coreutils_rs/uniq/

core.rs

use std::io::{self, BufRead, BufReader, BufWriter, Read, Write};

/// Write a large contiguous buffer, retrying on partial writes.
#[inline]
fn write_all_raw(writer: &mut impl Write, buf: &[u8]) -> io::Result<()> {
    writer.write_all(buf)
}

/// How to delimit groups when using --all-repeated
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AllRepeatedMethod {
    None,
    Prepend,
    Separate,
}

/// How to delimit groups when using --group
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum GroupMethod {
    Separate,
    Prepend,
    Append,
    Both,
}

/// Output mode for uniq
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum OutputMode {
    /// Default: print unique lines and first of each duplicate group
    Default,
    /// -d: print only first line of duplicate groups
    RepeatedOnly,
    /// -D / --all-repeated: print ALL duplicate lines
    AllRepeated(AllRepeatedMethod),
    /// -u: print only lines that are NOT duplicated
    UniqueOnly,
    /// --group: show all items with group separators
    Group(GroupMethod),
}

/// Configuration for uniq processing
#[derive(Debug, Clone)]
pub struct UniqConfig {
    pub mode: OutputMode,
    pub count: bool,
    pub ignore_case: bool,
    pub skip_fields: usize,
    pub skip_chars: usize,
    pub check_chars: Option<usize>,
    pub zero_terminated: bool,
}

impl Default for UniqConfig {
    fn default() -> Self {
        Self {
            mode: OutputMode::Default,
            count: false,
            ignore_case: false,
            skip_fields: 0,
            skip_chars: 0,
            check_chars: None,
            zero_terminated: false,
        }
    }
}

/// Extract the comparison key from a line according to skip_fields, skip_chars, check_chars.
/// Matches GNU uniq field-skip semantics exactly: for each field, skip blanks then non-blanks.
#[inline(always)]
fn get_compare_slice<'a>(line: &'a [u8], config: &UniqConfig) -> &'a [u8] {
    let mut start = 0;
    let len = line.len();

    // Skip N fields (GNU: each field = run of blanks + run of non-blanks)
    for _ in 0..config.skip_fields {
        // Skip blanks (space and tab)
        while start < len && (line[start] == b' ' || line[start] == b'\t') {
            start += 1;
        }
        // Skip non-blanks (field content)
        while start < len && line[start] != b' ' && line[start] != b'\t' {
            start += 1;
        }
    }

    // Skip N characters
    if config.skip_chars > 0 {
        let remaining = len - start;
        let skip = config.skip_chars.min(remaining);
        start += skip;
    }

    let slice = &line[start..];

    // Limit comparison to N characters
    if let Some(w) = config.check_chars {
        if w < slice.len() {
            return &slice[..w];
        }
    }

    slice
}

/// Compare two lines (without terminators) using the config's comparison rules.
#[inline(always)]
fn lines_equal(a: &[u8], b: &[u8], config: &UniqConfig) -> bool {
    let sa = get_compare_slice(a, config);
    let sb = get_compare_slice(b, config);

    if config.ignore_case {
        sa.eq_ignore_ascii_case(sb)
    } else {
        sa == sb
    }
}

/// Check if config requires field/char skipping or char limiting.
#[inline(always)]
fn needs_key_extraction(config: &UniqConfig) -> bool {
    config.skip_fields > 0 || config.skip_chars > 0 || config.check_chars.is_some()
}

/// Fast path comparison: no field/char extraction needed, no case folding.
/// Uses pointer+length equality shortcut and 8-byte prefix rejection.
#[inline(always)]
fn lines_equal_fast(a: &[u8], b: &[u8]) -> bool {
    let alen = a.len();
    if alen != b.len() {
        return false;
    }
    if alen == 0 {
        return true;
    }
    // 8-byte prefix check: reject most non-equal lines without full memcmp
    if alen >= 8 {
        let a8 = unsafe { (a.as_ptr() as *const u64).read_unaligned() };
        let b8 = unsafe { (b.as_ptr() as *const u64).read_unaligned() };
        if a8 != b8 {
            return false;
        }
    }
    a == b
}

/// Write a count-prefixed line in GNU uniq format.
/// GNU format: "%7lu " — right-aligned in 7-char field, followed by space.
/// Combines prefix + line + term into a single write for short lines (< 240 bytes).
#[inline(always)]
fn write_count_line(out: &mut impl Write, count: u64, line: &[u8], term: u8) -> io::Result<()> {
    // Build prefix "     N " in a stack buffer (max 21 bytes for u64 + spaces)
    let mut prefix = [b' '; 28]; // Enough for u64 max + padding + space
    let digits = itoa_right_aligned_into(&mut prefix, count);
    let width = digits.max(7); // minimum 7 chars
    let prefix_len = width + 1; // +1 for trailing space
    prefix[width] = b' ';

    // Single write for short lines (common case) — avoids 3 separate BufWriter calls
    let total = prefix_len + line.len() + 1;
    if total <= 256 {
        let mut buf = [0u8; 256];
        buf[..prefix_len].copy_from_slice(&prefix[..prefix_len]);
        buf[prefix_len..prefix_len + line.len()].copy_from_slice(line);
        buf[prefix_len + line.len()] = term;
        out.write_all(&buf[..total])
    } else {
        out.write_all(&prefix[..prefix_len])?;
        out.write_all(line)?;
        out.write_all(&[term])
    }
}

/// Write u64 decimal right-aligned into prefix buffer.
/// Buffer is pre-filled with spaces. Returns number of digits written.
#[inline(always)]
fn itoa_right_aligned_into(buf: &mut [u8; 28], mut val: u64) -> usize {
    if val == 0 {
        buf[6] = b'0';
        return 7; // 6 spaces + '0' = 7 chars
    }
    // Write digits right-to-left from position 27 (leaving room for trailing space)
    let mut pos = 27;
    while val > 0 {
        pos -= 1;
        buf[pos] = b'0' + (val % 10) as u8;
        val /= 10;
    }
    let num_digits = 27 - pos;
    if num_digits >= 7 {
        // Number is wide enough, shift to front
        buf.copy_within(pos..27, 0);
        num_digits
    } else {
        // Right-align in 7-char field: spaces then digits
        let pad = 7 - num_digits;
        buf.copy_within(pos..27, pad);
        // buf[0..pad] is already spaces from initialization
        7
    }
}

// ============================================================================
// High-performance mmap-based processing (for byte slices, zero-copy)
// ============================================================================

/// Process uniq from a byte slice (mmap'd file). Zero-copy, no per-line allocation.
pub fn process_uniq_bytes(data: &[u8], output: impl Write, config: &UniqConfig) -> io::Result<()> {
    // 16MB output buffer for fewer flush syscalls on large inputs
    let mut writer = BufWriter::with_capacity(16 * 1024 * 1024, output);
    let term = if config.zero_terminated { b'\0' } else { b'\n' };

    match config.mode {
        OutputMode::Group(method) => {
            process_group_bytes(data, &mut writer, config, method, term)?;
        }
        OutputMode::AllRepeated(method) => {
            process_all_repeated_bytes(data, &mut writer, config, method, term)?;
        }
        _ => {
            process_standard_bytes(data, &mut writer, config, term)?;
        }
    }

    writer.flush()?;
    Ok(())
}

/// Iterator over lines in a byte slice, yielding (line_without_terminator, has_terminator).
/// Uses memchr for SIMD-accelerated line boundary detection.
struct LineIter<'a> {
    data: &'a [u8],
    pos: usize,
    term: u8,
}

impl<'a> LineIter<'a> {
    #[inline(always)]
    fn new(data: &'a [u8], term: u8) -> Self {
        Self { data, pos: 0, term }
    }
}

impl<'a> Iterator for LineIter<'a> {
    /// (line content without terminator, full line including terminator for output)
    type Item = (&'a [u8], &'a [u8]);

    #[inline(always)]
    fn next(&mut self) -> Option<Self::Item> {
        if self.pos >= self.data.len() {
            return None;
        }

        let remaining = &self.data[self.pos..];
        match memchr::memchr(self.term, remaining) {
            Some(idx) => {
                let line_start = self.pos;
                let line_end = self.pos + idx; // without terminator
                let full_end = self.pos + idx + 1; // with terminator
                self.pos = full_end;
                Some((
                    &self.data[line_start..line_end],
                    &self.data[line_start..full_end],
                ))
            }
            None => {
                // Last line without terminator
                let line_start = self.pos;
                self.pos = self.data.len();
                let line = &self.data[line_start..];
                Some((line, line))
            }
        }
    }
}

/// Get line content (without terminator) from pre-computed positions.
/// `content_end` is the end of actual content (excludes trailing terminator if present).
#[inline(always)]
fn line_content_at<'a>(
    data: &'a [u8],
    line_starts: &[usize],
    idx: usize,
    content_end: usize,
) -> &'a [u8] {
    let start = line_starts[idx];
    let end = if idx + 1 < line_starts.len() {
        line_starts[idx + 1] - 1 // exclude terminator
    } else {
        content_end // last line: pre-computed to exclude trailing terminator
    };
    &data[start..end]
}

/// Get full line (with terminator) from pre-computed positions.
#[inline(always)]
fn line_full_at<'a>(data: &'a [u8], line_starts: &[usize], idx: usize) -> &'a [u8] {
    let start = line_starts[idx];
    let end = if idx + 1 < line_starts.len() {
        line_starts[idx + 1] // include terminator
    } else {
        data.len()
    };
    &data[start..end]
}

/// Linear scan for the end of a duplicate group.
/// Returns the index of the first line that differs from line_starts[group_start].
/// Must use linear scan (not binary search) because uniq input may NOT be sorted —
/// equal lines can appear in non-adjacent groups separated by different lines.
#[inline]
fn linear_scan_group_end(
    data: &[u8],
    line_starts: &[usize],
    group_start: usize,
    num_lines: usize,
    content_end: usize,
) -> usize {
    let key = line_content_at(data, line_starts, group_start, content_end);
    let mut i = group_start + 1;
    while i < num_lines {
        if !lines_equal_fast(key, line_content_at(data, line_starts, i, content_end)) {
            return i;
        }
        i += 1;
    }
    i
}

/// Standard processing for Default, RepeatedOnly, UniqueOnly on byte slices.
/// Ultra-fast path: single-pass inline scanning with memchr, no line_starts Vec.
/// General path: pre-computed line positions with binary search for groups.
fn process_standard_bytes(
    data: &[u8],
    writer: &mut impl Write,
    config: &UniqConfig,
    term: u8,
) -> io::Result<()> {
    if data.is_empty() {
        return Ok(());
    }

    let fast = !needs_key_extraction(config) && !config.ignore_case;

    // Ultra-fast path: default mode, no count, no key extraction.
    // Single-pass: scan with memchr, compare adjacent lines inline.
    // Avoids the 20MB+ line_starts allocation + cache misses from random access.
    if fast && !config.count && matches!(config.mode, OutputMode::Default) {
        return process_default_fast_singlepass(data, writer, term);
    }

    // Ultra-fast path: repeated-only or unique-only, no count, no key extraction
    if fast
        && !config.count
        && matches!(
            config.mode,
            OutputMode::RepeatedOnly | OutputMode::UniqueOnly
        )
    {
        return process_filter_fast_singlepass(data, writer, config, term);
    }

    // General path: pre-computed line positions for binary search on groups
    let estimated_lines = (data.len() / 40).max(64);
    let mut line_starts: Vec<usize> = Vec::with_capacity(estimated_lines);
    line_starts.push(0);
    for pos in memchr::memchr_iter(term, data) {
        if pos + 1 < data.len() {
            line_starts.push(pos + 1);
        }
    }
    let num_lines = line_starts.len();
    if num_lines == 0 {
        return Ok(());
    }

    // Pre-compute content end: if data ends with terminator, exclude it for last line
    let content_end = if data.last() == Some(&term) {
        data.len() - 1
    } else {
        data.len()
    };

    // Ultra-fast path: default mode, no count, no key extraction
    if fast && !config.count && matches!(config.mode, OutputMode::Default) {
        // Write first line
        let first_full = line_full_at(data, &line_starts, 0);
        let first_content = line_content_at(data, &line_starts, 0, content_end);
        write_all_raw(writer, first_full)?;
        if first_full.len() == first_content.len() {
            writer.write_all(&[term])?;
        }

        let mut i = 1;
        while i < num_lines {
            let prev = line_content_at(data, &line_starts, i - 1, content_end);
            let cur = line_content_at(data, &line_starts, i, content_end);

            if lines_equal_fast(prev, cur) {
                // Duplicate detected — linear scan for end of group
                let group_end =
                    linear_scan_group_end(data, &line_starts, i - 1, num_lines, content_end);
                i = group_end;
                continue;
            }

            // Unique line — write it
            let cur_full = line_full_at(data, &line_starts, i);
            write_all_raw(writer, cur_full)?;
            if cur_full.len() == cur.len() {
                writer.write_all(&[term])?;
            }
            i += 1;
        }
        return Ok(());
    }

    // General path with count tracking
    let mut i = 0;
    while i < num_lines {
        let content = line_content_at(data, &line_starts, i, content_end);
        let full = line_full_at(data, &line_starts, i);

        let group_end = if fast
            && i + 1 < num_lines
            && lines_equal_fast(
                content,
                line_content_at(data, &line_starts, i + 1, content_end),
            ) {
            // Duplicate detected — linear scan for end
            linear_scan_group_end(data, &line_starts, i, num_lines, content_end)
        } else if !fast
            && i + 1 < num_lines
            && lines_equal(
                content,
                line_content_at(data, &line_starts, i + 1, content_end),
                config,
            )
        {
            // Slow path linear scan with key extraction
            let mut j = i + 2;
            while j < num_lines {
                if !lines_equal(
                    content,
                    line_content_at(data, &line_starts, j, content_end),
                    config,
                ) {
                    break;
                }
                j += 1;
            }
            j
        } else {
            i + 1
        };

        let count = (group_end - i) as u64;
        output_group_bytes(writer, content, full, count, config, term)?;
        i = group_end;
    }

    Ok(())
}

/// Ultra-fast single-pass default mode: scan with memchr, compare adjacent lines inline.
/// No pre-computed positions, no binary search, no Vec allocation.
/// Outputs each line that differs from the previous.
fn process_default_fast_singlepass(
    data: &[u8],
    writer: &mut impl Write,
    term: u8,
) -> io::Result<()> {
    // Batch output into a contiguous buffer for fewer write() syscalls.
    let mut outbuf = Vec::with_capacity(data.len());

    let mut prev_start: usize = 0;
    let mut prev_end: usize; // exclusive, without terminator

    // Find end of first line
    match memchr::memchr(term, data) {
        Some(pos) => {
            prev_end = pos;
            // Write first line (always output)
            outbuf.extend_from_slice(&data[0..pos + 1]);
        }
        None => {
            // Single line, no terminator
            outbuf.extend_from_slice(data);
            outbuf.push(term);
            return writer.write_all(&outbuf);
        }
    }

    let mut cur_start = prev_end + 1;

    while cur_start < data.len() {
        let cur_end = match memchr::memchr(term, &data[cur_start..]) {
            Some(offset) => cur_start + offset,
            None => data.len(), // last line without terminator
        };

        let prev_content = &data[prev_start..prev_end];
        let cur_content = &data[cur_start..cur_end];

        if !lines_equal_fast(prev_content, cur_content) {
            // Different line — output it
            if cur_end < data.len() {
                outbuf.extend_from_slice(&data[cur_start..cur_end + 1]);
            } else {
                outbuf.extend_from_slice(&data[cur_start..cur_end]);
                outbuf.push(term);
            }
            prev_start = cur_start;
            prev_end = cur_end;
        }

        if cur_end < data.len() {
            cur_start = cur_end + 1;
        } else {
            break;
        }
    }

    writer.write_all(&outbuf)
}

/// Fast single-pass for RepeatedOnly (-d) and UniqueOnly (-u) modes.
fn process_filter_fast_singlepass(
    data: &[u8],
    writer: &mut impl Write,
    config: &UniqConfig,
    term: u8,
) -> io::Result<()> {
    let repeated = matches!(config.mode, OutputMode::RepeatedOnly);
    let mut outbuf = Vec::with_capacity(data.len() / 2);

    let prev_start: usize = 0;
    let prev_end: usize = match memchr::memchr(term, data) {
        Some(pos) => pos,
        None => {
            // Single line: unique (count=1)
            if !repeated {
                outbuf.extend_from_slice(data);
                outbuf.push(term);
            }
            return writer.write_all(&outbuf);
        }
    };

    let mut prev_start_mut = prev_start;
    let mut prev_end_mut = prev_end;
    let mut count: u64 = 1;
    let mut cur_start = prev_end + 1;

    while cur_start < data.len() {
        let cur_end = match memchr::memchr(term, &data[cur_start..]) {
            Some(offset) => cur_start + offset,
            None => data.len(),
        };

        let prev_content = &data[prev_start_mut..prev_end_mut];
        let cur_content = &data[cur_start..cur_end];

        if lines_equal_fast(prev_content, cur_content) {
            count += 1;
        } else {
            // Output previous group based on mode
            let should_print = if repeated { count > 1 } else { count == 1 };
            if should_print {
                if prev_end_mut < data.len() && data.get(prev_end_mut) == Some(&term) {
                    outbuf.extend_from_slice(&data[prev_start_mut..prev_end_mut + 1]);
                } else {
                    outbuf.extend_from_slice(&data[prev_start_mut..prev_end_mut]);
                    outbuf.push(term);
                }
            }
            prev_start_mut = cur_start;
            prev_end_mut = cur_end;
            count = 1;
        }

        if cur_end < data.len() {
            cur_start = cur_end + 1;
        } else {
            break;
        }
    }

    // Output last group
    let should_print = if repeated { count > 1 } else { count == 1 };
    if should_print {
        if prev_end_mut < data.len() && data.get(prev_end_mut) == Some(&term) {
            outbuf.extend_from_slice(&data[prev_start_mut..prev_end_mut + 1]);
        } else {
            outbuf.extend_from_slice(&data[prev_start_mut..prev_end_mut]);
            outbuf.push(term);
        }
    }

    writer.write_all(&outbuf)
}

/// Output a group for standard modes (bytes path).
#[inline(always)]
fn output_group_bytes(
    writer: &mut impl Write,
    content: &[u8],
    full: &[u8],
    count: u64,
    config: &UniqConfig,
    term: u8,
) -> io::Result<()> {
    let should_print = match config.mode {
        OutputMode::Default => true,
        OutputMode::RepeatedOnly => count > 1,
        OutputMode::UniqueOnly => count == 1,
        _ => true,
    };

    if should_print {
        if config.count {
            write_count_line(writer, count, content, term)?;
        } else {
            writer.write_all(full)?;
            // Add terminator if the original line didn't have one
            if full.len() == content.len() {
                writer.write_all(&[term])?;
            }
        }
    }

    Ok(())
}

/// Process --all-repeated / -D mode on byte slices.
fn process_all_repeated_bytes(
    data: &[u8],
    writer: &mut impl Write,
    config: &UniqConfig,
    method: AllRepeatedMethod,
    term: u8,
) -> io::Result<()> {
    let mut lines = LineIter::new(data, term);

    let first = match lines.next() {
        Some(v) => v,
        None => return Ok(()),
    };

    // Collect groups as (start_offset, line_count, first_line_content, lines_vec)
    // For all-repeated we need to buffer group lines since we only print if count > 1
    let mut group_lines: Vec<(&[u8], &[u8])> = Vec::with_capacity(64);
    group_lines.push(first);
    let mut first_group_printed = false;

    let fast = !needs_key_extraction(config) && !config.ignore_case;

    for (cur_content, cur_full) in lines {
        let prev_content = group_lines.last().unwrap().0;
        let equal = if fast {
            lines_equal_fast(prev_content, cur_content)
        } else {
            lines_equal(prev_content, cur_content, config)
        };

        if equal {
            group_lines.push((cur_content, cur_full));
        } else {
            // Flush group
            flush_all_repeated_bytes(writer, &group_lines, method, &mut first_group_printed, term)?;
            group_lines.clear();
            group_lines.push((cur_content, cur_full));
        }
    }

    // Flush last group
    flush_all_repeated_bytes(writer, &group_lines, method, &mut first_group_printed, term)?;

    Ok(())
}

/// Flush a group for --all-repeated mode (bytes path).
fn flush_all_repeated_bytes(
    writer: &mut impl Write,
    group: &[(&[u8], &[u8])],
    method: AllRepeatedMethod,
    first_group_printed: &mut bool,
    term: u8,
) -> io::Result<()> {
    if group.len() <= 1 {
        return Ok(()); // Not a duplicate group
    }

    match method {
        AllRepeatedMethod::Prepend => {
            writer.write_all(&[term])?;
        }
        AllRepeatedMethod::Separate => {
            if *first_group_printed {
                writer.write_all(&[term])?;
            }
        }
        AllRepeatedMethod::None => {}
    }

    for &(content, full) in group {
        writer.write_all(full)?;
        if full.len() == content.len() {
            writer.write_all(&[term])?;
        }
    }

    *first_group_printed = true;
    Ok(())
}

/// Process --group mode on byte slices.
fn process_group_bytes(
    data: &[u8],
    writer: &mut impl Write,
    config: &UniqConfig,
    method: GroupMethod,
    term: u8,
) -> io::Result<()> {
    let mut lines = LineIter::new(data, term);

    let (prev_content, prev_full) = match lines.next() {
        Some(v) => v,
        None => return Ok(()),
    };

    // Prepend/Both: separator before first group
    if matches!(method, GroupMethod::Prepend | GroupMethod::Both) {
        writer.write_all(&[term])?;
    }

    // Write first line
    writer.write_all(prev_full)?;
    if prev_full.len() == prev_content.len() {
        writer.write_all(&[term])?;
    }

    let mut prev_content = prev_content;
    let fast = !needs_key_extraction(config) && !config.ignore_case;

    for (cur_content, cur_full) in lines {
        let equal = if fast {
            lines_equal_fast(prev_content, cur_content)
        } else {
            lines_equal(prev_content, cur_content, config)
        };

        if !equal {
            // New group — write separator
            writer.write_all(&[term])?;
        }

        writer.write_all(cur_full)?;
        if cur_full.len() == cur_content.len() {
            writer.write_all(&[term])?;
        }

        prev_content = cur_content;
    }

    // Append/Both: separator after last group
    if matches!(method, GroupMethod::Append | GroupMethod::Both) {
        writer.write_all(&[term])?;
    }

    Ok(())
}

// ============================================================================
// Streaming processing (for stdin / pipe input)
// ============================================================================

/// Main streaming uniq processor.
/// Reads from `input`, writes to `output`.
pub fn process_uniq<R: Read, W: Write>(input: R, output: W, config: &UniqConfig) -> io::Result<()> {
    let reader = BufReader::with_capacity(8 * 1024 * 1024, input);
    let mut writer = BufWriter::with_capacity(16 * 1024 * 1024, output);
    let term = if config.zero_terminated { b'\0' } else { b'\n' };

    match config.mode {
        OutputMode::Group(method) => {
            process_group_stream(reader, &mut writer, config, method, term)?;
        }
        OutputMode::AllRepeated(method) => {
            process_all_repeated_stream(reader, &mut writer, config, method, term)?;
        }
        _ => {
            process_standard_stream(reader, &mut writer, config, term)?;
        }
    }

    writer.flush()?;
    Ok(())
}

/// Standard processing for Default, RepeatedOnly, UniqueOnly modes (streaming).
fn process_standard_stream<R: BufRead, W: Write>(
    mut reader: R,
    writer: &mut W,
    config: &UniqConfig,
    term: u8,
) -> io::Result<()> {
    let mut prev_line: Vec<u8> = Vec::with_capacity(4096);
    let mut current_line: Vec<u8> = Vec::with_capacity(4096);

    // Read first line
    if read_line_term(&mut reader, &mut prev_line, term)? == 0 {
        return Ok(()); // empty input
    }
    let mut count: u64 = 1;

    loop {
        current_line.clear();
        let bytes_read = read_line_term(&mut reader, &mut current_line, term)?;

        if bytes_read == 0 {
            // End of input — output the last group
            output_group_stream(writer, &prev_line, count, config, term)?;
            break;
        }

        if compare_lines_stream(&prev_line, &current_line, config, term) {
            count += 1;
        } else {
            output_group_stream(writer, &prev_line, count, config, term)?;
            std::mem::swap(&mut prev_line, &mut current_line);
            count = 1;
        }
    }

    Ok(())
}

/// Compare two lines (with terminators) in streaming mode.
#[inline(always)]
fn compare_lines_stream(a: &[u8], b: &[u8], config: &UniqConfig, term: u8) -> bool {
    let a_stripped = strip_term(a, term);
    let b_stripped = strip_term(b, term);
    lines_equal(a_stripped, b_stripped, config)
}

/// Strip terminator from end of line.
#[inline(always)]
fn strip_term(line: &[u8], term: u8) -> &[u8] {
    if line.last() == Some(&term) {
        &line[..line.len() - 1]
    } else {
        line
    }
}

/// Output a group in streaming mode.
#[inline(always)]
fn output_group_stream(
    writer: &mut impl Write,
    line: &[u8],
    count: u64,
    config: &UniqConfig,
    term: u8,
) -> io::Result<()> {
    let should_print = match config.mode {
        OutputMode::Default => true,
        OutputMode::RepeatedOnly => count > 1,
        OutputMode::UniqueOnly => count == 1,
        _ => true,
    };

    if should_print {
        let content = strip_term(line, term);
        if config.count {
            write_count_line(writer, count, content, term)?;
        } else {
            writer.write_all(content)?;
            writer.write_all(&[term])?;
        }
    }

    Ok(())
}

/// Process --all-repeated / -D mode (streaming).
fn process_all_repeated_stream<R: BufRead, W: Write>(
    mut reader: R,
    writer: &mut W,
    config: &UniqConfig,
    method: AllRepeatedMethod,
    term: u8,
) -> io::Result<()> {
    let mut group: Vec<Vec<u8>> = Vec::new();
    let mut current_line: Vec<u8> = Vec::with_capacity(4096);
    let mut first_group_printed = false;

    current_line.clear();
    if read_line_term(&mut reader, &mut current_line, term)? == 0 {
        return Ok(());
    }
    group.push(current_line.clone());

    loop {
        current_line.clear();
        let bytes_read = read_line_term(&mut reader, &mut current_line, term)?;

        if bytes_read == 0 {
            flush_all_repeated_stream(writer, &group, method, &mut first_group_printed, term)?;
            break;
        }

        if compare_lines_stream(group.last().unwrap(), &current_line, config, term) {
            group.push(current_line.clone());
        } else {
            flush_all_repeated_stream(writer, &group, method, &mut first_group_printed, term)?;
            group.clear();
            group.push(current_line.clone());
        }
    }

    Ok(())
}

/// Flush a group for --all-repeated mode (streaming).
fn flush_all_repeated_stream(
    writer: &mut impl Write,
    group: &[Vec<u8>],
    method: AllRepeatedMethod,
    first_group_printed: &mut bool,
    term: u8,
) -> io::Result<()> {
    if group.len() <= 1 {
        return Ok(());
    }

    match method {
        AllRepeatedMethod::Prepend => {
            writer.write_all(&[term])?;
        }
        AllRepeatedMethod::Separate => {
            if *first_group_printed {
                writer.write_all(&[term])?;
            }
        }
        AllRepeatedMethod::None => {}
    }

    for line in group {
        let content = strip_term(line, term);
        writer.write_all(content)?;
        writer.write_all(&[term])?;
    }

    *first_group_printed = true;
    Ok(())
}

/// Process --group mode (streaming).
fn process_group_stream<R: BufRead, W: Write>(
    mut reader: R,
    writer: &mut W,
    config: &UniqConfig,
    method: GroupMethod,
    term: u8,
) -> io::Result<()> {
    let mut prev_line: Vec<u8> = Vec::with_capacity(4096);
    let mut current_line: Vec<u8> = Vec::with_capacity(4096);

    if read_line_term(&mut reader, &mut prev_line, term)? == 0 {
        return Ok(());
    }

    // Prepend/Both: separator before first group
    if matches!(method, GroupMethod::Prepend | GroupMethod::Both) {
        writer.write_all(&[term])?;
    }

    let content = strip_term(&prev_line, term);
    writer.write_all(content)?;
    writer.write_all(&[term])?;

    loop {
        current_line.clear();
        let bytes_read = read_line_term(&mut reader, &mut current_line, term)?;

        if bytes_read == 0 {
            if matches!(method, GroupMethod::Append | GroupMethod::Both) {
                writer.write_all(&[term])?;
            }
            break;
        }

        if !compare_lines_stream(&prev_line, &current_line, config, term) {
            writer.write_all(&[term])?;
        }

        let content = strip_term(&current_line, term);
        writer.write_all(content)?;
        writer.write_all(&[term])?;

        std::mem::swap(&mut prev_line, &mut current_line);
    }

    Ok(())
}

/// Read a line terminated by the given byte (newline or NUL).
/// Returns number of bytes read (0 = EOF).
#[inline(always)]
fn read_line_term<R: BufRead>(reader: &mut R, buf: &mut Vec<u8>, term: u8) -> io::Result<usize> {
    reader.read_until(term, buf)
}