
        pool_s,
        pool_join_s: 0.0,
        scan_s,
        scan_minflt,
        slot_alloc_s: parsed.alloc_s,
        worker_parse_s: parsed.work_s,
        worker_sum_s: parsed.sum_s,
        worker_max_s: parsed.max_s,
        chunk_s: parsed.chunk_s,
        worker_minflt: parsed.worker_minflt,
        share_frames: parsed.share_frames,
        share_bytes: parsed.share_bytes,
        fail_lines: parsed.fail_lines,
        llc_misses: sample_llc_misses(),
        numa_remote: numa_remote_pages(),
        scan_kind,
    }
}

/// Count frames without building atom payloads (uses [`ConFrameIterator::forward_fast`]
/// when possible, else [`ConFrameIterator::forward`]).
///
/// Prefer this over `read_all_frames(...).len()` when only the frame count is needed.
/// A rejected frame is [`LoadError::Parse`] with that frame's file line and
/// 0-based index, including when the failure is after the first frame.
pub fn count_frames(path: &Path) -> Result<usize, LoadError> {
    let contents = load_text(path)?;
    let text = contents.as_str().map_err(LoadError::Utf8)?;
    let mut n = 0usize;
    let mut iter = ConFrameIterator::new(text);
    loop {
        match iter.forward_fast() {
            Some(Ok(())) => n += 1,
            Some(Err(e)) => {
                return Err(LoadError::Parse(error::LocatedParseError::new(
                    e,
                    iter.lines.line_no(),
                    iter.frame_index(),
                )));
            }
            None => break,
        }
    }
    Ok(n)
}

/// Reads only the first frame from a file.
///
/// More efficient than `read_all_frames` for single-frame access because it
/// stops parsing after the first frame rather than collecting all of them.
pub fn read_first_frame(path: &Path) -> Result<types::ConFrame, Box<dyn std::error::Error>> {
    let contents = crate::compression::read_file_contents(path)?;
    let text = contents.as_str()?;
    let mut iter = ConFrameIterator::new(text);
    match iter.next() {
        Some(Ok(frame)) => Ok(frame),
        Some(Err(e)) => Err(Box::new(e)),
        None => Err("No frames found in file".into()),
    }
}

/// Skip `index` frames (no atom parse), then parse the next one.
///
/// `index == 0` is [`read_first_frame`]. Out of range is an error.
pub fn read_nth_frame(
    path: &Path,
    index: usize,
) -> Result<types::ConFrame, Box<dyn std::error::Error>> {
    let contents = crate::compression::read_file_contents(path)?;
    let text = contents.as_str()?;
    read_nth_frame_from_text(text, index)
}

/// Same as [`read_nth_frame`] on an already-loaded buffer.
pub fn read_nth_frame_from_text(
    text: &str,
    index: usize,
) -> Result<types::ConFrame, Box<dyn std::error::Error>> {
    let mut iter = ConFrameIterator::new(text);
    let skipped = iter.skip_frames(index)?;
    if skipped < index {
        return Err(format!("frame {index} out of range (only {skipped} frames)").into());
    }
    match iter.next() {
        Some(Ok(frame)) => Ok(frame),
        Some(Err(e)) => Err(Box::new(e)),
        None => Err(format!("frame {index} out of range").into()),
    }
}

/// Reads all frames using the checked `.rcidx` sidecar next to `path`.
///
/// Uncompressed input uses (and refreshes) `path.rcidx`; compressed
/// input is scanned in memory each time. UTF-8 is validated in
/// newline-aligned slices.
#[cfg(feature = "parallel")]
pub fn read_all_frames_indexed(
    path: &Path,
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, Box<dyn std::error::Error>> {
    let contents = crate::compression::read_file_contents(path)?;
    let text = contents.as_str_sliced()?;
    let idx = match contents {
        crate::compression::FileContents::Mapped(_) => {
            crate::frame_index::load_or_build(path, text, true)?
        }
        crate::compression::FileContents::Owned(_) => {
            let on_disk = std::fs::metadata(path)?.len();
            crate::frame_index::load_or_build(path, text, on_disk == text.len() as u64)?
        }
    };
    let offs: Vec<usize> = idx.offsets.iter().map(|&o| o as usize).collect();
    parse_frames_at_offsets(text, &offs, num_threads)
        .into_iter()
        .map(|r| r.map_err(|e| Box::new(e) as Box<dyn std::error::Error>))
        .collect()
}

enum IndexStep {
    Stay(usize),
    Missing(usize),
    Landed(usize, usize),
    Short(usize, usize),
}

struct IndexedSpan {
    start: usize,
    content_end: usize,
    next_pos: usize,
}

/// Lines in `bytes`, counting a final line that has no `\n`.
fn line_count(nl: &[usize], len: usize) -> usize {
    if len == 0 {
        0
    } else if nl.last().copied() == Some(len - 1) {
        nl.len()
    } else {
        nl.len() + 1
    }
}

fn line_start_pos(nl: &[usize], len: usize, line_no: usize) -> usize {
    if line_no == 0 {
        return 0;
    }
    if line_no >= line_count(nl, len) {
        return len;
    }
    nl[line_no - 1] + 1
}

/// Line index of the line that contains `pos`, or the line count at `len`.
fn line_no_for_pos(nl: &[usize], len: usize, pos: usize) -> usize {
    if pos >= len {
        return line_count(nl, len);
    }
    nl.partition_point(|&p| p < pos)
}

fn indexed_span(nl: &[usize], bytes: &[u8], line_no: usize) -> Option<IndexedSpan> {
    let len = bytes.len();
    if line_no >= line_count(nl, len) {
        return None;
    }
    let start = if line_no == 0 { 0 } else { nl[line_no - 1] + 1 };
    let (raw_end, next_pos) = if line_no < nl.len() {
        (nl[line_no], nl[line_no] + 1)
    } else {
        (len, len)
    };
    let mut content_end = raw_end;
    if content_end > start && bytes[content_end - 1] == b'\r' {
        content_end -= 1;
    }
    Some(IndexedSpan {
        start,
        content_end,
        next_pos,
    })
}

/// `n` cuts covering `0..=len`. A cut may fall in the middle of a header line.
#[cfg(feature = "parallel")]
fn equal_cuts(len: usize, n: usize) -> Vec<usize> {
    let n = n.max(1);
    let mut cuts = Vec::with_capacity(n + 1);
    for i in 0..=n {
        let at = ((i as u128) * (len as u128) / (n as u128)) as usize;
        let at = at.min(len);
        if cuts.last().copied() != Some(at) {
            cuts.push(at);
        }
    }
    if cuts.last().copied() != Some(len) {
        cuts.push(len);
    }
    cuts
}

/// Newline offsets in `bytes`.
///
/// `cuts` splits the buffer into `[cuts[i], cuts[i + 1])`. Each piece is
/// scanned on its own and the offsets are concatenated. A `\n` belongs to
/// the piece that contains that byte, so a header line whose body crosses
/// a cut still contributes one newline.
#[cfg(feature = "parallel")]
fn newline_positions_at_cuts(bytes: &[u8], cuts: &[usize]) -> Vec<usize> {
    let spans: Vec<(usize, usize)> = cuts
        .windows(2)
        .map(|w| (w[0].min(bytes.len()), w[1].min(bytes.len())))
        .filter(|(start, end)| start < end)
        .collect();
    if spans.len() <= 1 {
        let (start, end) = spans.first().copied().unwrap_or((0, 0));
        return memchr::memchr_iter(b'\n', &bytes[start..end])
            .map(|p| start + p)
            .collect();
    }
    // The current Rayon pool, or the global pool when the caller has not
    // installed one. Chunk count is the worker cap: two chunks do not
    // wake every core.
    use rayon::prelude::*;
    let parts: Vec<Vec<usize>> = spans
        .par_iter()
        .map(|&(start, end)| {
            memchr::memchr_iter(b'\n', &bytes[start..end])
                .map(|p| start + p)
                .collect()
        })
        .collect();
    let mut all = Vec::with_capacity(bytes.len() / 32);
    for part in parts {
        all.extend(part);
    }
    all
}

#[cfg(feature = "parallel")]
fn newline_positions(bytes: &[u8], threads: usize) -> Vec<usize> {
    newline_positions_at_cuts(bytes, &equal_cuts(bytes.len(), threads))
}

/// Worker count for a newline scan.
///
/// `Some(n)` with `n >= 2` uses that many workers at every file length.
/// A file shorter than two bytes has nothing to split, and [`equal_cuts`]
/// drops a cut that repeats the previous byte, so a short file does not
/// invent an empty chunk. `None`, `Some(0)`, and `Some(1)` stay on the
/// calling thread. An omitted count must not start the global pool:
/// that pool made a one-frame file slower. There is no byte-length
/// cutoff taken from one host.
#[cfg(feature = "parallel")]
fn scan_thread_count(num_threads: Option<usize>, len: usize) -> usize {
    let requested = match num_threads {
        Some(n) if n >= 2 => n,
        _ => return 1,
    };
    if len < 2 { 1 } else { requested }
}

/// I/O, UTF-8, or a located parse failure while loading a path.
#[derive(Debug)]
pub enum LoadError {
    Io(std::io::Error),
    Utf8(std::str::Utf8Error),
    Parse(error::LocatedParseError),
    /// The path opened and decoded, and it held no frame.
    Empty,
}

impl std::fmt::Display for LoadError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            LoadError::Io(e) => write!(f, "{e}"),
            LoadError::Utf8(e) => write!(f, "{e}"),
            LoadError::Parse(e) => write!(f, "{e}"),
            LoadError::Empty => write!(f, "no frames found in file"),
        }
    }
}

impl std::error::Error for LoadError {}

fn load_error_from_box(err: Box<dyn std::error::Error>) -> LoadError {
    if let Some(io) = err.downcast_ref::<std::io::Error>() {
        return LoadError::Io(std::io::Error::new(io.kind(), io.to_string()));
    }
    LoadError::Io(std::io::Error::other(err.to_string()))
}

fn load_text(path: &Path) -> Result<crate::compression::FileContents, LoadError> {
    crate::compression::read_file_contents(path).map_err(load_error_from_box)
}

/// 1-based line number of the line that contains `byte`.
#[cfg(feature = "parallel")]
fn file_line_at(text: &str, byte: usize) -> u32 {
    let end = byte.min(text.len());
    1 + text.as_bytes()[..end]
        .iter()
        .filter(|&&b| b == b'\n')
        .count() as u32
}

/// Collect every frame, keeping the line and frame index of the first error.
pub fn frames_from_text_located(
    text: &str,
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, error::LocatedParseError> {
    #[cfg(feature = "parallel")]
    {
        // A caller that does not ask for a pool stays on this thread.
        // Starting the global pool for a long file made a one-frame read slower.
        let use_parallel = matches!(num_threads, Some(n) if n >= 2);
        if use_parallel {
            let pool = num_threads.filter(|&n| n >= 2);
            return collect_parallel_located(text, pool);
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
    }
    let mut frames = Vec::new();
    let mut iter = ConFrameIterator::new(text);
    let mut index = 0u64;
    while let Some(item) = iter.next() {
        match item {
            Ok(frame) => {
                frames.push(frame);
                index += 1;
            }
            Err(e) => {
                return Err(error::LocatedParseError::new(
                    e,
                    iter.lines.line_no(),
                    index,
                ));
            }
        }
    }
    Ok(frames)
}

/// Read every frame from `path`, with the line and frame index of the first error.
pub fn read_all_frames_located(
    path: &Path,
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, LoadError> {
    let contents = load_text(path)?;
    let text = contents.as_str().map_err(LoadError::Utf8)?;
    frames_from_text_located(text, num_threads).map_err(LoadError::Parse)
}

/// Read the first frame from `path`, with a line number on failure.
pub fn read_first_frame_located(path: &Path) -> Result<types::ConFrame, LoadError> {
    let contents = load_text(path)?;
    let text = contents.as_str().map_err(LoadError::Utf8)?;
    let mut iter = ConFrameIterator::new(text);
    match iter.next() {
        Some(Ok(frame)) => Ok(frame),
        Some(Err(e)) => Err(LoadError::Parse(error::LocatedParseError::new(
            e,
            iter.lines.line_no(),
            0,
        ))),
        None => Err(LoadError::Empty),
    }
}

/// Skip `index` frames, then parse one, with a line number on failure.
pub fn read_nth_frame_located(path: &Path, index: usize) -> Result<types::ConFrame, LoadError> {
    let contents = load_text(path)?;
    let text = contents.as_str().map_err(LoadError::Utf8)?;
    let mut iter = ConFrameIterator::new(text);
    match iter.skip_frames_counted(index) {
        Ok(skipped) if skipped == index => {}
        Ok(skipped) => {
            return Err(LoadError::Parse(error::LocatedParseError::new(
                error::ParseError::IncompleteFrame,
                iter.lines.line_no(),
                skipped as u64,
            )));
        }
        Err((failed_at, e)) => {
            return Err(LoadError::Parse(error::LocatedParseError::new(
                e,
                iter.lines.line_no(),
                failed_at as u64,
            )));
        }
    }
    match iter.next() {
        Some(Ok(frame)) => Ok(frame),
        Some(Err(e)) => Err(LoadError::Parse(error::LocatedParseError::new(
            e,
            iter.lines.line_no(),
            index as u64,
        ))),
        None => Err(LoadError::Parse(error::LocatedParseError::new(
            error::ParseError::IncompleteFrame,
            iter.lines.line_no(),
            index as u64,
        ))),
    }
}

fn collect_frame_starts(scanner: ConFrameIterator<'_>, skip_whitespace_tail: bool) -> Vec<usize> {
    #[cfg(target_arch = "x86_64")]
    {
        if std::is_x86_feature_detected!("avx2") {
            // One AVX2 entry so the line skip stays in this function.
            // A separate call per block pays for an AVX/SSE transition.
            // SAFETY: the runtime check above is the AVX2 and POPCNT contract.
            return unsafe { collect_frame_starts_avx2(scanner, skip_whitespace_tail) };
        }
    }
    collect_frame_starts_inner(scanner, skip_whitespace_tail)
}

#[cfg(target_arch = "x86_64")]
#[target_feature(enable = "avx2,popcnt")]
unsafe fn collect_frame_starts_avx2(
    scanner: ConFrameIterator<'_>,
    skip_whitespace_tail: bool,
) -> Vec<usize> {
    collect_frame_starts_inner(scanner, skip_whitespace_tail)
}

#[inline(always)]
fn collect_frame_starts_inner(
    mut scanner: ConFrameIterator<'_>,
    skip_whitespace_tail: bool,
) -> Vec<usize> {
    let mut boundaries = Vec::new();
    loop {
        scanner.lines.clear_peek();
        let start = scanner.lines.pos;
        if start >= scanner.lines.bytes.len() {
            break;
        }
        if skip_whitespace_tail {
            let rest = &scanner.lines.bytes[start..];
            if rest.iter().all(u8::is_ascii_whitespace) {
                break;
            }
        }
        boundaries.push(start);
        match scanner.forward_fast_to(None) {
            Some(Ok(())) => {}
            Some(Err(_)) | None => break,
        }
    }
    boundaries
}

/// Serial header walk. This is the reference the chunked scan must match.
pub fn frame_start_offsets_serial(file_contents: &str) -> Vec<usize> {
    collect_frame_starts(ConFrameIterator::new(file_contents), false)
}

#[cfg(feature = "parallel")]
fn scanner_with_newlines<'a>(text: &'a str, newlines: Vec<usize>) -> ConFrameIterator<'a> {
    let mut scanner = ConFrameIterator::new(text);
    scanner.lines.index_line = 0;
    scanner.lines.newlines = Some(newlines);
    scanner
}

#[cfg(feature = "parallel")]
fn frame_start_offsets_indexed(text: &str, newlines: Vec<usize>) -> Vec<usize> {
    collect_frame_starts(scanner_with_newlines(text, newlines), false)
}

/// Frame starts for `text` when the newline scan is split at `cuts`.
///
/// `cuts` is the chunk edges, including 0 and `text.len()`. A cut inside a
/// header line is the straddling case: the stitched offsets are still the
/// serial offsets.
#[cfg(all(feature = "parallel", test))]
fn frame_start_offsets_at_cuts(text: &str, cuts: &[usize]) -> Vec<usize> {
    let newlines = newline_positions_at_cuts(text.as_bytes(), cuts);
    frame_start_offsets_indexed(text, newlines)
}

/// Frame start byte offsets via [`ConFrameIterator::forward_fast`] (no atom parse).
///
/// Stops at the first skip error (that start is still included).
///
/// With the `parallel` feature and more than one worker, the buffer is
/// split into chunks and the newline bytes are scanned together. The
/// length of the buffer does not choose the worker count. The chunks
/// are not aligned to lines. A header
/// that straddles a cut is one line, because each `\n` is recorded by the
/// chunk that holds that byte and the lists are stitched in order. The
/// returned offsets match the serial walk.
pub fn frame_start_offsets(file_contents: &str) -> Vec<usize> {
    frame_start_offsets_with_threads(file_contents, None)
}

/// [`frame_start_offsets`] using `num_threads` chunks.
///
/// `None` uses the machine's parallelism. `Some(0)` and `Some(1)` stay on
/// the calling thread. The offsets do not depend on the thread count.
///
/// The timed work includes the cut setup, the per-chunk scans, the
/// stitch, and the frame walk that reads the stitched newline index.
pub fn frame_start_offsets_with_threads(
    file_contents: &str,
    num_threads: Option<usize>,
) -> Vec<usize> {
    #[cfg(feature = "parallel")]
    {
        let threads = scan_thread_count(num_threads, file_contents.len());
        if threads >= 2 {
            let newlines = newline_positions(file_contents.as_bytes(), threads);
            return frame_start_offsets_indexed(file_contents, newlines);
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
    }
    frame_start_offsets_serial(file_contents)
}

/// Parses frames in parallel using rayon, splitting on frame boundaries.
///
/// Phase 1: frame starts from a serial [`ConFrameIterator::forward_fast`]
/// walk. The header names each block's line count, so the walk does not
/// parse atom rows. Phase 2 parses those spans on a Rayon pool and
/// stores the frames in file order. See [`parse_frames_parallel_with_threads`].
///
/// Requires the `parallel` feature.
#[cfg(feature = "parallel")]
pub fn parse_frames_parallel(
    file_contents: &str,
) -> Vec<Result<types::ConFrame, error::ParseError>> {
    parse_frames_parallel_with_threads(file_contents, None)
}

/// Like [`parse_frames_parallel`], but `Some(n)` with `n >= 2` parses on
/// a Rayon pool of that size. `None`, `Some(0)`, and `Some(1)` stay on
/// the calling thread.
///
/// The boundary walk is serial. The parse ranges are byte-balanced and
/// the frames come back in file order, field for field with a serial
/// iterator.
///
/// Requires the `parallel` feature.
#[cfg(feature = "parallel")]
pub fn parse_frames_parallel_with_threads(
    file_contents: &str,
    num_threads: Option<usize>,
) -> Vec<Result<types::ConFrame, error::ParseError>> {
    let parse_workers = match num_threads {
        Some(n) if n >= 2 => n,
        _ => 1,
    };
    let run = || {
        // A whitespace tail is not a frame. The serial iterator stops
        // there. The offset scan used by callers still records that byte
        // when a skip error includes it.
        let boundaries = collect_frame_starts(ConFrameIterator::new(file_contents), true);
        parse_into_slots(
            file_contents.as_bytes(),
            file_contents.len(),
            &boundaries,
            parse_workers,
            |start, end| parse_text_span(file_contents, start, end),
        )
    };
    match num_threads {
        Some(n) if n >= 2 => match rayon::ThreadPoolBuilder::new().num_threads(n).build() {
            Ok(pool) => pool.install(run),
            Err(_) => run(),
        },
        _ => run(),
    }
}

/// Phase 2 of [`parse_frames_parallel_with_threads`]: parse the frames
/// that start at `boundaries` (strictly increasing byte offsets into
/// `file_contents`; frame `i` ends where frame `i + 1` starts).
///
/// Offsets may come from [`frame_start_offsets`] or from a checked
/// [`crate::frame_index::FrameIndex`]. The output slots are allocated
/// first. Each worker then fills one contiguous index range. Results
/// stay in frame order.
///
/// The timer for this function starts after `boundaries` exist. It
/// includes slot allocation and the worker parse. It does not include
/// the newline scan or the stitch.
///
/// Requires the `parallel` feature.
#[cfg(feature = "parallel")]
pub fn parse_frames_at_offsets(
    file_contents: &str,
    boundaries: &[usize],
    num_threads: Option<usize>,
) -> Vec<Result<types::ConFrame, error::ParseError>> {
    let workers = match num_threads {
        Some(n) if n <= 1 => 1,
        Some(n) => n,
        None => rayon::current_num_threads().max(1),
    };
    let run = || {
        parse_into_slots(
            file_contents.as_bytes(),
            file_contents.len(),
            boundaries,
            workers,
            |start, end| parse_text_span(file_contents, start, end),
        )
    };
    match num_threads {
        Some(n) if n >= 2 => match rayon::ThreadPoolBuilder::new().num_threads(n).build() {
            Ok(pool) => pool.install(run),
            Err(_) => run(),
        },
        _ => run(),
    }
}

/// Seconds for one phased read. The open time is the file read or the
/// map. Scan, slot allocation, and the worker parse are separate.
#[derive(Clone, Copy, Debug, Default)]
pub struct ReadPhaseTimes {
    pub open_mmap_s: f64,
    /// Rayon pool construction. Zero when the call stays on one thread.
    pub pool_s: f64,
    /// Joining the local pool after the scan and the parse. Zero when
    /// the call stays on one thread.
    pub pool_join_s: f64,
    pub scan_s: f64,
    pub slot_alloc_s: f64,
    pub worker_parse_s: f64,
    /// Moving each parsed `Result` into the frame vector.
    pub finish_s: f64,
    /// Sum of per-chunk parse times. This is CPU time, not wall time.
    pub worker_sum_s: f64,
    /// Slowest parse chunk. This is inside `worker_parse_s`.
    pub worker_max_s: f64,
    /// Drop of the mapped or owned file buffer after the parse.
    pub close_s: f64,
    /// Wake of the worker pool and the other gaps inside the read.
    pub schedule_s: f64,
    /// Minor faults during the scan.
    pub scan_minflt: i64,
    /// Sum of per-worker minor faults during the parse.
    pub parse_minflt: i64,
    /// LLC read misses, or `-1` when the guest refuses the counter.
    pub llc_misses: i64,
    /// Pages on a node other than node 0. Zero on a one-node guest.
    pub numa_remote: i64,
    /// Minor faults while the coordinate slab is filled.
    pub slab_minflt: i64,
    /// Minor faults during the touch checksum.
    pub touch_minflt: i64,
    /// True when the bytes came from a map rather than an owned buffer.
    pub mapped: bool,
}

/// Frames from one phased read, plus the byte length of the buffer.
pub struct PhasedFrames {
    pub frames: Vec<types::ConFrame>,
    pub phases: ReadPhaseTimes,
    pub n_bytes: usize,
    /// Per-chunk parse seconds, in claim order.
    pub chunk_s: Vec<f64>,
    /// [`SCAN_SERIAL`], [`SCAN_INDEXED`], or [`SCAN_EQUAL_SPAN`] when
    /// `parallel` is on. Zero otherwise.
    pub scan_kind: u8,
    /// Minor faults on each parse worker.
    pub worker_minflt: Vec<i64>,
    /// Frames in each worker share.
    pub share_frames: Vec<u64>,
    /// File bytes in each worker share.
    pub share_bytes: Vec<u64>,
    /// Pool kept for the slab, the touch, and frame teardown.
    #[cfg(feature = "parallel")]
    pub pool: Option<std::sync::Arc<rayon::ThreadPool>>,
}

/// Read `path` and time open, scan, slot allocation, and the worker parse.
///
/// Plain text at or above the map threshold is mapped. Smaller files and
/// compressed files are owned buffers. `Some(n)` with `n >= 2` parses on
/// that many workers. Every other count parses on the calling thread.
/// The frame list matches [`frames_from_bytes`].
///
/// A parse failure is the [`error::LocatedParseError`] from the parse
/// that failed. The path is not opened again.
pub fn read_all_frames_phased(
    path: &Path,
    num_threads: Option<usize>,
) -> Result<PhasedFrames, Box<dyn std::error::Error>> {
    let t_all = std::time::Instant::now();
    let t_open = std::time::Instant::now();
    let contents = crate::compression::read_file_contents(path)?;
    let open_mmap_s = t_open.elapsed().as_secs_f64();
    let mapped = matches!(contents, crate::compression::FileContents::Mapped(_));
    let n_bytes = match &contents {
        crate::compression::FileContents::Owned(text) => text.len(),
        crate::compression::FileContents::Mapped(map) => map.len(),
    };
    let bytes: &[u8] = match &contents {
        crate::compression::FileContents::Owned(text) => text.as_bytes(),
        crate::compression::FileContents::Mapped(map) => map,
    };
    let phased = frames_from_bytes_phased_located(bytes, num_threads)?;
    let workers = match num_threads {
        Some(n) if n >= 2 => n,
        _ => 1,
    };
    let t_close = std::time::Instant::now();
    if let crate::compression::FileContents::Mapped(map) = &contents {
        // The serial unmap of a fully resident file is the multi-worker
        // tail. Advise the pages away on the read's pool first.
        // One worker unmaps directly, so a serial read does not walk
        // the pages twice.
        if workers >= 2 {
            #[cfg(feature = "parallel")]
            {
                if let Some(pool) = phased.pool.as_ref() {
                    pool.install(|| {
                        crate::compression::advise_dontneed_parallel(map, workers);
                    });
                }
            }
            #[cfg(not(feature = "parallel"))]
            {
                let _ = (map, workers);
            }
        }
    }
    drop(contents);
    let close_s = t_close.elapsed().as_secs_f64();
    let elapsed = t_all.elapsed().as_secs_f64();
    let named = open_mmap_s
        + phased.pool_s
        + phased.pool_join_s
        + phased.scan_s
        + phased.slot_alloc_s
        + phased.worker_parse_s
        + phased.finish_s
        + close_s;
    let schedule_s = (elapsed - named).max(0.0);
    let parse_minflt = phased.worker_minflt.iter().sum();
    Ok(PhasedFrames {
        frames: phased.frames,
        n_bytes,
        chunk_s: phased.chunk_s,
        scan_kind: phased.scan_kind,
        worker_minflt: phased.worker_minflt,
        share_frames: phased.share_frames,
        share_bytes: phased.share_bytes,
        phases: ReadPhaseTimes {
            open_mmap_s,
            pool_s: phased.pool_s,
            pool_join_s: phased.pool_join_s,
            scan_s: phased.scan_s,
            slot_alloc_s: phased.slot_alloc_s,
            worker_parse_s: phased.worker_parse_s,
            finish_s: phased.finish_s,
            worker_sum_s: phased.worker_sum_s,
            worker_max_s: phased.worker_max_s,
            close_s,
            schedule_s,
            scan_minflt: phased.scan_minflt,
            parse_minflt,
            llc_misses: phased.llc_misses,
            numa_remote: phased.numa_remote,
            mapped,
            ..ReadPhaseTimes::default()
        },
        #[cfg(feature = "parallel")]
        pool: phased.pool,
    })
}

/// [`frames_from_bytes`] with scan, slot allocation, and worker-parse times.
///
/// `Some(n)` with `n >= 2` uses that pool. Every other count, including
/// `None`, stays on the calling thread. That is the `read_con` policy:
/// an omitted count does not start a pool.
pub struct PhasedByteTimes {
    pub frames: Vec<types::ConFrame>,
    pub pool_s: f64,
    pub pool_join_s: f64,
    pub scan_s: f64,
    pub slot_alloc_s: f64,
    pub worker_parse_s: f64,
    pub finish_s: f64,
    pub worker_sum_s: f64,
    pub worker_max_s: f64,
    /// Per-chunk parse seconds, in claim order.
    pub chunk_s: Vec<f64>,
    /// [`SCAN_SERIAL`], [`SCAN_INDEXED`], or [`SCAN_EQUAL_SPAN`] when
    /// `parallel` is on. Zero otherwise.
    pub scan_kind: u8,
    /// Minor faults during the scan.
    pub scan_minflt: i64,
    /// Minor faults on each parse worker, in worker order.
    pub worker_minflt: Vec<i64>,
    pub share_frames: Vec<u64>,
    pub share_bytes: Vec<u64>,
    /// LLC read misses, or `-1` when the guest refuses the counter.
    pub llc_misses: i64,
    /// Pages on a node other than node 0.
    pub numa_remote: i64,
    /// Pool kept for the slab, the touch, and frame teardown.
    #[cfg(feature = "parallel")]
    pub pool: Option<std::sync::Arc<rayon::ThreadPool>>,
}

pub fn frames_from_bytes_phased(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Result<PhasedByteTimes, error::ParseError> {
    frames_from_bytes_phased_located(bytes, num_threads).map_err(|err| err.error)
}

fn frames_from_bytes_phased_located(
    bytes: &[u8],
    num_threads: Option<usize>,
) -> Result<PhasedByteTimes, error::LocatedParseError> {
    #[cfg(feature = "parallel")]
    {
        // One thread walks the file once. A scan plus a second parse made
        // a short file slower than the plain iterator.
        if !matches!(num_threads, Some(n) if n >= 2) {
            let flt0 = thread_minflt();
            let t_work = std::time::Instant::now();
            let frames = collect_bytes_located(bytes)?;
            let worker_parse_s = t_work.elapsed().as_secs_f64();
            let worker_flt = thread_minflt().saturating_sub(flt0);
            let n_frames = frames.len() as u64;
            return Ok(PhasedByteTimes {
                frames,
                pool_s: 0.0,
                pool_join_s: 0.0,
                scan_s: 0.0,
                slot_alloc_s: 0.0,
                worker_parse_s,
                finish_s: 0.0,
                worker_sum_s: worker_parse_s,
                worker_max_s: worker_parse_s,
                chunk_s: vec![worker_parse_s],
                scan_kind: SCAN_SERIAL,
                scan_minflt: 0,
                worker_minflt: vec![worker_flt],
                share_frames: vec![n_frames],
                share_bytes: vec![bytes.len() as u64],
                llc_misses: sample_llc_misses(),
                numa_remote: numa_remote_pages(),
                pool: None,
            });
        }
        let requested = Some(num_threads.expect("workers"));
        let timed = parse_bytes_timed(bytes, requested);
        let t_finish = std::time::Instant::now();
        let frames = if timed.slots.iter().any(|item| item.is_err()) {
            collect_located_slots(timed.slots, &timed.fail_lines)?
        } else {
            finish_results(timed.pool.as_deref(), timed.slots)
                .map_err(|error| error::LocatedParseError::new(error, 0, 0))?
        };
        let finish_s = t_finish.elapsed().as_secs_f64();
        Ok(PhasedByteTimes {
            frames,
            pool_s: timed.pool_s,
            pool_join_s: timed.pool_join_s,
            scan_s: timed.scan_s,
            slot_alloc_s: timed.slot_alloc_s,
            worker_parse_s: timed.worker_parse_s,
            finish_s,
            worker_sum_s: timed.worker_sum_s,
            worker_max_s: timed.worker_max_s,
            chunk_s: timed.chunk_s,
            scan_kind: timed.scan_kind,
            scan_minflt: timed.scan_minflt,
            worker_minflt: timed.worker_minflt,
            share_frames: timed.share_frames,
            share_bytes: timed.share_bytes,
            llc_misses: timed.llc_misses,
            numa_remote: timed.numa_remote,
            pool: timed.pool,
        })
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = num_threads;
        let t_work = std::time::Instant::now();
        let frames = collect_bytes_located(bytes)?;
        let worker_parse_s = t_work.elapsed().as_secs_f64();
        Ok(PhasedByteTimes {
            frames,
            pool_s: 0.0,
            pool_join_s: 0.0,
            scan_s: 0.0,
            slot_alloc_s: 0.0,
            worker_parse_s,
            finish_s: 0.0,
            worker_sum_s: worker_parse_s,
            worker_max_s: worker_parse_s,
            chunk_s: vec![worker_parse_s],
            scan_kind: 0,
            scan_minflt: 0,
            worker_minflt: Vec::new(),
            share_frames: Vec::new(),
            share_bytes: Vec::new(),
            llc_misses: -1,
            numa_remote: 0,
        })
    }
}

/// Copy every frame's coordinates into one row-major buffer.
///
/// `None` means the frames do not share one atom count. `workers >= 2`
/// writes disjoint frame runs. The copy is bit-identical to reading
/// each frame's `f64` positions in order.
pub fn pack_position_slab(frames: &[types::ConFrame], workers: usize) -> Option<Vec<f64>> {
    if frames.is_empty() {
        return None;
    }
    let n_atoms = frames[0].positions.nrows();
    if frames
        .iter()
        .any(|frame| frame.positions.nrows() != n_atoms)
    {
        return None;
    }
    let stride = n_atoms.saturating_mul(3);
    let total = frames.len().saturating_mul(stride);
    let mut values = Vec::<f64>::with_capacity(total);
    if total > 0 {
        // The parallel writers are the first touch of these pages.
        // A zeroed allocation would fault the whole slab on this thread.
        // SAFETY: every index is overwritten before the buffer is read.
        unsafe {
            values.set_len(total);
        }
    }
    if stride == 0 {
        return Some(values);
    }
    let write = |frame: &types::ConFrame, dest: &mut [f64]| {
        if let Some(src) = frame.positions.f64_slice() {
            dest.copy_from_slice(src);
        } else {
            for (row, slot) in dest.as_chunks_mut::<3>().0.iter_mut().enumerate() {
                slot.copy_from_slice(&frame.positions.as_f64_row(row));
            }
        }
    };
    #[cfg(feature = "parallel")]
    {
        if workers >= 2 {
            let chunk_frames = frames.len().div_ceil(workers).max(1);
            let chunk_vals = chunk_frames.saturating_mul(stride);
            let copy_chunks = |dest_all: &mut [f64]| {
                use rayon::prelude::*;
                dest_all
                    .par_chunks_mut(chunk_vals)
                    .enumerate()
                    .for_each(|(chunk_index, dest)| {
                        let base = chunk_index * chunk_frames;
                        for (offset, slot) in dest.chunks_exact_mut(stride).enumerate() {
                            let index = base + offset;
                            if index < frames.len() {
                                write(&frames[index], slot);
                            }
                        }
                    });
            };
            // `par_chunks_mut` follows the installed pool. Outside one,
            // a private scope of `workers` threads keeps the global pool
            // out of this phase.
            if rayon::current_thread_index().is_some() {
                first_touch_f64(&mut values, workers);
                copy_chunks(&mut values);
            } else {
                first_touch_f64(&mut values, workers);
                std::thread::scope(|scope| {
                    let mut rest = &mut values[..];
                    let mut base = 0usize;
                    let mut handles = Vec::new();
                    while base < frames.len() {
                        let take_frames = chunk_frames.min(frames.len() - base);
                        let take_vals = take_frames * stride;
                        let (dest, tail) = rest.split_at_mut(take_vals);
                        rest = tail;
                        let frame_slice = &frames[base..base + take_frames];
                        handles.push(scope.spawn(move || {
                            for (frame, slot) in
                                frame_slice.iter().zip(dest.chunks_exact_mut(stride))
                            {
                                write(frame, slot);
                            }
                        }));
                        base += take_frames;
                    }
                    for handle in handles {
                        handle.join().expect("slab worker");
                    }
                });
            }
            return Some(values);
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = workers;
    }
    for (frame, dest) in frames.iter().zip(values.chunks_exact_mut(stride)) {
        write(frame, dest);
    }
    Some(values)
}

fn mix_touch(hash: u64, value: u64) -> u64 {
    hash.wrapping_mul(0x9E3779B185EBCA87) ^ value
}

fn one_frame_touch(frame: &types::ConFrame, coords: &[f64], index: u64) -> u64 {
    let mut hash = index;
    for value in frame.header.boxl {
        hash = mix_touch(hash, value.to_bits());
    }
    for atom in &frame.atom_data {
        hash = mix_touch(hash, atom.symbol.len() as u64);
        for byte in atom.symbol.as_bytes() {
            hash = mix_touch(hash, u64::from(*byte));
        }
    }
    for value in coords {
        hash = mix_touch(hash, value.to_bits());
    }
    hash
}

/// XOR of positions, cell lengths, and atom-type symbols.
///
/// `positions` is frame-major `[frames, atoms, 3]`. `None` means the
/// slice does not match the frames. `workers >= 2` folds disjoint
/// frame runs on that many threads. Each frame's hash includes its
/// index, and the returned word is the XOR of those hashes.
pub fn touch_frame_checksum(
    frames: &[types::ConFrame],
    positions: &[f64],
    workers: usize,
) -> Option<u64> {
    if frames.is_empty() {
        return Some(0);
    }
    let n_atoms = frames[0].atom_data.len();
    let stride = n_atoms.saturating_mul(3);
    if stride == 0 || positions.len() != frames.len().saturating_mul(stride) {
        return None;
    }
    if frames.iter().any(|frame| frame.atom_data.len() != n_atoms) {
        return None;
    }
    let one = |index: usize| {
        let start = index * stride;
        one_frame_touch(
            &frames[index],
            &positions[start..start + stride],
            index as u64,
        )
    };
    Some(reduce_touch(frames.len(), workers, one))
}

/// [`touch_frame_checksum`] for frames still sitting in optional slots.
///
/// `None` means a slot is empty or the position slice does not match.
pub fn touch_option_checksum(
    frames: &[Option<types::ConFrame>],
    positions: &[f64],
    workers: usize,
) -> Option<u64> {
    if frames.is_empty() {
        return Some(0);
    }
    if frames.iter().any(Option::is_none) {
        return None;
    }
    let n_atoms = frames[0].as_ref()?.atom_data.len();
    let stride = n_atoms.saturating_mul(3);
    if stride == 0 || positions.len() != frames.len().saturating_mul(stride) {
        return None;
    }
    if frames
        .iter()
        .any(|frame| frame.as_ref().map(|item| item.atom_data.len()) != Some(n_atoms))
    {
        return None;
    }
    let one = |index: usize| {
        let start = index * stride;
        one_frame_touch(
            frames[index].as_ref().expect("slot"),
            &positions[start..start + stride],
            index as u64,
        )
    };
    Some(reduce_touch(frames.len(), workers, one))
}

fn reduce_touch(n: usize, workers: usize, one: impl Fn(usize) -> u64 + Sync) -> u64 {
    #[cfg(feature = "parallel")]
    if workers >= 2 {
        let fold_range = |first: usize, last: usize| {
            let mut acc = 0u64;
            for index in first..last {
                acc ^= one(index);
            }
            acc
        };
        let chunk = n.div_ceil(workers).max(1);
        // Reuse the installed read pool. A fresh pool per touch is the
        // build this phase used to pay on every call.
        if rayon::current_thread_index().is_some() {
            use rayon::prelude::*;
            return (0..workers)
                .into_par_iter()
                .map(|worker| {
                    let first = worker * chunk;
                    let last = (first + chunk).min(n);
                    fold_range(first, last)
                })
                .reduce(|| 0u64, |left, right| left ^ right);
        }
        let mut acc = 0u64;
        std::thread::scope(|scope| {
            let mut handles = Vec::with_capacity(workers);
            for worker in 0..workers {
                handles.push(scope.spawn(move || {
                    let first = worker * chunk;
                    let last = (first + chunk).min(n);
                    fold_range(first, last)
                }));
            }
            for handle in handles {
                acc ^= handle.join().unwrap_or(0);
            }
        });
        return acc;
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = workers;
    }
    (0..n).map(one).fold(0, |acc, item| acc ^ item)
}

/// Time a newline scan of `text`.
///
/// `chunked` with `threads >= 2` splits the buffer into that many
/// chunks, stitches the newline index, and walks frame starts. The
/// pool is built before the timer starts. The serial walk is
/// [`frame_start_offsets_serial`]. The returned offsets are the frame
/// starts. The caller chooses the worker count. File length does not.
pub fn measure_newline_scan(text: &str, threads: usize, chunked: bool) -> (Vec<usize>, f64) {
    #[cfg(feature = "parallel")]
    {
        if chunked && threads >= 2 {
            let pool = rayon::ThreadPoolBuilder::new().num_threads(threads).build();
            let t_scan = std::time::Instant::now();
            let offsets = match pool {
                Ok(pool) => pool.install(|| {
                    let newlines = newline_positions(text.as_bytes(), threads);
                    frame_start_offsets_indexed(text, newlines)
                }),
                Err(_) => {
                    let newlines = newline_positions(text.as_bytes(), threads);
                    frame_start_offsets_indexed(text, newlines)
                }
            };
            return (offsets, t_scan.elapsed().as_secs_f64());
        }
    }
    #[cfg(not(feature = "parallel"))]
    {
        let _ = (threads, chunked);
    }
    let t_scan = std::time::Instant::now();
    let offsets = frame_start_offsets_serial(text);
    (offsets, t_scan.elapsed().as_secs_f64())
}

/// Parallel parse that keeps the file line of the first error.
/// Workers return the error. They do not write thread-local error state.
#[cfg(feature = "parallel")]
fn collect_parallel_located(
    file_contents: &str,
    num_threads: Option<usize>,
) -> Result<Vec<types::ConFrame>, error::LocatedParseError> {
    use rayon::prelude::*;

    let boundaries = frame_start_offsets(file_contents);
    let parse_chunks = || -> Result<Vec<types::ConFrame>, error::LocatedParseError> {
        let num_frames = boundaries.len();
        let parts: Vec<Result<types::ConFrame, error::LocatedParseError>> = (0..num_frames)
            .into_par_iter()
            .map(|i| {
                let start = boundaries[i];
                let end = if i + 1 < num_frames {
                    boundaries[i + 1]
                } else {
                    file_contents.len()
                };
                let chunk = &file_contents[start..end];
                let mut iter = ConFrameIterator::new(chunk);
                match iter.next() {
                    Some(Ok(frame)) => Ok(frame),
                    Some(Err(e)) => {
                        let local = iter.lines.line_no();
                        let base = file_line_at(file_contents, start);
                        let cursor = if local == 0 { base } else { base + local - 1 };
                        Err(error::LocatedParseError::new(e, cursor, i as u64))
                    }
                    None => Err(error::LocatedParseError::new(
                        error::ParseError::IncompleteFrame,
                        file_line_at(file_contents, start),
                        i as u64,
                    )),
                }
            })
            .collect();
        let mut frames = Vec::with_capacity(parts.len());
        for part in parts {
            frames.push(part?);
        }
        Ok(frames)
    };
    match num_threads {
        None => parse_chunks(),
        Some(n) => {
            let n = n.max(1);
            match rayon::ThreadPoolBuilder::new().num_threads(n).build() {
                Ok(pool) => pool.install(parse_chunks),
                Err(_) => parse_chunks(),
            }
        }
    }
}

#[cfg(all(test, feature = "parallel"))]
mod parallel_strong_scale_tests {
    use super::*;
    use std::path::PathBuf;

    fn multi_frame_fixture() -> String {
        let p = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("resources/test/tiny_cuh2.con");
        let one = std::fs::read_to_string(p).expect("fixture");
        // Enough frames for >1 worker to exercise the pool.
        one.repeat(8)
    }

    fn sequential_frames(text: &str) -> Vec<types::ConFrame> {
        ConFrameIterator::new(text)
            .map(|r| r.expect("seq frame"))
            .collect()
    }

    fn frames_payload_key(f: &types::ConFrame) -> (usize, Vec<(String, f64, f64, f64)>) {
        let atoms: Vec<_> = f
            .atom_data
            .iter()
            .map(|a| (a.symbol.to_string(), a.x, a.y, a.z))
            .collect();
        (f.atom_data.len(), atoms)
    }

    #[test]
    fn parallel_workers_match_sequential_payloads() {
        let text = multi_frame_fixture();
        let seq = sequential_frames(&text);
        assert!(seq.len() >= 8);
        let seq_keys: Vec<_> = seq.iter().map(frames_payload_key).collect();

        for workers in [1usize, 2, 4] {
            let par = parse_frames_parallel_with_threads(&text, Some(workers));
            assert_eq!(par.len(), seq.len(), "workers={workers}");
            let par_keys: Vec<_> = par
                .into_iter()
                .map(|r| frames_payload_key(&r.expect("par frame")))
                .collect();
            assert_eq!(par_keys, seq_keys, "workers={workers} frame payloads");
        }

        // Global pool path agrees too.
        let par_default = parse_frames_parallel(&text);
        let def_keys: Vec<_> = par_default
            .into_iter()
            .map(|r| frames_payload_key(&r.expect("par")))
            .collect();
        assert_eq!(def_keys, seq_keys);
    }

    fn live_threads() -> usize {
        std::fs::read_to_string("/proc/self/status")
            .unwrap()
            .lines()
            .find(|line| line.starts_with("Threads:"))
            .unwrap()
            .split_whitespace()
            .nth(1)
            .unwrap()
            .parse()
            .unwrap()
    }

    /// A file past the old map and pool gates stays on this thread.
    #[test]
    fn default_read_of_a_wide_file_stays_on_the_calling_thread() {
        const CHILD: &str = "READCON_DEFAULT_THREAD_CHILD";
        const NAME: &str = "iterators::parallel_strong_scale_tests::default_read_of_a_wide_file_stays_on_the_calling_thread";
        if std::env::var_os(CHILD).is_none() {
            let out = std::process::Command::new(std::env::current_exe().unwrap())
                .args([NAME, "--exact", "--test-threads=1"])
                .env(CHILD, "1")
                .output()
                .unwrap();
            let stdout = String::from_utf8_lossy(&out.stdout);
            assert!(
                out.status.success() && stdout.contains("test result: ok. 1 passed;"),
                "{stdout}{}",
                String::from_utf8_lossy(&out.stderr)
            );
            return;
        }
        let dir = tempfile::tempdir().unwrap();
        let path = dir.path().join("wide.con");
        let one = std::fs::read_to_string(
            PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("resources/test/tiny_cuh2.con"),
        )
        .unwrap();
        std::fs::write(&path, one.repeat(200)).unwrap();
        assert!(std::fs::metadata(&path).unwrap().len() > 64 * 1024);
        let before = live_threads();
        let frames = super::read_all_frames(&path).unwrap();
        let after = live_threads();
        assert_eq!(before, after, "default read started worker threads");
        assert!(frames.len() > 1);
        let text = std::fs::read_to_string(&path).unwrap();
        let sequential = super::frames_from_text(&text, None).unwrap();
        assert_eq!(sequential.len(), frames.len());
        assert_eq!(
            sequential[0].atom_data[0].x, frames[0].atom_data[0].x,
            "default read changed a coordinate"
        );
    }
}

#[cfg(all(test, feature = "parallel"))]
mod parallel_header_scan_tests {
    use super::*;
    use std::path::PathBuf;

    fn fixture(name: &str) -> String {
        let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
            .join("resources/test")
            .join(name);
        std::fs::read_to_string(path).expect(name)
    }

    /// A cut on the byte between `\r` and `\n`. The `\n` is in the next chunk.
    fn assert_crlf_gap_cut(text: &str) {
        let bytes = text.as_bytes();
        let mut found = 0usize;
        let mut i = 0usize;
        while i + 1 < bytes.len() && found < 6 {
            if bytes[i] == b'\r' && bytes[i + 1] == b'\n' {
                let cut = i + 1;
                assert_eq!(bytes[cut - 1], b'\r');
                assert_eq!(bytes[cut], b'\n');
                assert_cuts_match(text, &[0, cut, bytes.len()]);
                found += 1;
                i += 2;
                continue;
            }
            i += 1;
        }
        assert!(found > 0, "fixture has no CRLF pair");
    }

    fn assert_cuts_match(text: &str, cuts: &[usize]) {
        let serial_nl: Vec<usize> = memchr::memchr_iter(b'\n', text.as_bytes()).collect();
        let stitched = newline_positions_at_cuts(text.as_bytes(), cuts);
        assert_eq!(stitched, serial_nl, "newline stitch at {cuts:?}");
        let serial = frame_start_offsets_serial(text);
        let parallel = frame_start_offsets_at_cuts(text, cuts);
        assert_eq!(parallel, serial, "frame offsets at {cuts:?}");
    }

    fn header_straddle_cuts(text: &str) -> Vec<Vec<usize>> {
        let serial = frame_start_offsets_serial(text);
        let bytes = text.as_bytes();
        let mut out = Vec::new();
        // Equal splits, plus a cut forced into a header line of every frame.
        for n in [2usize, 3, 4, 8] {
            out.push(equal_cuts(bytes.len(), n));
        }
        for (index, &start) in serial.iter().enumerate() {
            let next = serial.get(index + 1).copied().unwrap_or(bytes.len());
            // The header ends at the coordinate label. The window is that
            // header, not a fixed count of bytes past the frame start.
            let window = super::header_window_end(bytes, start, next);
            let mut line = start;
            while line < window {
                let Some(rel) = memchr::memchr(b'\n', &bytes[line..window]) else {
                    break;
                };
                let end = line + rel;
                if end - line > 1 {
                    let mid = line + (end - line) / 2;
                    // mid is inside the line, so the header byte sits across two chunks.
                    assert!(mid > line && mid < end);
                    out.push(vec![0, mid, bytes.len()]);
                }
                line = end + 1;
            }
        }
        out
    }

    fn assert_slab(text: &str) {
        assert!(!text.is_empty());
        for cuts in header_straddle_cuts(text) {
            assert_cuts_match(text, &cuts);
        }
        let bare = text.trim_end_matches(['\n', '\r']);
        if !bare.is_empty() {
            assert_cuts_match(bare, &equal_cuts(bare.len(), 4));
            let mid = bare.len() / 2;
            let line_start = bare[..mid].rfind('\n').map(|i| i + 1).unwrap_or(0);
            let line_end = bare[mid..]
                .find('\n')
                .map(|i| mid + i)
                .unwrap_or(bare.len());
            if line_end - line_start > 1 {
                let cut = line_start + (line_end - line_start) / 2;
                assert_cuts_match(bare, &[0, cut, bare.len()]);
            }
        }
        let tailed = format!("{text}\n   \n\t\n");
        assert_cuts_match(
            &tailed,
            &[0, 3, tailed.len() / 2, tailed.len() - 1, tailed.len()],
        );
        let crlf = text.replace('\n', "\r\n");
        assert_cuts_match(&crlf, &equal_cuts(crlf.len(), 5));
        assert_crlf_gap_cut(&crlf);
        let serial = frame_start_offsets_serial(text);
        if serial.len() >= 2 {
            let second = serial[1];
            let next = serial.get(2).copied().unwrap_or(text.len());
            let broken_at = second + (next - second) / 2;
            let broken = &text[..broken_at.max(1)];
            assert_cuts_match(broken, &[0, 1.max(broken.len() / 3), broken.len()]);
        }
    }

    #[test]
    fn chunked_header_scan_matches_serial_offsets() {
        assert_eq!(frame_start_offsets_serial(""), frame_start_offsets(""));
        assert_cuts_match("\n", &[0, 1]);
        assert_cuts_match("   \n\n", &[0, 2, 5]);
        for name in [
            "tiny_cuh2.con",
            "tiny_cuh2.convel",
            "tiny_cuh2_forces.con",
            "tiny_cuh2_vel_forces.con",
            "cuh2.con",
        ] {
            let one = fixture(name);
            assert_slab(&one);
            assert_slab(&one.repeat(3));
        }
        // The bytes parser (the read_con path) uses the same stitch.
        // The single frame is the short file. The repeat is a wider file.
        // Both use the requested worker count. There is no length cutoff.
        let one = fixture("cuh2.con");
        for wide in [one.clone(), one.repeat(8)] {
            let serial = frame_start_offsets_serial(&wide);
            for workers in [2usize, 3, 4, 8] {
                let parallel = frame_start_offsets_with_threads(&wide, Some(workers));
                assert_eq!(parallel, serial, "workers={workers} bytes={}", wide.len());
                let parsed =
                    frames_from_bytes(wide.as_bytes(), Some(workers)).expect("bytes parse");
                assert_eq!(parsed.len(), serial.len());
                let seq = frames_from_text(&wide, Some(1)).expect("serial parse");
                assert_eq!(parsed.len(), seq.len());
                assert_eq!(parsed[0].atom_data[0].x, seq[0].atom_data[0].x);
                assert_eq!(
                    parsed.last().unwrap().atom_data.last().unwrap().z,
                    seq.last().unwrap().atom_data.last().unwrap().z
                );
            }
        }
    }
}

/// End of the header window used by straddle cuts.
///
/// The header runs up to the line that starts the coordinate block.
/// When that label is absent the window is the next frame start.
#[cfg(all(test, feature = "parallel"))]
fn header_window_end(bytes: &[u8], start: usize, limit: usize) -> usize {
    let limit = limit.min(bytes.len());
    let marker = b"Coordinates";
    let mut line = start;
    while line < limit {
        let rest = &bytes[line..limit];
        let rel = memchr::memchr(b'\n', rest).unwrap_or(rest.len());
        let content_end = line + rel;
        let content = &bytes[line..content_end];
        let trimmed = match content.iter().position(|byte| !byte.is_ascii_whitespace()) {
            Some(index) => &content[index..],
            None => b"",
        };
        if trimmed.starts_with(marker) {
            return line;
        }
        if content_end >= limit {
            break;
        }
        line = content_end + 1;
    }
    limit
}

#[cfg(all(test, feature = "parallel"))]
mod parallel_frame_parse_tests {
    use super::*;
    use std::path::PathBuf;

    const ZERO_ATOM: &str = "\
zero-atom frame
{\"con_spec_version\":3,\"con_spec_minor\":1,\"units\":{\"length\":\"angstrom\",\"energy\":\"eV\"}}
10.0 10.0 10.0
90.0 90.0 90.0


0


";

    fn fixture(name: &str) -> String {
        let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
            .join("resources/test")
            .join(name);
        std::fs::read_to_string(path).expect(name)
    }

    fn serial_results(text: &str) -> Vec<Result<types::ConFrame, error::ParseError>> {
        ConFrameIterator::new(text).collect()
    }

    fn written(frame: &types::ConFrame) -> Vec<u8> {
        let mut out = Vec::new();
        {
            let mut writer = crate::writer::ConFrameWriter::new(&mut out);
            writer.write_frame(frame).expect("write");
            writer.flush().expect("flush");
        }
        out
    }

    fn assert_same_results(
        label: &str,
        serial: Vec<Result<types::ConFrame, error::ParseError>>,
        parallel: Vec<Result<types::ConFrame, error::ParseError>>,
    ) {
        assert_eq!(serial.len(), parallel.len(), "{label} count");
        for (i, (left, right)) in serial.into_iter().zip(parallel).enumerate() {
            match (left, right) {
                (Ok(a), Ok(b)) => {
                    assert_eq!(written(&a), written(&b), "{label} bytes frame {i}");
                    assert_eq!(a, b, "{label} fields frame {i}");
                }
                (Err(a), Err(b)) => {
                    assert_eq!(
                        format!("{a:?}"),
                        format!("{b:?}"),
                        "{label} error frame {i}"
                    );
                }
                (Ok(_), Err(err)) => panic!("{label} frame {i} serial ok parallel {err:?}"),
                (Err(err), Ok(_)) => panic!("{label} frame {i} serial {err:?} parallel ok"),
            }
        }
    }

    fn assert_workers(label: &str, text: &str) {
        let serial_ok = serial_results(text).iter().all(|item| item.is_ok());
        for workers in [1usize, 2, 3, 4, 8] {
            let parallel = parse_frames_parallel_with_threads(text, Some(workers));
            assert_same_results(
                &format!("{label} workers {workers}"),
                serial_results(text),
                parallel,
            );
            let bounds = collect_frame_starts(ConFrameIterator::new(text), true);
            let at = parse_frames_at_offsets(text, &bounds, Some(workers));
            assert_same_results(
                &format!("{label} offsets workers {workers}"),
                serial_results(text),
                at,
            );
        }
        if serial_ok {
            for workers in [2usize, 3, 8] {
                let bytes = frames_from_bytes(text.as_bytes(), Some(workers)).expect(label);
                let seq = frames_from_text(text, Some(1)).expect(label);
                assert_eq!(bytes.len(), seq.len(), "{label} byte count");
                for (i, (a, b)) in bytes.iter().zip(seq.iter()).enumerate() {
                    assert_eq!(written(a), written(b), "{label} byte path frame {i}");
                    assert_eq!(a, b, "{label} byte path fields frame {i}");
                }
            }
        }
    }

    fn crlf_gap_offsets(text: &str) -> Vec<usize> {
        let bytes = text.as_bytes();
        let mut cut = None;
        let mut i = 0usize;
        while i + 1 < bytes.len() {
            if bytes[i] == b'\r' && bytes[i + 1] == b'\n' {
                cut = Some(i + 1);
                break;
            }
            i += 1;
        }
        let cut = cut.expect("crlf");
        frame_start_offsets_at_cuts(text, &[0, cut, bytes.len()])
    }

    #[test]
    fn parallel_slots_match_serial_frames() {
        assert_workers("empty", "");
        assert_workers("blank", "\n");
        assert_workers("space", "   \n\n");
        assert_workers("zero", ZERO_ATOM);
        assert_workers("zero-repeat", &ZERO_ATOM.repeat(5));
        for name in [
            "tiny_cuh2.con",
            "tiny_cuh2.convel",
            "tiny_cuh2_forces.con",
            "tiny_cuh2_vel_forces.con",
            "tiny_cuh2_charges_spins_magmoms.con",
            "tiny_cuh2_displacements.con",
            "tiny_cuh2_spreads.con",
            "cuh2.con",
        ] {
            let one = fixture(name);
            assert_workers(name, &one);
            assert_workers(&format!("{name} x5"), &one.repeat(5));
            let bare = one.trim_end_matches(['\n', '\r']);
            assert_workers(&format!("{name} bare"), bare);
            let tailed = format!("{one}\n   \n\t\n");
            assert_workers(&format!("{name} tail"), &tailed);
            let crlf = one.replace('\n', "\r\n");
            assert_workers(&format!("{name} crlf"), &crlf);
            let gap = crlf_gap_offsets(&crlf);
            assert_eq!(
                gap,
                frame_start_offsets_serial(&crlf),
                "{name} crlf gap offsets"
            );
            let parsed = parse_frames_at_offsets(&crlf, &gap, Some(4));
            assert_same_results(
                &format!("{name} crlf gap parse"),
                serial_results(&crlf),
                parsed,
            );
            let serial_offs = frame_start_offsets_serial(&one);
            let broken: &str = if serial_offs.len() >= 2 {
                let second = serial_offs[1];
                let next = serial_offs.get(2).copied().unwrap_or(one.len());
                &one[..second + (next - second) / 2]
            } else if one.len() > 8 {
                &one[..one.len() / 2]
            } else {
                ""
            };
            if !broken.is_empty() {
                let serial_err = frames_from_text(broken, Some(1))
                    .err()
                    .map(|e| format!("{e:?}"));
                assert!(serial_err.is_some(), "{name} chopped still parsed");
                for workers in [1usize, 2, 4, 8] {
                    let parallel_err = frames_from_text(broken, Some(workers))
                        .err()
                        .map(|e| format!("{e:?}"));
                    assert_eq!(parallel_err, serial_err, "{name} chopped workers {workers}");
                    let byte_err = frames_from_bytes(broken.as_bytes(), Some(workers))
                        .err()
                        .map(|e| format!("{e:?}"));
                    assert_eq!(
                        byte_err, serial_err,
                        "{name} chopped bytes workers {workers}"
                    );
                }
            }
        }
    }
}

#[cfg(all(test, feature = "parallel"))]
mod phased_read_tests {
    use super::*;
    use std::path::PathBuf;

    fn fixture(name: &str) -> String {
        let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
            .join("resources/test")
            .join(name);
        std::fs::read_to_string(path).expect(name)
    }

    #[test]
    fn phased_bytes_match_serial_frames() {
        let text = fixture("cuh2.con").repeat(4);
        let serial = frames_from_text(&text, Some(1)).expect("serial");
        for workers in [1usize, 2, 4, 8] {
            let phased = frames_from_bytes_phased(text.as_bytes(), Some(workers)).expect("phased");
            let parallel = phased.frames;
            assert_eq!(parallel.len(), serial.len(), "workers={workers}");
            assert!(
                phased.scan_s >= 0.0 && phased.slot_alloc_s >= 0.0 && phased.worker_parse_s >= 0.0
            );
            assert_eq!(parallel[0], serial[0], "workers={workers} first");
            assert_eq!(
                parallel.last().unwrap(),
                serial.last().unwrap(),
                "workers={workers} last"
            );
            let slab = pack_position_slab(&serial, workers).expect("slab");
            let stride = serial[0].positions.nrows() * 3;
            let src = serial[0].positions.f64_slice().expect("f64");
            assert_eq!(&slab[..stride], src);
            let (chunked, _) = measure_newline_scan(&text, workers.max(2), true);
            let (plain, _) = measure_newline_scan(&text, 1, false);
            if workers >= 2 {
                assert_eq!(chunked, plain);
            }
        }
    }

    #[test]
    fn phased_file_matches_serial_and_maps_a_wide_file() {
        let text = fixture("cuh2.con").repeat(5);
        let dir = std::env::temp_dir();
        let path = dir.join("readcon-phased-wide.con");
        std::fs::write(&path, &text).expect("write");
        let phased = read_all_frames_phased(&path, Some(4)).expect("phased");
        let serial = frames_from_text(&text, Some(1)).expect("serial");
        assert_eq!(phased.frames, serial);
        assert!(phased.phases.mapped, "a file past 64 KiB is mapped");
        assert!(phased.phases.worker_parse_s >= 0.0);
        assert_eq!(phased.n_bytes, text.len());
        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn equal_span_boundaries_match_the_serial_walk() {
        let text = fixture("cuh2.con").repeat(8);
        let bytes = text.as_bytes();
        let got = equal_span_boundaries(bytes, 4).expect("equal span");
        let expect = collect_frame_starts(ConFrameIterator::from_bytes(bytes), true);
        assert_eq!(got, expect);
        assert_eq!(got.len(), 8);
        let phased = frames_from_bytes_phased(bytes, Some(4)).expect("phased");
        assert_eq!(phased.scan_kind, SCAN_EQUAL_SPAN);
        assert_eq!(
            phased.frames,
            frames_from_text(&text, Some(1)).expect("serial")
        );
        let tailed = format!("{text}\n\n  \n");
        let tail_phased = frames_from_bytes_phased(tailed.as_bytes(), Some(4)).expect("tail");
        assert_eq!(tail_phased.scan_kind, SCAN_EQUAL_SPAN);
        assert_eq!(
            tail_phased.frames,
            frames_from_text(&tailed, Some(1)).expect("tail serial")
        );
    }

    #[test]
    fn a_whitespace_tail_uses_the_equal_span_walk() {
        let mut text = fixture("cuh2.con").repeat(3);
        text.push('\n');
        assert!(equal_span_boundaries(text.as_bytes(), 4).is_some());
        let serial = frames_from_text(&text, Some(1)).expect("serial");
        let parallel = frames_from_bytes_phased(text.as_bytes(), Some(4)).expect("phased");
        assert_eq!(parallel.scan_kind, SCAN_EQUAL_SPAN);
        assert_eq!(parallel.frames, serial);
    }

    #[test]
    fn touch_checksum_matches_across_worker_counts() {
        let text = fixture("cuh2.con").repeat(4);
        let serial = frames_from_bytes_phased(text.as_bytes(), Some(1)).expect("serial");
        let parallel = frames_from_bytes_phased(text.as_bytes(), Some(4)).expect("parallel");
        let slab_s = pack_position_slab(&serial.frames, 1).expect("slab");
        let slab_p = pack_position_slab(&parallel.frames, 4).expect("slab");
        let left = touch_frame_checksum(&serial.frames, &slab_s, 1).expect("touch");
        let right = touch_frame_checksum(&parallel.frames, &slab_p, 4).expect("touch");
        assert_eq!(left, right);
        assert_ne!(left, 0);
    }
}

#[cfg(test)]
mod later_frame_location_tests {
    use super::*;
    use std::fs;

    fn one_atom(coord: &str) -> String {
        format!(
            "seed\n{{\"con_spec_version\":2}}\n10 10 10\n90 90 90\n0 0\n0 0 0\n1\n1\n1.0\nH\nCoordinates of Component 1\n{coord}\n"
        )
    }

    fn bad_count_frame() -> String {
        "seed\n{\"con_spec_version\":2}\n10 10 10\n90 90 90\n0 0\n0 0 0\n1\n-1\n1.0\nH\nCoordinates of Component 1\n0 0 0 0 0\n".into()
    }

    fn two_frames() -> String {
        format!("{}{}", one_atom("0 0 0 0 0"), bad_count_frame())
    }

    fn assert_later_count(
        err: &error::ParseError,
        line: u32,
        frame: u64,
        iter: &ConFrameIterator<'_>,
    ) {
        assert_eq!(err.class_name(), "InvalidCount", "{err}");
        assert_eq!(iter.lines.line_no(), line, "{err}");
        assert_eq!(iter.frame_index(), frame, "{err}");
    }

    fn assert_first_pass(err: Box<dyn std::error::Error>) {
        let located = err
            .downcast_ref::<error::LocatedParseError>()
            .unwrap_or_else(|| panic!("first pass lost the location: {err}"));
        assert_eq!(located.error.class_name(), "InvalidCount", "{located:?}");
        assert_eq!(located.line, 20, "{located:?}");
        assert_eq!(located.frame_index, 1, "{located:?}");
    }

    fn take_err<T>(result: Result<T, Box<dyn std::error::Error>>) -> Box<dyn std::error::Error> {
        match result {
            Err(err) => err,
            Ok(_) => panic!("expected a parse error"),
        }
    }

    /// The batch readers report the later frame from the parse that failed.
    /// A second open of the path is not required.
    #[test]
    fn batch_readers_keep_the_later_frame_on_the_first_pass() {
        let path = std::env::temp_dir().join(format!(
            "readcon-later-frame-first-{}.con",
            std::process::id()
        ));
        fs::write(&path, two_frames()).unwrap();
        for threads in [None, Some(1), Some(2)] {
            assert_first_pass(take_err(read_all_frames_with_threads(&path, threads)));
            assert_first_pass(take_err(read_all_frames_phased(&path, threads)));
            assert_first_pass(take_err(read_positions_with_threads(&path, threads)));
        }
        let _ = fs::remove_file(&path);
    }

    /// An error in the second frame keeps that frame's index and file line.
    #[test]
    fn next_forward_and_skip_locate_the_later_frame() {
        let text = two_frames();

        let mut iter = ConFrameIterator::new(&text);
        assert!(iter.next().unwrap().is_ok());
        let err = iter.next().unwrap().unwrap_err();
        assert_later_count(&err, 20, 1, &iter);

        let mut iter = ConFrameIterator::new(&text);
        assert!(iter.forward().unwrap().is_ok());
        let err = iter.forward().unwrap().unwrap_err();
        assert_later_count(&err, 20, 1, &iter);

        let mut iter = ConFrameIterator::new(&text);
        let err = iter.skip_frames(3).unwrap_err();
        assert_later_count(&err, 20, 1, &iter);
    }

    /// The blank line that opens a velocity block is a file line. A later
    /// frame's error must include it. 12 coordinate lines, the separator,
    /// and 3 velocity lines put the next frame's line 8 at file line 24.
    #[test]
    fn skip_after_a_velocity_frame_keeps_the_separator_line() {
        let text = format!(
            "{}\nH\nVelocities of Component 1\n0 0 0 0 0\n{}",
            one_atom("0 0 0 0 0"),
            bad_count_frame()
        );
        let mut iter = ConFrameIterator::new(&text);
        assert!(iter.forward().unwrap().is_ok());
        let err = iter.forward().unwrap().unwrap_err();
        assert_later_count(&err, 24, 1, &iter);
    }

    #[test]
    fn count_frames_locates_the_later_frame() {
        let path = std::env::temp_dir().join(format!(
            "readcon-later-frame-count-{}.con",
            std::process::id()
        ));
        fs::write(&path, two_frames()).unwrap();
        let err = count_frames(&path).unwrap_err();
        let _ = fs::remove_file(&path);
        match err {
            LoadError::Parse(located) => {
                assert_eq!(located.error.class_name(), "InvalidCount");
                assert_eq!(located.line, 20, "{located}");
                assert_eq!(located.frame_index, 1, "{located}");
            }
            other => panic!("expected a located parse error, got {other}"),
        }
    }

    /// Metadata on line 2 of the second frame is file line 14, not line 2.
    /// This is the serial contract the collective reader matches.
    #[test]
    fn metadata_in_a_later_frame_reports_that_frames_line() {
        let bad = "seed\n{not-json\n10 10 10\n90 90 90\n0 0\n0 0 0\n1\n1\n1.0\nH\nCoordinates of Component 1\n0 0 0 0 0\n";
        let text = format!("{}{}", one_atom("0 0 0 0 0"), bad);
        for threads in [Some(1usize), Some(2)] {
            let err = frames_from_text_located(&text, threads).unwrap_err();
            assert_eq!(
                err.error.class_name(),
                "InvalidMetadataJson",
                "{threads:?}: {err}"
            );
            assert_eq!(err.frame_index, 1, "{threads:?}");
            assert_eq!(err.line, 14, "{threads:?}: {err}");
        }
    }

    /// Several spans after the first frame fail. Serial collection keeps
    /// the earliest slot. The parallel position reader must report that
    /// same line and frame index.
    ///
    /// The first frame is large. A later worker can fail before the
    /// calling thread reaches the earliest bad span, which is the case
    /// that used to return the later slot.
    #[test]
    fn parallel_positions_report_the_earliest_failing_slot() {
        const SLOW_ATOMS: usize = 20_000;
        let slow = counted_frame(SLOW_ATOMS);
        let bad = counted_frame(1);
        let text = format!("{slow}{bad}{bad}{bad}");
        let earliest_line = 1 + slow.bytes().filter(|byte| *byte == b'\n').count() as u32;
        let path =
            std::env::temp_dir().join(format!("readcon-earliest-slot-{}.con", std::process::id()));
        fs::write(&path, &text).unwrap();
        assert_earliest_slot(
            take_err(read_positions_with_threads(&path, Some(1))),
            earliest_line,
        );
        assert_earliest_slot(
            take_err(read_positions_with_threads(&path, Some(2))),
            earliest_line,
        );
        let _ = fs::remove_file(&path);
    }

    fn counted_frame(natoms: usize) -> String {
        let mut body = String::with_capacity(natoms * 10 + 96);
        body.push_str("seed\n{\"con_spec_version\":2}\n10 10 10\n90 90 90\n0 0\n0 0 0\n1\n");
        body.push_str(&natoms.to_string());
        body.push_str("\n1.0\nH\nCoordinates of Component 1\n");
        for _ in 0..natoms {
            body.push_str("0 0 0 0 0\n");
        }
        body
    }

    fn assert_earliest_slot(err: Box<dyn std::error::Error>, line: u32) {
        let located = err
            .downcast_ref::<error::LocatedParseError>()
            .unwrap_or_else(|| panic!("position read lost the location: {err}"));
        assert_eq!(located.error.class_name(), "InvalidCount", "{located:?}");
        assert_eq!(located.frame_index, 1, "{located:?}");
        assert_eq!(located.line, line, "{located:?}");
    }
}

#[cfg(test)]
mod frame_equality_tests {
    use super::ConFrameIterator;

    fn sample_con(frames: usize, long: bool) -> String {
        let mut out = String::new();
        for f in 0..frames {
            let shift = (f as f64) * 1.0e-4;
            out.push_str("Generated by benchmark\n");
            out.push_str("{\"con_spec_version\":2}\n");
            out.push_str("15.3456 21.702 100.0\n");
            out.push_str("90.0 90.0 90.0\n");
            out.push_str("0 0\n");
            out.push_str(&format!("{f} 0 1\n"));
            out.push_str("2\n6 2\n63.546 1.00793\n");
            out.push_str("Cu\nCoordinates of Component 1\n");
            for i in 0..6 {
                let x = (i as f64) * 2.557 + 0.6394 + shift;
                let y = 0.9045 + shift * 0.5;
                let z = (f as f64) * 1.0e-6 + if i == 3 { -1.25 } else { 0.0 };
                if long {
                    out.push_str(&format!("   {x:.17}    {y:.17}    {z:.17} 0    {i}\n"));
                } else {
                    out.push_str(&format!("   {x:.6}    {y:.6}    {z:.6} 0    {i}\n"));
                }
            }
            out.push_str("H\nCoordinates of Component 2\n");
            out.push_str("   8.6823    9.947   11.733 0  6\n");
            out.push_str(&format!("  -7.9421    9.947   {shift:.6} 0  7\n"));
        }
        out
    }

    fn sample_convel() -> String {
        let mut out = String::new();
        for f in 0..3 {
            let shift = (f as f64) * 1.0e-4;
            out.push_str("Generated by benchmark\n");
            out.push_str("{\"con_spec_version\":2}\n");
            out.push_str("15.3456 21.702 100.0\n");
            out.push_str("90.0 90.0 90.0\n");
            out.push_str("0 0\n");
            out.push_str(&format!("{f} 0 1\n"));
            out.push_str("2\n4 1\n63.546 1.00793\n");
            out.push_str("Cu\nCoordinates of Component 1\n");
            for i in 0..4 {
                let x = (i as f64) * 2.557 + shift;
                out.push_str(&format!("   {x:.17}    0.9045    1.0 0    {i}\n"));
            }
            out.push_str("H\nCoordinates of Component 2\n");
            out.push_str("   1.5    2.5    3.5 0  4\n");
            out.push_str("\nCu\nVelocities of Component 1\n");
            for i in 0..4 {
                let vx = (i as f64) * 1.0e-4 - shift;
                out.push_str(&format!("   {vx:.17}    -0.5    0.25 0    {i}\n"));
            }
            out.push_str("H\nVelocities of Component 2\n");
            out.push_str("   0.1    -0.2    0.3 0  4\n");
        }
        out
    }

    fn reference_f64(token: &str) -> f64 {
        let (val, n) = fast_float2::parse_partial::<f64, _>(token).expect(token);
        assert_eq!(n, token.len(), "{token}");
        val
    }

    fn assert_rows_match(text: &str, positions: bool) {
        let frames: Vec<_> = ConFrameIterator::new(text)
            .map(|frame| frame.expect("parse"))
            .collect();
        assert!(frames.len() >= 2);
        let mut frame_i = 0usize;
        let mut row = 0usize;
        let mut in_section = false;
        for line in text.lines() {
            let token = line.trim();
            if token.is_empty() {
                in_section = true;
                row = 0;
                continue;
            }
            if token.starts_with("Generated") {
                if row > 0 || in_section {
                    frame_i += 1;
                }
                row = 0;
                in_section = false;
                continue;
            }
            let cols: Vec<&str> = token.split_whitespace().collect();
            if cols.len() < 4 || reference_ok(cols[0]).is_none() {
                continue;
            }
            if cols[3].parse::<u64>().is_err() {
                continue;
            }
            let got = if in_section {
                frames[frame_i].atom_data[row].velocity().expect("velocity")
            } else if positions {
                let p = frames[frame_i].positions.as_f64_row(row);
                [p[0], p[1], p[2]]
            } else {
                continue;
            };
            for (k, token) in cols.iter().take(3).enumerate() {
                assert_eq!(
                    got[k].to_bits(),
                    reference_f64(token).to_bits(),
                    "frame {frame_i} row {row} col {k} token {token} section {in_section}"
                );
            }
            let aos = if in_section {
                frames[frame_i].atom_data[row].velocity().unwrap()
            } else {
                let atom = &frames[frame_i].atom_data[row];
                [atom.x, atom.y, atom.z]
            };
            for k in 0..3 {
                assert_eq!(aos[k].to_bits(), got[k].to_bits());
            }
            row += 1;
        }
    }

    fn reference_ok(token: &str) -> Option<f64> {
        let (val, n) = fast_float2::parse_partial::<f64, _>(token).ok()?;
        (n == token.len()).then_some(val)
    }

    #[test]
    fn frames_match_general_decimal_parser() {
        assert_rows_match(&sample_con(4, true), true);
        assert_rows_match(&sample_con(4, false), true);
        let text = sample_convel();
        let frames: Vec<_> = ConFrameIterator::new(&text)
            .map(|frame| frame.expect("convel"))
            .collect();
        assert_eq!(frames.len(), 3);
        assert!(
            frames
                .iter()
                .all(|frame| frame.atom_data[0].velocity().is_some())
        );
        assert_eq!(frames[0].positions.nrows(), 5);
        // Every stored coordinate and velocity bit matches the general parser.
        let mut frame_i = 0usize;
        let mut coord_row = 0usize;
        let mut vel_row = 0usize;
        let mut velocities = false;
        for line in text.lines() {
            let token = line.trim();
            if token.starts_with("Generated") && (coord_row > 0 || velocities) {
                frame_i += 1;
                coord_row = 0;
                vel_row = 0;
                velocities = false;
            }
            if token.starts_with("Velocities") {
                velocities = true;
                continue;
            }
            let cols: Vec<&str> = token.split_whitespace().collect();
            if cols.len() < 5 {
                continue;
            }
            if reference_ok(cols[0]).is_none() || cols[3].parse::<u64>().is_err() {
                continue;
            }
            let row = if velocities { vel_row } else { coord_row };
            let got = if velocities {
                frames[frame_i].atom_data[row].velocity().unwrap()
            } else {
                let p = frames[frame_i].positions.as_f64_row(row);
                [p[0], p[1], p[2]]
            };
            for (k, col) in cols.iter().take(3).enumerate() {
                assert_eq!(got[k].to_bits(), reference_f64(col).to_bits(), "{token}");
            }
            if velocities {
                let soa = frames[frame_i].velocities.as_f64_row(row);
                for k in 0..3 {
                    assert_eq!(soa[k].to_bits(), got[k].to_bits());
                }
                vel_row += 1;
            } else {
                coord_row += 1;
            }
        }
        assert_eq!(frame_i, 2);
        assert_eq!(vel_row, 5);
    }
}

#[cfg(test)]
mod newline_skip_tests {
    use super::ConFrameIterator;

    /// The per-line search [`ConFrameIterator::advance_lines`] used before
    /// the counted skip. Positions and errors are the oracle.
    fn naive(bytes: &[u8], n: usize) -> Result<usize, ()> {
        let mut pos = 0usize;
        for i in 0..n {
            let rest = &bytes[pos..];
            match memchr::memchr(b'\n', rest) {
                Some(at) => pos += at + 1,
                None => {
                    if rest.is_empty() {
                        return Err(());
                    }
                    pos = bytes.len();
                    if i + 1 == n {
                        return Ok(pos);
                    }
                    return Err(());
                }
            }
        }
        Ok(pos)
    }

    #[test]
    fn counted_skip_matches_per_line_memchr() {
        let mut samples = vec![
            String::new(),
            "a".into(),
            "a\n".into(),
            "a\nb".into(),
            "\n".into(),
            "\n\n".into(),
            "abc\ndef\nghi".into(),
            "a\r\nb\r\n".into(),
            "x".repeat(100) + "\n" + &"y".repeat(40),
        ];
        let path =
            std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("resources/test/cuh2.con");
        let one = std::fs::read_to_string(&path).unwrap();
        samples.push(one.repeat(2));
        let bare = one.trim_end_matches(['\n', '\r']).to_string();
        samples.push(bare);
        for text in samples {
            let bytes = text.as_bytes();
            for n in [0usize, 1, 2, 3, 6, 7, 9, 40, 200, 450, 1000, 5000] {
                let expect = naive(bytes, n);
                let mut it = ConFrameIterator::from_bytes(bytes);
                let got = it.advance_lines(n).map(|_| it.byte_pos()).map_err(|_| ());
                assert_eq!(got, expect, "n={n} len={}", bytes.len());
            }
        }
    }
}

#[cfg(test)]
mod cached_unit_table_read_paths {
    use super::*;

    const FRAME: &str = "\
v3
{\"con_spec_version\":3,\"con_spec_minor\":1,\"units\":{\"length\":\"angstrom\",\"energy\":\"eV\"}}
10 10 10
90 90 90


1
1
1.0
H
Coordinates of Component 1
0.5 0.25 0.125 0 7
";

    fn coords(frame: &types::ConFrame) -> [f64; 3] {
        frame.positions.as_f64_row(0)
    }

    /// The serial iterator, the byte reader behind the C API, and the phased
    /// reader behind `read_con` all validate units through the one table.
    #[test]
    fn read_paths_share_the_cached_unit_table() {
        let addr = crate::units::cached_unit_table_addr();
        let serial = ConFrameIterator::new(FRAME)
            .next()
            .expect("frame")
            .expect("parse");
        let from_bytes = frames_from_bytes(FRAME.as_bytes(), Some(1)).expect("bytes");
        let phased = frames_from_bytes_phased(FRAME.as_bytes(), Some(1)).expect("phased");
        assert_eq!(from_bytes.len(), 1);
        assert_eq!(phased.frames.len(), 1);
        assert_eq!(coords(&from_bytes[0]), coords(&serial));
        assert_eq!(coords(&phased.frames[0]), coords(&serial));
        assert_eq!(crate::units::cached_unit_table_addr(), addr);
        let bad = FRAME.replace("angstrom", "not-a-unit");
        assert!(ConFrameIterator::new(&bad).next().unwrap().is_err());
        assert!(frames_from_bytes(bad.as_bytes(), Some(1)).is_err());
        assert!(frames_from_bytes_phased(bad.as_bytes(), Some(1)).is_err());
        assert_eq!(crate::units::cached_unit_table_addr(), addr);
    }
}