coreutils_rs/tac/

core.rs

use std::io::{self, IoSlice, Write};

use rayon::prelude::*;

/// Threshold for parallel processing (64MB).
/// Each benchmark invocation is a fresh process, so rayon pool init (~0.5-1ms)
/// is paid every time. For 10MB files, single-threaded scan (0.3ms) is faster
/// than rayon init + parallel scan. Only use parallelism for genuinely large
/// files where multi-core scanning and copying pays off.
const PARALLEL_THRESHOLD: usize = 64 * 1024 * 1024;

/// Maximum IoSlice entries per write_vectored batch.
/// Used by string/regex separator paths.
const IOSLICE_BATCH_SIZE: usize = 1024;

/// Reverse records separated by a single byte.
/// For large data (>= 8MB): parallel chunk-local reversal with contiguous buffers.
/// For small data: single-threaded forward SIMD scan + contiguous output buffer.
pub fn tac_bytes(data: &[u8], separator: u8, before: bool, out: &mut impl Write) -> io::Result<()> {
    if data.is_empty() {
        return Ok(());
    }
    if data.len() >= PARALLEL_THRESHOLD {
        if !before {
            tac_bytes_after_contiguous(data, separator, out)
        } else {
            tac_bytes_before_contiguous(data, separator, out)
        }
    } else if !before {
        tac_bytes_after(data, separator, out)
    } else {
        tac_bytes_before(data, separator, out)
    }
}

/// Reverse records of an owned Vec. Delegates to tac_bytes.
pub fn tac_bytes_owned(
    data: &mut [u8],
    separator: u8,
    before: bool,
    out: &mut impl Write,
) -> io::Result<()> {
    tac_bytes(data, separator, before, out)
}

/// Collect multi-byte separator positions with pre-allocated Vec.
#[inline]
fn collect_positions_str(data: &[u8], separator: &[u8]) -> Vec<usize> {
    let estimated = data.len() / 40 + 64;
    let mut positions = Vec::with_capacity(estimated);
    for pos in memchr::memmem::find_iter(data, separator) {
        positions.push(pos);
    }
    positions
}

/// Parallel chunk-local reversal for after-separator mode.
/// Splits data into N chunks at newline boundaries, each chunk independently
/// scans forward and builds a reversed output buffer in parallel, then writes
/// chunk buffers in reverse order. Eliminates IoSlice overhead and reduces
/// syscalls to N (one per chunk).
fn tac_bytes_after_contiguous(data: &[u8], sep: u8, out: &mut impl Write) -> io::Result<()> {
    let n_threads = rayon::current_num_threads().max(1);
    let chunk_size = data.len() / n_threads;

    // Find chunk boundaries at separator positions
    let mut boundaries = Vec::with_capacity(n_threads + 1);
    boundaries.push(0);
    for i in 1..n_threads {
        let target = i * chunk_size;
        if target >= data.len() {
            break;
        }
        // In after mode, separator ends a record; boundary is right after separator
        let boundary = memchr::memchr(sep, &data[target..])
            .map(|p| target + p + 1)
            .unwrap_or(data.len());
        if boundary < data.len() {
            boundaries.push(boundary);
        }
    }
    boundaries.push(data.len());
    boundaries.dedup();
    let n_chunks = boundaries.len() - 1;

    // Each chunk: forward scan for positions, build reversed output buffer
    let reversed_chunks: Vec<Vec<u8>> = (0..n_chunks)
        .into_par_iter()
        .map(|i| {
            let start = boundaries[i];
            let end = boundaries[i + 1];
            let chunk = &data[start..end];
            if chunk.is_empty() {
                return Vec::new();
            }

            // Collect separator positions within chunk
            let mut positions: Vec<usize> = Vec::with_capacity(chunk.len() / 40 + 64);
            for pos in memchr::memchr_iter(sep, chunk) {
                positions.push(pos);
            }

            // Build reversed output buffer
            let mut buf = Vec::with_capacity(chunk.len());
            let mut end_pos = chunk.len();
            for &pos in positions.iter().rev() {
                let rec_start = pos + 1;
                if rec_start < end_pos {
                    buf.extend_from_slice(&chunk[rec_start..end_pos]);
                }
                end_pos = rec_start;
            }
            if end_pos > 0 {
                buf.extend_from_slice(&chunk[..end_pos]);
            }
            buf
        })
        .collect();

    // Write chunks in reverse order (last chunk first = correct tac order)
    for chunk in reversed_chunks.iter().rev() {
        if !chunk.is_empty() {
            out.write_all(chunk)?;
        }
    }
    Ok(())
}

/// Parallel chunk-local reversal for before-separator mode.
/// Same approach as after mode but separator attaches to the START of each record.
fn tac_bytes_before_contiguous(data: &[u8], sep: u8, out: &mut impl Write) -> io::Result<()> {
    let n_threads = rayon::current_num_threads().max(1);
    let chunk_size = data.len() / n_threads;

    // Find chunk boundaries at separator positions
    let mut boundaries = Vec::with_capacity(n_threads + 1);
    boundaries.push(0);
    for i in 1..n_threads {
        let target = i * chunk_size;
        if target >= data.len() {
            break;
        }
        // In before mode, separator starts a record; boundary is AT the separator
        let boundary = memchr::memchr(sep, &data[target..])
            .map(|p| target + p)
            .unwrap_or(data.len());
        if boundary > 0 && boundary < data.len() {
            boundaries.push(boundary);
        }
    }
    boundaries.push(data.len());
    boundaries.dedup();
    let n_chunks = boundaries.len() - 1;

    // Each chunk: forward scan for positions, build reversed output buffer
    let reversed_chunks: Vec<Vec<u8>> = (0..n_chunks)
        .into_par_iter()
        .map(|i| {
            let start = boundaries[i];
            let end = boundaries[i + 1];
            let chunk = &data[start..end];
            if chunk.is_empty() {
                return Vec::new();
            }

            // Collect separator positions within chunk
            let mut positions: Vec<usize> = Vec::with_capacity(chunk.len() / 40 + 64);
            for pos in memchr::memchr_iter(sep, chunk) {
                positions.push(pos);
            }

            // Build reversed output buffer (before mode: separator at start of record)
            let mut buf = Vec::with_capacity(chunk.len());
            let mut end_pos = chunk.len();
            for &pos in positions.iter().rev() {
                if pos < end_pos {
                    buf.extend_from_slice(&chunk[pos..end_pos]);
                }
                end_pos = pos;
            }
            if end_pos > 0 {
                buf.extend_from_slice(&chunk[..end_pos]);
            }
            buf
        })
        .collect();

    // Write chunks in reverse order
    for chunk in reversed_chunks.iter().rev() {
        if !chunk.is_empty() {
            out.write_all(chunk)?;
        }
    }
    Ok(())
}

/// After-separator mode for small files: backward memrchr scan + streaming output.
/// Zero extra allocation: scans backward with memrchr, writes records directly.
fn tac_bytes_after(data: &[u8], sep: u8, out: &mut impl Write) -> io::Result<()> {
    if data.is_empty() {
        return Ok(());
    }

    let mut prev_end = data.len();
    let mut search_end = data.len();

    loop {
        match memchr::memrchr(sep, &data[..search_end]) {
            Some(pos) => {
                let rec_start = pos + 1;
                if rec_start < prev_end {
                    out.write_all(&data[rec_start..prev_end])?;
                }
                prev_end = rec_start;
                search_end = pos;
            }
            None => {
                if prev_end > 0 {
                    out.write_all(&data[..prev_end])?;
                }
                break;
            }
        }
        if search_end == 0 {
            if prev_end > 0 {
                out.write_all(&data[..prev_end])?;
            }
            break;
        }
    }

    Ok(())
}

/// Before-separator mode for small files: backward memrchr scan + streaming output.
/// Zero extra allocation: scans backward with memrchr, writes records directly.
fn tac_bytes_before(data: &[u8], sep: u8, out: &mut impl Write) -> io::Result<()> {
    if data.is_empty() {
        return Ok(());
    }

    let mut prev_end = data.len();
    let mut search_end = data.len();

    loop {
        match memchr::memrchr(sep, &data[..search_end]) {
            Some(pos) => {
                if pos < prev_end {
                    out.write_all(&data[pos..prev_end])?;
                }
                prev_end = pos;
                if pos == 0 {
                    break;
                }
                search_end = pos;
            }
            None => {
                if prev_end > 0 {
                    out.write_all(&data[..prev_end])?;
                }
                break;
            }
        }
    }

    Ok(())
}

/// Reverse records using a multi-byte string separator.
/// Uses SIMD-accelerated memmem + write_all output.
///
/// For single-byte separators, delegates to tac_bytes which uses memchr (faster).
pub fn tac_string_separator(
    data: &[u8],
    separator: &[u8],
    before: bool,
    out: &mut impl Write,
) -> io::Result<()> {
    if data.is_empty() {
        return Ok(());
    }

    if separator.len() == 1 {
        return tac_bytes(data, separator[0], before, out);
    }

    let sep_len = separator.len();

    if !before {
        tac_string_after(data, separator, sep_len, out)
    } else {
        tac_string_before(data, separator, sep_len, out)
    }
}

/// Multi-byte string separator, after mode. Uses writev batching.
fn tac_string_after(
    data: &[u8],
    separator: &[u8],
    sep_len: usize,
    out: &mut impl Write,
) -> io::Result<()> {
    let positions = collect_positions_str(data, separator);

    if positions.is_empty() {
        return out.write_all(data);
    }

    let mut slices: Vec<IoSlice<'_>> = Vec::with_capacity(IOSLICE_BATCH_SIZE);
    let mut end = data.len();

    for &pos in positions.iter().rev() {
        let rec_start = pos + sep_len;
        if rec_start < end {
            slices.push(IoSlice::new(&data[rec_start..end]));
            if slices.len() >= IOSLICE_BATCH_SIZE {
                write_all_vectored(out, &slices)?;
                slices.clear();
            }
        }
        end = rec_start;
    }
    if end > 0 {
        slices.push(IoSlice::new(&data[..end]));
    }
    if !slices.is_empty() {
        write_all_vectored(out, &slices)?;
    }
    Ok(())
}

/// Multi-byte string separator, before mode. Uses writev batching.
fn tac_string_before(
    data: &[u8],
    separator: &[u8],
    _sep_len: usize,
    out: &mut impl Write,
) -> io::Result<()> {
    let positions = collect_positions_str(data, separator);

    if positions.is_empty() {
        return out.write_all(data);
    }

    let mut slices: Vec<IoSlice<'_>> = Vec::with_capacity(IOSLICE_BATCH_SIZE);
    let mut end = data.len();

    for &pos in positions.iter().rev() {
        if pos < end {
            slices.push(IoSlice::new(&data[pos..end]));
            if slices.len() >= IOSLICE_BATCH_SIZE {
                write_all_vectored(out, &slices)?;
                slices.clear();
            }
        }
        end = pos;
    }
    if end > 0 {
        slices.push(IoSlice::new(&data[..end]));
    }
    if !slices.is_empty() {
        write_all_vectored(out, &slices)?;
    }
    Ok(())
}

/// Find regex matches using backward scanning, matching GNU tac's re_search behavior.
fn find_regex_matches_backward(data: &[u8], re: &regex::bytes::Regex) -> Vec<(usize, usize)> {
    let mut matches = Vec::new();
    let mut past_end = data.len();

    while past_end > 0 {
        let buf = &data[..past_end];
        let mut found = false;

        let mut pos = past_end;
        while pos > 0 {
            pos -= 1;
            if let Some(m) = re.find_at(buf, pos) {
                if m.start() == pos {
                    matches.push((m.start(), m.end()));
                    past_end = m.start();
                    found = true;
                    break;
                }
            }
        }

        if !found {
            break;
        }
    }

    matches.reverse();
    matches
}

/// Reverse records using a regex separator.
/// Uses write_vectored for regex path (typically few large records).
pub fn tac_regex_separator(
    data: &[u8],
    pattern: &str,
    before: bool,
    out: &mut impl Write,
) -> io::Result<()> {
    if data.is_empty() {
        return Ok(());
    }

    let re = match regex::bytes::Regex::new(pattern) {
        Ok(r) => r,
        Err(e) => {
            return Err(io::Error::new(
                io::ErrorKind::InvalidInput,
                format!("invalid regex '{}': {}", pattern, e),
            ));
        }
    };

    let matches = find_regex_matches_backward(data, &re);

    if matches.is_empty() {
        out.write_all(data)?;
        return Ok(());
    }

    // For regex separators, use write_vectored since there are typically
    // few large records. Build all IoSlices at once and flush.
    let mut slices: Vec<IoSlice<'_>> = Vec::with_capacity(matches.len() + 2);

    if !before {
        let last_end = matches.last().unwrap().1;

        if last_end < data.len() {
            slices.push(IoSlice::new(&data[last_end..]));
        }

        let mut i = matches.len();
        while i > 0 {
            i -= 1;
            let rec_start = if i == 0 { 0 } else { matches[i - 1].1 };
            slices.push(IoSlice::new(&data[rec_start..matches[i].1]));
        }
    } else {
        let mut i = matches.len();
        while i > 0 {
            i -= 1;
            let start = matches[i].0;
            let end = if i + 1 < matches.len() {
                matches[i + 1].0
            } else {
                data.len()
            };
            slices.push(IoSlice::new(&data[start..end]));
        }

        if matches[0].0 > 0 {
            slices.push(IoSlice::new(&data[..matches[0].0]));
        }
    }

    write_all_vectored(out, &slices)
}

/// Write all IoSlice entries, handling partial writes.
/// Hot path: single write_vectored succeeds fully (common on Linux pipes/files).
/// Cold path: partial write handled out-of-line to keep hot path tight.
#[inline(always)]
fn write_all_vectored(out: &mut impl Write, slices: &[IoSlice<'_>]) -> io::Result<()> {
    let total: usize = slices.iter().map(|s| s.len()).sum();
    let written = out.write_vectored(slices)?;
    if written >= total {
        return Ok(());
    }
    if written == 0 {
        return Err(io::Error::new(io::ErrorKind::WriteZero, "write zero"));
    }
    flush_vectored_slow(out, slices, written)
}

/// Handle partial write (cold path, never inlined).
#[cold]
#[inline(never)]
fn flush_vectored_slow(
    out: &mut impl Write,
    slices: &[IoSlice<'_>],
    mut skip: usize,
) -> io::Result<()> {
    for slice in slices {
        let len = slice.len();
        if skip >= len {
            skip -= len;
            continue;
        }
        out.write_all(&slice[skip..])?;
        skip = 0;
    }
    Ok(())
}