mkit-core 0.4.0

//! Staging-area index.
//!
//! On-disk layout per `docs/specs/SPEC-INDEX.md`:
//!
//! ```text
//! [4B magic "MKIX"][1B version][4B LE entry_count][entries...]
//! entry := [1B status][32B object_hash][8B LE mtime_ns][8B LE size]
//!          [8B LE ino][8B LE ctime_ns][2B LE path_len][path_len UTF-8 bytes]
//! ```
//!
//! `mtime_ns`/`size`/`ino`/`ctime_ns` are the stat cache (SPEC-INDEX
//! §"stat cache"): when a worktree file's live `stat` matches them,
//! `add`/`status` may reuse `object_hash` without re-reading or
//! re-hashing the content — O(stat) instead of O(content) for unchanged
//! files. `mtime_ns == 0` is the sentinel for "no cache, always
//! re-hash". Writers smudge (zero) the cache of any entry
//! whose mtime falls within the racy window of the index write itself
//! — see [`write_index`].
//!
//! SPEC-INDEX §2 is normative on the magic value — readers MUST reject
//! any other magic.
//!
//! Path rules (SPEC-INDEX §2): non-empty, no leading `/`, no `.`/`..`
//! segments, no NULs/backslashes, and never under `.mkit/` or `.git/`.

use std::collections::BTreeMap;
use std::fs;
use std::io;
use std::path::PathBuf;

use crate::atomic::write_atomic;
use crate::hash::{self, HASH_LEN, Hash};
use crate::layout::RepoLayout;
use crate::object::{EntryMode, Object};
use crate::store::{MAX_TREE_DEPTH, ObjectStore, StoreError};

/// Magic bytes — ASCII `"MKIX"`.
pub const MAGIC: [u8; 4] = *b"MKIX";
/// The current (and only supported) format version.
pub const FORMAT_VERSION: u8 = 0x02;
/// Hard cap on a serialised index file (64 MiB), per SPEC-INDEX §4.
pub const MAX_INDEX_BYTES: u64 = 64 * 1024 * 1024;
/// Hard cap on a single entry's path length (SPEC-INDEX §2).
pub const MAX_PATH_LEN: usize = 4096;

/// Default location of the index file relative to the worktree root.
pub const INDEX_FILE: &str = ".mkit/index";

/// Status byte for an index entry. Values match SPEC-INDEX §3.
#[repr(u8)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum EntryStatus {
    /// `0x00` — path scheduled for deletion in the next commit.
    Removed = 0x00,
    /// `0x01` — regular file blob.
    Blob = 0x01,
    /// `0x02` — reserved for subtree staging; currently unused.
    Tree = 0x02,
    /// `0x03` — symbolic link, blob payload is the target string.
    Symlink = 0x03,
    /// `0x04` — executable blob (mode bit per SPEC-OBJECTS §4.2).
    Executable = 0x04,
}

impl EntryStatus {
    /// Decode a status byte. Returns `None` on unknown values.
    #[must_use]
    pub fn from_byte(b: u8) -> Option<Self> {
        match b {
            0x00 => Some(Self::Removed),
            0x01 => Some(Self::Blob),
            0x02 => Some(Self::Tree),
            0x03 => Some(Self::Symlink),
            0x04 => Some(Self::Executable),
            _ => None,
        }
    }
}

/// One staged entry.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct IndexEntry {
    /// Repo-relative path with `/` separators.
    pub path: String,
    /// Status byte.
    pub status: EntryStatus,
    /// Object hash; `[0;32]` for removed entries.
    pub object_hash: Hash,
    /// Stat cache: worktree mtime (nanoseconds since the Unix epoch,
    /// saturating) observed when `object_hash` was computed. `0` =
    /// no cache — the file must be re-read and re-hashed to compare.
    pub mtime_ns: u64,
    /// Stat cache: file size in bytes observed when `object_hash` was
    /// computed. Only meaningful when `mtime_ns != 0`.
    pub size: u64,
    /// Stat cache: inode number (0 on platforms without one, or when
    /// uncached). Catches replace-by-rename swaps that preserve
    /// mtime+size — the replacement file has a different inode.
    pub ino: u64,
    /// Stat cache: status-change time (ctime) in saturating ns. ctime
    /// cannot be set from userspace, so it catches `touch -r`-style
    /// timestamp restoration after an edit. 0 = don't check.
    pub ctime_ns: u64,
}

/// In-memory staging index.
///
/// `entries` stays `pub` for the many read-only call sites across the CLI
/// (`.iter()`, indexing, `.len()`) that predate the path index below and
/// have no reason to route through a method. Any code that *mutates*
/// `entries` — inserting, removing, or reordering — MUST go through
/// [`Index::upsert_entry`], [`Index::remove_entry_at`],
/// [`Index::remove_path`], or [`Index::retain_entries`] instead of touching
/// the `Vec` directly, or `by_path` silently goes stale (see those methods'
/// docs). In-place field mutation of an entry already at a known position
/// (status/hash/stat-cache updates that leave `path` unchanged) is fine
/// either way, since it never moves or renames anything `by_path` tracks.
#[derive(Debug, Default, Clone)]
pub struct Index {
    /// Entries in insertion order.
    pub entries: Vec<IndexEntry>,
    /// `path -> position in entries`, maintained by every mutation that
    /// goes through this type's own methods (issue #708). Lets
    /// `find_entry`/`tracks_path_or_descendant`/`has_tracked_file_at` —
    /// each called up to three times per staged file by `mkit add -A` —
    /// answer in `O(log n)` instead of the `O(n)` linear scan that made
    /// staging N files cost `O(N^2)` overall. A `BTreeMap` (not a
    /// `HashMap`) so it can *also* answer `tracks_path_or_descendant`'s
    /// ancestor/descendant prefix query via a sorted range scan, without a
    /// second structure to keep in sync.
    by_path: BTreeMap<String, usize>,
}

/// Value equality compares staged content only — `by_path` is a derived
/// lookup cache, not user-visible state, and two indexes with identical
/// entries are equal regardless of whether their caches happen to be
/// populated (e.g. one built via [`deserialize`], the other via
/// `entries.push` in a test fixture that never queries it).
impl PartialEq for Index {
    fn eq(&self, other: &Self) -> bool {
        self.entries == other.entries
    }
}

impl Eq for Index {}

impl Index {
    /// Construct an empty index.
    #[must_use]
    pub const fn new() -> Self {
        Self {
            entries: Vec::new(),
            by_path: BTreeMap::new(),
        }
    }

    /// Build an index from an already path-unique entry vec (e.g. freshly
    /// parsed by [`deserialize`]), populating `by_path` in one `O(n log n)`
    /// pass. `pub(crate)` so other `mkit-core` modules that assemble an
    /// `Index` directly from a `Vec<IndexEntry>` (test fixtures in
    /// `worktree`, `ops::stash`, `ops::gc`) don't need to duplicate this,
    /// now that `by_path` makes the old `Index { entries }` struct-literal
    /// construction unavailable outside this module.
    ///
    /// # Panics
    /// Panics (via the debug-only consistency check) if `entries` contains
    /// duplicate paths — callers must pre-validate uniqueness, same
    /// contract as `deserialize`'s `seen_paths` check.
    pub(crate) fn from_entries(entries: Vec<IndexEntry>) -> Self {
        let mut idx = Self {
            entries,
            by_path: BTreeMap::new(),
        };
        idx.rebuild_path_index();
        idx
    }

    /// Find an entry by path. `O(log n)`.
    #[must_use]
    pub fn find_entry(&self, path: &str) -> Option<usize> {
        self.by_path.get(path).copied()
    }

    /// `true` if `path` is itself tracked (a non-removed entry) or is an
    /// ancestor directory of a tracked path. Used to decide whether an
    /// ignored worktree path must still be visited because it (or its
    /// subtree) holds tracked content. `O(log n + k)`, `k` = number of
    /// tracked entries directly under `path`.
    #[must_use]
    pub fn tracks_path_or_descendant(&self, path: &str) -> bool {
        if let Some(&pos) = self.by_path.get(path)
            && self.entries[pos].status != EntryStatus::Removed
        {
            return true;
        }
        let mut prefix = String::with_capacity(path.len() + 1);
        prefix.push_str(path);
        prefix.push('/');
        self.by_path
            .range(prefix.clone()..)
            .take_while(|(p, _)| p.starts_with(prefix.as_str()))
            .any(|(_, &pos)| self.entries[pos].status != EntryStatus::Removed)
    }

    /// `true` if a tracked (non-removed) entry exists at *exactly* `path`.
    ///
    /// Because the index stores only leaf paths (files / symlinks / exec
    /// files, never directories), a hit means `path` is tracked as a
    /// non-directory object. Used by the untracked-discovery walks to detect
    /// a worktree directory that shadows a tracked file: git suppresses the
    /// directory's contents as untracked in that case (#288), reporting only
    /// the tracked-side deletion. A `Removed` tombstone does **not** count —
    /// the path is no longer tracked, so its replacement is genuinely
    /// untracked. `O(log n)`.
    #[must_use]
    pub fn has_tracked_file_at(&self, path: &str) -> bool {
        self.find_entry(path)
            .is_some_and(|i| self.entries[i].status != EntryStatus::Removed)
    }

    /// Count non-removed entries.
    #[must_use]
    pub fn staged_count(&self) -> usize {
        self.entries
            .iter()
            .filter(|e| e.status != EntryStatus::Removed)
            .count()
    }

    /// Insert `entry`, replacing any existing entry at the same path.
    /// `O(log n)`. The sanctioned way to add or wholesale-replace an
    /// entry — unlike a direct `entries.push`/`entries[i] = entry`, this
    /// keeps `by_path` in lockstep.
    pub fn upsert_entry(&mut self, entry: IndexEntry) {
        if let Some(&pos) = self.by_path.get(entry.path.as_str()) {
            self.entries[pos] = entry;
        } else {
            let pos = self.entries.len();
            self.by_path.insert(entry.path.clone(), pos);
            self.entries.push(entry);
        }
        self.debug_assert_consistent();
    }

    /// Remove and return the entry at `pos`. `O(n)` — same asymptotic cost
    /// as the underlying `Vec::remove` shift (every later entry's position
    /// changes), so `by_path` is fully rebuilt. Intended for single-path
    /// removals (`rm`/`restore`/conflict abort), not a per-file staging
    /// loop.
    ///
    /// # Panics
    /// Panics if `pos >= self.entries.len()` (same as `Vec::remove`).
    pub fn remove_entry_at(&mut self, pos: usize) -> IndexEntry {
        let removed = self.entries.remove(pos);
        self.rebuild_path_index();
        removed
    }

    /// Remove the entry at `path`, if any tracked or tombstoned entry
    /// exists there. `O(log n)` to find it, `O(n)` to remove (see
    /// [`Index::remove_entry_at`]).
    pub fn remove_path(&mut self, path: &str) -> Option<IndexEntry> {
        self.find_entry(path).map(|pos| self.remove_entry_at(pos))
    }

    /// Retain only entries matching `keep`, rebuilding `by_path` in one
    /// pass afterward. `O(n)` — the same cost `Vec::retain` already pays.
    pub fn retain_entries(&mut self, keep: impl FnMut(&IndexEntry) -> bool) {
        self.entries.retain(keep);
        self.rebuild_path_index();
    }

    /// Remove any entry that conflicts with staging `path` as a file leaf:
    /// a tracked ancestor directory-as-file (blocks descending into it), or
    /// a tracked descendant nested under `path` treated as a directory (the
    /// reverse conflict). An entry at exactly `path` is left untouched —
    /// callers replace/insert it separately via [`Index::upsert_entry`].
    ///
    /// `O(depth)` in the common case where staging `path` has no conflict
    /// (checked via `by_path` rather than a full scan of the index); falls
    /// back to an `O(n)` retain + rebuild only when a conflict is actually
    /// found, which is rare relative to the number of files staged.
    pub fn remove_directory_conflicts(&mut self, path: &str) {
        let has_ancestor_conflict =
            ancestor_prefixes(path).any(|anc| self.by_path.contains_key(anc));
        let descendant_prefix = format!("{path}/");
        let has_descendant_conflict = self
            .by_path
            .range(descendant_prefix.clone()..)
            .next()
            .is_some_and(|(p, _)| p.starts_with(descendant_prefix.as_str()));
        if !has_ancestor_conflict && !has_descendant_conflict {
            return;
        }
        self.entries.retain(|entry| {
            entry.path == path
                || !(path_descends_from(&entry.path, path) || path_descends_from(path, &entry.path))
        });
        self.rebuild_path_index();
    }

    /// Rebuild `by_path` from `entries` in one `O(n log n)` pass. Called by
    /// every mutation method that can move/remove entries at more than one
    /// position at once.
    fn rebuild_path_index(&mut self) {
        self.by_path.clear();
        for (i, e) in self.entries.iter().enumerate() {
            self.by_path.insert(e.path.clone(), i);
        }
        self.debug_assert_consistent();
    }

    /// Debug-only consistency check for `by_path`: same length as
    /// `entries`, and every entry's path maps back to its own position. A
    /// mismatch means some code mutated `entries` directly instead of going
    /// through `upsert_entry`/`remove_entry_at`/`remove_path`/
    /// `retain_entries`/`remove_directory_conflicts`. Exercised by the
    /// `by_path_*` tests below and by every other index test indirectly
    /// (each mutation call re-checks itself).
    #[cfg(debug_assertions)]
    fn debug_assert_consistent(&self) {
        debug_assert_eq!(
            self.by_path.len(),
            self.entries.len(),
            "Index path map desynced from entries (missed upsert/remove/retain?)"
        );
        for (i, e) in self.entries.iter().enumerate() {
            debug_assert_eq!(
                self.by_path.get(e.path.as_str()),
                Some(&i),
                "Index path map has a stale/dangling position for '{}'",
                e.path
            );
        }
    }

    #[cfg(not(debug_assertions))]
    fn debug_assert_consistent(&self) {}

    /// Serialise to the on-disk byte form per SPEC-INDEX §2.
    ///
    /// # Panics
    /// Panics if any entry's path exceeds `u16::MAX` bytes; callers
    /// should reject such paths via [`validate_index_path`] earlier.
    #[must_use]
    pub fn serialize(&self) -> Vec<u8> {
        // Pre-compute capacity: header + per-entry fixed overhead +
        // path lengths.
        let body: usize = self
            .entries
            .iter()
            .map(|e| 1 + HASH_LEN + 8 + 8 + 8 + 8 + 2 + e.path.len())
            .sum();
        let mut out = Vec::with_capacity(9 + body);
        out.extend_from_slice(&MAGIC);
        out.push(FORMAT_VERSION);
        let count = u32::try_from(self.entries.len()).expect("index entry count fits in u32");
        out.extend_from_slice(&count.to_le_bytes());
        for entry in &self.entries {
            out.push(entry.status as u8);
            out.extend_from_slice(&entry.object_hash);
            out.extend_from_slice(&entry.mtime_ns.to_le_bytes());
            out.extend_from_slice(&entry.size.to_le_bytes());
            out.extend_from_slice(&entry.ino.to_le_bytes());
            out.extend_from_slice(&entry.ctime_ns.to_le_bytes());
            let path_len =
                u16::try_from(entry.path.len()).expect("index entry path length fits in u16");
            out.extend_from_slice(&path_len.to_le_bytes());
            out.extend_from_slice(entry.path.as_bytes());
        }
        out
    }
}

/// Yield every strict ancestor directory prefix of `path`, shallowest
/// first — e.g. `"a/b/c.txt"` yields `"a"`, then `"a/b"` (never `path`
/// itself). Used by [`Index::remove_directory_conflicts`] to check for a
/// tracked ancestor-as-file in `O(depth)` instead of scanning the index.
fn ancestor_prefixes(path: &str) -> impl Iterator<Item = &str> {
    path.match_indices('/').map(move |(i, _)| &path[..i])
}

/// `true` if `path` is a strict descendant of `base` (`base` followed by a
/// `/` and at least one more byte). Mirrors
/// `mkit_cli::commands::index_path_descends_from` — duplicated here rather
/// than shared across the crate boundary, since `mkit-core` cannot depend
/// on `mkit-cli`.
fn path_descends_from(path: &str, base: &str) -> bool {
    path.len() > base.len()
        && path.starts_with(base)
        && path.as_bytes().get(base.len()) == Some(&b'/')
}

/// Errors returned by the index subsystem.
#[derive(Debug, thiserror::Error)]
pub enum IndexError {
    /// Magic bytes were not `"MKIX"`.
    #[error("index file has wrong magic (expected MKIX)")]
    BadMagic,
    /// `version` byte was not the current `FORMAT_VERSION`.
    #[error("unsupported index version: {0:#x}")]
    UnsupportedVersion(u8),
    /// Status byte was outside the documented {0x00..=0x04} range.
    #[error("index entry has unknown status byte {0:#x}")]
    BadStatus(u8),
    /// Truncated or otherwise malformed entry.
    #[error("index file is corrupt")]
    Corrupt,
    /// File exceeded [`MAX_INDEX_BYTES`].
    #[error("index file too large (>{MAX_INDEX_BYTES} bytes)")]
    TooLarge,
    /// Path failed [`validate_index_path`].
    #[error("invalid index path '{0}'")]
    InvalidPath(String),
    /// Path appeared more than once in the same index.
    #[error("duplicate index path '{0}'")]
    DuplicatePath(String),
    /// A `Removed` entry carried a nonzero `object_hash` (SPEC-INDEX §3
    /// requires the all-zero hash for removals).
    #[error("removed index entry '{0}' has nonzero object_hash")]
    RemovedHasHash(String),
    /// Path UTF-8 decoding failed.
    #[error("index path is not valid UTF-8")]
    InvalidPathEncoding,
    /// Underlying I/O failure.
    #[error(transparent)]
    Io(#[from] io::Error),
    /// Object store lookup/decoding failed while deriving an index from a tree.
    #[error(transparent)]
    Store(#[from] StoreError),
    /// A tree walk found a non-tree object where a tree hash was expected.
    #[error("object is not a tree")]
    NotTree,
    /// A tree walk exceeded [`MAX_TREE_DEPTH`] nesting levels — likely a
    /// crafted untrusted repo trying to overflow the native stack.
    #[error("tree nesting exceeds {} levels", MAX_TREE_DEPTH)]
    TreeTooDeep,
}

/// Result alias used throughout this module.
pub type IndexResult<T> = Result<T, IndexError>;

/// Deserialise bytes into an [`Index`].
///
/// # Errors
/// See [`IndexError`].
///
/// # Panics
/// Panics only if internal fixed-width slicing is wrong, which is
/// impossible by construction (lengths are bounds-checked first).
pub fn deserialize(data: &[u8]) -> IndexResult<Index> {
    if data.len() < 9 {
        return Err(IndexError::Corrupt);
    }
    if data[0..4] != MAGIC {
        return Err(IndexError::BadMagic);
    }
    let version = data[4];
    if version != FORMAT_VERSION {
        return Err(IndexError::UnsupportedVersion(version));
    }
    // Entries carry mtime_ns(8) + size(8) + ino(8) + ctime_ns(8) before
    // path_len.
    let stat_cache_len: usize = 32;
    // Fixed bytes per entry: status(1) + hash(32) + stat cache + path_len(2).
    let min_entry_len = 1 + HASH_LEN + stat_cache_len + 2;
    let count = u32::from_le_bytes([data[5], data[6], data[7], data[8]]) as usize;
    // Reject an attacker-supplied `count` that is impossible given the
    // remaining bytes. The minimum wire-length of an entry is 67 bytes
    // (empty path). Without this up-front check the loop would walk
    // `count` iterations before failing — trivially triggered with a
    // 9-byte buffer declaring `count = u32::MAX`.
    // Mirrors the same up-front bound used in `serialize.rs`.
    if (count as u64).saturating_mul(min_entry_len as u64) > data.len() as u64 {
        return Err(IndexError::Corrupt);
    }
    let mut entries = Vec::with_capacity(count.min(1024)); // bound initial alloc
    let mut seen_paths = std::collections::HashSet::with_capacity(count.min(1024));
    let mut offset = 9usize;
    for _ in 0..count {
        if offset + min_entry_len > data.len() {
            return Err(IndexError::Corrupt);
        }
        let status =
            EntryStatus::from_byte(data[offset]).ok_or(IndexError::BadStatus(data[offset]))?;
        offset += 1;
        let mut object_hash = [0u8; HASH_LEN];
        object_hash.copy_from_slice(&data[offset..offset + HASH_LEN]);
        offset += HASH_LEN;
        let (mtime_ns, size, ino, ctime_ns) = {
            let mut next_u64 = || {
                let v = u64::from_le_bytes(data[offset..offset + 8].try_into().expect("8 bytes"));
                offset += 8;
                v
            };
            (next_u64(), next_u64(), next_u64(), next_u64())
        };
        let path_len = u16::from_le_bytes([data[offset], data[offset + 1]]) as usize;
        offset += 2;
        if path_len > MAX_PATH_LEN {
            return Err(IndexError::Corrupt);
        }
        if offset + path_len > data.len() {
            return Err(IndexError::Corrupt);
        }
        let path_bytes = &data[offset..offset + path_len];
        let path = core::str::from_utf8(path_bytes)
            .map_err(|_| IndexError::InvalidPathEncoding)?
            .to_string();
        offset += path_len;
        if !validate_index_path(&path) {
            return Err(IndexError::InvalidPath(path));
        }
        if !seen_paths.insert(path.clone()) {
            return Err(IndexError::DuplicatePath(path));
        }
        if status == EntryStatus::Removed && object_hash != hash::ZERO {
            return Err(IndexError::RemovedHasHash(path));
        }
        entries.push(IndexEntry {
            path,
            status,
            object_hash,
            mtime_ns,
            size,
            ino,
            ctime_ns,
        });
    }
    if offset != data.len() {
        return Err(IndexError::Corrupt);
    }
    Ok(Index::from_entries(entries))
}

/// Read this worktree's staging index. Returns an empty index if the
/// file is absent or zero-length. The index is per-worktree state —
/// see [`crate::layout`].
pub fn read_index(layout: &RepoLayout) -> IndexResult<Index> {
    let path = layout.index_file();
    let meta = match fs::metadata(&path) {
        Ok(m) => m,
        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(Index::new()),
        Err(e) => return Err(IndexError::Io(e)),
    };
    if meta.len() == 0 {
        return Ok(Index::new());
    }
    if meta.len() > MAX_INDEX_BYTES {
        return Err(IndexError::TooLarge);
    }
    let bytes = fs::read(&path)?;
    let mut idx = deserialize(&bytes)?;
    // git's racy-clean rule, applied at read time: an entry whose
    // cached mtime is not safely OLDER than the index file itself may
    // have been modified after hashing without its stat changing —
    // within the filesystem timestamp granularity the modification is
    // invisible to stat. Treat such entries as uncached (zero
    // sentinel) so callers re-hash them; the next index write (whose
    // file mtime is then newer) heals the cache.
    // Same conversion (incl. 0-sentinel + saturation semantics) as the
    // entry mtimes it is compared against — one implementation only.
    let index_mtime_ns = crate::worktree::mtime_nanos(&meta);
    // Window sizing, like git's USE_NSEC — but judged PER ENTRY: the
    // tight 10ms window is only safe when BOTH the index file's mtime
    // and the entry's recorded worktree mtime show sub-second
    // precision. A worktree file whose mtime is whole-second (vfat/
    // SMB/NFS mounts, tar/touch -t/rsync-truncated timestamps) could
    // be rewritten within its coarse tick without the stat changing,
    // so such entries keep the conservative 1s window.
    let index_ns_precise = !index_mtime_ns.is_multiple_of(1_000_000_000);
    for e in &mut idx.entries {
        if e.mtime_ns == 0 {
            continue;
        }
        let window = if index_ns_precise && !e.mtime_ns.is_multiple_of(1_000_000_000) {
            RACY_WINDOW_NS / 100
        } else {
            RACY_WINDOW_NS
        };
        if e.mtime_ns >= index_mtime_ns.saturating_sub(window) {
            e.mtime_ns = 0;
            e.size = 0;
            e.ino = 0;
            e.ctime_ns = 0;
        }
    }
    Ok(idx)
}

/// The racy-clean window: an entry whose cached mtime is within this
/// span of the index file's own mtime may have been modified after
/// hashing without its stat changing (filesystem timestamp granularity
/// can be as coarse as 1s), so its cache cannot be trusted. One second
/// is the conservative bound git uses for second-granularity
/// filesystems.
const RACY_WINDOW_NS: u64 = 1_000_000_000;

/// Write this worktree's staging index atomically. The containing
/// directory is created if absent.
///
/// Stat-cache fields are written verbatim; the racy-clean rule is
/// applied at READ time against the index file's own mtime (see
/// [`read_index`]). Note a read-modify-write command that loads a
/// racy-marked entry persists the zeroed cache for it — sound (zero
/// always re-hashes) and healed by the next add/status touching the
/// path; only the racy window's worth of entries is affected.
pub fn write_index(layout: &RepoLayout, idx: &Index) -> IndexResult<()> {
    let path = layout.index_file();
    write_atomic(&path, &idx.serialize(), true)?;
    Ok(())
}

/// Materialize a staging index from a committed tree.
///
/// This is used after commands that move `HEAD` and restore the
/// worktree so the index keeps matching the new commit snapshot. Tree
/// entries are recursively flattened into leaf paths; removed entries
/// are not represented because a committed tree has no tombstones.
///
/// # Errors
/// Propagates object-store errors and returns [`IndexError::NotTree`]
/// if `tree_hash` does not point at a tree object.
pub fn from_tree(store: &ObjectStore, tree_hash: Hash) -> IndexResult<Index> {
    let mut entries = Vec::new();
    push_tree_entries(store, tree_hash, "", &mut entries, 0)?;
    Ok(Index::from_entries(entries))
}

fn push_tree_entries(
    store: &ObjectStore,
    tree_hash: Hash,
    prefix: &str,
    entries: &mut Vec<IndexEntry>,
    depth: usize,
) -> IndexResult<()> {
    if depth > MAX_TREE_DEPTH {
        return Err(IndexError::TreeTooDeep);
    }
    let Object::Tree(tree) = store.read_object(&tree_hash)? else {
        return Err(IndexError::NotTree);
    };
    for entry in tree.entries {
        let name = String::from_utf8(entry.name).map_err(|_| IndexError::InvalidPathEncoding)?;
        let path = if prefix.is_empty() {
            name
        } else {
            format!("{prefix}/{name}")
        };
        match entry.mode {
            EntryMode::Tree => {
                push_tree_entries(store, entry.object_hash, &path, entries, depth + 1)?;
            }
            EntryMode::Blob | EntryMode::Executable | EntryMode::Symlink => {
                if !validate_index_path(&path) {
                    return Err(IndexError::InvalidPath(path));
                }
                let status = match entry.mode {
                    EntryMode::Blob => EntryStatus::Blob,
                    EntryMode::Executable => EntryStatus::Executable,
                    EntryMode::Symlink => EntryStatus::Symlink,
                    EntryMode::Tree => unreachable!("handled above"),
                };
                entries.push(IndexEntry {
                    path,
                    status,
                    object_hash: entry.object_hash,
                    // A tree-derived entry has no observed worktree
                    // stat — zero sentinel means "re-hash to compare".
                    mtime_ns: 0,
                    size: 0,
                    ino: 0,
                    ctime_ns: 0,
                });
            }
        }
    }
    Ok(())
}

/// Compute the absolute path of this worktree's index file.
#[must_use]
pub fn index_path(layout: &RepoLayout) -> PathBuf {
    layout.index_file()
}

/// Validate a staged path: non-empty, relative, no traversal, no NUL,
/// no backslash, never under `.mkit/` or `.git/`.
#[must_use]
pub fn validate_index_path(path: &str) -> bool {
    if path.is_empty() {
        return false;
    }
    if path.starts_with('/') {
        return false;
    }
    if path.len() > MAX_PATH_LEN {
        return false;
    }
    if path == ".mkit" || path == ".git" {
        return false;
    }
    if path.starts_with(".mkit/") || path.starts_with(".git/") {
        return false;
    }
    for part in path.split('/') {
        if part.is_empty() {
            return false;
        }
        if part == "." || part == ".." {
            return false;
        }
        for &c in part.as_bytes() {
            if c == 0 || c == b'\\' {
                return false;
            }
        }
    }
    true
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::hash;
    use tempfile::TempDir;

    fn seed_hash(s: &str) -> Hash {
        hash::hash(s.as_bytes())
    }

    #[test]
    fn empty_index_round_trip() {
        let idx = Index::new();
        let bytes = idx.serialize();
        // 4 magic + 1 version + 4 count = 9 bytes.
        assert_eq!(bytes.len(), 9);
        assert_eq!(&bytes[0..4], &MAGIC);
        assert_eq!(bytes[4], FORMAT_VERSION);
        assert_eq!(&bytes[5..9], &0u32.to_le_bytes());
        let parsed = deserialize(&bytes).unwrap();
        assert_eq!(parsed, idx);
    }

    // ---- stat cache -----------------------------------------------------

    /// Pinned vector: header(9) + status(1) + hash(32) +
    /// `mtime_ns`(8) + `size`(8) + `ino`(8) + `ctime_ns`(8) +
    /// `path_len`(2) + "hello.txt"(9) = 85 bytes.
    #[test]
    fn single_entry_pinned_bytes() {
        let h = seed_hash("hello");
        let idx = Index::from_entries(vec![IndexEntry {
            path: "hello.txt".to_string(),
            status: EntryStatus::Blob,
            object_hash: h,
            mtime_ns: 0x0102_0304_0506_0708,
            size: 11,
            ino: 0x0A0B_0C0D_0E0F_1011,
            ctime_ns: 0x1112_1314_1516_1718,
        }]);
        let bytes = idx.serialize();
        assert_eq!(bytes.len(), 85);
        let mut expected = Vec::new();
        expected.extend_from_slice(b"MKIX");
        expected.push(0x02); // version
        expected.extend_from_slice(&1u32.to_le_bytes());
        expected.push(0x01); // Blob
        expected.extend_from_slice(&h);
        expected.extend_from_slice(&0x0102_0304_0506_0708u64.to_le_bytes());
        expected.extend_from_slice(&11u64.to_le_bytes());
        expected.extend_from_slice(&0x0A0B_0C0D_0E0F_1011u64.to_le_bytes());
        expected.extend_from_slice(&0x1112_1314_1516_1718u64.to_le_bytes());
        expected.extend_from_slice(&9u16.to_le_bytes());
        expected.extend_from_slice(b"hello.txt");
        assert_eq!(bytes, expected, "byte layout is pinned");
        assert_eq!(deserialize(&bytes).unwrap(), idx);
    }

    #[test]
    fn rejects_count_overflow_at_min_entry_bytes() {
        // 9-byte header declaring u32::MAX entries: the minimum entry is
        // 67 bytes, so this must fail fast, before looping.
        let mut bytes = Vec::new();
        bytes.extend_from_slice(b"MKIX");
        bytes.push(0x02);
        bytes.extend_from_slice(&u32::MAX.to_le_bytes());
        assert!(matches!(deserialize(&bytes), Err(IndexError::Corrupt)));
        // One entry declared, only 60 bytes of body: still corrupt.
        let mut short = Vec::new();
        short.extend_from_slice(b"MKIX");
        short.push(0x02);
        short.extend_from_slice(&1u32.to_le_bytes());
        short.extend_from_slice(&[0u8; 60]);
        assert!(matches!(deserialize(&short), Err(IndexError::Corrupt)));
    }

    #[test]
    fn rejects_unknown_version_0x03() {
        let mut bytes = Vec::new();
        bytes.extend_from_slice(b"MKIX");
        bytes.push(0x03);
        bytes.extend_from_slice(&0u32.to_le_bytes());
        assert!(matches!(
            deserialize(&bytes),
            Err(IndexError::UnsupportedVersion(0x03))
        ));
    }

    /// The old pre-stat-cache format (version `0x01`) is rejected like any
    /// other unsupported version — mkit does not carry dual-version
    /// read-compat for its local, advisory index file (SPEC-CONVENTIONS
    /// §4: no version-suffixed compatibility eras).
    #[test]
    fn rejects_old_version_0x01() {
        let mut bytes = Vec::new();
        bytes.extend_from_slice(b"MKIX");
        bytes.push(0x01);
        bytes.extend_from_slice(&0u32.to_le_bytes());
        assert!(matches!(
            deserialize(&bytes),
            Err(IndexError::UnsupportedVersion(0x01))
        ));
    }

    /// git's racy-clean rule: an entry whose mtime is within the
    /// filesystem-timestamp granularity of the index file's mtime may
    /// have been modified after hashing without the stat changing —
    /// its cache must be ignored on read so the caller re-hashes.
    #[test]
    fn read_index_invalidates_racy_entries() {
        let dir = TempDir::new().unwrap();
        let layout = RepoLayout::single(dir.path());
        let now_ns = u64::try_from(
            std::time::SystemTime::now()
                .duration_since(std::time::UNIX_EPOCH)
                .unwrap()
                .as_nanos(),
        )
        .unwrap();
        let idx = Index::from_entries(vec![
            IndexEntry {
                path: "racy.txt".to_string(),
                status: EntryStatus::Blob,
                object_hash: seed_hash("racy"),
                mtime_ns: now_ns,
                size: 4,
                ino: 0,
                ctime_ns: 0,
            },
            IndexEntry {
                path: "settled.txt".to_string(),
                status: EntryStatus::Blob,
                object_hash: seed_hash("settled"),
                mtime_ns: now_ns - 10_000_000_000, // 10s ago
                size: 7,
                ino: 0,
                ctime_ns: 0,
            },
        ]);
        write_index(&layout, &idx).unwrap();
        // Pin the index FILE's mtime to exactly the racy entry's time so
        // the test is deterministic regardless of scheduling delays and
        // the granularity-derived window size: an entry whose mtime
        // equals the index mtime is racy under any window.
        let f = fs::File::options()
            .write(true)
            .open(index_path(&layout))
            .unwrap();
        f.set_times(
            fs::FileTimes::new()
                .set_modified(std::time::UNIX_EPOCH + std::time::Duration::from_nanos(now_ns)),
        )
        .unwrap();
        drop(f);
        let read = read_index(&layout).unwrap();
        let racy = &read.entries[read.find_entry("racy.txt").unwrap()];
        let settled = &read.entries[read.find_entry("settled.txt").unwrap()];
        assert_eq!(
            racy.mtime_ns, 0,
            "an entry touched within the racy window must lose its cache"
        );
        assert_eq!(racy.size, 0);
        assert_eq!(settled.mtime_ns, now_ns - 10_000_000_000);
        assert_eq!(settled.size, 7);
    }

    /// A whole-second entry mtime (vfat/SMB/tar-truncated timestamps)
    /// must keep the conservative 1s racy window even when the index
    /// file itself has nanosecond precision — the file could be
    /// rewritten within its coarse tick without the stat changing.
    #[test]
    fn coarse_entry_mtime_keeps_one_second_window() {
        let dir = TempDir::new().unwrap();
        let layout = RepoLayout::single(dir.path());
        let base_ns: u64 = 1_700_000_000_000_000_000; // whole-second tick
        let idx = Index::from_entries(vec![
            IndexEntry {
                path: "coarse.txt".to_string(),
                status: EntryStatus::Blob,
                object_hash: seed_hash("coarse"),
                // 500ms before the index mtime, WHOLE-second value:
                // inside the 1s window, outside the 10ms one.
                mtime_ns: base_ns - 1_000_000_000,
                size: 4,
                ino: 0,
                ctime_ns: 0,
            },
            IndexEntry {
                path: "precise.txt".to_string(),
                status: EntryStatus::Blob,
                object_hash: seed_hash("precise"),
                // Same age but ns-precise: the 10ms window applies
                // and it is safely older than the floor.
                mtime_ns: base_ns - 1_000_000_000 + 123,
                size: 7,
                ino: 0,
                ctime_ns: 0,
            },
        ]);
        write_index(&layout, &idx).unwrap();
        // Index file mtime: ns-precise, 500ms after the coarse entry.
        let f = fs::File::options()
            .write(true)
            .open(index_path(&layout))
            .unwrap();
        f.set_times(fs::FileTimes::new().set_modified(
            std::time::UNIX_EPOCH + std::time::Duration::from_nanos(base_ns - 500_000_000 + 777),
        ))
        .unwrap();
        drop(f);

        let read = read_index(&layout).unwrap();
        let coarse = &read.entries[read.find_entry("coarse.txt").unwrap()];
        let precise = &read.entries[read.find_entry("precise.txt").unwrap()];
        assert_eq!(
            coarse.mtime_ns, 0,
            "coarse-mtime entry within 1s of the index write must be racy"
        );
        assert_ne!(
            precise.mtime_ns, 0,
            "ns-precise entry outside the 10ms window keeps its cache"
        );
    }

    #[test]
    fn tracks_path_or_descendant_matches_self_and_ancestors() {
        let mut idx = Index::new();
        idx.upsert_entry(IndexEntry {
            path: "src/lib.rs".to_string(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("lib"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.upsert_entry(IndexEntry {
            path: "removed.txt".to_string(),
            status: EntryStatus::Removed,
            object_hash: hash::ZERO,
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        // Exact tracked path and its ancestor directory both match.
        assert!(idx.tracks_path_or_descendant("src/lib.rs"));
        assert!(idx.tracks_path_or_descendant("src"));
        // A prefix that is not a path-segment boundary does not match.
        assert!(!idx.tracks_path_or_descendant("sr"));
        // Unrelated and removed-only paths do not match.
        assert!(!idx.tracks_path_or_descendant("docs"));
        assert!(!idx.tracks_path_or_descendant("removed.txt"));
    }

    #[test]
    fn has_tracked_file_at_exact_only_and_not_removed() {
        let mut idx = Index::new();
        idx.upsert_entry(IndexEntry {
            path: "f".to_string(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("f"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.upsert_entry(IndexEntry {
            path: "gone".to_string(),
            status: EntryStatus::Removed,
            object_hash: hash::ZERO,
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        // Exact tracked file matches.
        assert!(idx.has_tracked_file_at("f"));
        // Unlike `tracks_path_or_descendant`, an ancestor directory does NOT
        // match — only an exact tracked leaf does (the collision predicate).
        idx.upsert_entry(IndexEntry {
            path: "dir/inner.txt".to_string(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("inner"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        assert!(!idx.has_tracked_file_at("dir"));
        assert!(idx.has_tracked_file_at("dir/inner.txt"));
        // A `Removed` tombstone must NOT suppress — the path is no longer
        // tracked, so a replacement at that path is genuinely untracked.
        assert!(!idx.has_tracked_file_at("gone"));
        // Unrelated path.
        assert!(!idx.has_tracked_file_at("other"));
    }

    #[test]
    fn multi_entry_round_trip_with_all_statuses() {
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "a.txt".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("a"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "b/sub".into(),
            status: EntryStatus::Tree,
            object_hash: seed_hash("b"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "c.link".into(),
            status: EntryStatus::Symlink,
            object_hash: seed_hash("c"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "scripts/build".into(),
            status: EntryStatus::Executable,
            object_hash: seed_hash("d"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "old.txt".into(),
            status: EntryStatus::Removed,
            object_hash: [0u8; HASH_LEN],
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        let bytes = idx.serialize();
        let parsed = deserialize(&bytes).unwrap();
        assert_eq!(parsed, idx);
    }

    #[test]
    fn rejects_bad_magic() {
        let mut bytes = Index::new().serialize();
        bytes[0] = b'X';
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::BadMagic));
    }

    #[test]
    fn rejects_zmix_magic_explicitly() {
        // SPEC-INDEX §5: readers MUST reject `"ZMIX"`-prefixed files. We
        // construct the rejected magic as ASCII bytes here rather than
        // embedding the wrong literal string.
        let bytes = [
            0x5A,
            0x4D,
            0x49,
            0x58, // "ZMIX"
            FORMAT_VERSION,
            0,
            0,
            0,
            0,
        ];
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::BadMagic));
    }

    #[test]
    fn rejects_unsupported_version() {
        let mut bytes = Index::new().serialize();
        bytes[4] = 0xFF;
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::UnsupportedVersion(0xFF)));
    }

    #[test]
    fn rejects_truncated_header() {
        let err = deserialize(b"MKIX").unwrap_err();
        assert!(matches!(err, IndexError::Corrupt));
    }

    #[test]
    fn rejects_truncated_entry() {
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "a".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("a"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        let mut bytes = idx.serialize();
        bytes.truncate(bytes.len() - 1); // drop the trailing path byte
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::Corrupt));
    }

    #[test]
    fn rejects_trailing_bytes_after_declared_entries() {
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "a".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("a"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        let mut bytes = idx.serialize();
        bytes.extend_from_slice(b"junk");
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::Corrupt));
    }

    #[test]
    fn rejects_invalid_path_on_deserialize() {
        let mut bytes = Vec::new();
        bytes.extend_from_slice(&MAGIC);
        bytes.push(FORMAT_VERSION);
        bytes.extend_from_slice(&1u32.to_le_bytes());
        bytes.push(EntryStatus::Blob as u8);
        bytes.extend_from_slice(&[0u8; HASH_LEN]);
        bytes.extend_from_slice(&[0u8; 32]); // stat cache
        let path = b"../escape";
        let path_len = u16::try_from(path.len()).unwrap();
        bytes.extend_from_slice(&path_len.to_le_bytes());
        bytes.extend_from_slice(path);
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::InvalidPath(path) if path == "../escape"));
    }

    #[test]
    fn rejects_duplicate_paths_on_deserialize() {
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "same.txt".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("a"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "same.txt".into(),
            status: EntryStatus::Executable,
            object_hash: seed_hash("b"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        let err = deserialize(&idx.serialize()).unwrap_err();
        assert!(matches!(err, IndexError::DuplicatePath(path) if path == "same.txt"));
    }

    #[test]
    fn rejects_path_len_overflow() {
        // Hand-roll: path_len = 1000 but only 1 byte available.
        let mut bytes = Vec::new();
        bytes.extend_from_slice(&MAGIC);
        bytes.push(FORMAT_VERSION);
        bytes.extend_from_slice(&1u32.to_le_bytes());
        bytes.push(EntryStatus::Blob as u8);
        bytes.extend_from_slice(&[0u8; HASH_LEN]);
        bytes.extend_from_slice(&1000u16.to_le_bytes());
        bytes.push(b'a');
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::Corrupt));
    }

    #[test]
    fn rejects_unknown_status_byte() {
        let mut bytes = Vec::new();
        bytes.extend_from_slice(&MAGIC);
        bytes.push(FORMAT_VERSION);
        bytes.extend_from_slice(&1u32.to_le_bytes());
        bytes.push(0x77); // bogus status
        bytes.extend_from_slice(&[0u8; HASH_LEN]);
        bytes.extend_from_slice(&[0u8; 32]); // stat cache
        bytes.extend_from_slice(&0u16.to_le_bytes());
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::BadStatus(0x77)));
    }

    #[test]
    fn rejects_removed_entry_with_nonzero_hash() {
        // Hand-roll a Removed (0x00) entry carrying a nonzero object_hash —
        // SPEC-INDEX §3 requires the all-zero hash for removals.
        let mut bytes = Vec::new();
        bytes.extend_from_slice(&MAGIC);
        bytes.push(FORMAT_VERSION);
        bytes.extend_from_slice(&1u32.to_le_bytes());
        bytes.push(EntryStatus::Removed as u8);
        bytes.extend_from_slice(&seed_hash("nonzero")); // MUST be all-zero
        bytes.extend_from_slice(&[0u8; 32]); // stat cache
        let path = b"removed.txt";
        bytes.extend_from_slice(&11u16.to_le_bytes());
        bytes.extend_from_slice(path);
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::RemovedHasHash(p) if p == "removed.txt"));
    }

    #[test]
    fn removed_entry_with_zero_hash_round_trips() {
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "gone.txt".into(),
            status: EntryStatus::Removed,
            object_hash: hash::ZERO,
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        let round_tripped = deserialize(&idx.serialize()).unwrap();
        assert_eq!(round_tripped.entries[0].status, EntryStatus::Removed);
        assert_eq!(round_tripped.entries[0].object_hash, hash::ZERO);
    }

    #[test]
    fn write_and_read_round_trip_via_disk() {
        let dir = TempDir::new().unwrap();
        fs::create_dir_all(dir.path().join(".mkit")).unwrap();
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "test.txt".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("c"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        let layout = RepoLayout::single(dir.path());
        write_index(&layout, &idx).unwrap();
        let read = read_index(&layout).unwrap();
        assert_eq!(read, idx);
    }

    #[test]
    fn read_missing_file_returns_empty_index() {
        let dir = TempDir::new().unwrap();
        let idx = read_index(&RepoLayout::single(dir.path())).unwrap();
        assert!(idx.entries.is_empty());
    }

    #[test]
    fn read_zero_length_file_returns_empty_index() {
        let dir = TempDir::new().unwrap();
        fs::create_dir_all(dir.path().join(".mkit")).unwrap();
        fs::write(dir.path().join(INDEX_FILE), b"").unwrap();
        let idx = read_index(&RepoLayout::single(dir.path())).unwrap();
        assert!(idx.entries.is_empty());
    }

    #[test]
    fn read_oversize_file_rejected() {
        let dir = TempDir::new().unwrap();
        fs::create_dir_all(dir.path().join(".mkit")).unwrap();
        let path = dir.path().join(INDEX_FILE);
        // Sparse-extend beyond the cap; allocates effectively no blocks.
        let f = fs::OpenOptions::new()
            .write(true)
            .create(true)
            .truncate(true)
            .open(&path)
            .unwrap();
        f.set_len(MAX_INDEX_BYTES + 1).unwrap();
        drop(f);
        let err = read_index(&RepoLayout::single(dir.path())).unwrap_err();
        assert!(matches!(err, IndexError::TooLarge));
    }

    #[test]
    fn staged_count_excludes_removed() {
        let mut idx = Index::new();
        idx.entries.push(IndexEntry {
            path: "a".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("a"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "b".into(),
            status: EntryStatus::Removed,
            object_hash: [0u8; HASH_LEN],
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        idx.entries.push(IndexEntry {
            path: "c".into(),
            status: EntryStatus::Blob,
            object_hash: seed_hash("c"),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        });
        assert_eq!(idx.staged_count(), 2);
    }

    #[test]
    fn rejects_bogus_huge_count_before_loop() {
        // G11 regression: a 13-byte buffer whose header declares
        // count = u32::MAX must be rejected up-front — the
        // deserializer must NOT spin through u32::MAX iterations
        // (or allocate Vec::with_capacity(count)).
        let mut bytes = Vec::new();
        bytes.extend_from_slice(&MAGIC);
        bytes.push(FORMAT_VERSION);
        bytes.extend_from_slice(&u32::MAX.to_le_bytes());
        // No entries follow — buffer is just the 9-byte header.
        let err = deserialize(&bytes).unwrap_err();
        assert!(matches!(err, IndexError::Corrupt));
    }

    #[test]
    fn validate_path_basic() {
        assert!(validate_index_path("a.txt"));
        assert!(validate_index_path("src/main.rs"));
        assert!(validate_index_path(".mkitignore"));
        assert!(!validate_index_path(""));
        assert!(!validate_index_path("/abs"));
        assert!(!validate_index_path("../escape"));
        assert!(!validate_index_path("a/../b"));
        assert!(!validate_index_path(".mkit"));
        assert!(!validate_index_path(".git"));
        assert!(!validate_index_path(".mkit/objects"));
        assert!(!validate_index_path(".git/HEAD"));
        assert!(!validate_index_path("a\\b"));
        assert!(!validate_index_path("a//b"));
    }

    #[test]
    fn from_tree_flattens_tree_entries() {
        use crate::object::{Blob, EntryMode, Object, Tree, TreeEntry};
        use crate::serialize;
        use crate::store::ObjectStore;

        fn put(store: &ObjectStore, obj: &Object) -> Hash {
            let bytes = serialize::serialize(obj).unwrap();
            store.write(&bytes).unwrap()
        }

        let dir = TempDir::new().unwrap();
        let store = ObjectStore::init(&RepoLayout::single(dir.path())).unwrap();
        let file = put(
            &store,
            &Object::Blob(Blob {
                data: b"file".to_vec(),
            }),
        );
        let exec = put(
            &store,
            &Object::Blob(Blob {
                data: b"exec".to_vec(),
            }),
        );
        let link = put(
            &store,
            &Object::Blob(Blob {
                data: b"target".to_vec(),
            }),
        );
        let sub = put(
            &store,
            &Object::Tree(Tree {
                entries: vec![TreeEntry {
                    name: b"run".to_vec(),
                    mode: EntryMode::Executable,
                    object_hash: exec,
                }],
            }),
        );
        let root = put(
            &store,
            &Object::Tree(Tree {
                entries: vec![
                    TreeEntry {
                        name: b"file.txt".to_vec(),
                        mode: EntryMode::Blob,
                        object_hash: file,
                    },
                    TreeEntry {
                        name: b"link".to_vec(),
                        mode: EntryMode::Symlink,
                        object_hash: link,
                    },
                    TreeEntry {
                        name: b"sub".to_vec(),
                        mode: EntryMode::Tree,
                        object_hash: sub,
                    },
                ],
            }),
        );

        let idx = from_tree(&store, root).unwrap();
        assert_eq!(idx.entries.len(), 3);
        assert_eq!(idx.entries[0].path, "file.txt");
        assert_eq!(idx.entries[0].status, EntryStatus::Blob);
        assert_eq!(idx.entries[1].path, "link");
        assert_eq!(idx.entries[1].status, EntryStatus::Symlink);
        assert_eq!(idx.entries[2].path, "sub/run");
        assert_eq!(idx.entries[2].status, EntryStatus::Executable);
    }

    #[test]
    fn from_tree_round_trips_through_worktree_builder() {
        use crate::object::{Blob, EntryMode, Object, Tree, TreeEntry};
        use crate::serialize;
        use crate::store::ObjectStore;

        fn put(store: &ObjectStore, obj: &Object) -> Hash {
            let bytes = serialize::serialize(obj).unwrap();
            store.write(&bytes).unwrap()
        }

        let dir = TempDir::new().unwrap();
        let store = ObjectStore::init(&RepoLayout::single(dir.path())).unwrap();
        let blob = put(
            &store,
            &Object::Blob(Blob {
                data: b"content".to_vec(),
            }),
        );
        let tree = put(
            &store,
            &Object::Tree(Tree {
                entries: vec![TreeEntry {
                    name: b"a.txt".to_vec(),
                    mode: EntryMode::Blob,
                    object_hash: blob,
                }],
            }),
        );

        let idx = from_tree(&store, tree).unwrap();
        let rebuilt = crate::worktree::build_tree_from_index(&store, &idx).unwrap();
        assert_eq!(rebuilt, tree);
    }

    // ---- by_path (issue #708) --------------------------------------------

    fn blob_entry(path: &str) -> IndexEntry {
        IndexEntry {
            path: path.to_string(),
            status: EntryStatus::Blob,
            object_hash: seed_hash(path),
            mtime_ns: 0,
            size: 0,
            ino: 0,
            ctime_ns: 0,
        }
    }

    /// `upsert_entry` on a fresh path appends and registers it; `find_entry`
    /// then answers via the map, not a scan. Each call self-checks
    /// consistency in debug builds (see `debug_assert_consistent`), so this
    /// test alone exercises the map on every one of its mutations.
    #[test]
    fn upsert_entry_inserts_new_path() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("a.txt"));
        idx.upsert_entry(blob_entry("b.txt"));
        assert_eq!(idx.entries.len(), 2);
        assert_eq!(idx.find_entry("a.txt"), Some(0));
        assert_eq!(idx.find_entry("b.txt"), Some(1));
        assert_eq!(idx.find_entry("missing"), None);
    }

    /// `upsert_entry` on an already-tracked path replaces in place — same
    /// position, no growth, and the map still resolves it.
    #[test]
    fn upsert_entry_replaces_existing_path() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("a.txt"));
        idx.upsert_entry(blob_entry("b.txt"));
        let mut replacement = blob_entry("a.txt");
        replacement.status = EntryStatus::Executable;
        idx.upsert_entry(replacement);
        assert_eq!(idx.entries.len(), 2, "replace must not grow the index");
        let pos = idx.find_entry("a.txt").unwrap();
        assert_eq!(idx.entries[pos].status, EntryStatus::Executable);
        // The unrelated entry's position is untouched.
        assert_eq!(idx.find_entry("b.txt"), Some(1));
    }

    /// `remove_path` drops the entry and shifts later positions in the map
    /// to match the `Vec::remove` shift.
    #[test]
    fn remove_path_updates_positions_of_later_entries() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("a.txt"));
        idx.upsert_entry(blob_entry("b.txt"));
        idx.upsert_entry(blob_entry("c.txt"));
        let removed = idx.remove_path("a.txt").expect("a.txt was tracked");
        assert_eq!(removed.path, "a.txt");
        assert_eq!(idx.entries.len(), 2);
        assert_eq!(idx.find_entry("a.txt"), None);
        assert_eq!(idx.entries[idx.find_entry("b.txt").unwrap()].path, "b.txt");
        assert_eq!(idx.entries[idx.find_entry("c.txt").unwrap()].path, "c.txt");
        // Removing an absent path is a documented no-op.
        assert!(idx.remove_path("a.txt").is_none());
    }

    /// `retain_entries` rebuilds the map so positions stay correct for
    /// every survivor, regardless of how many entries were dropped.
    #[test]
    fn retain_entries_rebuilds_positions() {
        let mut idx = Index::new();
        for p in ["a.txt", "b.txt", "c.txt", "d.txt"] {
            idx.upsert_entry(blob_entry(p));
        }
        idx.retain_entries(|e| e.path != "b.txt" && e.path != "c.txt");
        assert_eq!(idx.entries.len(), 2);
        assert_eq!(idx.entries[idx.find_entry("a.txt").unwrap()].path, "a.txt");
        assert_eq!(idx.entries[idx.find_entry("d.txt").unwrap()].path, "d.txt");
        assert_eq!(idx.find_entry("b.txt"), None);
        assert_eq!(idx.find_entry("c.txt"), None);
    }

    /// The common `add -A` case: staging an unrelated file has no ancestor
    /// or descendant conflict, so nothing is removed.
    #[test]
    fn remove_directory_conflicts_is_noop_without_conflict() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("src/lib.rs"));
        idx.remove_directory_conflicts("src/main.rs");
        assert_eq!(idx.entries.len(), 1);
        assert!(idx.find_entry("src/lib.rs").is_some());
    }

    /// Staging `a/b` when `a` is already tracked as a file evicts the
    /// ancestor entry.
    #[test]
    fn remove_directory_conflicts_evicts_tracked_ancestor() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("a"));
        idx.remove_directory_conflicts("a/b");
        assert_eq!(idx.find_entry("a"), None);
        assert_eq!(idx.entries.len(), 0);
    }

    /// Staging `a` when `a/b` is already tracked as a file evicts the
    /// descendant entry, leaving unrelated entries alone.
    #[test]
    fn remove_directory_conflicts_evicts_tracked_descendant() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("a/b"));
        idx.upsert_entry(blob_entry("a/c"));
        idx.upsert_entry(blob_entry("unrelated.txt"));
        idx.remove_directory_conflicts("a");
        assert_eq!(idx.find_entry("a/b"), None);
        assert_eq!(idx.find_entry("a/c"), None);
        assert!(idx.find_entry("unrelated.txt").is_some());
        assert_eq!(idx.entries.len(), 1);
    }

    /// An entry at exactly the staged path is left alone — the caller
    /// upserts it separately.
    #[test]
    fn remove_directory_conflicts_keeps_exact_match() {
        let mut idx = Index::new();
        idx.upsert_entry(blob_entry("a.txt"));
        idx.remove_directory_conflicts("a.txt");
        assert!(idx.find_entry("a.txt").is_some());
    }

    /// `deserialize` (and therefore `read_index`) populates `by_path`
    /// without any caller having to call `upsert_entry` — `find_entry` on
    /// the result is immediately map-backed.
    #[test]
    fn deserialize_populates_path_index() {
        let mut built = Index::new();
        built.upsert_entry(blob_entry("a.txt"));
        built.upsert_entry(blob_entry("dir/b.txt"));
        let round_tripped = deserialize(&built.serialize()).unwrap();
        assert_eq!(round_tripped.find_entry("a.txt"), Some(0));
        assert_eq!(round_tripped.find_entry("dir/b.txt"), Some(1));
        assert!(round_tripped.tracks_path_or_descendant("dir"));
    }

    /// Two indexes with identical entries compare equal regardless of
    /// whether their `by_path` cache happens to be populated — equality is
    /// about staged content, not internal cache state.
    #[test]
    fn equality_ignores_path_index_population() {
        let mut via_field_push = Index::new();
        via_field_push.entries.push(blob_entry("a.txt"));
        let mut via_upsert = Index::new();
        via_upsert.upsert_entry(blob_entry("a.txt"));
        assert_eq!(via_field_push, via_upsert);
    }
}