// This file is part of the uutils diffutils package.
//
// For the full copyright and license information, please view the LICENSE-*
// files that was distributed with this source code.

use std::collections::VecDeque;
use std::io::Write;

use crate::params::Params;
use crate::utils::do_write_line;
use crate::utils::get_modification_time;

#[derive(Debug, PartialEq)]
pub enum DiffLine {
    Context(Vec<u8>),
    Expected(Vec<u8>),
    Actual(Vec<u8>),
    MissingNL,
}

#[derive(Debug, PartialEq)]
struct Mismatch {
    pub line_number_expected: u32,
    pub line_number_actual: u32,
    pub lines: Vec<DiffLine>,
}

impl Mismatch {
    fn new(line_number_expected: u32, line_number_actual: u32) -> Mismatch {
        Mismatch {
            line_number_expected,
            line_number_actual,
            lines: Vec::new(),
        }
    }
}

// Produces a diff between the expected output and actual output.
fn make_diff(
    expected: &[u8],
    actual: &[u8],
    context_size: usize,
    stop_early: bool,
) -> Vec<Mismatch> {
    let mut line_number_expected = 1;
    let mut line_number_actual = 1;
    let mut context_queue: VecDeque<&[u8]> = VecDeque::with_capacity(context_size);
    // Like GNU diff, keep changes in one hunk while at most twice the context
    // size of unchanged lines separates them.
    let max_gap = 2 * context_size;
    let mut lines_since_mismatch = max_gap + 1;
    let mut results = Vec::new();
    let mut mismatch = Mismatch::new(0, 0);

    let mut expected_lines: Vec<&[u8]> = expected.split(|&c| c == b'\n').collect();
    let mut actual_lines: Vec<&[u8]> = actual.split(|&c| c == b'\n').collect();

    debug_assert_eq!(b"".split(|&c| c == b'\n').count(), 1);
    // ^ means that underflow here is impossible
    let expected_lines_count = expected_lines.len() as u32 - 1;
    let actual_lines_count = actual_lines.len() as u32 - 1;

    if expected_lines.last() == Some(&&b""[..]) {
        expected_lines.pop();
    }

    if actual_lines.last() == Some(&&b""[..]) {
        actual_lines.pop();
    }

    for result in diff::slice(&expected_lines, &actual_lines) {
        match result {
            diff::Result::Left(str) => {
                if lines_since_mismatch > max_gap {
                    results.push(mismatch);
                    mismatch = Mismatch::new(
                        line_number_expected - context_queue.len() as u32,
                        line_number_actual - context_queue.len() as u32,
                    );
                }

                while let Some(line) = context_queue.pop_front() {
                    mismatch.lines.push(DiffLine::Context(line.to_vec()));
                }

                if mismatch.lines.last() == Some(&DiffLine::MissingNL) {
                    mismatch.lines.pop();
                    match mismatch.lines.pop() {
                        Some(DiffLine::Actual(res)) => {
                            // We have to make sure that Actual (the + lines)
                            // always come after Expected (the - lines)
                            mismatch.lines.push(DiffLine::Expected(str.to_vec()));
                            if line_number_expected > expected_lines_count {
                                mismatch.lines.push(DiffLine::MissingNL);
                            }
                            mismatch.lines.push(DiffLine::Actual(res));
                            mismatch.lines.push(DiffLine::MissingNL);
                        }
                        _ => unreachable!("unterminated Left and Common lines shouldn't be followed by more Left lines"),
                    }
                } else {
                    mismatch.lines.push(DiffLine::Expected(str.to_vec()));
                    if line_number_expected > expected_lines_count {
                        mismatch.lines.push(DiffLine::MissingNL);
                    }
                }
                line_number_expected += 1;
                lines_since_mismatch = 0;
            }
            diff::Result::Right(str) => {
                if lines_since_mismatch > max_gap {
                    results.push(mismatch);
                    mismatch = Mismatch::new(
                        line_number_expected - context_queue.len() as u32,
                        line_number_actual - context_queue.len() as u32,
                    );
                }

                while let Some(line) = context_queue.pop_front() {
                    debug_assert!(mismatch.lines.last() != Some(&DiffLine::MissingNL));
                    mismatch.lines.push(DiffLine::Context(line.to_vec()));
                }

                mismatch.lines.push(DiffLine::Actual(str.to_vec()));
                if line_number_actual > actual_lines_count {
                    mismatch.lines.push(DiffLine::MissingNL);
                }
                line_number_actual += 1;
                lines_since_mismatch = 0;
            }
            diff::Result::Both(str, _) => {
                // if one of them is missing a newline and the other isn't, then they don't actually match
                if (line_number_actual > actual_lines_count)
                    && (line_number_expected > expected_lines_count)
                {
                    if context_queue.len() < context_size {
                        while let Some(line) = context_queue.pop_front() {
                            debug_assert!(mismatch.lines.last() != Some(&DiffLine::MissingNL));
                            mismatch.lines.push(DiffLine::Context(line.to_vec()));
                        }
                        if lines_since_mismatch < context_size {
                            mismatch.lines.push(DiffLine::Context(str.to_vec()));
                            mismatch.lines.push(DiffLine::MissingNL);
                        }
                    }
                    lines_since_mismatch = 0;
                } else if line_number_actual > actual_lines_count {
                    if lines_since_mismatch > max_gap {
                        results.push(mismatch);
                        mismatch = Mismatch::new(
                            line_number_expected - context_queue.len() as u32,
                            line_number_actual - context_queue.len() as u32,
                        );
                    }
                    while let Some(line) = context_queue.pop_front() {
                        debug_assert!(mismatch.lines.last() != Some(&DiffLine::MissingNL));
                        mismatch.lines.push(DiffLine::Context(line.to_vec()));
                    }
                    mismatch.lines.push(DiffLine::Expected(str.to_vec()));
                    mismatch.lines.push(DiffLine::Actual(str.to_vec()));
                    mismatch.lines.push(DiffLine::MissingNL);
                    lines_since_mismatch = 0;
                } else if line_number_expected > expected_lines_count {
                    if lines_since_mismatch > max_gap {
                        results.push(mismatch);
                        mismatch = Mismatch::new(
                            line_number_expected - context_queue.len() as u32,
                            line_number_actual - context_queue.len() as u32,
                        );
                    }
                    while let Some(line) = context_queue.pop_front() {
                        debug_assert!(mismatch.lines.last() != Some(&DiffLine::MissingNL));
                        mismatch.lines.push(DiffLine::Context(line.to_vec()));
                    }
                    mismatch.lines.push(DiffLine::Expected(str.to_vec()));
                    mismatch.lines.push(DiffLine::MissingNL);
                    mismatch.lines.push(DiffLine::Actual(str.to_vec()));
                    lines_since_mismatch = 0;
                } else {
                    debug_assert!(context_queue.len() <= context_size);
                    if context_queue.len() >= context_size {
                        let _ = context_queue.pop_front();
                    }
                    if lines_since_mismatch < context_size {
                        mismatch.lines.push(DiffLine::Context(str.to_vec()));
                    } else if context_size > 0 {
                        context_queue.push_back(str);
                    }
                    lines_since_mismatch += 1;
                }
                line_number_expected += 1;
                line_number_actual += 1;
            }
        }
        if stop_early && !results.is_empty() {
            // Optimization: stop analyzing the files as soon as there are any differences
            return results;
        }
    }

    results.push(mismatch);
    results.remove(0);

    if results.is_empty() && expected_lines_count != actual_lines_count {
        let mut mismatch = Mismatch::new(expected_lines.len() as u32, actual_lines.len() as u32);
        // empty diff and only expected lines has a missing line at end
        if expected_lines_count != expected_lines.len() as u32 {
            mismatch.lines.push(DiffLine::Expected(
                expected_lines
                    .pop()
                    .expect("can't be empty; produced by split()")
                    .to_vec(),
            ));
            mismatch.lines.push(DiffLine::MissingNL);
            mismatch.lines.push(DiffLine::Actual(
                actual_lines
                    .pop()
                    .expect("can't be empty; produced by split()")
                    .to_vec(),
            ));
            results.push(mismatch);
        } else if actual_lines_count != actual_lines.len() as u32 {
            mismatch.lines.push(DiffLine::Expected(
                expected_lines
                    .pop()
                    .expect("can't be empty; produced by split()")
                    .to_vec(),
            ));
            mismatch.lines.push(DiffLine::Actual(
                actual_lines
                    .pop()
                    .expect("can't be empty; produced by split()")
                    .to_vec(),
            ));
            mismatch.lines.push(DiffLine::MissingNL);
            results.push(mismatch);
        }
    }

    results
}

#[must_use]
pub fn diff(expected: &[u8], actual: &[u8], params: &Params) -> Vec<u8> {
    let from_modified_time = get_modification_time(&params.from.to_string_lossy());
    let to_modified_time = get_modification_time(&params.to.to_string_lossy());
    let mut output = format!(
        "--- {0}\t{1}\n+++ {2}\t{3}\n",
        params.from.to_string_lossy(),
        from_modified_time,
        params.to.to_string_lossy(),
        to_modified_time
    )
    .into_bytes();
    let diff_results = make_diff(expected, actual, params.context_count, params.brief);
    if diff_results.is_empty() {
        return Vec::new();
    }
    if params.brief {
        return output;
    }
    for result in diff_results {
        let mut line_number_expected = result.line_number_expected;
        let mut line_number_actual = result.line_number_actual;
        let mut expected_count = 0;
        let mut actual_count = 0;
        for line in &result.lines {
            match line {
                DiffLine::Expected(_) => {
                    expected_count += 1;
                }
                DiffLine::Context(_) => {
                    expected_count += 1;
                    actual_count += 1;
                }
                DiffLine::Actual(_) => {
                    actual_count += 1;
                }
                DiffLine::MissingNL => {}
            }
        }
        // Let's imagine this diff file
        //
        // --- a/something
        // +++ b/something
        // @@ -2,0 +3,1 @@
        // + x
        //
        // In the unified diff format as implemented by GNU diff and patch,
        // this is an instruction to insert the x *after* the preexisting line 2,
        // not before. You can demonstrate it this way:
        //
        // $ echo -ne '--- a/something\t\n+++ b/something\t\n@@ -2,0 +3,1 @@\n+ x\n' > diff
        // $ echo -ne 'a\nb\nc\nd\n' > something
        // $ patch -p1 < diff
        // patching file something
        // $ cat something
        // a
        // b
        //  x
        // c
        // d
        //
        // Notice how the x winds up at line 3, not line 2. This requires contortions to
        // work with our diffing algorithm, which keeps track of the "intended destination line",
        // not a line that things are supposed to be placed after. It's changing the first number,
        // not the second, that actually affects where the x goes.
        //
        // # change the first number from 2 to 3, and now the x is on line 4 (it's placed after line 3)
        // $ echo -ne '--- a/something\t\n+++ b/something\t\n@@ -3,0 +3,1 @@\n+ x\n' > diff
        // $ echo -ne 'a\nb\nc\nd\n' > something
        // $ patch -p1 < diff
        // patching file something
        // $ cat something
        // a
        // b
        // c
        //  x
        // d
        // # change the third number from 3 to 1000, and it's obvious that it's the first number that's
        // # actually being read
        // $ echo -ne '--- a/something\t\n+++ b/something\t\n@@ -2,0 +1000,1 @@\n+ x\n' > diff
        // $ echo -ne 'a\nb\nc\nd\n' > something
        // $ patch -p1 < diff
        // patching file something
        // $ cat something
        // a
        // b
        //  x
        // c
        // d
        //
        // Now watch what happens if I add a context line:
        //
        // $ echo -ne '--- a/something\t\n+++ b/something\t\n@@ -2,1 +3,2 @@\n+ x\n c\n' > diff
        // $ echo -ne 'a\nb\nc\nd\n' > something
        // $ patch -p1 < diff
        // patching file something
        // Hunk #1 succeeded at 3 (offset 1 line).
        //
        // It technically "succeeded", but this is a warning. We want to produce clean diffs.
        // Now that I have a context line, I'm supposed to say what line it's actually on, which is the
        // line that the x will wind up on, and not the line immediately before.
        //
        // $ echo -ne '--- a/something\t\n+++ b/something\t\n@@ -3,1 +3,2 @@\n+ x\n c\n' > diff
        // $ echo -ne 'a\nb\nc\nd\n' > something
        // $ patch -p1 < diff
        // patching file something
        // $ cat something
        // a
        // b
        //  x
        // c
        // d
        //
        // I made this comment because this stuff is not obvious from GNU's
        // documentation on the format at all.
        if expected_count == 0 {
            line_number_expected -= 1;
        }
        if actual_count == 0 {
            line_number_actual -= 1;
        }
        let exp_ct = if expected_count == 1 {
            String::new()
        } else {
            format!(",{expected_count}")
        };
        let act_ct = if actual_count == 1 {
            String::new()
        } else {
            format!(",{actual_count}")
        };
        writeln!(
            output,
            "@@ -{line_number_expected}{exp_ct} +{line_number_actual}{act_ct} @@"
        )
        .expect("write to Vec is infallible");
        for line in result.lines {
            match line {
                DiffLine::Expected(e) => {
                    write!(output, "-").expect("write to Vec is infallible");
                    do_write_line(&mut output, &e, params.expand_tabs, params.tabsize)
                        .expect("write to Vec is infallible");
                    writeln!(output).unwrap();
                }
                DiffLine::Context(c) => {
                    write!(output, " ").expect("write to Vec is infallible");
                    do_write_line(&mut output, &c, params.expand_tabs, params.tabsize)
                        .expect("write to Vec is infallible");
                    writeln!(output).unwrap();
                }
                DiffLine::Actual(r) => {
                    write!(output, "+",).expect("write to Vec is infallible");
                    do_write_line(&mut output, &r, params.expand_tabs, params.tabsize)
                        .expect("write to Vec is infallible");
                    writeln!(output).unwrap();
                }
                DiffLine::MissingNL => {
                    writeln!(output, r"\ No newline at end of file")
                        .expect("write to Vec is infallible");
                }
            }
        }
    }
    output
}

#[cfg(test)]
mod tests {
    use super::*;
    use pretty_assertions::assert_eq;

    use crate::utils::testcmds::PATCH_CMD;

    #[test]
    fn test_permutations() {
        let target = "target/unified-diff/";
        // test all possible six-line files.
        let _ = std::fs::create_dir(target);
        for &a in &[0, 1, 2] {
            for &b in &[0, 1, 2] {
                for &c in &[0, 1, 2] {
                    for &d in &[0, 1, 2] {
                        for &e in &[0, 1, 2] {
                            for &f in &[0, 1, 2] {
                                use std::fs::{self, File};
                                use std::io::Write;
                                let mut alef = Vec::new();
                                let mut bet = Vec::new();
                                alef.write_all(if a == 0 { b"a\n" } else { b"b\n" })
                                    .unwrap();
                                if a != 2 {
                                    bet.write_all(b"b\n").unwrap();
                                }
                                alef.write_all(if b == 0 { b"c\n" } else { b"d\n" })
                                    .unwrap();
                                if b != 2 {
                                    bet.write_all(b"d\n").unwrap();
                                }
                                alef.write_all(if c == 0 { b"e\n" } else { b"f\n" })
                                    .unwrap();
                                if c != 2 {
                                    bet.write_all(b"f\n").unwrap();
                                }
                                alef.write_all(if d == 0 { b"g\n" } else { b"h\n" })
                                    .unwrap();
                                if d != 2 {
                                    bet.write_all(b"h\n").unwrap();
                                }
                                alef.write_all(if e == 0 { b"i\n" } else { b"j\n" })
                                    .unwrap();
                                if e != 2 {
                                    bet.write_all(b"j\n").unwrap();
                                }
                                alef.write_all(if f == 0 { b"k\n" } else { b"l\n" })
                                    .unwrap();
                                if f != 2 {
                                    bet.write_all(b"l\n").unwrap();
                                }
                                // This test diff is intentionally reversed.
                                // We want it to turn the alef into bet.
                                let patched = &format!("{target}/alef");
                                let diff = diff(
                                    &alef,
                                    &bet,
                                    &Params {
                                        from: patched.into(),
                                        to: patched.into(),
                                        context_count: 2,
                                        ..Default::default()
                                    },
                                );
                                File::create(format!("{target}/ab.diff"))
                                    .unwrap()
                                    .write_all(&diff)
                                    .unwrap();
                                let mut fa = File::create(format!("{target}/alef")).unwrap();
                                fa.write_all(&alef[..]).unwrap();
                                let mut fb = File::create(format!("{target}/bet")).unwrap();
                                fb.write_all(&bet[..]).unwrap();
                                let _ = fa;
                                let _ = fb;
                                println!(
                                    "diff: {:?}",
                                    String::from_utf8(diff.clone())
                                        .unwrap_or_else(|_| String::from("[Invalid UTF-8]"))
                                );
                                println!(
                                    "alef: {:?}",
                                    String::from_utf8(alef.clone())
                                        .unwrap_or_else(|_| String::from("[Invalid UTF-8]"))
                                );
                                println!(
                                    "bet: {:?}",
                                    String::from_utf8(bet.clone())
                                        .unwrap_or_else(|_| String::from("[Invalid UTF-8]"))
                                );

                                use crate::utils::testcmds::PATCH_CMD;

                                let output = PATCH_CMD
                                    .new()
                                    .arg("-p0")
                                    .stdin(File::open(format!("{target}/ab.diff")).unwrap())
                                    .output()
                                    .unwrap();
                                println!("{}", String::from_utf8_lossy(&output.stdout));
                                println!("{}", String::from_utf8_lossy(&output.stderr));
                                assert!(output.status.success(), "{output:?}");
                                let alef = fs::read(format!("{target}/alef")).unwrap();
                                assert_eq!(alef, bet);
                            }
                        }
                    }
                }
            }
        }
    }

    #[test]
    fn test_permutations_missing_line_ending() {
        let target = "target/unified-diff/";
        // test all possible six-line files with missing newlines.
        let _ = std::fs::create_dir(target);
        for &a in &[0, 1, 2] {
            for &b in &[0, 1, 2] {
                for &c in &[0, 1, 2] {
                    for &d in &[0, 1, 2] {
                        for &e in &[0, 1, 2] {
                            for &f in &[0, 1, 2] {
                                for &g in &[0, 1, 2] {
                                    use std::fs::{self, File};
                                    use std::io::Write;
                                    let mut alef = Vec::new();
                                    let mut bet = Vec::new();
                                    alef.write_all(if a == 0 { b"a\n" } else { b"b\n" })
                                        .unwrap();
                                    if a != 2 {
                                        bet.write_all(b"b\n").unwrap();
                                    }
                                    alef.write_all(if b == 0 { b"c\n" } else { b"d\n" })
                                        .unwrap();
                                    if b != 2 {
                                        bet.write_all(b"d\n").unwrap();
                                    }
                                    alef.write_all(if c == 0 { b"e\n" } else { b"f\n" })
                                        .unwrap();
                                    if c != 2 {
                                        bet.write_all(b"f\n").unwrap();
                                    }
                                    alef.write_all(if d == 0 { b"g\n" } else { b"h\n" })
                                        .unwrap();
                                    if d != 2 {
                                        bet.write_all(b"h\n").unwrap();
                                    }
                                    alef.write_all(if e == 0 { b"i\n" } else { b"j\n" })
                                        .unwrap();
                                    if e != 2 {
                                        bet.write_all(b"j\n").unwrap();
                                    }
                                    alef.write_all(if f == 0 { b"k\n" } else { b"l\n" })
                                        .unwrap();
                                    if f != 2 {
                                        bet.write_all(b"l\n").unwrap();
                                    }
                                    match g {
                                        0 => {
                                            alef.pop();
                                        }
                                        1 => {
                                            bet.pop();
                                        }
                                        2 => {
                                            alef.pop();
                                            bet.pop();
                                        }
                                        _ => unreachable!(),
                                    }
                                    // This test diff is intentionally reversed.
                                    // We want it to turn the alef into bet.
                                    let patched = &format!("{target}/alefn");
                                    let diff = diff(
                                        &alef,
                                        &bet,
                                        &Params {
                                            from: patched.into(),
                                            to: patched.into(),
                                            context_count: 2,
                                            ..Default::default()
                                        },
                                    );
                                    File::create(format!("{target}/abn.diff"))
                                        .unwrap()
                                        .write_all(&diff)
                                        .unwrap();
                                    let mut fa = File::create(format!("{target}/alefn")).unwrap();
                                    fa.write_all(&alef[..]).unwrap();
                                    let mut fb = File::create(format!("{target}/betn")).unwrap();
                                    fb.write_all(&bet[..]).unwrap();
                                    let _ = fa;
                                    let _ = fb;
                                    let output = PATCH_CMD
                                        .new()
                                        .arg("-p0")
                                        .stdin(File::open(format!("{target}/abn.diff")).unwrap())
                                        .output()
                                        .unwrap();
                                    assert!(output.status.success(), "{output:?}");
                                    //println!("{}", String::from_utf8_lossy(&output.stdout));
                                    //println!("{}", String::from_utf8_lossy(&output.stderr));
                                    let alef = fs::read(format!("{target}/alefn")).unwrap();
                                    assert_eq!(alef, bet);
                                }
                            }
                        }
                    }
                }
            }
        }
    }

    #[test]
    fn test_permutations_empty_lines() {
        let target = "target/unified-diff/";
        // test all possible six-line files with missing newlines.
        let _ = std::fs::create_dir(target);
        for &a in &[0, 1, 2] {
            for &b in &[0, 1, 2] {
                for &c in &[0, 1, 2] {
                    for &d in &[0, 1, 2] {
                        for &e in &[0, 1, 2] {
                            for &f in &[0, 1, 2] {
                                for &g in &[0, 1, 2, 3] {
                                    use std::fs::{self, File};
                                    use std::io::Write;
                                    let mut alef = Vec::new();
                                    let mut bet = Vec::new();
                                    alef.write_all(if a == 0 { b"\n" } else { b"b\n" }).unwrap();
                                    if a != 2 {
                                        bet.write_all(b"b\n").unwrap();
                                    }
                                    alef.write_all(if b == 0 { b"\n" } else { b"d\n" }).unwrap();
                                    if b != 2 {
                                        bet.write_all(b"d\n").unwrap();
                                    }
                                    alef.write_all(if c == 0 { b"\n" } else { b"f\n" }).unwrap();
                                    if c != 2 {
                                        bet.write_all(b"f\n").unwrap();
                                    }
                                    alef.write_all(if d == 0 { b"\n" } else { b"h\n" }).unwrap();
                                    if d != 2 {
                                        bet.write_all(b"h\n").unwrap();
                                    }
                                    alef.write_all(if e == 0 { b"\n" } else { b"j\n" }).unwrap();
                                    if e != 2 {
                                        bet.write_all(b"j\n").unwrap();
                                    }
                                    alef.write_all(if f == 0 { b"\n" } else { b"l\n" }).unwrap();
                                    if f != 2 {
                                        bet.write_all(b"l\n").unwrap();
                                    }
                                    match g {
                                        0 => {
                                            alef.pop();
                                        }
                                        1 => {
                                            bet.pop();
                                        }
                                        2 => {
                                            alef.pop();
                                            bet.pop();
                                        }
                                        3 => {}
                                        _ => unreachable!(),
                                    }
                                    // This test diff is intentionally reversed.
                                    // We want it to turn the alef into bet.
                                    let patched = &format!("{target}/alef_");
                                    let diff = diff(
                                        &alef,
                                        &bet,
                                        &Params {
                                            from: patched.into(),
                                            to: patched.into(),
                                            context_count: 2,
                                            ..Default::default()
                                        },
                                    );
                                    File::create(format!("{target}/ab_.diff"))
                                        .unwrap()
                                        .write_all(&diff)
                                        .unwrap();
                                    let mut fa = File::create(format!("{target}/alef_")).unwrap();
                                    fa.write_all(&alef[..]).unwrap();
                                    let mut fb = File::create(format!("{target}/bet_")).unwrap();
                                    fb.write_all(&bet[..]).unwrap();
                                    let _ = fa;
                                    let _ = fb;
                                    let output = PATCH_CMD
                                        .new()
                                        .arg("-p0")
                                        .stdin(File::open(format!("{target}/ab_.diff")).unwrap())
                                        .output()
                                        .unwrap();
                                    assert!(output.status.success(), "{output:?}");
                                    //println!("{}", String::from_utf8_lossy(&output.stdout));
                                    //println!("{}", String::from_utf8_lossy(&output.stderr));
                                    let alef = fs::read(format!("{target}/alef_")).unwrap();
                                    assert_eq!(alef, bet);
                                }
                            }
                        }
                    }
                }
            }
        }
    }

    #[test]
    fn test_permutations_missing_lines() {
        let target = "target/unified-diff/";
        // test all possible six-line files.
        let _ = std::fs::create_dir(target);
        for &a in &[0, 1, 2] {
            for &b in &[0, 1, 2] {
                for &c in &[0, 1, 2] {
                    for &d in &[0, 1, 2] {
                        for &e in &[0, 1, 2] {
                            for &f in &[0, 1, 2] {
                                use std::fs::{self, File};
                                use std::io::Write;
                                let mut alef = Vec::new();
                                let mut bet = Vec::new();
                                alef.write_all(if a == 0 { b"a\n" } else { b"" }).unwrap();
                                if a != 2 {
                                    bet.write_all(b"b\n").unwrap();
                                }
                                alef.write_all(if b == 0 { b"c\n" } else { b"" }).unwrap();
                                if b != 2 {
                                    bet.write_all(b"d\n").unwrap();
                                }
                                alef.write_all(if c == 0 { b"e\n" } else { b"" }).unwrap();
                                if c != 2 {
                                    bet.write_all(b"f\n").unwrap();
                                }
                                alef.write_all(if d == 0 { b"g\n" } else { b"" }).unwrap();
                                if d != 2 {
                                    bet.write_all(b"h\n").unwrap();
                                }
                                alef.write_all(if e == 0 { b"i\n" } else { b"" }).unwrap();
                                if e != 2 {
                                    bet.write_all(b"j\n").unwrap();
                                }
                                alef.write_all(if f == 0 { b"k\n" } else { b"" }).unwrap();
                                if f != 2 {
                                    bet.write_all(b"l\n").unwrap();
                                }
                                // This test diff is intentionally reversed.
                                // We want it to turn the alef into bet.
                                let patched = &format!("{target}/alefx");
                                let diff = diff(
                                    &alef,
                                    &bet,
                                    &Params {
                                        from: patched.into(),
                                        to: patched.into(),
                                        context_count: 2,
                                        ..Default::default()
                                    },
                                );
                                File::create(format!("{target}/abx.diff"))
                                    .unwrap()
                                    .write_all(&diff)
                                    .unwrap();
                                let mut fa = File::create(format!("{target}/alefx")).unwrap();
                                fa.write_all(&alef[..]).unwrap();
                                let mut fb = File::create(format!("{target}/betx")).unwrap();
                                fb.write_all(&bet[..]).unwrap();
                                let _ = fa;
                                let _ = fb;
                                let output = PATCH_CMD
                                    .new()
                                    .arg("-p0")
                                    .stdin(File::open(format!("{target}/abx.diff")).unwrap())
                                    .output()
                                    .unwrap();
                                assert!(output.status.success(), "{output:?}");
                                //println!("{}", String::from_utf8_lossy(&output.stdout));
                                //println!("{}", String::from_utf8_lossy(&output.stderr));
                                let alef = fs::read(format!("{target}/alefx")).unwrap();
                                assert_eq!(alef, bet);
                            }
                        }
                    }
                }
            }
        }
    }

    #[test]
    fn test_permutations_reverse() {
        let target = "target/unified-diff/";
        // test all possible six-line files.
        let _ = std::fs::create_dir(target);
        for &a in &[0, 1, 2] {
            for &b in &[0, 1, 2] {
                for &c in &[0, 1, 2] {
                    for &d in &[0, 1, 2] {
                        for &e in &[0, 1, 2] {
                            for &f in &[0, 1, 2] {
                                use std::fs::{self, File};
                                use std::io::Write;
                                let mut alef = Vec::new();
                                let mut bet = Vec::new();
                                alef.write_all(if a == 0 { b"a\n" } else { b"f\n" })
                                    .unwrap();
                                if a != 2 {
                                    bet.write_all(b"a\n").unwrap();
                                }
                                alef.write_all(if b == 0 { b"b\n" } else { b"e\n" })
                                    .unwrap();
                                if b != 2 {
                                    bet.write_all(b"b\n").unwrap();
                                }
                                alef.write_all(if c == 0 { b"c\n" } else { b"d\n" })
                                    .unwrap();
                                if c != 2 {
                                    bet.write_all(b"c\n").unwrap();
                                }
                                alef.write_all(if d == 0 { b"d\n" } else { b"c\n" })
                                    .unwrap();
                                if d != 2 {
                                    bet.write_all(b"d\n").unwrap();
                                }
                                alef.write_all(if e == 0 { b"e\n" } else { b"b\n" })
                                    .unwrap();
                                if e != 2 {
                                    bet.write_all(b"e\n").unwrap();
                                }
                                alef.write_all(if f == 0 { b"f\n" } else { b"a\n" })
                                    .unwrap();
                                if f != 2 {
                                    bet.write_all(b"f\n").unwrap();
                                }
                                // This test diff is intentionally reversed.
                                // We want it to turn the alef into bet.
                                let patched = &format!("{target}/alefr");
                                let diff = diff(
                                    &alef,
                                    &bet,
                                    &Params {
                                        from: patched.into(),
                                        to: patched.into(),
                                        context_count: 2,
                                        ..Default::default()
                                    },
                                );
                                File::create(format!("{target}/abr.diff"))
                                    .unwrap()
                                    .write_all(&diff)
                                    .unwrap();
                                let mut fa = File::create(format!("{target}/alefr")).unwrap();
                                fa.write_all(&alef[..]).unwrap();
                                let mut fb = File::create(format!("{target}/betr")).unwrap();
                                fb.write_all(&bet[..]).unwrap();
                                let _ = fa;
                                let _ = fb;
                                let output = PATCH_CMD
                                    .new()
                                    .arg("-p0")
                                    .stdin(File::open(format!("{target}/abr.diff")).unwrap())
                                    .output()
                                    .unwrap();
                                assert!(output.status.success(), "{output:?}");
                                //println!("{}", String::from_utf8_lossy(&output.stdout));
                                //println!("{}", String::from_utf8_lossy(&output.stderr));
                                let alef = fs::read(format!("{target}/alefr")).unwrap();
                                assert_eq!(alef, bet);
                            }
                        }
                    }
                }
            }
        }
    }

    #[test]
    fn test_stop_early() {
        use crate::assert_diff_eq;

        let from_filename = "foo";
        let from = ["a", "b", "c", ""].join("\n");
        let to_filename = "bar";
        let to = ["a", "d", "c", ""].join("\n");

        let diff_full = diff(
            from.as_bytes(),
            to.as_bytes(),
            &Params {
                from: from_filename.into(),
                to: to_filename.into(),
                ..Default::default()
            },
        );

        let expected_full = [
            "--- foo\tTIMESTAMP",
            "+++ bar\tTIMESTAMP",
            "@@ -1,3 +1,3 @@",
            " a",
            "-b",
            "+d",
            " c",
            "",
        ]
        .join("\n");
        assert_diff_eq!(diff_full, expected_full);

        let diff_brief = diff(
            from.as_bytes(),
            to.as_bytes(),
            &Params {
                from: from_filename.into(),
                to: to_filename.into(),
                brief: true,
                ..Default::default()
            },
        );

        let expected_brief = ["--- foo\tTIMESTAMP", "+++ bar\tTIMESTAMP", ""].join("\n");
        assert_diff_eq!(diff_brief, expected_brief);

        let nodiff_full = diff(
            from.as_bytes(),
            from.as_bytes(),
            &Params {
                from: from_filename.into(),
                to: to_filename.into(),
                ..Default::default()
            },
        );
        assert!(nodiff_full.is_empty());

        let nodiff_brief = diff(
            from.as_bytes(),
            from.as_bytes(),
            &Params {
                from: from_filename.into(),
                to: to_filename.into(),
                brief: true,
                ..Default::default()
            },
        );
        assert!(nodiff_brief.is_empty());
    }

    #[test]
    fn test_hunk_merging() {
        use crate::assert_diff_eq;

        let params = Params {
            from: "foo".into(),
            to: "bar".into(),
            context_count: 1,
            ..Default::default()
        };

        // Changes separated by up to twice the context size share one hunk.
        let from = ["a", "b", "c", "d", "e", "f", ""].join("\n");
        let to = ["a", "B", "c", "d", "E", "f", ""].join("\n");
        let expected = [
            "--- foo\tTIMESTAMP",
            "+++ bar\tTIMESTAMP",
            "@@ -1,6 +1,6 @@",
            " a",
            "-b",
            "+B",
            " c",
            " d",
            "-e",
            "+E",
            " f",
            "",
        ]
        .join("\n");
        let output = diff(from.as_bytes(), to.as_bytes(), &params);
        assert_diff_eq!(output, expected);

        // One more unchanged line between them splits the hunk.
        let from = ["a", "b", "c", "d", "e", "f", "g", ""].join("\n");
        let to = ["a", "B", "c", "d", "e", "F", "g", ""].join("\n");
        let expected = [
            "--- foo\tTIMESTAMP",
            "+++ bar\tTIMESTAMP",
            "@@ -1,3 +1,3 @@",
            " a",
            "-b",
            "+B",
            " c",
            "@@ -5,3 +5,3 @@",
            " e",
            "-f",
            "+F",
            " g",
            "",
        ]
        .join("\n");
        let output = diff(from.as_bytes(), to.as_bytes(), &params);
        assert_diff_eq!(output, expected);

        // An insertion before a nearby change gives the two sides different
        // line counts; the hunks must still be merged (issue #268).
        let from = ["milk", "eggs", "bread", "butter", "apples", ""].join("\n");
        let to = ["milk", "eggs", "yogurt", "bread", "honey", "apples", ""].join("\n");
        let expected = [
            "--- foo\tTIMESTAMP",
            "+++ bar\tTIMESTAMP",
            "@@ -2,4 +2,5 @@",
            " eggs",
            "+yogurt",
            " bread",
            "-butter",
            "+honey",
            " apples",
            "",
        ]
        .join("\n");
        let output = diff(from.as_bytes(), to.as_bytes(), &params);
        assert_diff_eq!(output, expected);
    }
}