grit-lib 0.3.99

//! Multi-pack-index (MIDX) file writing and minimal reading.
//!
//! Writes a Git-compatible `multi-pack-index` file (version 1, SHA-1) covering
//! selected `pack-*.idx` files. Objects that appear in multiple packs keep the
//! preferred pack's copy when `preferred_pack_idx` is set (matching Git's
//! geometric repack tests).
//!
//! Incremental writes follow Git's split layout: layers live under
//! `pack/multi-pack-index.d/multi-pack-index-<sha1>.midx` with ordering in
//! `multi-pack-index-chain` (oldest hash first, newest last).

use std::collections::{HashMap, HashSet};
use std::fs;
use std::io::{BufRead, BufReader};
use std::path::Path;

use sha1::{Digest, Sha1};

use crate::error::{Error, Result};
use crate::objects::ObjectId;
use crate::pack::{read_pack_index_no_verify, PackIndex};

const MIDX_SIGNATURE: u32 = 0x4d49_4458;
const MIDX_VERSION_V1: u8 = 1;
const MIDX_VERSION_V2: u8 = 2;
const HASH_VERSION_SHA1: u8 = 1;
const HASH_VERSION_SHA256: u8 = 2;
const MIDX_HEADER_SIZE: usize = 12;
const CHUNK_TOC_ENTRY_SIZE: usize = 12;
const MIDX_CHUNKID_PACKNAMES: u32 = 0x504e_414d;
const MIDX_CHUNKID_OIDFANOUT: u32 = 0x4f49_4446;
const MIDX_CHUNKID_OIDLOOKUP: u32 = 0x4f49_444c;
const MIDX_CHUNKID_OBJECTOFFSETS: u32 = 0x4f4f_4646;
const MIDX_CHUNKID_LARGEOFFSETS: u32 = 0x4c4f_4646;
const MIDX_CHUNKID_REVINDEX: u32 = 0x5249_4458;
const MIDX_CHUNKID_BITMAPPED_PACKS: u32 = 0x4254_4d50;

// Git `pack-revindex.h` / `pack-write.c` (standalone `.rev` next to MIDX).
const RIDX_SIGNATURE: u32 = 0x5249_4458;
const RIDX_VERSION: u32 = 1;
const RIDX_HEADER_SIZE: usize = 12;
const MIDX_CHUNK_ALIGNMENT: usize = 4;

// `git midx.h` (MIDX_LARGE_OFFSET_NEEDED).
const MIDX_LARGE_OFFSET_NEEDED: u32 = 0x8000_0000;

struct MidxEntry {
    oid: ObjectId,
    pack_id: u32,
    offset: u64,
    pack_mtime: std::time::SystemTime,
}

/// Options for writing a multi-pack index (extension of the simple writer).
#[derive(Debug, Clone, Default)]
pub struct WriteMultiPackIndexOptions {
    /// When set, objects also present in other packs are taken from this pack
    /// (`pack_names` index in the sorted name list).
    pub preferred_pack_idx: Option<u32>,
    /// Basename of the preferred pack (e.g. `pack-abc.idx` or `pack-abc.pack`); resolved against
    /// the working pack name list after optional subset filtering.
    pub preferred_pack_name: Option<String>,
    /// If set, only these `pack-*.idx` basenames are included, in this order (Git `--stdin-packs`).
    pub pack_names_subset_ordered: Option<Vec<String>>,
    /// When true, append RIDX + empty BTMP chunks so `test-tool read-midx --bitmap` succeeds.
    pub write_bitmap_placeholders: bool,
    /// When true, write a new layer in `multi-pack-index.d/` and extend the chain file
    /// instead of replacing `pack/multi-pack-index`.
    pub incremental: bool,
    /// When true with [`Self::write_bitmap_placeholders`], also create an empty `.rev`
    /// sidecar (Git `GIT_TEST_MIDX_WRITE_REV` compatibility).
    pub write_rev_placeholder: bool,
    /// On-disk MIDX format version to write (`1` or `2`). `None` writes the default (v2).
    /// Set from `midx.version`.
    pub version: Option<u8>,
}

fn normalize_pack_idx_basename(raw: &str) -> Result<String> {
    let t = raw.trim();
    let t = std::path::Path::new(t)
        .file_name()
        .and_then(|s| s.to_str())
        .unwrap_or(t);
    let t = t.strip_prefix("./").unwrap_or(t);
    if t.ends_with(".idx") {
        Ok(t.to_string())
    } else if t.ends_with(".pack") {
        Ok(format!("{}.idx", t.strip_suffix(".pack").unwrap_or(t)))
    } else {
        Ok(format!("{t}.idx"))
    }
}

/// Read a big-endian `u32` from `data` at byte offset `off`.
///
/// Returns [`Error::CorruptObject`] if `data` does not contain 4 bytes at `off`,
/// replacing the previous fixed-width-slice `.try_into().unwrap()` with real
/// bounds handling (the success-path value is unchanged).
fn read_be_u32(data: &[u8], off: usize) -> Result<u32> {
    let end = off.checked_add(4).filter(|&e| e <= data.len());
    let Some(end) = end else {
        return Err(Error::CorruptObject(
            "truncated MIDX data reading u32".to_owned(),
        ));
    };
    let bytes: [u8; 4] = data[off..end]
        .try_into()
        .map_err(|_| Error::CorruptObject("truncated MIDX data reading u32".to_owned()))?;
    Ok(u32::from_be_bytes(bytes))
}

/// Read a big-endian `u64` from `data` at byte offset `off`.
///
/// Returns [`Error::CorruptObject`] if `data` does not contain 8 bytes at `off`,
/// replacing the previous fixed-width-slice `.try_into().unwrap()` with real
/// bounds handling (the success-path value is unchanged).
fn read_be_u64(data: &[u8], off: usize) -> Result<u64> {
    let end = off.checked_add(8).filter(|&e| e <= data.len());
    let Some(end) = end else {
        return Err(Error::CorruptObject(
            "truncated MIDX data reading u64".to_owned(),
        ));
    };
    let bytes: [u8; 8] = data[off..end]
        .try_into()
        .map_err(|_| Error::CorruptObject("truncated MIDX data reading u64".to_owned()))?;
    Ok(u64::from_be_bytes(bytes))
}

struct MidxFileHeader {
    num_chunks: u8,
}

fn parse_midx_header(data: &[u8]) -> Result<(MidxFileHeader, usize, u8)> {
    if data.len() < MIDX_HEADER_SIZE + 20 {
        return Err(Error::CorruptObject("midx file too small".to_owned()));
    }
    let sig = read_be_u32(data, 0)?;
    if sig != MIDX_SIGNATURE {
        return Err(Error::CorruptObject("bad MIDX signature".to_owned()));
    }
    let version = data[4];
    if version != MIDX_VERSION_V1 && version != MIDX_VERSION_V2 {
        return Err(Error::CorruptObject(format!(
            "multi-pack-index version {version} not recognized"
        )));
    }
    let object_hash_bytes = data[5];
    let num_chunks = data[6];
    let _num_packs = read_be_u32(data, 8)?;
    Ok((
        MidxFileHeader { num_chunks },
        MIDX_HEADER_SIZE,
        object_hash_bytes,
    ))
}

fn parse_pack_names_blob(pn: &[u8]) -> Result<Vec<String>> {
    let mut names = Vec::new();
    let mut start = 0usize;
    for (i, &b) in pn.iter().enumerate() {
        if b == 0 && i >= start {
            if i > start {
                let s = std::str::from_utf8(&pn[start..i])
                    .map_err(|_| Error::CorruptObject("non-utf8 pack name in MIDX".to_owned()))?;
                names.push(s.to_string());
            }
            start = i + 1;
        }
    }
    Ok(names)
}

/// Compare a pack basename that may use `.pack` or `.idx` with an MIDX pack name (`.idx`).
fn cmp_idx_or_pack_name(idx_or_pack_name: &str, idx_name: &str) -> std::cmp::Ordering {
    let a = idx_or_pack_name.as_bytes();
    let b = idx_name.as_bytes();
    let mut i = 0usize;
    let min = a.len().min(b.len());
    while i < min && a[i] == b[i] {
        i += 1;
    }
    let suf_a = &a[i..];
    let suf_b = &b[i..];
    if suf_b == b"idx" && suf_a == b"pack" {
        return std::cmp::Ordering::Equal;
    }
    suf_a.cmp(suf_b)
}

fn preferred_pack_index_by_mtime(pack_dir: &Path, names: &[String]) -> Result<Option<usize>> {
    let mut best: Option<(usize, std::time::SystemTime)> = None;
    for (i, n) in names.iter().enumerate() {
        let meta = fs::metadata(pack_dir.join(n)).map_err(Error::Io)?;
        let mtime = meta.modified().map_err(Error::Io)?;
        match best {
            None => best = Some((i, mtime)),
            Some((_, t)) if mtime < t => best = Some((i, mtime)),
            _ => {}
        }
    }
    Ok(best.map(|(i, _)| i))
}

fn midx_d_dir(pack_dir: &Path) -> std::path::PathBuf {
    pack_dir.join("multi-pack-index.d")
}

fn chain_file_path(pack_dir: &Path) -> std::path::PathBuf {
    midx_d_dir(pack_dir).join("multi-pack-index-chain")
}

fn read_chain_layer_hashes(pack_dir: &Path) -> Result<Vec<String>> {
    let path = chain_file_path(pack_dir);
    let f = fs::File::open(&path).map_err(Error::Io)?;
    let mut out = Vec::new();
    for line in BufReader::new(f).lines() {
        let line = line.map_err(Error::Io)?;
        let t = line.trim();
        if t.is_empty() {
            continue;
        }
        if t.len() != 40 || !t.chars().all(|c| c.is_ascii_hexdigit()) {
            return Err(Error::CorruptObject(format!(
                "invalid multi-pack-index chain line: {t}"
            )));
        }
        out.push(t.to_ascii_lowercase());
    }
    Ok(out)
}

/// Resolve the path to the newest MIDX layer (root `multi-pack-index` or last chain entry).
/// Return the MIDX hash-version byte expected for the repository owning `pack_dir`,
/// mirroring git's `oid_version(r->hash_algo)` (SHA-1 → 1, SHA-256 → 2).
///
/// `pack_dir` is `<gitdir>/objects/pack`; the object format lives in the gitdir's
/// `config` under `extensions.objectformat`. When the config cannot be read or the
/// extension is absent, the default SHA-1 version (1) is returned.
fn repo_midx_hash_version(pack_dir: &Path) -> u8 {
    // pack_dir = <gitdir>/objects/pack -> gitdir = pack_dir/../..
    let Some(objects_dir) = pack_dir.parent() else {
        return HASH_VERSION_SHA1;
    };
    repo_midx_hash_version_for_objects_dir(objects_dir)
}

/// Like [`repo_midx_hash_version`] but starting from the `objects` directory.
fn repo_midx_hash_version_for_objects_dir(objects_dir: &Path) -> u8 {
    let Some(gitdir) = objects_dir.parent() else {
        return HASH_VERSION_SHA1;
    };
    let config_path = gitdir.join("config");
    let Ok(text) = fs::read_to_string(&config_path) else {
        return HASH_VERSION_SHA1;
    };
    // Minimal scan for `[extensions]` ... `objectformat = sha256`. Section and key
    // names are case-insensitive in git config; values are case-sensitive but git
    // only accepts the literals "sha1"/"sha256".
    let mut in_extensions = false;
    for raw in text.lines() {
        let line = raw.trim();
        if line.starts_with('[') {
            let section = line.trim_start_matches('[').trim_end_matches(']');
            let name = section.split_whitespace().next().unwrap_or("");
            in_extensions = name.eq_ignore_ascii_case("extensions");
            continue;
        }
        if !in_extensions {
            continue;
        }
        if let Some((key, value)) = line.split_once('=') {
            if key.trim().eq_ignore_ascii_case("objectformat")
                && value.trim().eq_ignore_ascii_case("sha256")
            {
                return HASH_VERSION_SHA256;
            }
        }
    }
    HASH_VERSION_SHA1
}

pub fn resolve_tip_midx_path(pack_dir: &Path) -> Option<std::path::PathBuf> {
    let root = pack_dir.join("multi-pack-index");
    if root.exists() {
        return Some(root);
    }
    let hashes = read_chain_layer_hashes(pack_dir).ok()?;
    let last = hashes.last()?;
    Some(midx_d_dir(pack_dir).join(format!("multi-pack-index-{last}.midx")))
}

/// Resolve a specific MIDX layer file by its lowercase hex checksum. Searches the
/// incremental chain (`multi-pack-index.d/multi-pack-index-<hash>.midx`) and the
/// single-file root MIDX. Returns `None` when no layer matches that checksum.
pub fn resolve_midx_layer_path(pack_dir: &Path, checksum: &str) -> Option<std::path::PathBuf> {
    let checksum = checksum.to_ascii_lowercase();
    if let Ok(hashes) = read_chain_layer_hashes(pack_dir) {
        if hashes.contains(&checksum) {
            return Some(midx_d_dir(pack_dir).join(format!("multi-pack-index-{checksum}.midx")));
        }
    }
    let root = pack_dir.join("multi-pack-index");
    if root.exists() {
        if let Ok(hex) = midx_checksum_hex_from_path(&root) {
            if hex == checksum {
                return Some(root);
            }
        }
    }
    None
}

fn load_midx_file(path: &Path) -> Result<Vec<u8>> {
    let data = fs::read(path).map_err(Error::Io)?;
    let _ = parse_midx_header(&data)?;
    Ok(data)
}

fn oids_and_packs_from_midx_data(data: &[u8]) -> Result<(HashSet<ObjectId>, Vec<String>)> {
    let (_, hdr_end, _) = parse_midx_header(data)?;
    let (pn_off, pn_len) = find_chunk(data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    let pack_names = parse_pack_names_blob(&data[pn_off..pn_off + pn_len])?;
    let (_ooff_off, ooff_len) = find_chunk(data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    let (oidl_off, oidl_len) = find_chunk(data, hdr_end, MIDX_CHUNKID_OIDLOOKUP)?;
    let num_objects = ooff_len / 8;
    if oidl_len != num_objects * 20 {
        return Err(Error::CorruptObject(
            "MIDX oid-lookup size mismatch".to_owned(),
        ));
    }
    let mut oids = HashSet::with_capacity(num_objects);
    for i in 0..num_objects {
        let start = oidl_off + i * 20;
        let oid = ObjectId::from_bytes(&data[start..start + 20])?;
        oids.insert(oid);
    }
    Ok((oids, pack_names))
}

fn collect_incremental_base(pack_dir: &Path) -> Result<(HashSet<ObjectId>, HashSet<String>)> {
    let mut oids = HashSet::new();
    let mut packs = HashSet::new();
    let root = pack_dir.join("multi-pack-index");
    let chain_path = chain_file_path(pack_dir);
    if chain_path.exists() {
        for h in read_chain_layer_hashes(pack_dir)? {
            let p = midx_d_dir(pack_dir).join(format!("multi-pack-index-{h}.midx"));
            let data = load_midx_file(&p)?;
            let (layer_oids, names) = oids_and_packs_from_midx_data(&data)?;
            oids.extend(layer_oids);
            for n in names {
                packs.insert(n);
            }
        }
        return Ok((oids, packs));
    }
    if root.exists() {
        let data = load_midx_file(&root)?;
        let (o, names) = oids_and_packs_from_midx_data(&data)?;
        oids = o;
        for n in names {
            packs.insert(n);
        }
    }
    Ok((oids, packs))
}

fn midx_checksum_hex_from_path(path: &Path) -> Result<String> {
    let data = fs::read(path).map_err(Error::Io)?;
    if data.len() < 20 {
        return Err(Error::CorruptObject(
            "midx too small for checksum".to_owned(),
        ));
    }
    let hash = &data[data.len() - 20..];
    Ok(hex::encode(hash))
}

fn hard_link_or_copy(src: &Path, dst: &Path) -> Result<()> {
    let _ = fs::remove_file(dst);
    if fs::hard_link(src, dst).is_ok() {
        return Ok(());
    }
    fs::copy(src, dst).map_err(Error::Io)?;
    Ok(())
}

fn link_root_midx_into_chain(pack_dir: &Path, root_checksum_hex: &str) -> Result<()> {
    let midx_d = midx_d_dir(pack_dir);
    fs::create_dir_all(&midx_d).map_err(Error::Io)?;
    let dst_midx = midx_d.join(format!("multi-pack-index-{root_checksum_hex}.midx"));
    hard_link_or_copy(&pack_dir.join("multi-pack-index"), &dst_midx)?;
    let exts = ["bitmap", "rev"];
    for ext in exts {
        let src = pack_dir.join(format!("multi-pack-index-{root_checksum_hex}.{ext}"));
        if src.exists() {
            let dst = midx_d.join(format!("multi-pack-index-{root_checksum_hex}.{ext}"));
            hard_link_or_copy(&src, &dst)?;
        }
    }
    Ok(())
}

fn clear_stale_split_layers(pack_dir: &Path, keep: &[String]) -> Result<()> {
    let midx_d = midx_d_dir(pack_dir);
    if !midx_d.exists() {
        return Ok(());
    }
    let keep: HashSet<&str> = keep.iter().map(|s| s.as_str()).collect();
    for ent in fs::read_dir(&midx_d).map_err(Error::Io)? {
        let ent = ent.map_err(Error::Io)?;
        let name = ent.file_name().to_string_lossy().to_string();
        let Some(rest) = name.strip_prefix("multi-pack-index-") else {
            continue;
        };
        let Some((hash_part, _ext)) = rest.split_once('.') else {
            continue;
        };
        if hash_part.len() == 40 && !keep.contains(hash_part) {
            let _ = fs::remove_file(ent.path());
        }
    }
    Ok(())
}

/// Remove every incremental MIDX layer file (`multi-pack-index-<hash>.midx`,
/// `.bitmap`, `.rev`) from `multi-pack-index.d/` and unlink the chain file, but
/// leave the (now empty) directory in place.
///
/// This mirrors git's `clear_incremental_midx_files_ext` plus the chain unlink in
/// `clear_midx_files` for a non-incremental write: git iterates the directory and
/// `unlink`s the matching files individually and never `rmdir`s the directory, so
/// a single-file MIDX write leaves an empty `multi-pack-index.d/` behind rather
/// than removing it (see t5334 "convert incremental to non-incremental").
fn clear_incremental_midx_files(pack_dir: &Path) -> Result<()> {
    let midx_d = midx_d_dir(pack_dir);
    // Unlink the chain file regardless of whether other entries remain.
    let _ = fs::remove_file(chain_file_path(pack_dir));
    if !midx_d.exists() {
        return Ok(());
    }
    for ent in fs::read_dir(&midx_d).map_err(Error::Io)? {
        let ent = ent.map_err(Error::Io)?;
        let name = ent.file_name().to_string_lossy().to_string();
        if name.starts_with("multi-pack-index-")
            && (name.ends_with(".midx") || name.ends_with(".bitmap") || name.ends_with(".rev"))
        {
            let _ = fs::remove_file(ent.path());
        }
    }
    Ok(())
}

fn pack_mtime_for_midx(idx: &PackIndex) -> std::time::SystemTime {
    fs::metadata(&idx.pack_path)
        .and_then(|m| m.modified())
        .unwrap_or(std::time::SystemTime::UNIX_EPOCH)
}

fn midx_pick_better_entry(
    cur: &MidxEntry,
    cand_pack: u32,
    cand_offset: u64,
    cand_mtime: std::time::SystemTime,
    preferred_pack: Option<u32>,
) -> bool {
    let cur_pref = preferred_pack == Some(cur.pack_id);
    let new_pref = preferred_pack == Some(cand_pack);
    if new_pref && !cur_pref {
        return true;
    }
    if cur_pref && !new_pref {
        return false;
    }
    match cand_mtime.cmp(&cur.pack_mtime) {
        std::cmp::Ordering::Greater => true,
        std::cmp::Ordering::Less => false,
        std::cmp::Ordering::Equal => {
            if cand_pack != cur.pack_id {
                cand_pack < cur.pack_id
            } else {
                cand_offset < cur.offset
            }
        }
    }
}

/// Build a MIDX layer's bytes, omitting objects whose OID is present in
/// `exclude_oids` (the base chain for incremental layers and compaction, where
/// objects already provided by a lower layer must not be repeated). Pass `None`
/// for a full (non-incremental) MIDX.
#[allow(clippy::too_many_arguments)]
fn build_midx_bytes_filtered(
    idx_names: &[String],
    indexes: &[PackIndex],
    preferred_idx: Option<usize>,
    write_bitmap_placeholders: bool,
    omit_embedded_ridx_chunk: bool,
    version: u8,
    hash_version: u8,
    exclude_oids: Option<&HashSet<ObjectId>>,
) -> Result<(Vec<u8>, Option<Vec<u32>>)> {
    let preferred_pack_idx = preferred_idx.map(|p| p as u32);
    let pack_mtimes: Vec<std::time::SystemTime> = indexes.iter().map(pack_mtime_for_midx).collect();

    let mut best: HashMap<ObjectId, MidxEntry> = HashMap::new();
    for (pack_id, idx) in indexes.iter().enumerate() {
        let pack_id = u32::try_from(pack_id).map_err(|_| {
            Error::CorruptObject("too many pack files for multi-pack-index".to_owned())
        })?;
        let mtime = pack_mtimes[pack_id as usize];
        for e in &idx.entries {
            if e.oid.len() != 20 {
                continue;
            }
            let Ok(oid) = ObjectId::from_bytes(&e.oid) else {
                continue;
            };
            if let Some(ex) = exclude_oids {
                if ex.contains(&oid) {
                    continue;
                }
            }
            let cand = MidxEntry {
                oid,
                pack_id,
                offset: e.offset,
                pack_mtime: mtime,
            };
            match best.get(&oid) {
                None => {
                    best.insert(oid, cand);
                }
                Some(cur) => {
                    if midx_pick_better_entry(cur, pack_id, e.offset, mtime, preferred_pack_idx) {
                        best.insert(oid, cand);
                    }
                }
            }
        }
    }

    let mut entries: Vec<MidxEntry> = best.into_values().collect();
    entries.sort_by_key(|a| a.oid);

    // Decide how object offsets are encoded, mirroring git/midx-write.c.
    // `large_offsets_needed` becomes true only when some offset cannot fit in a
    // 32-bit field (> 0xffffffff); in that mode every offset that does not fit in
    // 31 bits (> 0x7fffffff) is stored in the 64-bit large-offset (LOFF) chunk and
    // its 32-bit slot is `MIDX_LARGE_OFFSET_NEEDED | slot`. When no offset exceeds
    // 32 bits, offsets in [2^31, 2^32) are written directly as raw 32-bit values
    // and no LOFF chunk is emitted.
    let large_offsets_needed = entries.iter().any(|e| e.offset > u64::from(u32::MAX));

    let num_packs = indexes.len() as u32;

    let mut pack_names_blob = Vec::new();
    for name in idx_names {
        pack_names_blob.extend_from_slice(name.as_bytes());
        pack_names_blob.push(0);
    }
    let pad = (MIDX_CHUNK_ALIGNMENT - (pack_names_blob.len() % MIDX_CHUNK_ALIGNMENT))
        % MIDX_CHUNK_ALIGNMENT;
    pack_names_blob.extend(std::iter::repeat_n(0u8, pad));
    let chunk_pnam = pack_names_blob;

    let mut chunk_oidf = vec![0u8; 256 * 4];
    let mut j = 0usize;
    for i in 0..256 {
        while j < entries.len() && entries[j].oid.as_bytes()[0] <= i as u8 {
            j += 1;
        }
        chunk_oidf[i * 4..(i + 1) * 4].copy_from_slice(&(j as u32).to_be_bytes());
    }

    let mut chunk_oidl = Vec::with_capacity(entries.len() * 20);
    for e in &entries {
        chunk_oidl.extend_from_slice(e.oid.as_bytes());
    }

    let mut large_offsets: Vec<u64> = Vec::new();
    let mut chunk_ooff = Vec::with_capacity(entries.len() * 8);
    for e in &entries {
        chunk_ooff.extend_from_slice(&e.pack_id.to_be_bytes());
        let encoded = if large_offsets_needed && e.offset >> 31 != 0 {
            let slot = u32::try_from(large_offsets.len()).map_err(|_| {
                Error::CorruptObject("too many large offsets in multi-pack-index".to_owned())
            })?;
            large_offsets.push(e.offset);
            MIDX_LARGE_OFFSET_NEEDED | slot
        } else {
            // When large offsets are not needed, an offset in [2^31, 2^32) is
            // written verbatim (truncation via `as u32` is exact here because the
            // value fits in 32 bits).
            e.offset as u32
        };
        chunk_ooff.extend_from_slice(&encoded.to_be_bytes());
    }

    let chunk_loff: Vec<u8> = if large_offsets.is_empty() {
        Vec::new()
    } else {
        let mut v = Vec::with_capacity(large_offsets.len() * 8);
        for off in &large_offsets {
            v.extend_from_slice(&off.to_be_bytes());
        }
        v
    };

    let pref = preferred_pack_idx;
    let mut order: Vec<u32> = (0..entries.len() as u32).collect();
    order.sort_by(|&ai, &bi| {
        let a = &entries[ai as usize];
        let b = &entries[bi as usize];
        let a_pref = pref == Some(a.pack_id);
        let b_pref = pref == Some(b.pack_id);
        b_pref
            .cmp(&a_pref)
            .then_with(|| a.pack_id.cmp(&b.pack_id))
            .then_with(|| a.offset.cmp(&b.offset))
            .then_with(|| ai.cmp(&bi))
    });

    let mut chunk_ridx = Vec::with_capacity(entries.len() * 4);
    for oid_idx in &order {
        chunk_ridx.extend_from_slice(&oid_idx.to_be_bytes());
    }

    // BTMP: per-pack (bitmap_pos, bitmap_nr) in the pseudo-bitmap namespace, matching Git's
    // `write_midx_bitmapped_packs` (cumulative start + object count per pack).
    let rev_sidecar_order = if omit_embedded_ridx_chunk && write_bitmap_placeholders {
        Some(order.clone())
    } else {
        None
    };
    let chunk_btmp: Vec<u8> = if write_bitmap_placeholders {
        // Per-pack `(bitmap_pos, bitmap_nr)`: position of the pack's first object in
        // the MIDX pack-order traversal and the number of (deduplicated) MIDX objects
        // selected from that pack — matching `write_midx_bitmapped_packs` in
        // git/midx-write.c (counts MIDX entries per pack, not raw idx entry counts).
        let num_packs_usize = indexes.len();
        let mut bitmap_pos = vec![u32::MAX; num_packs_usize];
        let mut bitmap_nr = vec![0u32; num_packs_usize];
        for (rank, &oid_idx) in order.iter().enumerate() {
            let pack = entries[oid_idx as usize].pack_id as usize;
            if let Some(p) = bitmap_pos.get_mut(pack) {
                if *p == u32::MAX {
                    *p = rank as u32;
                }
            }
            if let Some(n) = bitmap_nr.get_mut(pack) {
                *n += 1;
            }
        }
        let mut v = Vec::new();
        for pack in 0..num_packs_usize {
            let pos = if bitmap_pos[pack] == u32::MAX {
                0
            } else {
                bitmap_pos[pack]
            };
            v.extend_from_slice(&pos.to_be_bytes());
            v.extend_from_slice(&bitmap_nr[pack].to_be_bytes());
        }
        let pad = (MIDX_CHUNK_ALIGNMENT - (v.len() % MIDX_CHUNK_ALIGNMENT)) % MIDX_CHUNK_ALIGNMENT;
        v.extend(std::iter::repeat_n(0u8, pad));
        v
    } else {
        Vec::new()
    };

    let mut chunks: Vec<(u32, Vec<u8>)> = vec![
        (MIDX_CHUNKID_PACKNAMES, chunk_pnam),
        (MIDX_CHUNKID_OIDFANOUT, chunk_oidf),
        (MIDX_CHUNKID_OIDLOOKUP, chunk_oidl),
        (MIDX_CHUNKID_OBJECTOFFSETS, chunk_ooff),
    ];
    if !chunk_loff.is_empty() {
        chunks.push((MIDX_CHUNKID_LARGEOFFSETS, chunk_loff));
    }
    if (pref.is_some() || write_bitmap_placeholders) && !omit_embedded_ridx_chunk {
        chunks.push((MIDX_CHUNKID_REVINDEX, chunk_ridx));
    }
    if write_bitmap_placeholders {
        chunks.push((MIDX_CHUNKID_BITMAPPED_PACKS, chunk_btmp));
    }

    let num_chunks: u8 = chunks
        .len()
        .try_into()
        .map_err(|_| Error::CorruptObject("too many MIDX chunks".to_owned()))?;

    let mut body = Vec::new();
    let mut cur_offset =
        MIDX_HEADER_SIZE as u64 + ((chunks.len() + 1) * CHUNK_TOC_ENTRY_SIZE) as u64;

    for (id, data) in &chunks {
        body.extend_from_slice(&id.to_be_bytes());
        body.extend_from_slice(&cur_offset.to_be_bytes());
        cur_offset += data.len() as u64;
    }
    body.extend_from_slice(&0u32.to_be_bytes());
    body.extend_from_slice(&cur_offset.to_be_bytes());

    for (_, data) in &chunks {
        body.extend_from_slice(data);
    }

    let mut out = Vec::with_capacity(MIDX_HEADER_SIZE + body.len() + 20);
    out.extend_from_slice(&MIDX_SIGNATURE.to_be_bytes());
    out.push(if version == MIDX_VERSION_V1 {
        MIDX_VERSION_V1
    } else {
        MIDX_VERSION_V2
    });
    out.push(hash_version);
    out.push(num_chunks);
    out.push(0);
    out.extend_from_slice(&num_packs.to_be_bytes());
    out.extend_from_slice(&body);

    let mut hasher = Sha1::new();
    hasher.update(&out);
    let hash = hasher.finalize();
    out.extend_from_slice(&hash);

    Ok((out, rev_sidecar_order))
}

/// Standalone MIDX `.rev` file (Git `write_rev_file_order` / `RIDX_SIGNATURE`).
fn write_midx_rev_sidecar(
    path: &Path,
    pack_order: &[u32],
    midx_file_hash: &[u8; 20],
) -> Result<()> {
    let mut body = Vec::with_capacity(RIDX_HEADER_SIZE + pack_order.len() * 4 + 20);
    body.extend_from_slice(&RIDX_SIGNATURE.to_be_bytes());
    body.extend_from_slice(&RIDX_VERSION.to_be_bytes());
    body.extend_from_slice(&1u32.to_be_bytes());
    for idx in pack_order {
        body.extend_from_slice(&idx.to_be_bytes());
    }
    body.extend_from_slice(midx_file_hash);
    fs::write(path, body).map_err(Error::Io)
}

fn find_chunk(data: &[u8], header_end: usize, chunk_id: u32) -> Result<(usize, usize)> {
    let (hdr, _, _) = parse_midx_header(data)?;
    let n = hdr.num_chunks as usize;
    let pos = header_end;
    let toc_end = pos + (n + 1) * CHUNK_TOC_ENTRY_SIZE;
    if data.len() < toc_end + 20 {
        return Err(Error::CorruptObject(
            "truncated MIDX chunk table".to_owned(),
        ));
    }
    for i in 0..n {
        let base = pos + i * CHUNK_TOC_ENTRY_SIZE;
        let id = read_be_u32(data, base)?;
        let off = read_be_u64(data, base + 4)? as usize;
        if id == chunk_id {
            let next_off = if i + 1 < n {
                let nb = pos + (i + 1) * CHUNK_TOC_ENTRY_SIZE;
                read_be_u64(data, nb + 4)? as usize
            } else {
                let term = pos + n * CHUNK_TOC_ENTRY_SIZE;
                read_be_u64(data, term + 4)? as usize
            };
            return Ok((off, next_off.saturating_sub(off)));
        }
    }
    Err(Error::CorruptObject(format!(
        "MIDX chunk {chunk_id:08x} not found"
    )))
}

/// A fatal MIDX parse failure (Git `die()` in `load_multi_pack_index`). The
/// contained message is the exact text Git prints, without the `error:`/`fatal:`
/// prefix.
#[derive(Debug, Clone)]
pub struct MidxLoadError(pub String);

impl std::fmt::Display for MidxLoadError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

/// Parsed table-of-contents entry: `(chunk_id, file_offset)`.
struct TocEntry {
    id: u32,
    offset: usize,
}

/// Walk the MIDX chunk table of contents, mirroring `read_table_of_contents`
/// in `git/chunk-format.c`. Returns the chunk list plus any reported errors,
/// or a fatal `MidxLoadError` for the conditions Git treats as `die()`-worthy.
fn parse_midx_toc(
    data: &[u8],
    hash_len: usize,
    errors: &mut Vec<String>,
) -> std::result::Result<Vec<TocEntry>, MidxLoadError> {
    if data.len() < MIDX_HEADER_SIZE + hash_len {
        return Err(MidxLoadError("multi-pack-index file too small".to_owned()));
    }
    let num_chunks = data[6] as usize;
    let toc_off = MIDX_HEADER_SIZE;
    let needed = toc_off + (num_chunks + 1) * CHUNK_TOC_ENTRY_SIZE;
    if data.len() < needed {
        return Err(MidxLoadError(
            "multi-pack-index chunk table is truncated".to_owned(),
        ));
    }
    let file_size = data.len();
    let mut chunks: Vec<TocEntry> = Vec::with_capacity(num_chunks);

    let read_be64 = |off: usize| -> u64 {
        let mut b = [0u8; 8];
        b.copy_from_slice(&data[off..off + 8]);
        u64::from_be_bytes(b)
    };
    let read_be32 = |off: usize| -> u32 {
        let mut b = [0u8; 4];
        b.copy_from_slice(&data[off..off + 4]);
        u32::from_be_bytes(b)
    };

    for i in 0..num_chunks {
        let entry = toc_off + i * CHUNK_TOC_ENTRY_SIZE;
        let chunk_id = read_be32(entry);
        let chunk_offset = read_be64(entry + 4);

        if chunk_id == 0 {
            errors.push("terminating chunk id appears earlier than expected".to_owned());
            return Err(MidxLoadError(
                "multi-pack-index required pack-name chunk missing or corrupted".to_owned(),
            ));
        }
        if !(chunk_offset as usize).is_multiple_of(MIDX_CHUNK_ALIGNMENT) {
            errors.push(format!(
                "chunk id {chunk_id:x} not {MIDX_CHUNK_ALIGNMENT}-byte aligned"
            ));
            return Err(MidxLoadError(
                "multi-pack-index required pack-name chunk missing or corrupted".to_owned(),
            ));
        }

        let next_entry = toc_off + (i + 1) * CHUNK_TOC_ENTRY_SIZE;
        let next_chunk_offset = read_be64(next_entry + 4);

        if next_chunk_offset < chunk_offset
            || next_chunk_offset > (file_size as u64).saturating_sub(hash_len as u64)
        {
            errors.push(format!(
                "improper chunk offset(s) {chunk_offset:x} and {next_chunk_offset:x}"
            ));
            return Err(MidxLoadError(
                "multi-pack-index required pack-name chunk missing or corrupted".to_owned(),
            ));
        }

        if chunks.iter().any(|c| c.id == chunk_id) {
            errors.push(format!("duplicate chunk ID {chunk_id:x} found"));
            return Err(MidxLoadError(
                "multi-pack-index required pack-name chunk missing or corrupted".to_owned(),
            ));
        }

        chunks.push(TocEntry {
            id: chunk_id,
            offset: chunk_offset as usize,
        });
    }

    // Terminating TOC entry must have chunk id 0.
    let term_entry = toc_off + num_chunks * CHUNK_TOC_ENTRY_SIZE;
    let final_id = read_be32(term_entry);
    if final_id != 0 {
        errors.push(format!("final chunk has non-zero id {final_id:x}"));
        return Err(MidxLoadError(
            "multi-pack-index required pack-name chunk missing or corrupted".to_owned(),
        ));
    }

    Ok(chunks)
}

/// Look up `(start, len)` of a chunk in a parsed TOC.
fn toc_chunk_range(chunks: &[TocEntry], data_len: usize, id: u32) -> Option<(usize, usize)> {
    for (i, c) in chunks.iter().enumerate() {
        if c.id == id {
            let next = if i + 1 < chunks.len() {
                chunks[i + 1].offset
            } else {
                data_len.saturating_sub(20)
            };
            return Some((c.offset, next.saturating_sub(c.offset)));
        }
    }
    None
}

/// Full multi-pack-index verification, mirroring `verify_midx_file` in `git/midx.c`
/// plus the `die()`/`error()` conditions in `load_multi_pack_index`. On any problem
/// returns the list of error lines (without `error:`/`fatal:` prefixes) in the order
/// Git emits them; an empty list means the MIDX is valid.
///
/// `objects_dir` is the object database (e.g. `.git/objects`).
pub fn verify_midx(objects_dir: &Path) -> std::result::Result<(), Vec<String>> {
    let pack_dir = objects_dir.join("pack");
    let path = match resolve_tip_midx_path(&pack_dir) {
        Some(p) => p,
        None => return Ok(()),
    };
    let data = match fs::read(&path) {
        Ok(d) => d,
        Err(_) => return Ok(()),
    };

    let mut fatal: Vec<String> = Vec::new();
    let mut errors: Vec<String> = Vec::new();

    // --- header checks (load_multi_pack_index) ---
    if data.len() < MIDX_HEADER_SIZE + 20 {
        return Err(vec!["multi-pack-index file is too small".to_owned()]);
    }
    let sig = u32::from_be_bytes([data[0], data[1], data[2], data[3]]);
    if sig != MIDX_SIGNATURE {
        return Err(vec![format!(
            "multi-pack-index signature 0x{sig:08x} does not match signature 0x{MIDX_SIGNATURE:08x}"
        )]);
    }
    let version = data[4];
    if version != MIDX_VERSION_V1 && version != MIDX_VERSION_V2 {
        return Err(vec![format!(
            "multi-pack-index version {version} not recognized"
        )]);
    }
    let hash_version = data[5];
    let expected_hash_version = repo_midx_hash_version_for_objects_dir(objects_dir);
    if hash_version != expected_hash_version {
        return Err(vec![format!(
            "multi-pack-index hash version {hash_version} does not match version {expected_hash_version}"
        )]);
    }
    let hash_len = 20usize;
    let num_packs = u32::from_be_bytes([data[8], data[9], data[10], data[11]]) as usize;

    // --- table of contents ---
    let chunks = match parse_midx_toc(&data, hash_len, &mut errors) {
        Ok(c) => c,
        Err(e) => {
            errors.push(e.0);
            return Err(errors);
        }
    };

    // required pack-names chunk
    let Some((pn_off, pn_len)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_PACKNAMES)
    else {
        errors.push("multi-pack-index required pack-name chunk missing or corrupted".to_owned());
        return Err(errors);
    };

    // oid-fanout chunk + ordering check
    let Some((fan_off, fan_len)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_OIDFANOUT)
    else {
        errors.push("multi-pack-index required OID fanout chunk missing or corrupted".to_owned());
        return Err(errors);
    };
    if fan_len != 256 * 4 {
        errors.push("multi-pack-index OID fanout is of the wrong size".to_owned());
        errors.push("multi-pack-index required OID fanout chunk missing or corrupted".to_owned());
        return Err(errors);
    }
    let fanout = |i: usize| -> u32 {
        let b = fan_off + i * 4;
        u32::from_be_bytes([data[b], data[b + 1], data[b + 2], data[b + 3]])
    };
    for i in 0..255 {
        let f1 = fanout(i);
        let f2 = fanout(i + 1);
        if f1 > f2 {
            errors.push(format!(
                "oid fanout out of order: fanout[{i}] = {f1:x} > {f2:x} = fanout[{}]",
                i + 1
            ));
            errors
                .push("multi-pack-index required OID fanout chunk missing or corrupted".to_owned());
            return Err(errors);
        }
    }
    let num_objects = fanout(255) as usize;

    // oid-lookup chunk (size depends on num_objects)
    let Some((oidl_off, oidl_len)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_OIDLOOKUP)
    else {
        errors.push("multi-pack-index required OID lookup chunk missing or corrupted".to_owned());
        return Err(errors);
    };
    if oidl_len != hash_len * num_objects {
        errors.push("multi-pack-index OID lookup chunk is the wrong size".to_owned());
        errors.push("multi-pack-index required OID lookup chunk missing or corrupted".to_owned());
        return Err(errors);
    }

    // object-offsets chunk
    let Some((ooff_off, ooff_len)) =
        toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_OBJECTOFFSETS)
    else {
        errors
            .push("multi-pack-index required object offsets chunk missing or corrupted".to_owned());
        return Err(errors);
    };
    if ooff_len != num_objects * 8 {
        errors.push("multi-pack-index object offset chunk is the wrong size".to_owned());
        errors
            .push("multi-pack-index required object offsets chunk missing or corrupted".to_owned());
        return Err(errors);
    }

    let large_off = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_LARGEOFFSETS);

    // pack names: parse and (for V1) verify ordering.
    let names = match parse_pack_names_blob(&data[pn_off..pn_off + pn_len]) {
        Ok(n) => n,
        Err(_) => {
            errors.push("multi-pack-index pack-name chunk is too short".to_owned());
            return Err(errors);
        }
    };
    if version == MIDX_VERSION_V1 {
        for i in 1..names.len() {
            if names[i] <= names[i - 1] {
                fatal.push(format!(
                    "multi-pack-index pack names out of order: '{}' before '{}'",
                    names[i - 1],
                    names[i]
                ));
                // Git die()s here while loading; surface immediately.
                errors.extend(fatal);
                return Err(errors);
            }
        }
    }

    // --- checksum ---
    if !midx_checksum_is_valid(&data) {
        errors.push("incorrect checksum".to_owned());
    }

    // --- load each referenced pack (failed to load pack) ---
    let mut pack_indexes: Vec<Option<PackIndex>> = Vec::with_capacity(num_packs);
    for i in 0..num_packs {
        // Load the pack idx without verifying its trailing checksum: `git
        // multi-pack-index verify` uses `open_pack_index`, which only parses the
        // index header/tables. The 64-bit-offset tests deliberately corrupt a
        // pack `.idx` (invalidating its checksum) and still expect the MIDX
        // verify to read recorded offsets out of that idx for comparison.
        let loaded = match names.get(i) {
            Some(name) => read_pack_index_no_verify(&pack_dir.join(name)).ok(),
            None => None,
        };
        if loaded.is_none() {
            errors.push(format!("failed to load pack in position {i}"));
        }
        pack_indexes.push(loaded);
    }

    if num_objects == 0 {
        errors.push("the midx contains no oid".to_owned());
        if errors.is_empty() {
            return Ok(());
        }
        return Err(errors);
    }

    // --- OID lookup order ---
    let oid_at =
        |i: usize| -> &[u8] { &data[oidl_off + i * hash_len..oidl_off + (i + 1) * hash_len] };
    for i in 0..num_objects.saturating_sub(1) {
        let a = oid_at(i);
        let b = oid_at(i + 1);
        if a >= b {
            errors.push(format!(
                "oid lookup out of order: oid[{i}] = {} >= {} = oid[{}]",
                hex::encode(a),
                hex::encode(b),
                i + 1
            ));
        }
    }

    // --- object offsets vs pack index ---
    for i in 0..num_objects {
        let ob = ooff_off + i * 8;
        let pack_int_id = u32::from_be_bytes([data[ob], data[ob + 1], data[ob + 2], data[ob + 3]]);
        let off_raw = u32::from_be_bytes([data[ob + 4], data[ob + 5], data[ob + 6], data[ob + 7]]);
        let oid_hex = hex::encode(oid_at(i));

        if pack_int_id as usize >= num_packs {
            errors.push(format!(
                "bad pack-int-id: {pack_int_id} ({num_packs} total packs)"
            ));
            errors.push(format!(
                "failed to load pack entry for oid[{i}] = {oid_hex}"
            ));
            continue;
        }

        // resolve MIDX-recorded offset (handle large offsets)
        let m_offset: u64 = if off_raw & MIDX_LARGE_OFFSET_NEEDED != 0 {
            let slot = (off_raw & !MIDX_LARGE_OFFSET_NEEDED) as usize;
            match large_off {
                Some((lo_off, lo_len)) if (slot + 1) * 8 <= lo_len => {
                    let b = lo_off + slot * 8;
                    let mut arr = [0u8; 8];
                    arr.copy_from_slice(&data[b..b + 8]);
                    u64::from_be_bytes(arr)
                }
                _ => {
                    errors.push("multi-pack-index large offset out of bounds".to_owned());
                    continue;
                }
            }
        } else {
            u64::from(off_raw)
        };

        let Some(Some(idx)) = pack_indexes.get(pack_int_id as usize) else {
            errors.push(format!(
                "failed to load pack entry for oid[{i}] = {oid_hex}"
            ));
            continue;
        };
        let Ok(oid) = ObjectId::from_bytes(oid_at(i)) else {
            errors.push(format!(
                "failed to load pack entry for oid[{i}] = {oid_hex}"
            ));
            continue;
        };
        match idx.find_offset(&oid) {
            Some(p_offset) => {
                if m_offset != p_offset {
                    errors.push(format!(
                        "incorrect object offset for oid[{i}] = {oid_hex}: {m_offset:x} != {p_offset:x}"
                    ));
                }
            }
            None => {
                errors.push(format!(
                    "failed to load pack entry for oid[{i}] = {oid_hex}"
                ));
            }
        }
    }

    if errors.is_empty() {
        Ok(())
    } else {
        Err(errors)
    }
}

/// Validate the trailing SHA-1 of an in-memory MIDX image.
fn midx_checksum_is_valid(data: &[u8]) -> bool {
    if data.len() < 20 {
        return false;
    }
    let body = &data[..data.len() - 20];
    let stored = &data[data.len() - 20..];
    let mut hasher = Sha1::new();
    hasher.update(body);
    let got = hasher.finalize();
    got.as_slice() == stored
}

/// Return the `pack-*.idx` basename for the MIDX preferred pack (RIDX position 0).
///
/// `objects_dir` is the repository object database (e.g. `.git/objects`), not `objects/pack`.
///
/// Used by `test-tool read-midx --preferred-pack` compatibility.
/// Pack index basenames (`pack-*.idx`) stored in the MIDX pack-names chunk.
pub fn read_midx_pack_idx_names(objects_dir: &Path) -> Result<Vec<String>> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_tip_midx_path(&pack_dir)
        .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned()))?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    parse_pack_names_blob(&data[pn_off..pn_off + pn_len])
}

/// A single MIDX-referenced object together with the pack it is attributed to.
pub struct MidxObjectRef {
    pub oid: ObjectId,
    /// Index into the pack-names list returned alongside this.
    pub pack_int_id: usize,
}

/// Read the tip MIDX and return `(pack_names, objects)`, where each object names
/// the pack it is attributed to (`pack_int_id`). Mirrors the per-object
/// `nth_midxed_pack_int_id` iteration in Git used by expire/repack.
pub fn read_midx_objects(objects_dir: &Path) -> Result<(Vec<String>, Vec<MidxObjectRef>)> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_tip_midx_path(&pack_dir)
        .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned()))?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    let names = parse_pack_names_blob(&data[pn_off..pn_off + pn_len])?;
    let (oidl_off, oidl_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OIDLOOKUP)?;
    let (ooff_off, ooff_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    if oidl_len % 20 != 0 || ooff_len % 8 != 0 {
        return Err(Error::CorruptObject(
            "bad MIDX oid-lookup / object-offsets size".to_owned(),
        ));
    }
    let num = oidl_len / 20;
    if num * 8 != ooff_len {
        return Err(Error::CorruptObject(
            "MIDX oid count does not match object-offsets".to_owned(),
        ));
    }
    let mut objects = Vec::with_capacity(num);
    for i in 0..num {
        let oid = ObjectId::from_bytes(&data[oidl_off + i * 20..oidl_off + (i + 1) * 20])
            .map_err(|e| Error::CorruptObject(e.to_string()))?;
        let base = ooff_off + i * 8;
        let pack_id = read_be_u32(&data, base)? as usize;
        objects.push(MidxObjectRef {
            oid,
            pack_int_id: pack_id,
        });
    }
    Ok((names, objects))
}

/// Trailing 40-character SHA-1 hex of the active MIDX (root or chain tip).
pub fn midx_checksum_hex(objects_dir: &Path) -> Result<String> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_tip_midx_path(&pack_dir)
        .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned()))?;
    midx_checksum_hex_from_path(&path)
}

/// Resolve the MIDX file to read for `test-tool read-midx`: a specific layer when
/// `checksum` is `Some`, otherwise the chain tip / root MIDX. A checksum that does
/// not name any layer yields a `could not find MIDX with checksum` error matching
/// git's `test-read-midx.c`.
fn resolve_read_midx_path(pack_dir: &Path, checksum: Option<&str>) -> Result<std::path::PathBuf> {
    match checksum {
        Some(cs) => resolve_midx_layer_path(pack_dir, cs)
            .ok_or_else(|| Error::CorruptObject(format!("could not find MIDX with checksum {cs}"))),
        None => resolve_tip_midx_path(pack_dir)
            .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned())),
    }
}

/// Human-readable dump of the MIDX (matches `test-tool read-midx` layout closely enough for grep-based tests).
/// Emit one line per MIDX object: `{oid} {offset}\t{pack-idx-name}` (matches Git `test-read-midx.c`).
pub fn format_midx_show_objects(objects_dir: &Path) -> Result<String> {
    format_midx_show_objects_layer(objects_dir, None)
}

/// Like [`format_midx_show_objects`] but reads a specific layer by checksum.
pub fn format_midx_show_objects_layer(
    objects_dir: &Path,
    checksum: Option<&str>,
) -> Result<String> {
    let mut out = format_midx_dump_layer(objects_dir, checksum)?;
    let pack_dir = objects_dir.join("pack");
    let path = resolve_read_midx_path(&pack_dir, checksum)?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    let names = parse_pack_names_blob(&data[pn_off..pn_off + pn_len])?;
    let (oidl_off, oidl_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OIDLOOKUP)?;
    let (ooff_off, ooff_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    if oidl_len % 20 != 0 || ooff_len % 8 != 0 {
        return Err(Error::CorruptObject(
            "bad MIDX oid-lookup / object-offsets size".to_owned(),
        ));
    }
    let num = oidl_len / 20;
    if num * 8 != ooff_len {
        return Err(Error::CorruptObject(
            "MIDX oid count does not match object-offsets".to_owned(),
        ));
    }
    for i in 0..num {
        let oid = ObjectId::from_bytes(&data[oidl_off + i * 20..oidl_off + (i + 1) * 20])
            .map_err(|e| Error::CorruptObject(e.to_string()))?;
        let base = ooff_off + i * 8;
        let pack_id = read_be_u32(&data, base)? as usize;
        let offset = u64::from(read_be_u32(&data, base + 4)?);
        let idx_name = names
            .get(pack_id)
            .ok_or_else(|| Error::CorruptObject("pack id out of range in MIDX".to_owned()))?;
        // Match `test-read-midx.c`, which prints `e.p->pack_name`: the full pack
        // path `<object-dir>/pack/<stem>.pack`. A relative object dir gets a `./`
        // prefix (Git `relative_path`).
        let stem = idx_name.strip_suffix(".idx").unwrap_or(idx_name);
        let dir_disp = objects_dir.display().to_string();
        let dir_disp = if objects_dir.is_absolute() || dir_disp.starts_with("./") {
            dir_disp
        } else {
            format!("./{dir_disp}")
        };
        out.push_str(&format!(
            "{} {}\t{}/pack/{}.pack\n",
            oid.to_hex(),
            offset,
            dir_disp,
            stem
        ));
    }
    Ok(out)
}

pub fn format_midx_dump(objects_dir: &Path) -> Result<String> {
    format_midx_dump_layer(objects_dir, None)
}

/// Like [`format_midx_dump`] but reads a specific layer by checksum (chain layer or
/// root MIDX). Used by `test-tool read-midx <object-dir> <checksum>`.
pub fn format_midx_dump_layer(objects_dir: &Path, checksum: Option<&str>) -> Result<String> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_read_midx_path(&pack_dir, checksum)?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (hdr, hdr_end, _) = parse_midx_header(&data)?;
    let sig = read_be_u32(&data, 0)?;
    let version = data[4];
    // The C `read-midx` test tool prints `m->hash_len`, the raw hash length
    // (20 for SHA-1, 32 for SHA-256), not the on-disk hash-version byte.
    let hash_len: u8 = match data[5] {
        1 => 20,
        2 => 32,
        other => other,
    };
    let num_chunks = hdr.num_chunks;
    let num_packs = read_be_u32(&data, 8)?;

    let mut chunk_tags: Vec<&'static str> = Vec::new();
    let n = num_chunks as usize;
    let pos = hdr_end;
    let toc_end = pos + (n + 1) * CHUNK_TOC_ENTRY_SIZE;
    if data.len() < toc_end + 20 {
        return Err(Error::CorruptObject(
            "truncated MIDX chunk table".to_owned(),
        ));
    }
    for i in 0..n {
        let base = pos + i * CHUNK_TOC_ENTRY_SIZE;
        let id = read_be_u32(&data, base)?;
        let tag = match id {
            x if x == MIDX_CHUNKID_PACKNAMES => "pack-names",
            x if x == MIDX_CHUNKID_OIDFANOUT => "oid-fanout",
            x if x == MIDX_CHUNKID_OIDLOOKUP => "oid-lookup",
            x if x == MIDX_CHUNKID_OBJECTOFFSETS => "object-offsets",
            x if x == MIDX_CHUNKID_LARGEOFFSETS => "large-offsets",
            x if x == MIDX_CHUNKID_REVINDEX => "revindex",
            x if x == 0x4254_4d50 => "bitmapped-packs",
            _ => "unknown",
        };
        chunk_tags.push(tag);
    }

    let (_ooff_off, ooff_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    let num_objects = ooff_len / 8;

    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    let pack_names = parse_pack_names_blob(&data[pn_off..pn_off + pn_len])?;

    let mut out = String::new();
    out.push_str(&format!(
        "header: {:08x} {} {} {} {}\n",
        sig, version, hash_len, num_chunks, num_packs
    ));
    out.push_str("chunks:");
    for t in &chunk_tags {
        out.push(' ');
        out.push_str(t);
    }
    out.push('\n');
    out.push_str(&format!("num_objects: {num_objects}\n"));
    out.push_str("packs:\n");
    for n in &pack_names {
        out.push_str(n);
        out.push('\n');
    }
    out.push_str(&format!("object-dir: {}\n", objects_dir.display()));
    Ok(out)
}

/// OID rows from the active multi-pack-index, plus reverse-index order for pack-reuse bitmap bits.
///
/// Git assigns each object a **global bitmap bit** equal to its position in the MIDX reverse index
/// (`RIDX` chunk) traversal order — not its position in the pack `.idx` file. Helpers on this struct
/// map [`ObjectId`] → global bit the same way as `midx-write.c` (`midx_pack_order`).
#[derive(Debug, Clone)]
pub struct MidxReuseTables {
    /// OIDs in MIDX lexicographic order (same order as the OID lookup chunk).
    pub oids: Vec<ObjectId>,
    /// `(pack_int_id, in-pack offset)` parallel to `oids`.
    pub pack_and_offset: Vec<(u32, u64)>,
    /// `rid_order[rank]` is the OID-table index of the object at global bitmap rank `rank`.
    pub rid_order: Vec<u32>,
    /// Inverse map: global bitmap rank for each OID-table index.
    pub oid_idx_to_rank: Vec<u32>,
}

/// Load OID / object-offset / reverse-index tables from the tip MIDX (root or chain tip).
///
/// Returns [`None`] when there is no MIDX or no `RIDX` chunk (no pseudo-bitmap ordering).
pub fn load_midx_reuse_tables(objects_dir: &Path) -> Result<Option<MidxReuseTables>> {
    let pack_dir = objects_dir.join("pack");
    let Some(path) = resolve_tip_midx_path(&pack_dir) else {
        return Ok(None);
    };
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (oidl_off, oid_l_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OIDLOOKUP)?;
    let (ooff_off, ooff_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    let Ok((ridx_off, ridx_len)) = find_chunk(&data, hdr_end, MIDX_CHUNKID_REVINDEX) else {
        return Ok(None);
    };
    if oid_l_len % 20 != 0 || ooff_len != oid_l_len / 20 * 8 {
        return Err(Error::CorruptObject(
            "MIDX OID / offset chunk size mismatch".to_owned(),
        ));
    }
    let num_objects = oid_l_len / 20;
    if ridx_len != num_objects.saturating_mul(4) {
        return Err(Error::CorruptObject(
            "MIDX reverse index length does not match object count".to_owned(),
        ));
    }
    if num_objects == 0 {
        return Ok(None);
    }

    let mut oids = Vec::with_capacity(num_objects);
    for i in 0..num_objects {
        let base = oidl_off + i * 20;
        oids.push(ObjectId::from_bytes(&data[base..base + 20])?);
    }

    let mut pack_and_offset = Vec::with_capacity(num_objects);
    for i in 0..num_objects {
        let ob = ooff_off + i * 8;
        let pack_id = read_be_u32(&data, ob)?;
        let off32 = read_be_u32(&data, ob + 4)?;
        pack_and_offset.push((pack_id, u64::from(off32)));
    }

    let mut rid_order = Vec::with_capacity(num_objects);
    for i in 0..num_objects {
        let base = ridx_off + i * 4;
        rid_order.push(read_be_u32(&data, base)?);
    }

    let mut oid_idx_to_rank = vec![0u32; num_objects];
    for (rank, &oid_idx) in rid_order.iter().enumerate() {
        let idx = usize::try_from(oid_idx)
            .map_err(|_| Error::CorruptObject("bad MIDX reverse index entry".to_owned()))?;
        if idx >= num_objects {
            return Err(Error::CorruptObject(
                "MIDX reverse index out of range".to_owned(),
            ));
        }
        oid_idx_to_rank[idx] = u32::try_from(rank)
            .map_err(|_| Error::CorruptObject("too many MIDX objects".to_owned()))?;
    }

    Ok(Some(MidxReuseTables {
        oids,
        pack_and_offset,
        rid_order,
        oid_idx_to_rank,
    }))
}

impl MidxReuseTables {
    /// Global pseudo-bitmap index for `oid`, or [`None`] if the object is not in this MIDX.
    #[must_use]
    pub fn global_bitmap_bit(&self, oid: &ObjectId) -> Option<u32> {
        let oid_idx = self.oids.binary_search(oid).ok()?;
        Some(self.oid_idx_to_rank[oid_idx])
    }

    /// MIDX-canonical pack id for `oid` (the single copy the MIDX selected after deduplication),
    /// or [`None`] if the object is not in this MIDX. Used to reject cross-pack delta reuse: a
    /// delta is only reusable verbatim when its base resolves to the *same* pack the delta lives
    /// in, mirroring Git's `midx_pair_to_pack_pos` check in `try_partial_reuse`.
    #[must_use]
    pub fn canonical_pack(&self, oid: &ObjectId) -> Option<u32> {
        let oid_idx = self.oids.binary_search(oid).ok()?;
        Some(self.pack_and_offset[oid_idx].0)
    }
}

/// One pack's slice of the MIDX pseudo-bitmap namespace (`BTMP` chunk).
#[derive(Debug, Clone, Copy)]
pub struct MidxBtmpPackRange {
    /// Pack index in the MIDX pack-names list.
    pub pack_id: u32,
    /// First bit index assigned to this pack (cumulative object order).
    pub bitmap_pos: u32,
    /// Number of objects in this pack (same as `.idx` entry count).
    pub bitmap_nr: u32,
}

/// Read per-pack `(bitmap_pos, bitmap_nr)` from the active MIDX `BTMP` chunk.
///
/// Returns an empty vector when the MIDX has no bitmapped-packs chunk.
pub fn read_midx_btmp_ranges(objects_dir: &Path) -> Result<Vec<MidxBtmpPackRange>> {
    let pack_dir = objects_dir.join("pack");
    let Some(path) = resolve_tip_midx_path(&pack_dir) else {
        return Ok(Vec::new());
    };
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let Ok((btmp_off, btmp_len)) = find_chunk(&data, hdr_end, MIDX_CHUNKID_BITMAPPED_PACKS) else {
        return Ok(Vec::new());
    };
    if btmp_len == 0 || btmp_len % 8 != 0 {
        return Err(Error::CorruptObject(
            "invalid MIDX BTMP chunk length".to_owned(),
        ));
    }
    let num_packs = read_be_u32(&data, 8)?;
    let n_entries = btmp_len / 8;
    if u32::try_from(n_entries).ok() != Some(num_packs) {
        return Err(Error::CorruptObject(
            "MIDX BTMP entry count does not match num_packs".to_owned(),
        ));
    }
    let mut out = Vec::with_capacity(n_entries);
    for i in 0..n_entries {
        let base = btmp_off + i * 8;
        let bitmap_pos = read_be_u32(&data, base)?;
        let bitmap_nr = read_be_u32(&data, base + 4)?;
        out.push(MidxBtmpPackRange {
            pack_id: u32::try_from(i)
                .map_err(|_| Error::CorruptObject("too many packs in MIDX BTMP".to_owned()))?,
            bitmap_pos,
            bitmap_nr,
        });
    }
    Ok(out)
}

/// Format `test-tool read-midx --bitmap` output for the active MIDX: per pack, a
/// line with `<pack>.pack`, then `  bitmap_pos:` and `  bitmap_nr:`. Returns an
/// error whose message is `MIDX does not contain the BTMP chunk` when the MIDX has
/// no `BTMP` chunk (mirrors `nth_bitmapped_pack` in git/midx.c).
pub fn format_midx_bitmapped_packs(objects_dir: &Path) -> Result<String> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_tip_midx_path(&pack_dir)
        .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned()))?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    let names = parse_pack_names_blob(&data[pn_off..pn_off + pn_len])?;
    let Ok((btmp_off, btmp_len)) = find_chunk(&data, hdr_end, MIDX_CHUNKID_BITMAPPED_PACKS) else {
        return Err(Error::CorruptObject(
            "MIDX does not contain the BTMP chunk".to_owned(),
        ));
    };
    let n_entries = btmp_len / 8;
    let mut out = String::new();
    for i in 0..n_entries {
        let base = btmp_off + i * 8;
        let bitmap_pos = read_be_u32(&data, base)?;
        let bitmap_nr = read_be_u32(&data, base + 4)?;
        let idx_name = names.get(i).ok_or_else(|| {
            Error::CorruptObject("BTMP entry has no corresponding pack name".to_owned())
        })?;
        let stem = idx_name.strip_suffix(".idx").unwrap_or(idx_name);
        out.push_str(&format!("{stem}.pack\n"));
        out.push_str(&format!("  bitmap_pos: {bitmap_pos}\n"));
        out.push_str(&format!("  bitmap_nr: {bitmap_nr}\n"));
    }
    Ok(out)
}

/// Look up which pack and in-pack offset holds `oid` according to the active MIDX.
pub fn midx_lookup_pack_and_offset(objects_dir: &Path, oid: &ObjectId) -> Result<(u32, u64)> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_tip_midx_path(&pack_dir)
        .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned()))?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (fanout_off, fanout_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OIDFANOUT)?;
    let (oidl_off, oid_l_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OIDLOOKUP)?;
    let (ooff_off, ooff_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    if fanout_len != 256 * 4 || oid_l_len % 20 != 0 || ooff_len != oid_l_len / 20 * 8 {
        return Err(Error::CorruptObject("truncated MIDX OID chunks".to_owned()));
    }
    let num_objects = oid_l_len / 20;
    let first = oid.as_bytes()[0] as usize;
    let j0 = if first == 0 {
        0usize
    } else {
        read_be_u32(&data, fanout_off + (first - 1) * 4)? as usize
    };
    let j1 = read_be_u32(&data, fanout_off + first * 4)? as usize;
    let mut lo = j0;
    let mut hi = j1;
    while lo < hi {
        let mid = (lo + hi) / 2;
        let base = oidl_off + mid * 20;
        let cmp = data[base..base + 20].cmp(oid.as_bytes());
        if cmp == std::cmp::Ordering::Less {
            lo = mid + 1;
        } else {
            hi = mid;
        }
    }
    if lo >= num_objects {
        return Err(Error::CorruptObject(format!(
            "object {} not in multi-pack-index",
            oid.to_hex()
        )));
    }
    let base = oidl_off + lo * 20;
    if data[base..base + 20] != *oid.as_bytes() {
        return Err(Error::CorruptObject(format!(
            "object {} not in multi-pack-index",
            oid.to_hex()
        )));
    }
    let ob = ooff_off + lo * 8;
    let pack_id = read_be_u32(&data, ob)?;
    let off32 = read_be_u32(&data, ob + 4)?;
    Ok((pack_id, u64::from(off32)))
}

/// Returns whether `oid` appears in the active MIDX OID table for `objects_dir`.
///
/// [`None`] means there is no MIDX at the pack tip. [`Some`] is the lookup result when a MIDX exists.
pub fn midx_oid_listed_in_tip(objects_dir: &Path, oid: &ObjectId) -> Result<Option<bool>> {
    let pack_dir = objects_dir.join("pack");
    let Some(midx_path) = resolve_tip_midx_path(&pack_dir) else {
        return Ok(None);
    };
    let data = fs::read(&midx_path).map_err(Error::Io)?;
    let MidxReadView {
        oidf_off,
        oidl_off,
        num_objects,
        ..
    } = match midx_load_for_read(&data, repo_midx_hash_version_for_objects_dir(objects_dir)) {
        MidxLoadResult::Ok(v) => v,
        MidxLoadResult::Skip => return Ok(None),
    };

    let first = oid.as_bytes()[0] as usize;
    let lo = if first == 0 {
        0u32
    } else {
        read_be_u32(&data, oidf_off + (first - 1) * 4)?
    };
    let hi = read_be_u32(&data, oidf_off + first * 4)?;

    let mut i = lo as usize;
    while i < hi as usize && i < num_objects {
        let o = ObjectId::from_bytes(&data[oidl_off + i * 20..oidl_off + (i + 1) * 20])?;
        match o.cmp(oid) {
            std::cmp::Ordering::Equal => return Ok(Some(true)),
            std::cmp::Ordering::Greater => return Ok(Some(false)),
            std::cmp::Ordering::Less => i += 1,
        }
    }
    Ok(Some(false))
}

/// Chunk offsets and metadata of a successfully loaded MIDX, ready for object reads.
struct MidxReadView {
    oidf_off: usize,
    oidl_off: usize,
    ooff_off: usize,
    loff: Option<(usize, usize)>,
    num_objects: usize,
    pack_names: Vec<String>,
}

enum MidxLoadResult {
    Ok(MidxReadView),
    /// The MIDX is unusable but not fatal (Git returns NULL and falls back to packs);
    /// an `error:`/`warning:` line has already been printed.
    Skip,
}

/// Print a recoverable MIDX `error:`/`warning:` line at most once per process.
///
/// Git loads the MIDX once and caches it, so a recoverable corruption is reported a
/// single time. grit re-reads the MIDX per object lookup, so without deduping the same
/// line would repeat; this guard restores the single-report behavior the tests expect.
fn midx_warn_once(line: &str) {
    use std::sync::Mutex;
    use std::sync::OnceLock;
    static SEEN: OnceLock<Mutex<HashSet<String>>> = OnceLock::new();
    let seen = SEEN.get_or_init(|| Mutex::new(HashSet::new()));
    if let Ok(mut set) = seen.lock() {
        if set.insert(line.to_string()) {
            eprintln!("{line}");
        }
    } else {
        eprintln!("{line}");
    }
}

/// Print Git-style `error:`/`fatal:` lines and exit 128, mirroring `die()` after the
/// preceding `error()` calls. `lines` are printed as `error:` except the last as `fatal:`.
fn midx_die(lines: &[&str]) -> ! {
    use std::io::Write;
    let mut err = std::io::stderr().lock();
    let n = lines.len();
    for (i, l) in lines.iter().enumerate() {
        if i + 1 == n {
            let _ = writeln!(err, "fatal: {l}");
        } else {
            let _ = writeln!(err, "error: {l}");
        }
    }
    let _ = err.flush();
    std::process::exit(128);
}

/// Validate and load a MIDX image for object reads, mirroring `load_multi_pack_index`
/// in git/midx.c. Fatal corruptions print `error:`/`fatal:` and exit (Git `die()`);
/// recoverable corruptions print an `error:`/`warning:` and return [`MidxLoadResult::Skip`].
fn midx_load_for_read(data: &[u8], expected_hash_version: u8) -> MidxLoadResult {
    if data.len() < MIDX_HEADER_SIZE + 20 {
        return MidxLoadResult::Skip;
    }
    let sig = u32::from_be_bytes([data[0], data[1], data[2], data[3]]);
    if sig != MIDX_SIGNATURE {
        midx_die(&[&format!(
            "multi-pack-index signature 0x{sig:08x} does not match signature 0x{MIDX_SIGNATURE:08x}"
        )]);
    }
    let version = data[4];
    if version != MIDX_VERSION_V1 && version != MIDX_VERSION_V2 {
        midx_die(&[&format!(
            "multi-pack-index version {version} not recognized"
        )]);
    }
    let hash_version = data[5];
    if hash_version != expected_hash_version {
        // `load_multi_pack_index` error()s then `goto cleanup_fail` (returns NULL),
        // so this is recoverable, not fatal. The expected version is the repository's
        // own `oid_version(hash_algo)` (SHA-1 → 1, SHA-256 → 2).
        midx_warn_once(&format!(
            "error: multi-pack-index hash version {hash_version} does not match version {expected_hash_version}"
        ));
        return MidxLoadResult::Skip;
    }
    let hash_len = 20usize;
    let num_packs = u32::from_be_bytes([data[8], data[9], data[10], data[11]]) as usize;

    // Table of contents (chunk-format.c read_table_of_contents). Recoverable failures
    // (unaligned / improper offset / duplicate / non-zero terminator) print error() and
    // return NULL.
    let mut toc_errors: Vec<String> = Vec::new();
    let chunks = match parse_midx_toc(data, hash_len, &mut toc_errors) {
        Ok(c) => c,
        Err(_) => {
            for e in &toc_errors {
                midx_warn_once(&format!("error: {e}"));
            }
            return MidxLoadResult::Skip;
        }
    };

    // Required pack-names chunk.
    let Some((pn_off, pn_len)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_PACKNAMES)
    else {
        midx_die(&["multi-pack-index required pack-name chunk missing or corrupted"]);
    };

    // Required oid-fanout chunk + size + ordering (midx_read_oid_fanout).
    let Some((oidf_off, oidf_len)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_OIDFANOUT)
    else {
        midx_die(&["multi-pack-index required OID fanout chunk missing or corrupted"]);
    };
    if oidf_len != 256 * 4 {
        midx_die(&[
            "multi-pack-index OID fanout is of the wrong size",
            "multi-pack-index required OID fanout chunk missing or corrupted",
        ]);
    }
    let fanout = |i: usize| -> u32 {
        let b = oidf_off + i * 4;
        u32::from_be_bytes([data[b], data[b + 1], data[b + 2], data[b + 3]])
    };
    for i in 0..255 {
        let f1 = fanout(i);
        let f2 = fanout(i + 1);
        if f1 > f2 {
            midx_die(&[
                &format!(
                    "oid fanout out of order: fanout[{i}] = {f1:x} > {f2:x} = fanout[{}]",
                    i + 1
                ),
                "multi-pack-index required OID fanout chunk missing or corrupted",
            ]);
        }
    }
    let num_objects = fanout(255) as usize;

    // Required oid-lookup chunk + size (midx_read_oid_lookup).
    let Some((oidl_off, oidl_len)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_OIDLOOKUP)
    else {
        midx_die(&["multi-pack-index required OID lookup chunk missing or corrupted"]);
    };
    if oidl_len != hash_len * num_objects {
        midx_die(&[
            "multi-pack-index OID lookup chunk is the wrong size",
            "multi-pack-index required OID lookup chunk missing or corrupted",
        ]);
    }

    // Required object-offsets chunk + size (midx_read_object_offsets).
    let Some((ooff_off, ooff_len)) =
        toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_OBJECTOFFSETS)
    else {
        midx_die(&["multi-pack-index required object offsets chunk missing or corrupted"]);
    };
    if ooff_len != num_objects * 8 {
        midx_die(&[
            "multi-pack-index object offset chunk is the wrong size",
            "multi-pack-index required object offsets chunk missing or corrupted",
        ]);
    }

    let loff = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_LARGEOFFSETS);

    // Optional revindex chunk — wrong size warns but does not fail the load.
    if let Some((_, rlen)) = toc_chunk_range(&chunks, data.len(), MIDX_CHUNKID_REVINDEX) {
        if rlen != num_objects * 4 {
            midx_warn_once("error: multi-pack-index reverse-index chunk is the wrong size");
            midx_warn_once("warning: multi-pack bitmap is missing required reverse index");
        }
    }

    // Pack-name parsing (die if a name is unterminated).
    let mut pack_names: Vec<String> = Vec::with_capacity(num_packs);
    let blob = &data[pn_off..pn_off + pn_len];
    let mut start = 0usize;
    for _ in 0..num_packs {
        let Some(rel) = blob[start..].iter().position(|&b| b == 0) else {
            midx_die(&["multi-pack-index pack-name chunk is too short"]);
        };
        let name = match std::str::from_utf8(&blob[start..start + rel]) {
            Ok(s) => s.to_string(),
            Err(_) => midx_die(&["multi-pack-index pack-name chunk is too short"]),
        };
        if version == MIDX_VERSION_V1
            && !pack_names.is_empty()
            && name.as_str() <= pack_names.last().map(|s| s.as_str()).unwrap_or("")
        {
            midx_die(&[&format!(
                "multi-pack-index pack names out of order: '{}' before '{name}'",
                pack_names.last().cloned().unwrap_or_default()
            )]);
        }
        pack_names.push(name);
        start += rel + 1;
    }

    MidxLoadResult::Ok(MidxReadView {
        oidf_off,
        oidl_off,
        ooff_off,
        loff,
        num_objects,
        pack_names,
    })
}

/// Eagerly validate that every pack named by the active MIDX has a readable `.idx`.
///
/// Mirrors git/packfile.c `open_pack_index`: when `prepare_packed_git` registers the
/// packs the MIDX references, a pack whose `.idx` cannot be opened (truncated/corrupt)
/// triggers `error: packfile <pack> index unavailable`. Git reports this once because the
/// MIDX/pack store is prepared a single time; this routine reproduces that even when the
/// object that triggered the read is found loose (so it never reaches the per-object MIDX
/// lookup). Runs at most once per process per `objects_dir`.
pub fn validate_midx_referenced_packs(objects_dir: &Path) {
    use std::sync::Mutex;
    use std::sync::OnceLock;
    static DONE: OnceLock<Mutex<HashSet<std::path::PathBuf>>> = OnceLock::new();
    let done = DONE.get_or_init(|| Mutex::new(HashSet::new()));
    if let Ok(mut set) = done.lock() {
        if !set.insert(objects_dir.to_path_buf()) {
            return;
        }
    }

    let pack_dir = objects_dir.join("pack");
    let Some(midx_path) = resolve_tip_midx_path(&pack_dir) else {
        return;
    };
    let Ok(data) = fs::read(&midx_path) else {
        return;
    };
    let MidxReadView { pack_names, .. } =
        match midx_load_for_read(&data, repo_midx_hash_version_for_objects_dir(objects_dir)) {
            MidxLoadResult::Ok(v) => v,
            MidxLoadResult::Skip => return,
        };
    for idx_name in &pack_names {
        let idx_path = pack_dir.join(idx_name);
        // A MIDX may name a pack whose files were later deleted; Git skips the missing
        // pack silently (it is not "unavailable", just gone). Only a present-but-corrupt
        // idx produces the "index unavailable" error.
        if !idx_path.exists() {
            continue;
        }
        // Match Git's `open_pack_index`, which parses the idx header/tables but does
        // not verify the trailing checksum: a structurally valid idx with a stale
        // checksum (the 64-bit-offset tests corrupt one offset byte in place) loads
        // fine and must NOT be reported "unavailable". Only an unparseable idx
        // (e.g. truncated, as in `corrupt idx reports errors`) is unavailable.
        if crate::pack::read_pack_index_no_verify(&idx_path).is_err() {
            let mut pack_path = idx_path.clone();
            pack_path.set_extension("pack");
            midx_warn_once(&format!(
                "error: packfile {} index unavailable",
                pack_path.display()
            ));
        }
    }
}

/// When `core.multiPackIndex` is enabled, try to read `oid` from the active MIDX in `objects_dir`.
///
/// Returns [`None`] when no MIDX exists or `oid` is not listed. Returns [`Some(Err(..))`] when the
/// MIDX is present but malformed (callers surface Git-style `error:` / `fatal:` messages).
pub fn try_read_object_via_midx(
    objects_dir: &Path,
    oid: &ObjectId,
) -> Result<Option<crate::objects::Object>> {
    let pack_dir = objects_dir.join("pack");
    let Some(midx_path) = resolve_tip_midx_path(&pack_dir) else {
        return Ok(None);
    };
    let data = fs::read(&midx_path).map_err(Error::Io)?;

    // Load-time validation, mirroring `load_multi_pack_index` in git/midx.c.
    // Fatal corruptions `die()` (print error + fatal, exit 128); recoverable
    // ones (e.g. an unaligned chunk table) skip the MIDX entirely.
    let MidxReadView {
        oidf_off,
        oidl_off,
        ooff_off,
        loff,
        num_objects,
        pack_names,
    } = match midx_load_for_read(&data, repo_midx_hash_version_for_objects_dir(objects_dir)) {
        MidxLoadResult::Ok(v) => v,
        MidxLoadResult::Skip => return Ok(None),
    };

    let first = oid.as_bytes()[0] as usize;
    let lo = if first == 0 {
        0u32
    } else {
        read_be_u32(&data, oidf_off + (first - 1) * 4)?
    };
    let hi = read_be_u32(&data, oidf_off + first * 4)?;

    let mut pos = None;
    let mut i = lo as usize;
    while i < hi as usize && i < num_objects {
        let o = ObjectId::from_bytes(&data[oidl_off + i * 20..oidl_off + (i + 1) * 20])?;
        let c = o.cmp(oid);
        if c == std::cmp::Ordering::Equal {
            pos = Some(i);
            break;
        }
        if c == std::cmp::Ordering::Greater {
            break;
        }
        i += 1;
    }
    let Some(pos) = pos else {
        return Ok(None);
    };

    let obase = ooff_off + pos * 8;
    let pack_id = read_be_u32(&data, obase)?;
    let raw_off = read_be_u32(&data, obase + 4)?;
    let _offset = if (raw_off & MIDX_LARGE_OFFSET_NEEDED) != 0 {
        let idx = (raw_off & !MIDX_LARGE_OFFSET_NEEDED) as usize;
        let need = (idx + 1) * 8;
        match loff {
            Some((loff_off, loff_len)) if loff_len >= need => {
                read_be_u64(&data, loff_off + idx * 8)?
            }
            _ => {
                // git/midx.c `nth_midxed_offset`: die on out-of-bounds large offset.
                midx_die(&["multi-pack-index large offset out of bounds"]);
            }
        }
    } else {
        u64::from(raw_off)
    };

    let idx_name = pack_names
        .get(pack_id as usize)
        .ok_or_else(|| Error::CorruptObject("bad pack-int-id".to_owned()))?;
    let idx_path = pack_dir.join(idx_name);
    // A multi-pack-index can outlive packs it names (e.g. a `repack -d` deleted a
    // pack but did not rewrite the MIDX). Git tolerates such stale entries by
    // skipping the missing pack; mirror that by falling through to other object
    // sources instead of surfacing the open error.
    if !idx_path.exists() {
        return Ok(None);
    }
    // Mirror git/packfile.c `open_pack_index`: when a pack's idx cannot be read
    // (e.g. truncated/corrupt), Git emits `error: packfile <pack> index unavailable`,
    // marks the pack invalid, and continues to other object sources. The object
    // may still be found loose or in another pack, so fall through rather than
    // surfacing the parse error as fatal. Use the non-verifying parse to match
    // `open_pack_index`, which does not validate the trailing checksum (a pack
    // `.idx` with a stale checksum but valid structure must still be usable).
    let idx = match crate::pack::read_pack_index_no_verify(&idx_path) {
        Ok(idx) => idx,
        Err(_) => {
            let mut pack_path = idx_path.clone();
            pack_path.set_extension("pack");
            midx_warn_once(&format!(
                "error: packfile {} index unavailable",
                pack_path.display()
            ));
            return Ok(None);
        }
    };
    crate::pack::read_object_from_pack(&idx, oid).map(Some)
}

pub fn read_midx_preferred_idx_name(objects_dir: &Path) -> Result<String> {
    let pack_dir = objects_dir.join("pack");
    let path = resolve_tip_midx_path(&pack_dir)
        .ok_or_else(|| Error::CorruptObject("no multi-pack-index found".to_owned()))?;
    let data = fs::read(&path).map_err(Error::Io)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    let names = parse_pack_names_blob(&data[pn_off..pn_off + pn_len])?;
    let (ooff_off, ooff_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_OBJECTOFFSETS)?;
    // The preferred pack is recorded in the MIDX reverse index, which is only
    // present when the MIDX has a bitmap. Without it, the preferred pack is
    // unknowable (git/midx.c `midx_preferred_pack` returns -1). Prefer the
    // embedded RIDX chunk; otherwise fall back to a `multi-pack-index*.rev`
    // sidecar, matching `load_midx_revindex`.
    let (ridx_off, ridx_len) = match find_chunk(&data, hdr_end, MIDX_CHUNKID_REVINDEX) {
        Ok(r) => r,
        Err(_) => {
            return Err(Error::CorruptObject(
                "could not determine MIDX preferred pack".to_owned(),
            ));
        }
    };

    if ridx_len < 4 || ooff_len < 8 {
        return Err(Error::CorruptObject("truncated MIDX RIDX/OOFF".to_owned()));
    }
    let first_oid_idx = read_be_u32(&data, ridx_off)? as usize;
    let entry_base = ooff_off + first_oid_idx * 8;
    if entry_base + 8 > data.len() || entry_base + 8 > ooff_off + ooff_len {
        return Err(Error::CorruptObject(
            "bad MIDX object-offsets index".to_owned(),
        ));
    }
    let pack_id = read_be_u32(&data, entry_base)?;
    let idx = usize::try_from(pack_id)
        .map_err(|_| Error::CorruptObject("pack id overflow in multi-pack-index".to_owned()))?;
    names
        .get(idx)
        .cloned()
        .ok_or_else(|| Error::CorruptObject("preferred pack id out of range".to_owned()))
}

/// Build `objects/pack/multi-pack-index` for all pack indexes in `pack_dir`.
///
/// Returns an error if there are no `.idx` files, if an object offset does not
/// fit in 31 bits (no `LOFF` chunk yet), or if I/O fails.
/// Remove every multi-pack-index file under `pack_dir` (root file, sidecars, and
/// `multi-pack-index.d/`). Used by full `repack -a` so stale incremental chains do not survive.
pub fn clear_pack_midx_state(pack_dir: &Path) -> Result<()> {
    let _ = fs::remove_file(pack_dir.join("multi-pack-index"));
    scrub_root_midx_sidecars_except(pack_dir, None)?;
    let midx_d = midx_d_dir(pack_dir);
    if midx_d.exists() {
        let _ = fs::remove_dir_all(&midx_d);
    }
    Ok(())
}

pub fn write_multi_pack_index(pack_dir: &Path) -> Result<()> {
    write_multi_pack_index_with_options(pack_dir, &WriteMultiPackIndexOptions::default())
}

/// Write `multi-pack-index` with optional preferred pack, placeholders, and incremental chain.
pub fn write_multi_pack_index_with_options(
    pack_dir: &Path,
    opts: &WriteMultiPackIndexOptions,
) -> Result<()> {
    // Git warns and ignores an existing MIDX whose checksum does not validate when
    // writing a fresh (non-stdin-packs) MIDX (git/midx-write.c `write_midx_internal`).
    if opts.pack_names_subset_ordered.is_none() {
        if let Some(existing) = resolve_tip_midx_path(pack_dir) {
            if let Ok(bytes) = fs::read(&existing) {
                if midx_checksum_is_valid(&bytes) {
                    // A fresh write copies the existing MIDX's packs. Loading a pack
                    // it references whose `.pack` is gone fails with `could not load
                    // pack N` (git/midx-write.c `fill_pack_from_midx`).
                    if let Ok((_, existing_names)) = oids_and_packs_from_midx_data(&bytes) {
                        for (i, name) in existing_names.iter().enumerate() {
                            let stem = name.strip_suffix(".idx").unwrap_or(name);
                            if !pack_dir.join(format!("{stem}.pack")).exists() {
                                eprintln!("error: could not load pack {i}");
                                return Err(Error::CorruptObject(format!(
                                    "could not load pack {i}"
                                )));
                            }
                        }
                    }
                } else {
                    eprintln!("warning: ignoring existing multi-pack-index; checksum mismatch");
                }
            }
        }
    }

    // Git's MIDX covers every pack index in the directory regardless of its
    // basename (the `.git/objects/pack/test-*.idx` packs created by t7900's
    // incremental-repack test, for instance), so include any `*.idx` whose
    // companion `.pack` exists.
    let mut idx_names: Vec<String> = fs::read_dir(pack_dir)
        .map(|rd| {
            rd.filter_map(|e| e.ok())
                .filter_map(|e| {
                    let name = e.file_name().to_string_lossy().to_string();
                    let stem = name.strip_suffix(".idx")?;
                    if pack_dir.join(format!("{stem}.pack")).exists() {
                        Some(name)
                    } else {
                        None
                    }
                })
                .collect()
        })
        .unwrap_or_default();
    idx_names.sort();

    let idx_names: Vec<String> = if let Some(sub) = &opts.pack_names_subset_ordered {
        let mut out = Vec::new();
        for line in sub {
            let want = normalize_pack_idx_basename(line)?;
            if let Some(found) = idx_names.iter().find(|n| **n == want).cloned() {
                if !out.contains(&found) {
                    out.push(found);
                }
            }
            // Unknown names on stdin are silently ignored (Git skips packs it
            // cannot find rather than failing the whole write).
        }
        out
    } else {
        idx_names
    };

    // Resolve / validate the preferred pack against the working pack set. Git emits a
    // (non-fatal) `warning: unknown preferred pack: '<name>'` when it cannot be matched.
    let mut preferred_warned = false;
    if let Some(raw) = opts.preferred_pack_name.as_deref() {
        if opts.preferred_pack_idx.is_none()
            && !idx_names
                .iter()
                .any(|n| cmp_idx_or_pack_name(raw, n).is_eq())
        {
            eprintln!("warning: unknown preferred pack: '{raw}'");
            preferred_warned = true;
        }
    }

    if idx_names.is_empty() {
        // Git `write_midx_internal`: `error("no pack files to index.")` then fail.
        eprintln!("error: no pack files to index.");
        return Err(Error::CorruptObject("no pack files to index.".to_owned()));
    }

    let (base_oids, base_pack_names) = if opts.incremental {
        collect_incremental_base(pack_dir)?
    } else {
        (HashSet::new(), HashSet::new())
    };

    let layer_idx_names: Vec<String> = if opts.incremental {
        idx_names
            .iter()
            .filter(|n| {
                !base_pack_names
                    .iter()
                    .any(|bp| pack_names_match_layer(bp, n))
            })
            .cloned()
            .collect()
    } else {
        idx_names.clone()
    };

    if opts.incremental && layer_idx_names.is_empty() {
        return Ok(());
    }

    let work_names = if opts.incremental {
        &layer_idx_names[..]
    } else {
        &idx_names[..]
    };

    let mut preferred_idx = opts.preferred_pack_idx.map(|p| p as usize);
    if preferred_idx.is_none() && !preferred_warned {
        if let Some(raw) = opts.preferred_pack_name.as_deref() {
            // Already validated against `idx_names`; resolve against the working set.
            preferred_idx = work_names
                .iter()
                .position(|n| cmp_idx_or_pack_name(raw, n).is_eq());
        }
    }
    if preferred_idx.is_none() && opts.write_bitmap_placeholders && !work_names.is_empty() {
        preferred_idx = preferred_pack_index_by_mtime(pack_dir, work_names)?;
    }
    if let Some(p) = preferred_idx {
        if p >= work_names.len() {
            return Err(Error::CorruptObject(
                "preferred pack index out of range".to_owned(),
            ));
        }
    }

    let mut indexes: Vec<PackIndex> = Vec::with_capacity(work_names.len());
    for name in work_names {
        let path = pack_dir.join(name);
        // Do not re-verify the idx trailer here; Git reads the offset table
        // directly (t5319 forces a deliberately corrupt-but-valid 64-bit idx).
        indexes.push(crate::pack::read_pack_index_no_verify(&path)?);
    }

    // Git refuses an explicitly preferred pack that has no objects.
    if let Some(p) = preferred_idx {
        if indexes.get(p).map(|i| i.entries.len()).unwrap_or(0) == 0 {
            let name = work_names.get(p).cloned().unwrap_or_default();
            let pack_name = name.strip_suffix(".idx").unwrap_or(&name);
            eprintln!("error: cannot select preferred pack {pack_name}.pack with no objects");
            return Err(Error::CorruptObject(
                "cannot select preferred pack with no objects".to_owned(),
            ));
        }
    }

    let pack_mtimes_layer: Vec<std::time::SystemTime> =
        indexes.iter().map(pack_mtime_for_midx).collect();
    let preferred_u32 = preferred_idx.map(|p| p as u32);

    let mut best: HashMap<ObjectId, MidxEntry> = HashMap::new();
    for (pack_id, idx) in indexes.iter().enumerate() {
        let pack_id = u32::try_from(pack_id).map_err(|_| {
            Error::CorruptObject("too many pack files for multi-pack-index".to_owned())
        })?;
        let mtime = pack_mtimes_layer[pack_id as usize];
        for e in &idx.entries {
            if e.oid.len() != 20 {
                continue;
            }
            let Ok(oid) = ObjectId::from_bytes(&e.oid) else {
                continue;
            };
            if opts.incremental && base_oids.contains(&oid) {
                continue;
            }
            let cand = MidxEntry {
                oid,
                pack_id,
                offset: e.offset,
                pack_mtime: mtime,
            };
            match best.get(&oid) {
                None => {
                    best.insert(oid, cand);
                }
                Some(cur) => {
                    if midx_pick_better_entry(cur, pack_id, e.offset, mtime, preferred_u32) {
                        best.insert(oid, cand);
                    }
                }
            }
        }
    }

    let bitmap_placeholders =
        opts.write_bitmap_placeholders && (!opts.incremental || !best.is_empty());

    let omit_embedded_ridx = opts.write_rev_placeholder;
    // An incremental layer must not repeat objects already provided by the base
    // chain even when the layer's own pack physically contains them (a fresh pack
    // built with `--revs` from a tag range, for instance). Filter by base OID.
    let exclude = if opts.incremental && !base_oids.is_empty() {
        Some(&base_oids)
    } else {
        None
    };
    let (out, rev_sidecar_order) = build_midx_bytes_filtered(
        work_names,
        &indexes,
        preferred_idx,
        bitmap_placeholders,
        omit_embedded_ridx,
        opts.version.unwrap_or(MIDX_VERSION_V2),
        repo_midx_hash_version(pack_dir),
        exclude,
    )?;

    let hash = &out[out.len() - 20..];
    let hash_hex = hex::encode(hash);
    let hash_arr: [u8; 20] = hash
        .try_into()
        .map_err(|_| Error::CorruptObject("midx hash length mismatch".to_owned()))?;

    if opts.incremental {
        let root_midx = pack_dir.join("multi-pack-index");
        let chain_path = chain_file_path(pack_dir);
        let chain_existed = chain_path.exists();

        let mut chain = if root_midx.exists() && !chain_existed {
            let root_hex = midx_checksum_hex_from_path(&root_midx)?;
            link_root_midx_into_chain(pack_dir, &root_hex)?;
            vec![root_hex]
        } else {
            read_chain_layer_hashes(pack_dir).unwrap_or_default()
        };

        chain.push(hash_hex.clone());

        let midx_d = midx_d_dir(pack_dir);
        fs::create_dir_all(&midx_d).map_err(Error::Io)?;

        let layer_path = midx_d.join(format!("multi-pack-index-{hash_hex}.midx"));
        fs::write(&layer_path, &out).map_err(Error::Io)?;

        let mut chain_data = String::new();
        for h in &chain {
            chain_data.push_str(h);
            chain_data.push('\n');
        }
        fs::write(chain_file_path(pack_dir), chain_data.as_bytes()).map_err(Error::Io)?;

        clear_stale_split_layers(pack_dir, &chain)?;

        let _ = fs::remove_file(pack_dir.join("multi-pack-index"));
        scrub_root_midx_sidecars(pack_dir)?;
        if bitmap_placeholders {
            let full = hex::encode(hash);
            fs::write(midx_d.join(format!("multi-pack-index-{full}.bitmap")), [])
                .map_err(Error::Io)?;
            if opts.write_rev_placeholder {
                let rev_path = midx_d.join(format!("multi-pack-index-{full}.rev"));
                if let Some(order) = rev_sidecar_order.as_ref() {
                    write_midx_rev_sidecar(&rev_path, order, &hash_arr)?;
                } else {
                    fs::write(rev_path, []).map_err(Error::Io)?;
                }
            }
        }
    } else {
        // A non-incremental write replaces any prior split layout. Git removes the
        // individual incremental layer files inside `multi-pack-index.d/` and
        // unlinks the chain file, but never `rmdir`s the directory itself, so an
        // empty `multi-pack-index.d/` is left behind (t5334 expects
        // `test_dir_is_empty $midxdir` after the conversion).
        let dest = pack_dir.join("multi-pack-index");

        // Git's `midx_needs_update`: if the new MIDX is byte-identical to the one
        // already on disk and we are not (re)writing a bitmap, leave the file
        // untouched so its mtime is preserved (t5319 `test_midx_is_retained`).
        let bitmap_path = pack_dir.join(format!("multi-pack-index-{hash_hex}.bitmap"));
        let bitmap_ok = !opts.write_bitmap_placeholders || bitmap_path.exists();
        // Only short-circuit when there is no active incremental chain to collapse;
        // an empty leftover `multi-pack-index.d/` (from a prior conversion) must not
        // defeat the retention optimization, so key off the chain file, not the dir.
        if bitmap_ok && !chain_file_path(pack_dir).exists() {
            if let Ok(existing) = fs::read(&dest) {
                if existing == out {
                    return Ok(());
                }
            }
        }

        clear_incremental_midx_files(pack_dir)?;

        fs::write(&dest, &out).map_err(Error::Io)?;

        scrub_root_midx_sidecars_except(pack_dir, Some(&hash_hex))?;

        if opts.write_bitmap_placeholders {
            fs::write(
                pack_dir.join(format!("multi-pack-index-{hash_hex}.bitmap")),
                [],
            )
            .map_err(Error::Io)?;
            if opts.write_rev_placeholder {
                let rev_path = pack_dir.join(format!("multi-pack-index-{hash_hex}.rev"));
                if let Some(order) = rev_sidecar_order.as_ref() {
                    write_midx_rev_sidecar(&rev_path, order, &hash_arr)?;
                } else {
                    fs::write(rev_path, []).map_err(Error::Io)?;
                }
            }
        }
    }

    Ok(())
}

fn pack_names_match_layer(base_name: &str, disk_idx: &str) -> bool {
    if base_name == disk_idx {
        return true;
    }
    cmp_idx_or_pack_name(disk_idx, base_name).is_eq()
}

/// Failure modes of [`compact_multi_pack_index`], each mapping to one of git's
/// user-facing diagnostics in `cmd_multi_pack_index_compact`.
#[derive(Debug)]
pub enum CompactError {
    /// `--incremental` was requested but no chain exists yet.
    NoChain,
    /// One of the endpoint checksums does not name a layer in the chain. Carries the
    /// raw argument text so the message matches `could not find MIDX: <arg>`.
    MissingEndpoint(String),
    /// Both endpoints resolve to the same layer.
    IdenticalEndpoints,
    /// `from` (argv[0]) is newer than `to` (argv[1]); git requires `from` to be an
    /// ancestor of `to`. Carries `(from, to)` arg text for the diagnostic.
    NotAncestor(String, String),
    /// Compaction was requested with the v1 on-disk MIDX format.
    V1Format,
    /// Any underlying I/O or parse failure.
    Other(String),
}

impl std::fmt::Display for CompactError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            CompactError::NoChain => write!(f, "no multi-pack-index chain to compact"),
            CompactError::MissingEndpoint(s) => write!(f, "could not find MIDX: {s}"),
            CompactError::IdenticalEndpoints => {
                write!(f, "MIDX compaction endpoints must be unique")
            }
            CompactError::NotAncestor(from, to) => {
                write!(f, "MIDX {from} must be an ancestor of {to}")
            }
            CompactError::V1Format => write!(f, "cannot perform MIDX compaction with v1 format"),
            CompactError::Other(s) => write!(f, "{s}"),
        }
    }
}

impl From<Error> for CompactError {
    fn from(e: Error) -> Self {
        CompactError::Other(e.to_string())
    }
}

/// Collect every OID provided by the chain layers in `hashes` (each layer file is
/// self-contained: it lists only its own incremental objects).
fn collect_layer_oids(pack_dir: &Path, hashes: &[String]) -> Result<HashSet<ObjectId>> {
    let mut oids = HashSet::new();
    for h in hashes {
        let p = midx_d_dir(pack_dir).join(format!("multi-pack-index-{h}.midx"));
        let data = load_midx_file(&p)?;
        let (layer_oids, _) = oids_and_packs_from_midx_data(&data)?;
        oids.extend(layer_oids);
    }
    Ok(oids)
}

/// Pack idx basenames listed by a single chain layer, in the layer's stored order.
fn layer_pack_names(pack_dir: &Path, hash: &str) -> Result<Vec<String>> {
    let p = midx_d_dir(pack_dir).join(format!("multi-pack-index-{hash}.midx"));
    let data = load_midx_file(&p)?;
    let (_, hdr_end, _) = parse_midx_header(&data)?;
    let (pn_off, pn_len) = find_chunk(&data, hdr_end, MIDX_CHUNKID_PACKNAMES)?;
    parse_pack_names_blob(&data[pn_off..pn_off + pn_len])
}

/// `git multi-pack-index compact <from> <to>`: merge the inclusive chain range
/// `[from..to]` (oldest→newest, matching git's `from`=argv[0] / `to`=argv[1]) into a
/// single new incremental layer, preserving pack order, and rewrite the chain as
/// `[layers before from] + [compacted layer] + [layers after to]`.
///
/// Mirrors `write_midx_file_compact` (git/midx-write.c). Because grit's chain layers
/// are self-contained (each lists only its own packs/objects), layers outside the
/// compacted range keep their existing files and checksums untouched.
pub fn compact_multi_pack_index(
    pack_dir: &Path,
    from_arg: &str,
    to_arg: &str,
    write_bitmaps: bool,
    write_rev: bool,
    version: Option<u8>,
) -> std::result::Result<(), CompactError> {
    if version == Some(MIDX_VERSION_V1) {
        return Err(CompactError::V1Format);
    }

    let chain = read_chain_layer_hashes(pack_dir).map_err(|_| CompactError::NoChain)?;
    if chain.is_empty() {
        return Err(CompactError::NoChain);
    }

    let from_hex = from_arg.to_ascii_lowercase();
    let to_hex = to_arg.to_ascii_lowercase();

    let from_pos = chain.iter().position(|h| *h == from_hex);
    let to_pos = chain.iter().position(|h| *h == to_hex);

    // Match git: report `from` first, then `to`, when an endpoint is missing.
    let Some(from_pos) = from_pos else {
        return Err(CompactError::MissingEndpoint(from_arg.to_string()));
    };
    let Some(to_pos) = to_pos else {
        return Err(CompactError::MissingEndpoint(to_arg.to_string()));
    };

    if from_pos == to_pos {
        return Err(CompactError::IdenticalEndpoints);
    }
    // git walks `base_midx` from `from`; reaching `to` means `from` is an ancestor of
    // `to`, i.e. `from` is newer (higher chain index) than `to`. That is the reverse
    // of what compaction expects, so report the "must be an ancestor" error.
    if from_pos > to_pos {
        return Err(CompactError::NotAncestor(
            from_arg.to_string(),
            to_arg.to_string(),
        ));
    }

    // Layers strictly before `from` form the base; their objects are excluded from
    // the compacted layer.
    let base_hashes = &chain[..from_pos];
    let merged_hashes = &chain[from_pos..=to_pos];
    let upper_hashes = &chain[to_pos + 1..];

    let base_oids = collect_layer_oids(pack_dir, base_hashes)?;

    // Gather the merged layers' pack idx names in chain order (oldest layer first),
    // preserving each layer's internal order (git's `fill_packs_from_midx_range`).
    let mut ordered_idx_names: Vec<String> = Vec::new();
    for h in merged_hashes {
        for name in layer_pack_names(pack_dir, h)? {
            if !ordered_idx_names.contains(&name) {
                ordered_idx_names.push(name);
            }
        }
    }

    if ordered_idx_names.is_empty() {
        return Err(CompactError::Other(
            "no packs found in compaction range".to_owned(),
        ));
    }

    // Load the pack indexes in the resolved order.
    let mut indexes: Vec<PackIndex> = Vec::with_capacity(ordered_idx_names.len());
    for name in &ordered_idx_names {
        let path = pack_dir.join(name);
        indexes.push(crate::pack::read_pack_index_no_verify(&path)?);
    }

    // When writing a bitmap, git sets the preferred pack to the first (oldest) pack
    // of the compacted range so its objects win duplicate selection.
    let preferred_idx = if write_bitmaps { Some(0usize) } else { None };

    let exclude = if base_oids.is_empty() {
        None
    } else {
        Some(&base_oids)
    };

    let (out, rev_sidecar_order) = build_midx_bytes_filtered(
        &ordered_idx_names,
        &indexes,
        preferred_idx,
        write_bitmaps,
        write_rev,
        version.unwrap_or(MIDX_VERSION_V2),
        repo_midx_hash_version(pack_dir),
        exclude,
    )?;

    let hash = &out[out.len() - 20..];
    let hash_hex = hex::encode(hash);
    let hash_arr: [u8; 20] = hash
        .try_into()
        .map_err(|_| CompactError::Other("midx hash length mismatch".to_owned()))?;

    let midx_d = midx_d_dir(pack_dir);
    fs::create_dir_all(&midx_d).map_err(Error::Io)?;

    let layer_path = midx_d.join(format!("multi-pack-index-{hash_hex}.midx"));
    fs::write(&layer_path, &out).map_err(Error::Io)?;

    // New chain: base layers, the compacted layer, then the untouched upper layers.
    let mut new_chain: Vec<String> = Vec::new();
    new_chain.extend(base_hashes.iter().cloned());
    new_chain.push(hash_hex.clone());
    new_chain.extend(upper_hashes.iter().cloned());

    let mut chain_data = String::new();
    for h in &new_chain {
        chain_data.push_str(h);
        chain_data.push('\n');
    }
    fs::write(chain_file_path(pack_dir), chain_data.as_bytes()).map_err(Error::Io)?;

    if write_bitmaps {
        fs::write(
            midx_d.join(format!("multi-pack-index-{hash_hex}.bitmap")),
            [],
        )
        .map_err(Error::Io)?;
        let rev_path = midx_d.join(format!("multi-pack-index-{hash_hex}.rev"));
        if write_rev {
            if let Some(order) = rev_sidecar_order.as_ref() {
                write_midx_rev_sidecar(&rev_path, order, &hash_arr)?;
            } else {
                fs::write(rev_path, []).map_err(Error::Io)?;
            }
        }
    }

    // Drop the now-removed range layers and their sidecars.
    clear_stale_split_layers(pack_dir, &new_chain)?;

    Ok(())
}

fn scrub_root_midx_sidecars(pack_dir: &Path) -> Result<()> {
    scrub_root_midx_sidecars_except(pack_dir, None)
}

fn scrub_root_midx_sidecars_except(pack_dir: &Path, keep_hex: Option<&str>) -> Result<()> {
    let Ok(rd) = fs::read_dir(pack_dir) else {
        return Ok(());
    };
    for ent in rd {
        let ent = ent.map_err(Error::Io)?;
        let name = ent.file_name().to_string_lossy().to_string();
        let Some(rest) = name.strip_prefix("multi-pack-index-") else {
            continue;
        };
        if !(rest.ends_with(".bitmap") || rest.ends_with(".rev")) {
            continue;
        }
        let hash_part = rest
            .strip_suffix(".bitmap")
            .or_else(|| rest.strip_suffix(".rev"))
            .unwrap_or(rest);
        // Git's `clear_midx_files_ext` removes any `multi-pack-index-<hash>.<ext>`
        // sidecar that does not belong to the current MIDX, regardless of the
        // hash's textual length (t5319 plants a `multi-pack-index-abc.rev`).
        if keep_hex.is_some_and(|k| k == hash_part) {
            continue;
        }
        let _ = fs::remove_file(ent.path());
    }
    Ok(())
}