//! Unified-diff parsing.
//!
//! The patch tier resolves a file through the rung ladder of `docs/installer/PLAN.md` §2.2.1, and
//! every rung is answered from the diff itself: a unified diff already carries the stock text of
//! each region it edits — the context lines plus the `-` lines **are** the pre-image, and the
//! context lines plus the `+` lines are the post-image. Nothing else has to be shipped alongside
//! the patch for the installer to know what it is looking for.
//!
//! Three properties of this parser are load-bearing rather than tidiness:
//!
//! - **Everything is bytes, never `String`.** The three ServUO files the tier edits are CRLF and
//!   are not guaranteed to be UTF-8; a lossy decode would corrupt bytes on write-back, and a strict
//!   one would refuse to patch a shard over a stray `0x92` in a comment. Line content is compared
//!   after normalization but written back verbatim.
//! - **The `index` line is optional and its absence is not a defect.** `commandlogging-event.patch`
//!   is a plain `---`/`+++` diff with no `diff --git` header at all, so rung 1 (whole-file
//!   pre-image hash) is structurally unavailable for it. That is fine: rung 2 matches on content
//!   and is the stronger guarantee anyway. A parser that required the header would have rejected a
//!   patch we ship.
//! - **Hunk line counts are checked against the lines actually present.** A truncated or
//!   hand-edited diff whose `@@` header promises more lines than it carries would otherwise
//!   reconstruct a short pre-image, which is exactly the sort of thing that then matches somewhere
//!   it should not.

use anyhow::{bail, Context, Result};

/// One line inside a hunk body.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum HunkLine {
    /// ` ` — present on both sides.
    Context(Vec<u8>),
    /// `-` — present in the stock file only.
    Removed(Vec<u8>),
    /// `+` — present in the patched file only.
    Added(Vec<u8>),
}

impl HunkLine {
    fn content(&self) -> &[u8] {
        match self {
            Self::Context(b) | Self::Removed(b) | Self::Added(b) => b,
        }
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Hunk {
    /// 1-based line number in the stock file, from the `@@` header. **Advisory only** — the match
    /// is made by content, since an insertion anywhere above shifts every number below it. It is
    /// used to prefer the nearest candidate when reporting, and nowhere else.
    pub old_start: usize,
    pub old_count: usize,
    pub new_start: usize,
    pub new_count: usize,
    pub lines: Vec<HunkLine>,
    /// The stock file's last line has no trailing newline (`\ No newline at end of file` after a
    /// `-` or context line).
    pub old_no_newline: bool,
    /// Likewise for the patched file.
    pub new_no_newline: bool,
}

impl Hunk {
    /// The stock text this hunk expects to find: context + removed, in order.
    pub fn pre_image(&self) -> Vec<&[u8]> {
        self.lines
            .iter()
            .filter(|l| !matches!(l, HunkLine::Added(_)))
            .map(HunkLine::content)
            .collect()
    }

    /// The text this hunk leaves behind: context + added, in order.
    pub fn post_image(&self) -> Vec<&[u8]> {
        self.lines
            .iter()
            .filter(|l| !matches!(l, HunkLine::Removed(_)))
            .map(HunkLine::content)
            .collect()
    }

    /// Whether this hunk changes anything. A hunk of pure context is a no-op and must not be
    /// counted as applied work — nor searched for, since its pre- and post-images are identical
    /// and rung 0 could never be distinguished from rung 2.
    pub fn is_noop(&self) -> bool {
        !self
            .lines
            .iter()
            .any(|l| matches!(l, HunkLine::Added(_) | HunkLine::Removed(_)))
    }
}

/// One file section of a patch. Our patches carry one each, but a diff may hold several and
/// silently applying the first would be a quiet way to half-patch a tree.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct FilePatch {
    /// From `+++ b/<path>`, with the `b/` prefix stripped and separators left as `/`.
    pub path: String,
    /// The abbreviated blob hash of the stock file, from `index <old>..<new>`. `None` when the
    /// diff has no `index` line, which makes rung 1 unavailable for this file — see the module
    /// docs.
    pub pre_blob: Option<String>,
    pub post_blob: Option<String>,
    pub hunks: Vec<Hunk>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Patch {
    pub files: Vec<FilePatch>,
}

impl Patch {
    /// The single file this patch edits.
    ///
    /// The tier declares one target per patch (`tier.json`), so a diff that turned out to edit two
    /// files would mean the declaration and the artifact disagree — and the installer would have
    /// checked only one of them against the rung ladder before writing both.
    pub fn single_file(&self) -> Result<&FilePatch> {
        match self.files.as_slice() {
            [only] => Ok(only),
            [] => bail!("this patch contains no file sections"),
            many => bail!(
                "this patch edits {} files ({}); the tier declares one target per patch",
                many.len(),
                many.iter()
                    .map(|f| f.path.as_str())
                    .collect::<Vec<_>>()
                    .join(", ")
            ),
        }
    }
}

/// Splits a buffer into lines **without** their terminators, tolerating CRLF, LF and a final line
/// with no terminator at all.
///
/// A trailing newline does not produce a final empty line: `b"a\n"` is one line, matching how every
/// diff tool counts them.
pub fn split_lines(data: &[u8]) -> Vec<&[u8]> {
    let mut out = Vec::new();
    let mut start = 0usize;
    for (i, b) in data.iter().enumerate() {
        if *b == b'\n' {
            let mut end = i;
            if end > start && data[end - 1] == b'\r' {
                end -= 1;
            }
            out.push(&data[start..end]);
            start = i + 1;
        }
    }
    if start < data.len() {
        let mut end = data.len();
        if end > start && data[end - 1] == b'\r' {
            end -= 1;
        }
        out.push(&data[start..end]);
    }
    out
}

/// The comparison form of a line: trailing whitespace removed.
///
/// This is the *whole* of the licence PLAN.md §2.2.1 grants — line-ending and trailing-whitespace
/// normalization, nothing else. There is no fuzz and no context reduction: dropping context to
/// force a match is precisely how a hunk lands in the wrong method. Line endings are already gone
/// by the time this runs, since [`split_lines`] strips them.
pub fn normalize(line: &[u8]) -> &[u8] {
    let mut end = line.len();
    while end > 0 && (line[end - 1] == b' ' || line[end - 1] == b'\t' || line[end - 1] == b'\r') {
        end -= 1;
    }
    &line[..end]
}

/// Parses a `.patch` file.
pub fn parse(data: &[u8]) -> Result<Patch> {
    let lines = split_lines(data);
    let mut files: Vec<FilePatch> = Vec::new();
    let mut pending_blobs: Option<(String, String)> = None;
    let mut i = 0usize;

    while i < lines.len() {
        let line = lines[i];

        if line.starts_with(b"index ") {
            // `index <old>..<new>[ <mode>]`. Abbreviated to 7+ hex characters by git, so rung 1
            // compares by prefix rather than for equality.
            pending_blobs = parse_index(line);
            i += 1;
            continue;
        }

        if line.starts_with(b"--- ") && i + 1 < lines.len() && lines[i + 1].starts_with(b"+++ ") {
            let path = header_path(lines[i + 1], b"+++ ")
                .or_else(|| header_path(lines[i], b"--- "))
                .with_context(|| {
                    format!(
                        "cannot read the target path from the diff header at line {}",
                        i + 2
                    )
                })?;
            let (pre_blob, post_blob) = match pending_blobs.take() {
                Some((a, b)) => (Some(a), Some(b)),
                None => (None, None),
            };
            i += 2;

            let mut hunks = Vec::new();
            while i < lines.len() && lines[i].starts_with(b"@@") {
                let (hunk, next) = parse_hunk(&lines, i)?;
                hunks.push(hunk);
                i = next;
            }
            if hunks.is_empty() {
                bail!("the diff section for {path} contains no hunks");
            }
            files.push(FilePatch {
                path,
                pre_blob,
                post_blob,
                hunks,
            });
            continue;
        }

        // `diff --git`, `new file mode`, `similarity index`, a covering-letter preamble — anything
        // outside a hunk body is skipped. Only `index` is worth keeping.
        i += 1;
    }

    if files.is_empty() {
        bail!("no unified-diff sections found — this file is not a patch");
    }
    Ok(Patch { files })
}

/// `--- a/Scripts/Commands/Logging.cs` → `Scripts/Commands/Logging.cs`.
///
/// The trailing tab-separated timestamp some tools append is dropped, and so is the one-letter
/// prefix directory git uses. `/dev/null` yields `None`, which makes a pure-creation diff fall back
/// to the other header rather than producing a file called `dev/null`.
fn header_path(line: &[u8], marker: &[u8]) -> Option<String> {
    let rest = line.strip_prefix(marker)?;
    let rest = match rest.iter().position(|b| *b == b'\t') {
        Some(tab) => &rest[..tab],
        None => rest,
    };
    let text = String::from_utf8_lossy(rest).trim().replace('\\', "/");
    if text.is_empty() || text == "/dev/null" {
        return None;
    }
    // git writes `a/` and `b/`; `-p1` semantics. A path with no prefix (`patch -p0` style) is left
    // alone rather than having its first directory eaten.
    for prefix in ["a/", "b/", "i/", "w/", "c/", "o/"] {
        if let Some(stripped) = text.strip_prefix(prefix) {
            return Some(stripped.to_string());
        }
    }
    Some(text)
}

fn parse_index(line: &[u8]) -> Option<(String, String)> {
    let rest = String::from_utf8_lossy(line.strip_prefix(b"index ")?).to_string();
    let head = rest.split_whitespace().next()?;
    let (old, new) = head.split_once("..")?;
    let hex = |s: &str| !s.is_empty() && s.chars().all(|c| c.is_ascii_hexdigit());
    if !hex(old) || !hex(new) {
        return None;
    }
    Some((old.to_ascii_lowercase(), new.to_ascii_lowercase()))
}

/// Parses one hunk, starting at the `@@` header. Returns the hunk and the index of the line after
/// it.
fn parse_hunk(lines: &[&[u8]], start: usize) -> Result<(Hunk, usize)> {
    let header = String::from_utf8_lossy(lines[start]).to_string();
    let (old_start, old_count, new_start, new_count) = parse_hunk_header(&header)
        .with_context(|| format!("cannot parse hunk header at line {}: {header}", start + 1))?;

    let mut body = Vec::new();
    let mut old_no_newline = false;
    let mut new_no_newline = false;
    let mut seen_old = 0usize;
    let mut seen_new = 0usize;
    let mut i = start + 1;

    while i < lines.len() {
        let line = lines[i];
        // The marker describes the line *above* it, and which side it applies to depends on that
        // line's kind: a `-` line means the stock file ended there, a `+` line the patched one, and
        // a context line both.
        if line.starts_with(b"\\ ") {
            match body.last() {
                Some(HunkLine::Removed(_)) => old_no_newline = true,
                Some(HunkLine::Added(_)) => new_no_newline = true,
                Some(HunkLine::Context(_)) => {
                    old_no_newline = true;
                    new_no_newline = true;
                }
                None => {}
            }
            i += 1;
            continue;
        }
        if seen_old >= old_count && seen_new >= new_count {
            break;
        }

        let (kind, rest) = match line.first() {
            Some(b' ') => (0u8, &line[1..]),
            Some(b'-') => (1, &line[1..]),
            Some(b'+') => (2, &line[1..]),
            // git emits a genuinely empty line for an empty context line rather than a lone space,
            // and trailing whitespace is routinely stripped in transit. Treating it as context is
            // what every patch tool does.
            None => (0, line),
            // Anything else ends the hunk — the next `@@`, `diff --git`, or trailing prose.
            Some(_) => break,
        };

        match kind {
            0 => {
                seen_old += 1;
                seen_new += 1;
                body.push(HunkLine::Context(rest.to_vec()));
            }
            1 => {
                seen_old += 1;
                body.push(HunkLine::Removed(rest.to_vec()));
            }
            _ => {
                seen_new += 1;
                body.push(HunkLine::Added(rest.to_vec()));
            }
        }
        i += 1;
    }

    // A header that promises more than the body delivers reconstructs a short pre-image, which is
    // then liable to match a place the author never meant. Refusing is the only safe reading.
    if seen_old != old_count || seen_new != new_count {
        bail!(
            "hunk at line {} declares -{old_start},{old_count} +{new_start},{new_count} but \
             carries {seen_old} old and {seen_new} new lines — the patch is truncated or malformed",
            start + 1
        );
    }

    Ok((
        Hunk {
            old_start,
            old_count,
            new_start,
            new_count,
            lines: body,
            old_no_newline,
            new_no_newline,
        },
        i,
    ))
}

/// `@@ -75,16 +75,27 @@ optional section heading` → `(75, 16, 75, 27)`.
///
/// A count may be omitted, which means 1 (`@@ -75 +75,2 @@`), and a count of 0 is legal for a pure
/// insertion or deletion.
fn parse_hunk_header(header: &str) -> Option<(usize, usize, usize, usize)> {
    let inner = header.strip_prefix("@@")?;
    let end = inner.find("@@")?;
    let mut parts = inner[..end].split_whitespace();
    let old = parts.next()?.strip_prefix('-')?;
    let new = parts.next()?.strip_prefix('+')?;

    let range = |s: &str| -> Option<(usize, usize)> {
        match s.split_once(',') {
            Some((a, b)) => Some((a.parse().ok()?, b.parse().ok()?)),
            None => Some((s.parse().ok()?, 1)),
        }
    };
    let (old_start, old_count) = range(old)?;
    let (new_start, new_count) = range(new)?;
    Some((old_start, old_count, new_start, new_count))
}

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

    fn parse_str(text: &str) -> Result<Patch> {
        parse(text.as_bytes())
    }

    const SIMPLE: &str = "\
diff --git a/Scripts/Commands/Logging.cs b/Scripts/Commands/Logging.cs
index 5dd3f54..9ab1c22 100644
--- a/Scripts/Commands/Logging.cs
+++ b/Scripts/Commands/Logging.cs
@@ -75,4 +75,6 @@ namespace Server.Commands
             return o;
         }

+        public static event Action<Mobile, string> OnWrite;
+
         public static void WriteLine(Mobile from, string text)
";

    #[test]
    fn a_git_format_patch_parses_whole() {
        let patch = parse_str(SIMPLE).unwrap();
        let file = patch.single_file().unwrap();
        assert_eq!(file.path, "Scripts/Commands/Logging.cs");
        assert_eq!(file.pre_blob.as_deref(), Some("5dd3f54"));
        assert_eq!(file.post_blob.as_deref(), Some("9ab1c22"));
        assert_eq!(file.hunks.len(), 1);

        let hunk = &file.hunks[0];
        assert_eq!((hunk.old_start, hunk.old_count), (75, 4));
        assert_eq!((hunk.new_start, hunk.new_count), (75, 6));
        assert_eq!(hunk.pre_image().len(), 4);
        assert_eq!(hunk.post_image().len(), 6);
        assert!(!hunk.is_noop());
    }

    #[test]
    fn a_plain_diff_without_a_git_header_parses_and_offers_no_blob() {
        // commandlogging-event.patch is exactly this shape. Rung 1 is unavailable for it, which is
        // a fact to report — not a parse error, and certainly not a reason to skip the patch.
        let text = SIMPLE
            .lines()
            .filter(|l| !l.starts_with("diff --git") && !l.starts_with("index "))
            .collect::<Vec<_>>()
            .join("\n");
        let patch = parse_str(&text).unwrap();
        let file = patch.single_file().unwrap();
        assert_eq!(file.path, "Scripts/Commands/Logging.cs");
        assert_eq!(file.pre_blob, None);
        assert_eq!(file.hunks.len(), 1);
    }

    #[test]
    fn a_crlf_patch_yields_the_same_lines_as_an_lf_one() {
        // Every .patch this tier ships is CRLF in a Windows checkout and LF in the tarball CI
        // builds. The two must parse identically or a patch would apply on one platform and not
        // the other.
        let crlf = SIMPLE.replace('\n', "\r\n");
        assert_eq!(parse_str(&crlf).unwrap(), parse_str(SIMPLE).unwrap());
    }

    #[test]
    fn pre_and_post_images_are_the_two_sides_of_the_hunk() {
        let patch = parse_str(
            "\
--- a/x
+++ b/x
@@ -1,3 +1,3 @@
 keep
-old
+new
 tail
",
        )
        .unwrap();
        let hunk = &patch.single_file().unwrap().hunks[0];
        assert_eq!(hunk.pre_image(), vec![&b"keep"[..], b"old", b"tail"]);
        assert_eq!(hunk.post_image(), vec![&b"keep"[..], b"new", b"tail"]);
    }

    #[test]
    fn the_no_newline_marker_attaches_to_the_side_it_describes() {
        // Three cases, because the marker follows the line it describes and the side depends on
        // that line's kind. Getting this wrong writes a spurious trailing newline into a file that
        // never had one — a one-byte change that shows up in every future hash comparison.
        let old_only = parse_str(
            "--- a/x\n+++ b/x\n@@ -1,1 +1,1 @@\n-old\n\\ No newline at end of file\n+new\n",
        )
        .unwrap();
        let hunk = &old_only.single_file().unwrap().hunks[0];
        assert!(hunk.old_no_newline && !hunk.new_no_newline);

        let new_only = parse_str(
            "--- a/x\n+++ b/x\n@@ -1,1 +1,1 @@\n-old\n+new\n\\ No newline at end of file\n",
        )
        .unwrap();
        let hunk = &new_only.single_file().unwrap().hunks[0];
        assert!(!hunk.old_no_newline && hunk.new_no_newline);

        let both = parse_str(
            "--- a/x\n+++ b/x\n@@ -1,1 +1,2 @@\n+added\n same\n\\ No newline at end of file\n",
        )
        .unwrap();
        let hunk = &both.single_file().unwrap().hunks[0];
        assert!(hunk.old_no_newline && hunk.new_no_newline);
    }

    #[test]
    fn several_hunks_and_several_files_are_all_kept() {
        let patch = parse_str(
            "\
diff --git a/one b/one
index aaaaaaa..bbbbbbb 100644
--- a/one
+++ b/one
@@ -1,1 +1,2 @@
 a
+b
@@ -10,1 +11,2 @@
 c
+d
diff --git a/two b/two
--- a/two
+++ b/two
@@ -5,1 +5,2 @@
 e
+f
",
        )
        .unwrap();
        assert_eq!(patch.files.len(), 2);
        assert_eq!(patch.files[0].hunks.len(), 2);
        assert_eq!(patch.files[1].hunks.len(), 1);
        // The second file has no index line of its own and must not inherit the first's.
        assert_eq!(patch.files[1].pre_blob, None);

        // And a multi-file patch is refused where the tier expects one target, rather than being
        // silently half-applied.
        let err = patch.single_file().unwrap_err().to_string();
        assert!(err.contains("edits 2 files"), "{err}");
    }

    #[test]
    fn an_omitted_count_means_one() {
        let patch = parse_str("--- a/x\n+++ b/x\n@@ -7 +7,2 @@\n a\n+b\n").unwrap();
        let hunk = &patch.single_file().unwrap().hunks[0];
        assert_eq!((hunk.old_start, hunk.old_count), (7, 1));
        assert_eq!((hunk.new_start, hunk.new_count), (7, 2));
    }

    #[test]
    fn a_truncated_hunk_is_rejected() {
        // The header promises four old lines; two are present. Reconstructing the short pre-image
        // and hunting for it is how a patch lands somewhere nobody intended.
        let err = parse_str("--- a/x\n+++ b/x\n@@ -1,4 +1,4 @@\n a\n b\n")
            .unwrap_err()
            .to_string();
        assert!(err.contains("truncated or malformed"), "{err}");
    }

    #[test]
    fn a_file_that_is_not_a_patch_is_rejected() {
        assert!(parse_str("# patches\n\nUnified diffs against stock ServUO.\n").is_err());
        assert!(parse_str("").is_err());
    }

    #[test]
    fn line_splitting_agrees_with_how_diffs_count_lines() {
        assert_eq!(split_lines(b"a\nb\n"), vec![&b"a"[..], b"b"]);
        assert_eq!(split_lines(b"a\r\nb"), vec![&b"a"[..], b"b"]);
        assert_eq!(split_lines(b""), Vec::<&[u8]>::new());
        assert_eq!(split_lines(b"\n"), vec![&b""[..]]);
        assert_eq!(split_lines(b"a"), vec![&b"a"[..]]);
    }

    #[test]
    fn normalization_covers_line_endings_and_trailing_space_and_nothing_else() {
        assert_eq!(normalize(b"code  \t"), b"code");
        assert_eq!(normalize(b"code\r"), b"code");
        // Leading indentation is content: two methods differing only in nesting are different
        // places, and collapsing them is how an anchor becomes ambiguous.
        assert_ne!(normalize(b"  code"), normalize(b"code"));
        assert_eq!(normalize(b"   "), b"");
    }

    #[test]
    fn a_pure_context_hunk_is_a_noop() {
        let patch = parse_str("--- a/x\n+++ b/x\n@@ -1,2 +1,2 @@\n a\n b\n").unwrap();
        assert!(patch.single_file().unwrap().hunks[0].is_noop());
    }

    #[test]
    fn an_empty_context_line_written_without_its_space_is_still_context() {
        // Mailers and editors strip the trailing space off a blank context line routinely, and
        // every patch tool tolerates it. Both spellings must produce the same pre-image.
        let padded = parse_str("--- a/x\n+++ b/x\n@@ -1,3 +1,4 @@\n a\n \n b\n+c\n").unwrap();
        let bare = parse_str("--- a/x\n+++ b/x\n@@ -1,3 +1,4 @@\n a\n\n b\n+c\n").unwrap();
        assert_eq!(
            padded.single_file().unwrap().hunks[0].pre_image(),
            bare.single_file().unwrap().hunks[0].pre_image()
        );
    }
}