znippy-plugin-git 0.1.1

//! Canonical git object bytes — the unit a `git`-format znippy archive stores.
//!
//! An archive entry's payload is the **canonical serialization** of a git
//! object: `"<type> <size>\0<content>"`. That is precisely the byte string a git
//! object id is the hash of, so
//!
//!   * `object_type` and `object_size` are derived from the stored bytes with no
//!     out-of-band state (P-2: the plugin writes only what it can see), and
//!   * a read-back is checkable *by construction* — re-hash the bytes and compare
//!     against the `relative_path`, which is the oid hex. That is an applied
//!     output, not a state round-trip (LAW 2).
//!
//! Nothing here allocates per object beyond the borrow it returns, and nothing
//! panics on malformed input (P-4).

use sha1::Digest as _;

/// The four git object types.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum GitObjectKind {
    Blob,
    Tree,
    Commit,
    Tag,
}

impl GitObjectKind {
    pub fn as_str(self) -> &'static str {
        match self {
            GitObjectKind::Blob => "blob",
            GitObjectKind::Tree => "tree",
            GitObjectKind::Commit => "commit",
            GitObjectKind::Tag => "tag",
        }
    }

    pub fn from_bytes(b: &[u8]) -> Option<Self> {
        match b {
            b"blob" => Some(GitObjectKind::Blob),
            b"tree" => Some(GitObjectKind::Tree),
            b"commit" => Some(GitObjectKind::Commit),
            b"tag" => Some(GitObjectKind::Tag),
            _ => None,
        }
    }
}

/// A parsed canonical git object: its type and the content after the NUL.
#[derive(Debug, Clone, Copy)]
pub struct GitObject<'a> {
    pub kind: GitObjectKind,
    pub payload: &'a [u8],
}

/// Parse `"<type> <size>\0<content>"`.
///
/// Returns `None` — never panics — when the header is absent, the type is not
/// one of the four, the size is not decimal, or the declared size disagrees with
/// the actual payload length. That last check is what makes a truncated or
/// padded entry a *miss* rather than a silently wrong `object_size`.
pub fn parse_canonical(data: &[u8]) -> Option<GitObject<'_>> {
    // Bound the header scan: a legal header is "<=6 type + 1 sp + <=20 digits + NUL".
    let scan = data.len().min(64);
    let nul = data[..scan].iter().position(|&b| b == 0)?;
    let header = &data[..nul];
    let sp = header.iter().position(|&b| b == b' ')?;
    let kind = GitObjectKind::from_bytes(&header[..sp])?;
    let size_txt = std::str::from_utf8(&header[sp + 1..]).ok()?;
    if size_txt.is_empty() || !size_txt.bytes().all(|b| b.is_ascii_digit()) {
        return None;
    }
    let size: usize = size_txt.parse().ok()?;
    let payload = &data[nul + 1..];
    if payload.len() != size {
        return None;
    }
    Some(GitObject { kind, payload })
}

/// Build canonical bytes from a type and content. Used by tests and by any
/// writer that has the content in hand.
pub fn canonical(kind: GitObjectKind, payload: &[u8]) -> Vec<u8> {
    let mut out = Vec::with_capacity(payload.len() + 32);
    out.extend_from_slice(kind.as_str().as_bytes());
    out.push(b' ');
    out.extend_from_slice(payload.len().to_string().as_bytes());
    out.push(0);
    out.extend_from_slice(payload);
    out
}

/// Which hash a repository's object ids use. gunnar defaults to sha256 (D11);
/// sha1 stays supported.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum GitHashKind {
    Sha1,
    Sha256,
}

impl GitHashKind {
    /// Raw oid length in bytes.
    pub fn oid_len(self) -> usize {
        match self {
            GitHashKind::Sha1 => 20,
            GitHashKind::Sha256 => 32,
        }
    }

    /// Hex oid length — the length of a `relative_path` in a git archive.
    pub fn hex_len(self) -> usize {
        self.oid_len() * 2
    }

    /// Infer the hash from a hex oid's length. `None` for anything else.
    pub fn from_hex_len(len: usize) -> Option<Self> {
        match len {
            40 => Some(GitHashKind::Sha1),
            64 => Some(GitHashKind::Sha256),
            _ => None,
        }
    }

    pub fn code(self) -> u8 {
        match self {
            GitHashKind::Sha1 => 1,
            GitHashKind::Sha256 => 2,
        }
    }

    pub fn from_code(c: u8) -> Option<Self> {
        match c {
            1 => Some(GitHashKind::Sha1),
            2 => Some(GitHashKind::Sha256),
            _ => None,
        }
    }

    /// The object id of these canonical bytes.
    pub fn oid_of(self, canonical_bytes: &[u8]) -> Vec<u8> {
        match self {
            GitHashKind::Sha1 => sha1::Sha1::digest(canonical_bytes).to_vec(),
            GitHashKind::Sha256 => sha2::Sha256::digest(canonical_bytes).to_vec(),
        }
    }

    /// The hex object id of these canonical bytes.
    pub fn oid_hex_of(self, canonical_bytes: &[u8]) -> String {
        hex::encode(self.oid_of(canonical_bytes))
    }
}

/// True when `path` is shaped like a git oid used as a `relative_path`: the
/// whole path is 40 or 64 **lowercase** hex characters. Deliberately strict —
/// git writes lowercase hex everywhere, and accepting mixed case would put two
/// spellings of one object in one archive.
pub fn is_oid_path(path: &str) -> bool {
    matches!(path.len(), 40 | 64) && path.bytes().all(|b| b.is_ascii_digit() || (b'a'..=b'f').contains(&b))
}

/// The two files a packed git object store is made of.
///
/// A `git`-format archive holds **either** loose canonical objects keyed by oid
/// **or** a repository's packs. The second is what gunnar's `repack()` produces
/// and what its cold tier seals: a pack keeps the client's own deflate and its
/// delta chains, both of which an inflated per-oid tier destroys forever.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum PackFileKind {
    /// `pack-<id>.pack` — the entries themselves, every one already deflated.
    Data,
    /// `pack-<id>.idx` — the pack's own oid index.
    Index,
}

impl PackFileKind {
    pub fn as_str(self) -> &'static str {
        match self {
            PackFileKind::Data => PACKFILE_TYPE,
            PackFileKind::Index => PACK_INDEX_TYPE,
        }
    }

    /// The magic every file of this kind starts with. Checking it is what makes
    /// a `.pack` name a *claim* rather than a fact.
    pub fn magic(self) -> &'static [u8] {
        match self {
            PackFileKind::Data => b"PACK",
            // pack index v2. v1 has no magic at all and gunnar never writes it.
            PackFileKind::Index => b"\xfftOc",
        }
    }
}

/// `object_type` for a `pack-<id>.pack`.
pub const PACKFILE_TYPE: &str = "packfile";

/// `object_type` for a `pack-<id>.idx`.
pub const PACK_INDEX_TYPE: &str = "pack-index";

/// True when `path` is `pack-<oid hex>.<ext>`, the name git itself gives the
/// files in `objects/pack`, and the name gunnar seals them under.
///
/// **The name matters, and this is why it is checked rather than ignored.**
/// znippy resolves "is this already compressed?" by extension first and by a
/// magic probe only as a fallback, and the probe reads offset 0 of *every*
/// chunk while a container's magic sits at offset 0 of chunk 0 alone. Storing a
/// pack under a name with no extension therefore costs a full codec pass over
/// all but the first chunk — measured at 35.6 s of CPU and 2.6 GB of RSS on
/// 200 MB, against 0.09 s and 184 MB once the name carried the extension. A
/// pack-tier archive keeps git's own file names for exactly that reason.
pub fn pack_path_kind(path: &str) -> Option<PackFileKind> {
    let name = path.rsplit('/').next().unwrap_or(path);
    let stem = name.strip_prefix("pack-")?;
    let (id, kind) = if let Some(id) = stem.strip_suffix(".pack") {
        (id, PackFileKind::Data)
    } else {
        (stem.strip_suffix(".idx")?, PackFileKind::Index)
    };
    is_oid_path(id).then_some(kind)
}

/// One entry of a tree object.
#[derive(Debug, Clone)]
pub struct TreeEntry<'a> {
    pub mode: &'a [u8],
    pub name: &'a [u8],
    pub oid: &'a [u8],
}

/// Parse a tree payload: a sequence of `<mode> <name>\0<raw oid>`.
///
/// Stops at the first malformed record and returns what it read so far — a
/// corrupt tree degrades the reachability closure, it never panics (P-4).
pub fn tree_entries(payload: &[u8], oid_len: usize) -> Vec<TreeEntry<'_>> {
    let mut out = Vec::new();
    let mut i = 0usize;
    while i < payload.len() {
        let Some(sp_rel) = payload[i..].iter().position(|&b| b == b' ') else { break };
        let sp = i + sp_rel;
        let Some(nul_rel) = payload[sp + 1..].iter().position(|&b| b == 0) else { break };
        let nul = sp + 1 + nul_rel;
        let end = nul + 1 + oid_len;
        if end > payload.len() {
            break;
        }
        out.push(TreeEntry {
            mode: &payload[i..sp],
            name: &payload[sp + 1..nul],
            oid: &payload[nul + 1..end],
        });
        i = end;
    }
    out
}

/// The header fields of a commit payload that the commit graph needs.
#[derive(Debug, Clone, Default)]
pub struct CommitHeader {
    pub tree: Option<String>,
    pub parents: Vec<String>,
    /// Committer timestamp, unix seconds. `None` when absent or unparsable.
    pub committer_time: Option<i64>,
}

/// Parse a commit payload's header block (everything before the first blank
/// line). Unknown headers are ignored; a malformed one is skipped (P-4).
pub fn parse_commit(payload: &[u8]) -> CommitHeader {
    let mut h = CommitHeader::default();
    let mut rest = payload;
    loop {
        let line_end = match rest.iter().position(|&b| b == b'\n') {
            Some(p) => p,
            None => rest.len(),
        };
        let line = &rest[..line_end];
        if line.is_empty() {
            break;
        }
        if let Some(v) = line.strip_prefix(b"tree ") {
            if let Ok(s) = std::str::from_utf8(v) {
                if is_oid_path(s) {
                    h.tree = Some(s.to_string());
                }
            }
        } else if let Some(v) = line.strip_prefix(b"parent ") {
            if let Ok(s) = std::str::from_utf8(v) {
                if is_oid_path(s) {
                    h.parents.push(s.to_string());
                }
            }
        } else if let Some(v) = line.strip_prefix(b"committer ") {
            h.committer_time = committer_timestamp(v);
        }
        if line_end >= rest.len() {
            break;
        }
        rest = &rest[line_end + 1..];
    }
    h
}

/// `"Name <email> 1712345678 +0200"` → `1712345678`.
fn committer_timestamp(v: &[u8]) -> Option<i64> {
    let s = std::str::from_utf8(v).ok()?;
    let gt = s.rfind('>')?;
    let mut it = s[gt + 1..].split_ascii_whitespace();
    it.next()?.parse::<i64>().ok()
}

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

    #[test]
    fn canonical_roundtrips_and_hashes_like_git() {
        // Ground truth: `printf '' | git hash-object -t blob --stdin` is the
        // well-known empty-blob sha1.
        let c = canonical(GitObjectKind::Blob, b"");
        assert_eq!(c, b"blob 0\0");
        assert_eq!(
            GitHashKind::Sha1.oid_hex_of(&c),
            "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"
        );
        // And the sha256 counterpart, from git's own sha256 test vectors.
        assert_eq!(
            GitHashKind::Sha256.oid_hex_of(&c),
            "473a0f4c3be8a93681a267e3b1e9a7dcda1185436fe141f7749120a303721813"
        );
    }

    #[test]
    fn hash_object_matches_git_for_nonempty_blob() {
        // `echo -n 'hello' | git hash-object --stdin`
        let c = canonical(GitObjectKind::Blob, b"hello");
        assert_eq!(
            GitHashKind::Sha1.oid_hex_of(&c),
            "b6fc4c620b67d95f953a5c1c1230aaab5db5a1b0"
        );
    }

    #[test]
    fn parse_rejects_size_mismatch_instead_of_lying() {
        // Declared 5, actual 4 — a truncated entry must be a miss, not a wrong size.
        assert!(parse_canonical(b"blob 5\0abcd").is_none());
        assert!(parse_canonical(b"blob 4\0abcd").is_some());
    }

    #[test]
    fn parse_never_panics_on_garbage() {
        for bad in [
            &b""[..],
            &b"\0"[..],
            &b"blob"[..],
            &b"blob \0"[..],
            &b"nope 3\0abc"[..],
            &b"blob -1\0abc"[..],
            &b"blob 99999999999999999999999999\0abc"[..],
            &[0xffu8; 300][..],
        ] {
            assert!(parse_canonical(bad).is_none(), "should not parse: {bad:?}");
        }
    }

    #[test]
    fn oid_path_shape_is_strict() {
        assert!(is_oid_path(&"a".repeat(40)));
        assert!(is_oid_path(&"0".repeat(64)));
        assert!(!is_oid_path(&"A".repeat(40)), "uppercase hex is not a git oid path");
        assert!(!is_oid_path(&"a".repeat(41)));
        assert!(!is_oid_path("objects/aa/bb"));
        assert!(!is_oid_path(&"g".repeat(40)));
    }

    #[test]
    fn tree_entries_parse_and_tolerate_truncation() {
        let mut payload = Vec::new();
        payload.extend_from_slice(b"100644 a.txt\0");
        payload.extend_from_slice(&[0x11u8; 20]);
        payload.extend_from_slice(b"40000 sub\0");
        payload.extend_from_slice(&[0x22u8; 20]);
        let e = tree_entries(&payload, 20);
        assert_eq!(e.len(), 2);
        assert_eq!(e[0].name, b"a.txt");
        assert_eq!(e[1].oid, &[0x22u8; 20]);

        // Truncated tail: the good record still comes back, no panic.
        let t = tree_entries(&payload[..payload.len() - 5], 20);
        assert_eq!(t.len(), 1);
    }

    #[test]
    fn commit_header_parses_tree_parents_and_time() {
        let payload = concat!(
            "tree 1111111111111111111111111111111111111111\n",
            "parent 2222222222222222222222222222222222222222\n",
            "parent 3333333333333333333333333333333333333333\n",
            "author A <a@x> 1700000000 +0100\n",
            "committer C <c@x> 1700000123 +0200\n",
            "\n",
            "message body\n",
        );
        let h = parse_commit(payload.as_bytes());
        assert_eq!(h.tree.as_deref(), Some("1111111111111111111111111111111111111111"));
        assert_eq!(h.parents.len(), 2);
        assert_eq!(h.committer_time, Some(1700000123));
    }
}