grit_lib/

commit_graph_file.rs

//! Parsing Git commit-graph files and Bloom filter lookup (`commit-graph.c` / `bloom.c` compatible).

use std::path::{Path, PathBuf};
use std::sync::{Arc, Mutex};

use crate::bloom::{
    bloom_filter_contains, bloom_keyvec_for_path, BloomBuildOutcome, BloomFilterSettings,
};
use crate::error::Error;
use crate::hash::verify_trailer;
use crate::objects::{HashAlgo, ObjectId};
use crate::odb::{hash_algo_for_objects_dir, Odb};
use crate::pack_map::PackData;

fn warn_once_for_disabled_bloom_layer(
    caches: Option<&crate::repo_caches::RepoCaches>,
    id: &str,
) -> bool {
    let key = format!("bloom-disabled:{id}");
    caches
        .map(|c| c.should_warn_commit_graph_once(&key))
        .unwrap_or(true)
}

fn warn_once_for_base_chunk_too_small(
    caches: Option<&crate::repo_caches::RepoCaches>,
    id: &str,
) -> bool {
    let key = format!("base-chunk:{id}");
    caches
        .map(|c| c.should_warn_commit_graph_once(&key))
        .unwrap_or(true)
}

const SIGNATURE: &[u8; 4] = b"CGPH";
const GRAPH_VERSION: u8 = 1;
const COMMIT_GRAPH_MIN_FILE_LEN: u64 = 28;

const CHUNK_OID_FANOUT: u32 = 0x4f49_4446; // OIDF
const CHUNK_OID_LOOKUP: u32 = 0x4f49_444c; // OIDL
const CHUNK_COMMIT_DATA: u32 = 0x4344_4154; // CDAT
const CHUNK_GENERATION_DATA: u32 = 0x4744_4132; // GDA2
const CHUNK_GENERATION_DATA_OVERFLOW: u32 = 0x4744_4f32; // GDO2
const CHUNK_EXTRA_EDGES: u32 = 0x4544_4745; // EDGE

/// CDAT parent word meaning "no parent in this slot" (Git `GRAPH_PARENT_NONE`).
const GRAPH_PARENT_NONE: u32 = 0x7000_0000;
/// CDAT parent-2 high bit meaning "remaining parents live in the EXTRA_EDGES
/// chunk" (octopus merge; Git `GRAPH_EXTRA_EDGES_NEEDED`).
const GRAPH_EXTRA_EDGES_NEEDED: u32 = 0x8000_0000;
const GRAPH_LAST_EDGE: u32 = 0x8000_0000;
const CHUNK_BLOOM_INDEXES: u32 = 0x4249_4458; // BIDX
const CHUNK_BLOOM_DATA: u32 = 0x4244_4154; // BDAT
const CHUNK_BASE_GRAPHS: u32 = 0x4241_5345; // BASE

const BLOOM_HEADER: usize = crate::bloom::BLOOMDATA_HEADER_LEN;

/// `CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW` — high bit marks GDA2 entries that index GDO2.
const CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW: u32 = 1u32 << 31;

fn warn_path_for_graph_file(path: &Path) -> String {
    let s = path.to_string_lossy();
    if let Some(idx) = s.find(".git/") {
        return s[idx..].replace('\\', "/");
    }
    s.replace('\\', "/")
}

/// One layer from `.git/objects/info/commit-graph` or `commit-graphs/<hash>.graph`.
#[derive(Debug, Clone)]
pub struct CommitGraphLayer {
    /// Repository-relative or filesystem path related to the failure.
    pub path: PathBuf,
    file: Arc<PackData>,
    body_len: usize,
    num_commits: u32,
    oid_lookup_off: usize,
    chunk_commit_data_off: usize,
    chunk_generation_data: Option<usize>,
    chunk_generation_overflow: Option<usize>,
    read_generation_data: bool,
    chunk_bloom_indexes: Option<usize>,
    chunk_bloom_data: Option<(usize, usize)>,
    bloom_settings: Option<BloomFilterSettings>,
    bloom_disabled: bool,
    /// Size in bytes of the BASE chunk (0 if absent). Used to bounds-check the
    /// base-graph list from the commit-graph header (Git `add_graph_to_chain`).
    base_chunk_size: usize,
    /// OID width in bytes implied by the header hash version (20 for SHA-1, 32 for SHA-256).
    hash_len: usize,
    /// Hash algorithm named by the header hash version.
    algo: HashAlgo,
    extra_edges_off: Option<usize>,
    extra_edges_end: Option<usize>,
}

/// OID width for a commit-graph header hash-version byte (`body[5]`).
fn commit_graph_hash_len(hash_version: u8) -> Option<usize> {
    HashAlgo::try_from(hash_version).ok().map(|a| a.len())
}

/// True when `h` is a lowercase/uppercase hex chain entry for `algo` (40 or 64 chars).
fn commit_graph_chain_hash_is_valid(h: &str, algo: HashAlgo) -> bool {
    h.len() == algo.hex_len() && h.chars().all(|c| c.is_ascii_hexdigit())
}

const OIDF_BUCKET_BYTES: usize = 256 * 4;

fn commit_graph_chunk_oob() -> Error {
    Error::CorruptObject("commit-graph chunk extends past end of file".to_owned())
}

fn commit_graph_offset_out_of_range() -> Error {
    Error::CorruptObject("commit-graph chunk offset out of range".to_owned())
}

fn checked_add_end(start: usize, len: usize, bound: usize) -> Result<usize, Error> {
    start
        .checked_add(len)
        .filter(|&end| end <= bound)
        .ok_or_else(commit_graph_chunk_oob)
}

fn body_slice(body: &[u8], start: usize, len: usize) -> Result<&[u8], Error> {
    let end = checked_add_end(start, len, body.len())?;
    body.get(start..end).ok_or_else(commit_graph_chunk_oob)
}

fn parse_toc_offset(body_len: usize, raw: [u8; 8]) -> Result<usize, Error> {
    let off = u64::from_be_bytes(raw);
    if off > body_len as u64 {
        return Err(commit_graph_offset_out_of_range());
    }
    usize::try_from(off).map_err(|_| commit_graph_offset_out_of_range())
}

fn fanout_num_commits(body: &[u8], fanout_off: usize) -> Result<u32, Error> {
    let fanout = body_slice(body, fanout_off, OIDF_BUCKET_BYTES)?;
    let word = fanout[255 * 4..256 * 4]
        .try_into()
        .map_err(|_| Error::CorruptObject("commit-graph fanout corrupt".to_owned()))?;
    Ok(u32::from_be_bytes(word))
}

#[allow(clippy::too_many_arguments)]
fn validate_single_layer_parent_indices(
    body: &[u8],
    num_commits: u32,
    commit_data_off: usize,
    graph_data_width: usize,
    hash_len: usize,
    extra_edges_off: Option<usize>,
    toc_entries: &[(u32, usize)],
    file_end: usize,
) -> Result<(), Error> {
    fn chunk_byte_range(
        start: usize,
        toc_entries: &[(u32, usize)],
        file_end: usize,
    ) -> Result<usize, Error> {
        let mut ends: Vec<usize> = toc_entries
            .iter()
            .map(|&(_, o)| o)
            .filter(|&o| o > start)
            .collect();
        ends.sort_unstable();
        Ok(ends.first().copied().unwrap_or(file_end))
    }

    for lex in 0..num_commits {
        let base = commit_data_off + lex as usize * graph_data_width;
        let p1_off = base + hash_len;
        let p1 =
            u32::from_be_bytes(body[p1_off..p1_off + 4].try_into().map_err(|_| {
                Error::CorruptObject("commit-graph commit data corrupt".to_owned())
            })?);
        let p2 =
            u32::from_be_bytes(body[p1_off + 4..p1_off + 8].try_into().map_err(|_| {
                Error::CorruptObject("commit-graph commit data corrupt".to_owned())
            })?);
        if p1 != GRAPH_PARENT_NONE
            && p1 & GRAPH_EXTRA_EDGES_NEEDED == 0
            && p1 as usize >= num_commits as usize
        {
            return Err(Error::CorruptObject(
                "commit-graph invalid parent index".to_owned(),
            ));
        }
        if p2 != GRAPH_PARENT_NONE {
            if p2 & GRAPH_EXTRA_EDGES_NEEDED != 0 {
                let start = (p2 & !GRAPH_EXTRA_EDGES_NEEDED) as usize;
                let Some(edge_off) = extra_edges_off else {
                    return Err(Error::CorruptObject(
                        "commit-graph extra edge list out of range".to_owned(),
                    ));
                };
                let edge_end = chunk_byte_range(edge_off, toc_entries, file_end)?;
                let edge_len = edge_end.saturating_sub(edge_off);
                if start * 4 >= edge_len {
                    return Err(Error::CorruptObject(
                        "commit-graph extra edge list out of range".to_owned(),
                    ));
                }
                let mut idx = start;
                loop {
                    if idx * 4 + 4 > edge_len {
                        return Err(Error::CorruptObject(
                            "commit-graph extra edge list corrupt".to_owned(),
                        ));
                    }
                    let ev = u32::from_be_bytes(
                        body[edge_off + idx * 4..edge_off + idx * 4 + 4]
                            .try_into()
                            .map_err(|_| {
                                Error::CorruptObject(
                                    "commit-graph extra edge list corrupt".to_owned(),
                                )
                            })?,
                    );
                    let parent = ev & !GRAPH_LAST_EDGE;
                    if parent != GRAPH_PARENT_NONE
                        && parent & GRAPH_EXTRA_EDGES_NEEDED == 0
                        && parent as usize >= num_commits as usize
                    {
                        return Err(Error::CorruptObject(
                            "commit-graph invalid parent index".to_owned(),
                        ));
                    }
                    if ev & GRAPH_LAST_EDGE != 0 {
                        break;
                    }
                    idx += 1;
                }
            } else if p2 as usize >= num_commits as usize {
                return Err(Error::CorruptObject(
                    "commit-graph invalid parent index".to_owned(),
                ));
            }
        }
    }
    Ok(())
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum CommitGraphParseMode {
    /// Normal read path: header, chunk table, and chunk bounds only (no checksum/OID scan).
    Open,
    /// Full on-disk verification (checksum, ordering, generation/parent consistency).
    Verify,
}

impl CommitGraphLayer {
    fn body(&self) -> &[u8] {
        &self.file[..self.body_len]
    }

    /// Map or read a commit-graph layer from disk (hot path; borrows mapped bytes).
    fn try_open(path: PathBuf) -> Result<Self, Error> {
        let storage = PackData::open_with_min_len(&path, COMMIT_GRAPH_MIN_FILE_LEN)?;
        Self::try_parse_storage(path, storage, CommitGraphParseMode::Open)
    }

    /// Parse a commit-graph layer; fails if generation overflow chunk is inconsistent with GDA2.
    pub fn try_parse(path: PathBuf, raw: Vec<u8>) -> Result<Self, Error> {
        Self::try_parse_storage(
            path,
            PackData::from_owned(raw),
            CommitGraphParseMode::Verify,
        )
    }

    fn try_parse_storage(
        path: PathBuf,
        storage: Arc<PackData>,
        mode: CommitGraphParseMode,
    ) -> Result<Self, Error> {
        let raw = storage.as_ref();
        if raw.len() < 28 {
            return Err(Error::CorruptObject(
                "commit-graph file too small".to_owned(),
            ));
        }
        // The hash version (byte 5) selects the OID width and trailing-checksum
        // width: 1 → SHA-1 (20 bytes), 2 → SHA-256 (32 bytes).
        let hash_len = match commit_graph_hash_len(raw[5]) {
            Some(n) => n,
            None => {
                return Err(Error::CorruptObject(format!(
                    "commit-graph version/hash not supported (version {} hash {})",
                    raw[4], raw[5]
                )))
            }
        };
        let algo = HashAlgo::try_from(raw[5]).map_err(|_| {
            Error::CorruptObject(format!(
                "commit-graph version/hash not supported (version {} hash {})",
                raw[4], raw[5]
            ))
        })?;
        if mode == CommitGraphParseMode::Verify {
            verify_trailer(algo, raw)
                .map_err(|_| Error::CorruptObject("commit-graph incorrect checksum".to_owned()))?;
        }
        let body_len = raw.len().saturating_sub(hash_len);
        let body = &raw[..body_len];
        if body.len() < 8 || &body[0..4] != SIGNATURE {
            return Err(Error::CorruptObject(
                "commit-graph has bad signature".to_owned(),
            ));
        }
        if body[4] != GRAPH_VERSION {
            return Err(Error::CorruptObject(format!(
                "commit-graph version/hash not supported (version {} hash {})",
                body[4], body[5]
            )));
        }
        let num_chunks = body[6] as usize;
        let toc_start: usize = 8;
        let toc_end = num_chunks
            .checked_add(1)
            .and_then(|n| n.checked_mul(12))
            .and_then(|n| toc_start.checked_add(n))
            .ok_or_else(|| {
                Error::CorruptObject("commit-graph truncated at chunk table".to_owned())
            })?;
        if body.len() < toc_end {
            return Err(Error::CorruptObject(
                "commit-graph truncated at chunk table".to_owned(),
            ));
        }

        let mut fanout_off = None;
        let mut oid_lookup_off = None;
        let mut commit_data_off = None;
        let mut generation_off = None;
        let mut generation_overflow_off = None;
        let mut bloom_idx_off = None;
        let mut bloom_data_range = None;
        let mut base_graphs_off = None;
        let mut extra_edges_off = None;
        let mut extra_edges_end = None;
        let mut chunk_offsets: Vec<usize> = Vec::new();
        let mut toc_entries: Vec<(u32, usize)> = Vec::with_capacity(num_chunks);

        for i in 0..num_chunks {
            let e = toc_start + i * 12;
            let id = u32::from_be_bytes(
                body[e..e + 4]
                    .try_into()
                    .map_err(|_| Error::CorruptObject("commit-graph bad TOC".to_owned()))?,
            );
            let off = parse_toc_offset(
                body.len(),
                body[e + 4..e + 12]
                    .try_into()
                    .map_err(|_| Error::CorruptObject("commit-graph bad TOC".to_owned()))?,
            )?;
            toc_entries.push((id, off));
            chunk_offsets.push(off);
            match id {
                CHUNK_OID_FANOUT => fanout_off = Some(off),
                CHUNK_OID_LOOKUP => oid_lookup_off = Some(off),
                CHUNK_COMMIT_DATA => commit_data_off = Some(off),
                CHUNK_GENERATION_DATA => generation_off = Some(off),
                CHUNK_GENERATION_DATA_OVERFLOW => generation_overflow_off = Some(off),
                CHUNK_BLOOM_INDEXES => bloom_idx_off = Some(off),
                CHUNK_EXTRA_EDGES => {
                    extra_edges_off = Some(off);
                    extra_edges_end = Some(if i + 1 < num_chunks {
                        let e2 = toc_start
                            .checked_add((i + 1).checked_mul(12).ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?)
                            .ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?;
                        parse_toc_offset(
                            body.len(),
                            body[e2 + 4..e2 + 12].try_into().map_err(|_| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?,
                        )?
                    } else {
                        let term = toc_start
                            .checked_add(num_chunks.checked_mul(12).ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?)
                            .ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?;
                        parse_toc_offset(
                            body.len(),
                            body[term + 4..term + 12].try_into().map_err(|_| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?,
                        )?
                    });
                }
                CHUNK_BASE_GRAPHS => base_graphs_off = Some(off),
                CHUNK_BLOOM_DATA => {
                    let end = if i + 1 < num_chunks {
                        let e2 = toc_start
                            .checked_add((i + 1).checked_mul(12).ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?)
                            .ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?;
                        parse_toc_offset(
                            body.len(),
                            body[e2 + 4..e2 + 12].try_into().map_err(|_| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?,
                        )?
                    } else {
                        let term = toc_start
                            .checked_add(num_chunks.checked_mul(12).ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?)
                            .ok_or_else(|| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?;
                        parse_toc_offset(
                            body.len(),
                            body[term + 4..term + 12].try_into().map_err(|_| {
                                Error::CorruptObject("commit-graph bad TOC".to_owned())
                            })?,
                        )?
                    };
                    let len = end.checked_sub(off).ok_or_else(|| {
                        Error::CorruptObject("commit-graph chunk layout invalid".to_owned())
                    })?;
                    bloom_data_range = Some((off, len));
                }
                _ => {}
            }
        }
        let file_end_pos = toc_start
            .checked_add(
                num_chunks
                    .checked_mul(12)
                    .ok_or_else(|| Error::CorruptObject("commit-graph bad file end".to_owned()))?,
            )
            .and_then(|t| t.checked_add(4))
            .ok_or_else(|| Error::CorruptObject("commit-graph bad file end".to_owned()))?;
        let file_end = parse_toc_offset(
            body.len(),
            body[file_end_pos..file_end_pos + 8]
                .try_into()
                .map_err(|_| Error::CorruptObject("commit-graph bad file end".to_owned()))?,
        )?;
        if file_end > body.len() {
            return Err(commit_graph_offset_out_of_range());
        }
        chunk_offsets.push(file_end);
        chunk_offsets.sort_unstable();
        chunk_offsets.dedup();

        fn chunk_byte_range(
            start: usize,
            toc_entries: &[(u32, usize)],
            file_end: usize,
        ) -> Result<usize, Error> {
            let mut ends: Vec<usize> = toc_entries
                .iter()
                .map(|&(_, o)| o)
                .filter(|&o| o > start)
                .collect();
            ends.sort_unstable();
            let end = ends.first().copied().unwrap_or(file_end);
            if end < start {
                return Err(Error::CorruptObject(
                    "commit-graph chunk layout invalid".to_owned(),
                ));
            }
            Ok(end)
        }

        if mode == CommitGraphParseMode::Verify {
            if let Some(gda) = generation_off {
                let gda_end = chunk_byte_range(gda, &toc_entries, file_end)?;
                let gda_len = gda_end.saturating_sub(gda);
                let fo = fanout_off.ok_or_else(|| {
                    Error::CorruptObject("commit-graph missing fanout".to_owned())
                })?;
                let num_commits_probe = fanout_num_commits(body, fo)?;
                let expected = num_commits_probe as usize * 4;
                if gda_len < expected {
                    return Err(Error::CorruptObject(
                        "commit-graph generation data chunk is too small".to_owned(),
                    ));
                }
                let gda_slice = body.get(gda..gda + expected).ok_or_else(|| {
                    Error::CorruptObject("commit-graph generation data OOB".to_owned())
                })?;
                let mut max_overflow_idx: Option<u32> = None;
                for w in 0..num_commits_probe as usize {
                    let v = u32::from_be_bytes(gda_slice[w * 4..w * 4 + 4].try_into().map_err(
                        |_| Error::CorruptObject("commit-graph GDA2 corrupt".to_owned()),
                    )?);
                    if v & CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW != 0 {
                        let pos = v ^ CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW;
                        max_overflow_idx = Some(match max_overflow_idx {
                            None => pos,
                            Some(m) => m.max(pos),
                        });
                    }
                }
                if let Some(pos) = max_overflow_idx {
                    let Some(gdo_start) = generation_overflow_off else {
                        return Err(Error::CorruptObject(
                            "commit-graph requires overflow generation data but has none"
                                .to_owned(),
                        ));
                    };
                    let gdo_end = chunk_byte_range(gdo_start, &toc_entries, file_end)?;
                    let overflow_bytes = gdo_end.saturating_sub(gdo_start);
                    let n_slots = overflow_bytes / 8;
                    if n_slots <= pos as usize {
                        return Err(Error::CorruptObject(
                            "commit-graph overflow generation data is too small".to_owned(),
                        ));
                    }
                }
            }
        }
        let bidx_len = bloom_idx_off.and_then(|b| {
            chunk_offsets
                .iter()
                .find(|&&o| o > b)
                .map(|&next| next.saturating_sub(b))
        });

        let fanout_off = fanout_off.ok_or_else(|| {
            Error::CorruptObject("commit-graph missing OID fanout chunk".to_owned())
        })?;
        let oid_lookup_off = oid_lookup_off.ok_or_else(|| {
            Error::CorruptObject("commit-graph missing OID lookup chunk".to_owned())
        })?;
        let commit_data_off = commit_data_off.ok_or_else(|| {
            Error::CorruptObject("commit-graph missing commit data chunk".to_owned())
        })?;
        checked_add_end(oid_lookup_off, 4, body.len())?;
        let num_commits = fanout_num_commits(body, fanout_off)?;
        let oid_lookup_bytes = (num_commits as usize)
            .checked_mul(hash_len)
            .ok_or_else(commit_graph_chunk_oob)?;
        checked_add_end(oid_lookup_off, oid_lookup_bytes, body.len())?;
        let graph_data_width = hash_len
            .checked_add(16)
            .ok_or_else(commit_graph_chunk_oob)?;
        let cdat_bytes = (num_commits as usize)
            .checked_mul(graph_data_width)
            .ok_or_else(commit_graph_chunk_oob)?;
        checked_add_end(commit_data_off, cdat_bytes, body.len())?;

        if mode == CommitGraphParseMode::Verify {
            for bucket in 0usize..255 {
                let a_off = fanout_off
                    .checked_add(bucket.checked_mul(4).ok_or_else(commit_graph_chunk_oob)?)
                    .ok_or_else(commit_graph_chunk_oob)?;
                let a =
                    u32::from_be_bytes(body_slice(body, a_off, 4)?.try_into().map_err(|_| {
                        Error::CorruptObject("commit-graph fanout corrupt".to_owned())
                    })?);
                let b_off = fanout_off
                    .checked_add(
                        (bucket + 1)
                            .checked_mul(4)
                            .ok_or_else(commit_graph_chunk_oob)?,
                    )
                    .ok_or_else(commit_graph_chunk_oob)?;
                let b =
                    u32::from_be_bytes(body_slice(body, b_off, 4)?.try_into().map_err(|_| {
                        Error::CorruptObject("commit-graph fanout corrupt".to_owned())
                    })?);
                if a > b {
                    return Err(Error::CorruptObject(
                        "commit-graph fanout value non-monotonic".to_owned(),
                    ));
                }
            }

            for lex in 1..num_commits {
                let off_a = oid_lookup_off
                    .checked_add(
                        (lex as usize - 1)
                            .checked_mul(hash_len)
                            .ok_or_else(commit_graph_chunk_oob)?,
                    )
                    .ok_or_else(commit_graph_chunk_oob)?;
                let off_b = oid_lookup_off
                    .checked_add(
                        (lex as usize)
                            .checked_mul(hash_len)
                            .ok_or_else(commit_graph_chunk_oob)?,
                    )
                    .ok_or_else(commit_graph_chunk_oob)?;
                let slice_a = body_slice(body, off_a, hash_len)?;
                let slice_b = body_slice(body, off_b, hash_len)?;
                if slice_a >= slice_b {
                    return Err(Error::CorruptObject(
                        "commit-graph incorrect OID order".to_owned(),
                    ));
                }
            }

            if base_graphs_off.is_none() {
                validate_single_layer_parent_indices(
                    body,
                    num_commits,
                    commit_data_off,
                    graph_data_width,
                    hash_len,
                    extra_edges_off,
                    &toc_entries,
                    file_end,
                )?;
            }
        }

        let read_generation_data = generation_off.is_some();
        let mut bloom_settings = None;
        let mut chunk_bloom_data = None;
        if let (Some(_bidx), Some((bdat_off, bdat_len))) = (bloom_idx_off, bloom_data_range) {
            if bdat_len < BLOOM_HEADER {
            } else if bdat_off + bdat_len <= body.len() {
                let hdr = &body[bdat_off..bdat_off + BLOOM_HEADER];
                let hash_version: [u8; 4] = hdr[0..4]
                    .try_into()
                    .map_err(|_| Error::CorruptObject("Bloom header corrupt".to_owned()))?;
                let num_hashes: [u8; 4] = hdr[4..8]
                    .try_into()
                    .map_err(|_| Error::CorruptObject("Bloom header corrupt".to_owned()))?;
                let bits_per_entry: [u8; 4] = hdr[8..12]
                    .try_into()
                    .map_err(|_| Error::CorruptObject("Bloom header corrupt".to_owned()))?;
                bloom_settings = Some(BloomFilterSettings {
                    hash_version: u32::from_be_bytes(hash_version),
                    num_hashes: u32::from_be_bytes(num_hashes),
                    bits_per_entry: u32::from_be_bytes(bits_per_entry),
                    max_changed_paths: 512,
                });
                chunk_bloom_data = Some((bdat_off, bdat_len));
            }
        }

        let bloom_indexes_ok = if let (Some(bidx), Some(bsize)) = (bloom_idx_off, bidx_len) {
            !(bsize / 4 != num_commits as usize || bidx + bsize > body.len())
        } else {
            false
        };
        let bloom_pair_ok = bloom_settings.is_some()
            && chunk_bloom_data.is_some()
            && bloom_indexes_ok
            && chunk_bloom_data.is_some_and(|(_, len)| len >= BLOOM_HEADER);

        let (chunk_bloom_indexes, bloom_settings) = if bloom_pair_ok {
            (bloom_idx_off, bloom_settings)
        } else {
            (None, None)
        };
        let chunk_bloom_data = if bloom_pair_ok {
            chunk_bloom_data
        } else {
            None
        };

        let base_chunk_size = match base_graphs_off {
            Some(off) => {
                let end = chunk_byte_range(off, &toc_entries, file_end)?;
                end.saturating_sub(off)
            }
            None => 0,
        };

        Ok(Self {
            path,
            file: storage,
            body_len,
            num_commits,
            oid_lookup_off,
            chunk_commit_data_off: commit_data_off,
            chunk_generation_data: generation_off,
            chunk_generation_overflow: generation_overflow_off,
            read_generation_data,
            chunk_bloom_indexes,
            chunk_bloom_data,
            bloom_settings,
            bloom_disabled: false,
            base_chunk_size,
            hash_len,
            algo,
            extra_edges_off,
            extra_edges_end,
        })
    }

    fn parse(path: PathBuf, raw: Vec<u8>) -> Option<Self> {
        Self::try_parse(path, raw).ok()
    }

    fn oid_at_lex(&self, lex_index: u32) -> Option<ObjectId> {
        if lex_index >= self.num_commits {
            return None;
        }
        let off = self.oid_lookup_off + lex_index as usize * self.hash_len;
        ObjectId::from_bytes(self.body().get(off..off + self.hash_len)?).ok()
    }

    /// Decode the CDAT (commit-data) record at `lex_index` into
    /// `(parent1_word, parent2_word, committer_time)`. The parent words are raw
    /// graph-position encodings (see [`GRAPH_PARENT_NONE`] /
    /// [`GRAPH_EXTRA_EDGES_NEEDED`]); the time is the low 34 bits of the trailing
    /// generation/commit-time field. Each record is `hash_len + 16` bytes:
    /// root-tree OID, two 4-byte parent words, then an 8-byte gen/time field.
    fn parent_global_positions(&self, lex_index: u32) -> Option<Vec<u32>> {
        let (p1, p2, _) = self.cdat_record(lex_index)?;
        let mut out = Vec::new();
        if p1 == GRAPH_PARENT_NONE {
            return Some(out);
        }
        if p1 & GRAPH_EXTRA_EDGES_NEEDED != 0 {
            return None;
        }
        out.push(p1);
        if p2 == GRAPH_PARENT_NONE {
            return Some(out);
        }
        if p2 & GRAPH_EXTRA_EDGES_NEEDED != 0 {
            let start = (p2 & !GRAPH_EXTRA_EDGES_NEEDED) as usize;
            let edge_off = self.extra_edges_off?;
            let edge_end = self.extra_edges_end?;
            let edge_len = edge_end.saturating_sub(edge_off);
            if start.saturating_mul(4) >= edge_len {
                return None;
            }
            let mut idx = start;
            loop {
                if idx.saturating_mul(4).saturating_add(4) > edge_len {
                    return None;
                }
                let ev = u32::from_be_bytes(
                    self.body()[edge_off + idx * 4..edge_off + idx * 4 + 4]
                        .try_into()
                        .ok()?,
                );
                let parent = ev & !GRAPH_LAST_EDGE;
                if parent != GRAPH_PARENT_NONE && parent & GRAPH_EXTRA_EDGES_NEEDED == 0 {
                    out.push(parent);
                }
                if ev & GRAPH_LAST_EDGE != 0 {
                    break;
                }
                idx += 1;
            }
            return Some(out);
        }
        out.push(p2);
        Some(out)
    }

    /// Root tree OID stored in the CDAT record for `lex_index`.
    fn cdat_root_tree(&self, lex_index: u32) -> Option<ObjectId> {
        if lex_index >= self.num_commits {
            return None;
        }
        let base = self.chunk_commit_data_off + lex_index as usize * (self.hash_len + 16);
        ObjectId::from_bytes(self.body().get(base..base + self.hash_len)?).ok()
    }

    fn cdat_record(&self, lex_index: u32) -> Option<(u32, u32, i64)> {
        if lex_index >= self.num_commits {
            return None;
        }
        let base = self.chunk_commit_data_off + lex_index as usize * (self.hash_len + 16);
        let p1_off = base + self.hash_len;
        let p1 = u32::from_be_bytes(self.body().get(p1_off..p1_off + 4)?.try_into().ok()?);
        let p2 = u32::from_be_bytes(self.body().get(p1_off + 4..p1_off + 8)?.try_into().ok()?);
        let gen_time =
            u64::from_be_bytes(self.body().get(p1_off + 8..p1_off + 16)?.try_into().ok()?);
        let commit_time = (gen_time & 0x3_FFFF_FFFF) as i64;
        Some((p1, p2, commit_time))
    }

    /// Corrected commit date (generation v2) from GDA2/GDO2, when the layer stores generation data.
    fn corrected_commit_generation(&self, lex_index: u32) -> Option<u64> {
        if !self.read_generation_data {
            return None;
        }
        let (_, _, commit_time) = self.cdat_record(lex_index)?;
        let gda_off = self.chunk_generation_data?;
        if lex_index >= self.num_commits {
            return None;
        }
        let start = gda_off + lex_index as usize * 4;
        let word = u32::from_be_bytes(self.body().get(start..start + 4)?.try_into().ok()?);
        let offset = if word & CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW != 0 {
            let slot = (word ^ CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW) as usize;
            let gdo_off = self.chunk_generation_overflow?;
            let off = gdo_off + slot * 8;
            let hi = u32::from_be_bytes(self.body().get(off..off + 4)?.try_into().ok()?);
            let lo = u32::from_be_bytes(self.body().get(off + 4..off + 8)?.try_into().ok()?);
            (u64::from(hi) << 32) | u64::from(lo)
        } else {
            u64::from(word)
        };
        Some((commit_time as u64).saturating_add(offset))
    }

    fn bsearch_oid(&self, oid: &ObjectId) -> Option<u32> {
        let mut lo = 0u32;
        let mut hi = self.num_commits;
        let bytes = oid.as_bytes();
        while lo < hi {
            let mid = (lo + hi) / 2;
            let off = self.oid_lookup_off + mid as usize * self.hash_len;
            let slice = &self.body()[off..off + self.hash_len];
            match slice.cmp(bytes) {
                std::cmp::Ordering::Less => lo = mid + 1,
                std::cmp::Ordering::Greater => hi = mid,
                std::cmp::Ordering::Equal => return Some(mid),
            }
        }
        None
    }

    fn disable_bloom(&mut self) {
        self.chunk_bloom_indexes = None;
        self.chunk_bloom_data = None;
        self.bloom_settings = None;
        self.bloom_disabled = true;
    }

    fn layer_display_id(&self) -> String {
        self.path
            .file_stem()
            .and_then(|s| s.to_str())
            .map(|s| s.strip_prefix("graph-").unwrap_or(s).to_string())
            .unwrap_or_else(|| "commit-graph".to_string())
    }

    fn bloom_filter_slice(&self, lex_index: u32) -> Option<&[u8]> {
        let _settings = self.bloom_settings.as_ref()?;
        let bidx_base = self.chunk_bloom_indexes?;
        let (bdat_off, bdat_total) = self.chunk_bloom_data?;
        let _graph_warn = warn_path_for_graph_file(self.path.as_path());
        if lex_index >= self.num_commits {
            return None;
        }
        let payload_len = bdat_total.saturating_sub(BLOOM_HEADER);
        let end_rel = u32::from_be_bytes(
            self.body()[bidx_base + lex_index as usize * 4..bidx_base + lex_index as usize * 4 + 4]
                .try_into()
                .ok()?,
        ) as usize;
        let start_rel = if lex_index == 0 {
            0usize
        } else {
            u32::from_be_bytes(
                self.body()[bidx_base + (lex_index as usize - 1) * 4
                    ..bidx_base + (lex_index as usize - 1) * 4 + 4]
                    .try_into()
                    .ok()?,
            ) as usize
        };
        // Git checks both offsets for being out of range *before* comparing them
        // (`load_bloom_filter_from_graph`: two `check_bloom_offset` calls joined by
        // `||`, then the decreasing-offset check). The end offset (at this position)
        // is checked first, short-circuiting the start offset (reported one position
        // back) when it fails.
        let max_payload = payload_len;
        if end_rel > max_payload {
            return None;
        }
        if start_rel > max_payload {
            return None;
        }
        if end_rel < start_rel {
            return None;
        }
        let data_base = bdat_off + BLOOM_HEADER;
        let abs_start = data_base + start_rel;
        let abs_end = data_base + end_rel;
        if abs_end > bdat_off + bdat_total || abs_start > abs_end {
            return None;
        }
        Some(&self.body()[abs_start..abs_end])
    }
}

/// Result of consulting Bloom filters before running a tree diff (matches `revision.c`).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum BloomPrecheck {
    /// No commit-graph, pathspecs disallow Bloom, or wrong parent count — no statistics.
    Inapplicable,
    /// Commit not in graph or generation unavailable — skip Bloom (`-1` without `filter_not_present`).
    NotInGraph,
    /// Bloom filter missing or unusable (`filter_not_present` in Git).
    FilterNotPresent,
    /// Bloom says path cannot be in this commit (`definitely_not`).
    DefinitelyNot,
    /// Bloom says maybe — caller must run diff and may count `false_positive`.
    Maybe,
}

/// Counters for `GIT_TRACE2_PERF` Bloom statistics (`revision.c` `trace2_bloom_filter_statistics_atexit`).
#[derive(Debug, Default, Clone)]
pub struct BloomWalkStats {
    /// Filter not present for this item.
    pub filter_not_present: u32,
    /// Maybe for this item.
    pub maybe: u32,
    /// Definitely not for this item.
    pub definitely_not: u32,
    /// False positive for this item.
    pub false_positive: u32,
}

impl BloomWalkStats {
    /// Record precheck.
    pub fn record_precheck(&mut self, pre: BloomPrecheck) {
        match pre {
            BloomPrecheck::Inapplicable | BloomPrecheck::NotInGraph => {}
            BloomPrecheck::FilterNotPresent => self.filter_not_present += 1,
            BloomPrecheck::DefinitelyNot => self.definitely_not += 1,
            BloomPrecheck::Maybe => self.maybe += 1,
        }
    }

    /// Record false positive.
    pub fn record_false_positive(&mut self) {
        self.false_positive += 1;
    }
}

/// Shared stats handle for [`crate::rev_list::RevListOptions::bloom_stats`].
pub type BloomWalkStatsHandle = Arc<Mutex<BloomWalkStats>>;

/// Generation reported for a commit the chain does not rank (absent, or no generation data).
/// It sorts above every real generation, so history walks expand such commits first.
pub(crate) const UNRANKED_GENERATION: u64 = (1u64 << 63) - 1;

/// A commit as read from the chain by [`CommitGraphChain::graph_walk_entry`].
#[derive(Debug, Clone)]
pub(crate) struct GraphWalkEntry {
    /// Root tree.
    pub(crate) tree: ObjectId,
    /// Parents, in commit order.
    pub(crate) parents: Vec<ObjectId>,
    /// Committer timestamp.
    pub(crate) time: i64,
    /// Corrected commit date, or [`UNRANKED_GENERATION`] when the chain has none.
    pub(crate) generation: u64,
}

/// Loaded commit-graph chain (newest layer first, matching `commit-graph-chain` file order).
#[derive(Debug, Clone)]
pub struct CommitGraphChain {
    layers: Vec<CommitGraphLayer>,
}

/// Test-only constructor (tip-first layer order).
#[doc(hidden)]
pub fn commit_graph_chain_from_layers_for_test(layers: Vec<CommitGraphLayer>) -> CommitGraphChain {
    CommitGraphChain { layers }
}

impl CommitGraphChain {
    /// Parse a complete, self-contained commit-graph file held in memory.
    ///
    /// `bytes` is what Git stores at `objects/info/commit-graph`, for example the output of
    /// [`crate::maintenance::build_commit_graph`] that a custom object store persisted in its
    /// own storage. Attach the result with
    /// [`OdbBuilder::commit_graph`](crate::odb::OdbBuilder::commit_graph) so history walks
    /// read parents, committer dates, generation numbers and changed-path Bloom filters from
    /// it instead of reading commit objects.
    ///
    /// The file is fully verified (trailing checksum, chunk table, object id order, parent
    /// positions). It is not checked against any object store: every commit it lists must
    /// exist in the store it is attached to.
    ///
    /// # Errors
    ///
    /// Returns [`Error::CorruptObject`] when the bytes are not a valid commit-graph file, or
    /// when the file is one layer of a split chain (it names base graphs it depends on).
    pub fn from_bytes(bytes: Vec<u8>) -> Result<Self, Error> {
        if bytes.get(7).is_some_and(|&base_graphs| base_graphs != 0) {
            return Err(Error::CorruptObject(
                "commit-graph layer depends on base graphs".to_owned(),
            ));
        }
        let layer = CommitGraphLayer::try_parse(PathBuf::from("commit-graph"), bytes)?;
        if layer.base_chunk_size != 0 {
            return Err(Error::CorruptObject(
                "commit-graph layer depends on base graphs".to_owned(),
            ));
        }
        Ok(Self {
            layers: vec![layer],
        })
    }

    /// Hash algorithm of the object ids this chain records.
    #[must_use]
    pub fn hash_algo(&self) -> HashAlgo {
        self.layers
            .first()
            .map_or(HashAlgo::Sha1, |layer| layer.algo)
    }

    /// Bloom settings from the newest layer, if that layer carries Bloom data.
    #[must_use]
    pub fn top_layer_bloom_settings(&self) -> Option<BloomFilterSettings> {
        self.layers.first()?.bloom_settings
    }

    /// Total commits across all layers (Git `num_commits_in_base` offset for new layers).
    #[must_use]
    pub fn total_commits(&self) -> u32 {
        self.layers.iter().map(|l| l.num_commits).sum()
    }

    /// Layer file paths from oldest base to newest (reverse of chain file order).
    #[must_use]
    pub fn layer_paths_oldest_first(&self) -> Vec<PathBuf> {
        self.layers.iter().rev().map(|l| l.path.clone()).collect()
    }

    /// Number of layers in the chain.
    #[must_use]
    pub fn num_layers(&self) -> usize {
        self.layers.len()
    }

    /// Commit counts per layer, tip-first (layer 0 is the newest tip).
    #[must_use]
    pub fn layer_commit_counts_tip_first(&self) -> Vec<u32> {
        self.layers.iter().map(|l| l.num_commits).collect()
    }

    /// Whether each layer carries a generation-data (GDA2) chunk, tip-first.
    #[must_use]
    pub fn layer_has_generation_data_tip_first(&self) -> Vec<bool> {
        self.layers
            .iter()
            .map(|l| l.chunk_generation_data.is_some())
            .collect()
    }

    /// Layer hex hashes (from `graph-<hash>.graph` file stem), tip-first.
    #[must_use]
    pub fn layer_hashes_tip_first(&self) -> Vec<String> {
        self.layers.iter().map(|l| l.layer_display_id()).collect()
    }

    /// Source object directory each layer was loaded from (its `.git/objects`),
    /// tip-first. Derived from the layer path: `<objdir>/info/commit-graphs/graph-*.graph`
    /// or `<objdir>/info/commit-graph`.
    #[must_use]
    pub fn layer_object_dirs_tip_first(&self) -> Vec<PathBuf> {
        self.layers
            .iter()
            .map(|l| {
                // .../objects/info/commit-graphs/graph-X.graph  -> .../objects
                // .../objects/info/commit-graph                 -> .../objects
                let p = l.path.as_path();
                let info = if p
                    .parent()
                    .and_then(|d| d.file_name())
                    .map(|n| n == "commit-graphs")
                    .unwrap_or(false)
                {
                    p.parent().and_then(|d| d.parent())
                } else {
                    p.parent()
                };
                info.and_then(|d| d.parent())
                    .map(Path::to_path_buf)
                    .unwrap_or_else(|| PathBuf::from("."))
            })
            .collect()
    }

    /// A sub-chain made of the layers at tip-first indices `start..end`
    /// (so `start` becomes the new tip). Used by the writer when only some base
    /// layers are kept after a split merge.
    #[must_use]
    pub fn sub_chain_tip_first(&self, start: usize, end: usize) -> Option<Self> {
        let end = end.min(self.layers.len());
        if start >= end {
            return None;
        }
        Some(Self {
            layers: self.layers[start..end].to_vec(),
        })
    }

    /// All commit OIDs in one layer (by tip-first index), in lexicographic order.
    #[must_use]
    pub fn layer_oids(&self, tip_first_idx: usize) -> Vec<ObjectId> {
        let Some(layer) = self.layers.get(tip_first_idx) else {
            return Vec::new();
        };
        let mut out = Vec::with_capacity(layer.num_commits as usize);
        for i in 0..layer.num_commits {
            if let Some(oid) = layer.oid_at_lex(i) {
                out.push(oid);
            }
        }
        out
    }

    /// Load from `objects/info/commit-graph` or `objects/info/commit-graphs/commit-graph-chain`.
    ///
    /// Returns `Ok(None)` when no commit-graph exists. Corrupt graphs (including invalid GDO2)
    /// return [`Err`].
    pub fn try_load(objects_dir: &Path) -> Result<Option<Self>, Error> {
        Self::try_load_with_caches(objects_dir, None)
    }

    /// Load a commit-graph chain, deduplicating warnings through `caches` when provided.
    pub fn try_load_with_caches(
        objects_dir: &Path,
        caches: Option<&crate::repo_caches::RepoCaches>,
    ) -> Result<Option<Self>, Error> {
        let info = objects_dir.join("info");
        let chain_path = info.join("commit-graphs").join("commit-graph-chain");
        if chain_path.is_file() {
            let algo =
                hash_algo_for_objects_dir(&crate::environment::Environment::empty(), objects_dir);
            let content = std::fs::read_to_string(&chain_path).map_err(Error::from)?;
            let mut layers = Vec::new();
            for line in content.lines() {
                let h = line.trim();
                if !commit_graph_chain_hash_is_valid(h, algo) {
                    continue;
                }
                let graph_path = info.join("commit-graphs").join(format!("graph-{h}.graph"));
                let layer = CommitGraphLayer::try_open(graph_path)?;
                // `add_graph_to_chain`: a layer that declares N base graphs must
                // carry a BASE chunk large enough to hold N hashes. If it is too
                // small, Git warns and refuses to add this layer (and anything
                // above it) to the chain, falling back to the object database for
                // those commits. `layers.len()` here is the number of base layers
                // already loaded below this one.
                let n = layers.len();
                if n > 0 && layer.base_chunk_size / layer.hash_len < n {
                    warn_once_for_base_chunk_too_small(caches, &layer.layer_display_id());
                    break;
                }
                layers.push(layer);
            }
            if layers.is_empty() {
                return Ok(None);
            }
            // The on-disk chain file lists layers base-first (Git order: line 1
            // is the base graph, the last line is the tip). Grit's internal
            // representation is tip-first, so reverse after reading.
            layers.reverse();
            let mut chain = Self { layers };
            chain.validate_bloom_compatibility(caches);
            return Ok(Some(chain));
        }
        let single = info.join("commit-graph");
        if single.is_file() {
            let layer = CommitGraphLayer::try_open(single.clone())?;
            let mut chain = Self {
                layers: vec![layer],
            };
            chain.validate_bloom_compatibility(caches);
            return Ok(Some(chain));
        }
        Ok(None)
    }

    /// Like [`Self::try_load`] but ignores parse errors (returns `None`).
    pub fn load(objects_dir: &Path) -> Option<Self> {
        Self::try_load(objects_dir).ok().flatten()
    }

    /// Full commit-graph verification (checksum, OID order, generation/parent checks).
    ///
    /// Mirrors `git commit-graph verify` — used by fsck, not the hot read path.
    ///
    /// # Errors
    ///
    /// Returns [`Error::CorruptObject`] when any layer fails verification.
    pub fn verify_on_disk(objects_dir: &Path) -> Result<(), Error> {
        let Some(chain) = Self::try_load(objects_dir)? else {
            return Ok(());
        };
        for path in chain.layer_paths_oldest_first() {
            let raw = std::fs::read(&path).map_err(Error::from)?;
            CommitGraphLayer::try_parse(path, raw)?;
        }
        Ok(())
    }

    /// Load a split commit-graph chain that may live in (and reference layers across) an
    /// alternate object directory.
    ///
    /// Git's commit-graph chain is owned by exactly one object directory, but its layer
    /// `graph-<hash>.graph` files may be split between that directory and the alternate(s) it
    /// inherits from. This is the situation created by a local clone whose `info/alternates`
    /// points at a source repository that already has a split chain: a new `commit-graph write
    /// --split` produces a chain file (in the local object dir) that lists the alternate's base
    /// layer hashes followed by the locally-written tip layer.
    ///
    /// The chain file itself is taken from `objects_dir` if present, otherwise from the first
    /// `alt_dirs` entry that has one. Each referenced `graph-<hash>.graph` layer is then resolved
    /// by searching `objects_dir` first and each alternate in turn.
    ///
    /// # Parameters
    /// - `objects_dir`: the primary object directory (e.g. the local `.git/objects`).
    /// - `alt_dirs`: alternate object directories, in search order.
    ///
    /// # Returns
    /// `Ok(None)` when no chain/single graph is found in any directory; `Ok(Some(_))` on success.
    ///
    /// # Errors
    /// Returns an error if a referenced layer file is missing/unreadable or fails to parse.
    pub fn try_load_across(
        objects_dir: &Path,
        alt_dirs: &[PathBuf],
    ) -> Result<Option<Self>, Error> {
        Self::try_load_across_with_caches(objects_dir, alt_dirs, None)
    }

    /// Like [`Self::try_load_across`] but reuses parsed commit-graph caches when provided.
    ///
    /// # Errors
    ///
    /// Returns an error if a referenced layer file is missing, unreadable, or fails to parse.
    pub fn try_load_across_with_caches(
        objects_dir: &Path,
        alt_dirs: &[PathBuf],
        caches: Option<&crate::repo_caches::RepoCaches>,
    ) -> Result<Option<Self>, Error> {
        let layer_dir = |dir: &Path| dir.join("info").join("commit-graphs");
        let resolve_layer = |hash: &str| -> Option<PathBuf> {
            let name = format!("graph-{hash}.graph");
            let local = layer_dir(objects_dir).join(&name);
            if local.is_file() {
                return Some(local);
            }
            alt_dirs
                .iter()
                .map(|d| layer_dir(d).join(&name))
                .find(|p| p.is_file())
        };

        // A split write only chains on an existing *split chain*. Precedence for the chain owner:
        //   1. the local dir's chain file (its layers may live across the alternate),
        //   2. the local dir's single (non-split) `info/commit-graph` (Git migrates it),
        //   3. a *chain file* in an alternate.
        // An alternate's single (non-split) graph file is never used as a base — Git refuses to
        // base a chain on a plain commit-graph file (t5324 "fork and fail to base a chain on a
        // commit-graph file").
        let local_chain = layer_dir(objects_dir).join("commit-graph-chain");
        let local_single = objects_dir.join("info").join("commit-graph");
        let chain_path = if local_chain.is_file() {
            local_chain
        } else if local_single.is_file() {
            return Self::try_load_with_caches(objects_dir, caches);
        } else {
            match alt_dirs
                .iter()
                .map(PathBuf::as_path)
                .find(|d| layer_dir(d).join("commit-graph-chain").is_file())
            {
                Some(owner) => layer_dir(owner).join("commit-graph-chain"),
                None => return Ok(None),
            }
        };

        let algo =
            hash_algo_for_objects_dir(&crate::environment::Environment::empty(), objects_dir);
        let content = std::fs::read_to_string(&chain_path).map_err(Error::from)?;
        let mut layers = Vec::new();
        for line in content.lines() {
            let h = line.trim();
            if !commit_graph_chain_hash_is_valid(h, algo) {
                continue;
            }
            let graph_path = resolve_layer(h).ok_or_else(|| {
                Error::from(std::io::Error::new(
                    std::io::ErrorKind::NotFound,
                    format!("commit-graph layer graph-{h}.graph not found"),
                ))
            })?;
            let layer = CommitGraphLayer::try_open(graph_path)?;
            let n = layers.len();
            if n > 0 && layer.base_chunk_size / layer.hash_len < n {
                warn_once_for_base_chunk_too_small(caches, &layer.layer_display_id());
                break;
            }
            layers.push(layer);
        }
        if layers.is_empty() {
            return Ok(None);
        }
        layers.reverse();
        let mut chain = Self { layers };
        chain.validate_bloom_compatibility(caches);
        Ok(Some(chain))
    }

    fn validate_bloom_compatibility(&mut self, caches: Option<&crate::repo_caches::RepoCaches>) {
        // Git walks the chain from the tip down to the base (`for (; g; g =
        // g->base_graph)` in validate_mixed_bloom_settings), so the *topmost*
        // (tip) layer's Bloom settings become the reference and any
        // incompatible *lower* (base) layer is disabled. `self.layers` is
        // stored tip-first internally, so iterate forward to match.
        let mut ref_settings: Option<BloomFilterSettings> = None;
        for layer in &mut self.layers {
            let Some(bs) = layer.bloom_settings else {
                continue;
            };
            match ref_settings {
                None => ref_settings = Some(bs),
                Some(r) => {
                    if r.hash_version != bs.hash_version
                        || r.num_hashes != bs.num_hashes
                        || r.bits_per_entry != bs.bits_per_entry
                    {
                        let id = layer.layer_display_id();
                        // Git loads the commit-graph chain once per process and caches
                        // it, so the "disabling Bloom filters" warning is emitted at
                        // most once per layer. Grit re-reads the chain from disk several
                        // times within a single command (settings probe, commit set,
                        // filter reuse), so dedupe the warning per layer id to match.
                        warn_once_for_disabled_bloom_layer(caches, &id);
                        layer.disable_bloom();
                    }
                }
            }
        }
    }

    /// Existing changed-path Bloom filter bytes for `oid`, if present in any layer
    /// whose Bloom settings are compatible with `want`. Returns `Some(bytes)` (possibly
    /// empty for an empty/no-change filter) when the filter can be reused verbatim, or
    /// `None` when no compatible filter exists. Used by the writer to backfill / reuse
    /// already-computed filters (Git counts these as `filter_not_computed`).
    pub fn existing_filter_bytes(
        &self,
        oid: &ObjectId,
        want: &BloomFilterSettings,
    ) -> Option<Vec<u8>> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        let layer = &self.layers[layer_idx];
        let settings = layer.bloom_settings.as_ref()?;
        if settings.hash_version != want.hash_version
            || settings.num_hashes != want.num_hashes
            || settings.bits_per_entry != want.bits_per_entry
        {
            return None;
        }
        // Git only reuses a loaded filter when its on-disk length is non-zero
        // (`get_or_compute_bloom_filter`: `if (filter->data && filter->len)`).
        // A zero-length entry means the filter was skipped (over the
        // `--max-new-filters` budget) and must be (re)computed. Empty-diff
        // filters are stored with length 1 (a single zero byte) and so are
        // reused. Truncated-large filters are length 1 (0xff) and reused too.
        match layer.bloom_filter_slice(lex) {
            Some(s) if !s.is_empty() => Some(s.to_vec()),
            _ => None,
        }
    }

    /// Existing non-empty filter bytes for `oid` whose stored `hash_version`
    /// differs from `want.hash_version` but is otherwise compatible (same
    /// `num_hashes`/`bits_per_entry`). Used to detect filters that may be
    /// *upgraded* (relabeled to the new version without recomputation) when the
    /// changed paths contain no high-bit bytes.
    pub fn upgradable_filter_bytes(
        &self,
        oid: &ObjectId,
        want: &BloomFilterSettings,
    ) -> Option<Vec<u8>> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        let layer = &self.layers[layer_idx];
        let settings = layer.bloom_settings.as_ref()?;
        if settings.hash_version == want.hash_version {
            return None;
        }
        if settings.num_hashes != want.num_hashes || settings.bits_per_entry != want.bits_per_entry
        {
            return None;
        }
        match layer.bloom_filter_slice(lex) {
            Some(s) if !s.is_empty() => Some(s.to_vec()),
            _ => None,
        }
    }

    /// Bloom filter settings stored for `oid`'s commit-graph layer, if any.
    pub(crate) fn bloom_settings_for_commit(&self, oid: &ObjectId) -> Option<BloomFilterSettings> {
        let (layer_idx, _) = self.find_commit(oid)?;
        self.layers[layer_idx].bloom_settings
    }

    /// Lexicographic position in the full chain, or `None` if not in any layer.
    pub fn find_commit(&self, oid: &ObjectId) -> Option<(usize, u32)> {
        for (i, layer) in self.layers.iter().enumerate() {
            if let Some(lex) = layer.bsearch_oid(oid) {
                return Some((i, lex));
            }
        }
        None
    }

    /// Global commit-graph position (Git `graph_pos`): base layers first, then newer layers.
    pub fn global_position(&self, oid: &ObjectId) -> Option<u32> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        let below: u32 = self.layers[layer_idx + 1..]
            .iter()
            .map(|l| l.num_commits)
            .sum();
        Some(below + lex)
    }

    /// Inverse of [`Self::global_position`]: the OID at a base-first global
    /// position (used to resolve CDAT parent words to OIDs).
    fn oid_at_global_position(&self, pos: u32) -> Option<ObjectId> {
        // `layers` is tip-first; global positions count base layers first.
        let mut remaining = pos;
        for layer in self.layers.iter().rev() {
            if remaining < layer.num_commits {
                return layer.oid_at_lex(remaining);
            }
            remaining -= layer.num_commits;
        }
        None
    }

    /// Commit parents and committer time read straight from the commit-graph file
    /// (no object decompression). Returns `None` when `oid` is not in the graph.
    pub fn graph_commit(&self, oid: &ObjectId) -> Option<(Vec<ObjectId>, i64)> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        self.graph_commit_at(layer_idx, lex)
    }

    fn graph_commit_at(&self, layer_idx: usize, lex: u32) -> Option<(Vec<ObjectId>, i64)> {
        let layer = &self.layers[layer_idx];
        let (_, _, commit_time) = layer.cdat_record(lex)?;
        let oids = layer
            .parent_global_positions(lex)?
            .into_iter()
            .filter_map(|pos| self.oid_at_global_position(pos))
            .collect();
        Some((oids, commit_time))
    }

    /// Root tree OID for `oid` from commit-graph CDAT (no object read).
    pub(crate) fn graph_commit_tree(&self, oid: &ObjectId) -> Option<ObjectId> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        self.layers[layer_idx].cdat_root_tree(lex)
    }

    /// Tree and parent OIDs for blame when `oid` is indexed in the chain.
    pub(crate) fn graph_commit_meta(&self, oid: &ObjectId) -> Option<(ObjectId, Vec<ObjectId>)> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        let tree = self.layers[layer_idx].cdat_root_tree(lex)?;
        let (parents, _) = self.graph_commit_at(layer_idx, lex)?;
        Some((tree, parents))
    }

    /// Everything a date- or generation-ordered history walk needs for `oid`, from a single
    /// lookup and without decompressing the commit. `None` when `oid` is not in the chain.
    pub(crate) fn graph_walk_entry(&self, oid: &ObjectId) -> Option<GraphWalkEntry> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        let layer = &self.layers[layer_idx];
        let tree = layer.cdat_root_tree(lex)?;
        let (parents, time) = self.graph_commit_at(layer_idx, lex)?;
        Some(GraphWalkEntry {
            tree,
            parents,
            time,
            generation: layer
                .corrected_commit_generation(lex)
                .unwrap_or(UNRANKED_GENERATION),
        })
    }

    /// Generation number for ordering merge-base walks (`2^63-1` when unknown).
    #[must_use]
    pub fn commit_generation_for_walk(&self, oid: &ObjectId) -> u64 {
        let Some((layer_idx, lex)) = self.find_commit(oid) else {
            return UNRANKED_GENERATION;
        };
        self.layers[layer_idx]
            .corrected_commit_generation(lex)
            .unwrap_or(UNRANKED_GENERATION)
    }

    /// Parent commit positions in global graph order (CDAT + EXTRA_EDGES), or `None` if `oid`
    /// is not in the chain.
    #[must_use]
    pub fn parent_global_positions(&self, oid: &ObjectId) -> Option<Vec<u32>> {
        let (layer_idx, lex) = self.find_commit(oid)?;
        self.layers[layer_idx].parent_global_positions(lex)
    }

    /// Count commits reachable from `tips` minus those reachable from `hide`, using only
    /// commit-graph parent links (no object reads). Returns `None` when any tip is missing
    /// from the graph or parent data cannot be decoded from the file.
    #[must_use]
    pub fn count_reachable_commits(
        &self,
        tips: &[ObjectId],
        hide: &[ObjectId],
        first_parent_only: bool,
        shallow: &std::collections::HashSet<ObjectId>,
    ) -> Option<usize> {
        if self.total_commits() == 0 {
            return None;
        }
        let mut seen = self.reachable_positions(tips, first_parent_only, shallow)?;
        if hide.is_empty() {
            return Some(seen.len());
        }
        let hidden = self.reachable_positions(hide, first_parent_only, shallow)?;
        seen.retain(|pos| !hidden.contains(pos));
        Some(seen.len())
    }

    fn reachable_positions(
        &self,
        tips: &[ObjectId],
        first_parent_only: bool,
        shallow: &std::collections::HashSet<ObjectId>,
    ) -> Option<std::collections::HashSet<u32>> {
        let total = self.total_commits();
        let mut stack: Vec<u32> = Vec::new();
        for tip in tips {
            stack.push(self.global_position(tip)?);
        }
        let mut seen = std::collections::HashSet::new();
        while let Some(pos) = stack.pop() {
            if pos >= total || !seen.insert(pos) {
                continue;
            }
            #[cfg(test)]
            crate::merge_base::intersect_walk_test_hooks::record_commit_parent_lookup();
            let oid = self.oid_at_global_position(pos)?;
            let parents = if shallow.contains(&oid) {
                Vec::new()
            } else {
                self.parent_global_positions(&oid)?
            };
            let parents = if first_parent_only {
                parents.into_iter().take(1).collect::<Vec<_>>()
            } else {
                parents
            };
            for p in parents {
                if p >= total {
                    return None;
                }
                stack.push(p);
            }
        }
        Some(seen)
    }

    /// All commit OIDs in the chain (oldest base first, then newer layers).
    pub fn all_oids_in_order(&self) -> Vec<ObjectId> {
        let mut out = Vec::new();
        for layer in self.layers.iter().rev() {
            for i in 0..layer.num_commits {
                if let Some(oid) = layer.oid_at_lex(i) {
                    out.push(oid);
                }
            }
        }
        out
    }

    /// Consult Bloom filters for a single-parent commit before diffing trees.
    pub fn bloom_precheck_for_paths(
        &self,
        _odb: &Odb,
        oid: ObjectId,
        pathspecs: &[String],
        bloom_cwd: Option<&str>,
        requested_hash_version: i32,
        read_changed_paths: bool,
    ) -> std::result::Result<BloomPrecheck, crate::error::Error> {
        if !read_changed_paths {
            return Ok(BloomPrecheck::Inapplicable);
        }
        let Some((layer_idx, lex)) = self.find_commit(&oid) else {
            return Ok(BloomPrecheck::NotInGraph);
        };
        let layer = &self.layers[layer_idx];
        let Some(settings) = layer.bloom_settings.as_ref() else {
            return Ok(BloomPrecheck::FilterNotPresent);
        };
        let effective_version = if requested_hash_version < 0 {
            settings.hash_version as i32
        } else {
            requested_hash_version
        };
        if effective_version != settings.hash_version as i32 {
            return Ok(BloomPrecheck::FilterNotPresent);
        }

        // Git computes the changed-path Bloom filter for every commit (including merges)
        // relative to its first parent, and `rev_compare_tree` consults it for the
        // first-parent comparison only (`nth_parent == 0`). The caller is responsible for
        // restricting the precheck to the first parent, so merge commits are handled here
        // exactly like single-parent commits.
        let filter = match layer.bloom_filter_slice(lex) {
            Some(s) => s,
            None => return Ok(BloomPrecheck::FilterNotPresent),
        };
        if filter.is_empty() {
            return Ok(BloomPrecheck::FilterNotPresent);
        }

        // Git `bloom_filter_contains_vec`: within one pathspec, every prefix key must match;
        // multiple pathspecs are ORed (`revision.c` loop over `bloom_keyvecs_nr`).
        let mut any_pathspec_maybe = false;
        let mut checked_any_keys = false;
        for spec in pathspecs {
            if spec.is_empty() || crate::pathspec::pathspec_is_exclude(spec) {
                continue;
            }
            let Some(norm) = crate::pathspec::bloom_lookup_prefix_with_cwd(spec, bloom_cwd) else {
                continue;
            };
            let keys = bloom_keyvec_for_path(norm.as_str(), settings);
            if keys.is_empty() {
                continue;
            }
            checked_any_keys = true;
            let mut all_keys_maybe = true;
            for key in &keys {
                match bloom_filter_contains(key, filter, settings) {
                    Ok(true) => {}
                    Ok(false) => {
                        all_keys_maybe = false;
                        break;
                    }
                    Err(crate::bloom::BloomFilterInvalid) => {
                        all_keys_maybe = true;
                        break;
                    }
                }
            }
            if all_keys_maybe {
                any_pathspec_maybe = true;
                break;
            }
        }
        if checked_any_keys && !any_pathspec_maybe {
            return Ok(BloomPrecheck::DefinitelyNot);
        }
        Ok(BloomPrecheck::Maybe)
    }

    /// Like [`Self::bloom_precheck_for_paths`], but reuses precomputed Bloom keys for one path.
    pub(crate) fn bloom_definitely_not_for_keys(
        &self,
        oid: ObjectId,
        path_keys: &[Vec<u32>],
    ) -> bool {
        if path_keys.is_empty() {
            return false;
        }
        let Some((layer_idx, lex)) = self.find_commit(&oid) else {
            return false;
        };
        let layer = &self.layers[layer_idx];
        let Some(settings) = layer.bloom_settings.as_ref() else {
            return false;
        };
        let Some(filter) = layer.bloom_filter_slice(lex) else {
            return false;
        };
        if filter.is_empty() {
            return false;
        }
        bloom_definitely_not_from_filter(filter, settings, path_keys)
    }
}

/// Git `bloom_filter_contains_vec` precheck for one path's prefix keys against a filter slice.
pub(crate) fn bloom_definitely_not_from_filter(
    filter: &[u8],
    settings: &crate::bloom::BloomFilterSettings,
    path_keys: &[Vec<u32>],
) -> bool {
    use crate::bloom::bloom_filter_contains;

    for key in path_keys {
        match bloom_filter_contains(key, filter, settings) {
            Ok(false) => return true,
            Ok(true) => {}
            Err(crate::bloom::BloomFilterInvalid) => return false,
        }
    }
    false
}

/// Compute changed paths between parent and commit trees (recursive diff, no rename detection).
pub fn diff_changed_paths_for_bloom(
    odb: &Odb,
    parent_tree: Option<ObjectId>,
    commit_tree: ObjectId,
) -> crate::error::Result<(Vec<String>, usize)> {
    use crate::diff::diff_trees;
    let entries = diff_trees(odb, parent_tree.as_ref(), Some(&commit_tree), "")?;
    let raw_len = entries.len();
    let mut paths = Vec::new();
    for e in entries {
        let p = e.path().to_string();
        if !p.is_empty() {
            paths.push(p);
        }
    }
    Ok((paths, raw_len))
}

/// Re-export for `commit-graph` write.
pub use crate::bloom::collect_changed_paths_for_bloom;

/// Build Bloom filter bytes for one commit; returns cumulative size contribution for BIDX.
pub fn bloom_filter_for_commit_write(
    odb: &Odb,
    parents: &[ObjectId],
    tree_oid: ObjectId,
    settings: &BloomFilterSettings,
) -> crate::error::Result<(Vec<u8>, BloomBuildOutcome)> {
    // Git computes the changed-path filter against the *first* parent only,
    // regardless of how many parents a commit has (see bloom.c:
    // `diff_tree_oid(&c->parents->item->object.oid, ...)`).
    let (changed_paths_vec, raw_count) = if let Some(first_parent) = parents.first() {
        let p = odb.commit_tree(first_parent)?;
        diff_changed_paths_for_bloom(odb, Some(p), tree_oid)?
    } else {
        diff_changed_paths_for_bloom(odb, None, tree_oid)?
    };
    let set = collect_changed_paths_for_bloom(&changed_paths_vec);
    Ok(crate::bloom::build_bloom_filter_data(
        &set, raw_count, settings,
    ))
}

/// Whether any path component in `tree` (recursively) contains a byte with the
/// high bit set (`& 0x80`). Mirrors `bloom.c:has_entries_with_high_bit`, used to
/// decide whether a v1 changed-path Bloom filter can be relabeled as v2 without
/// recomputation (v1/v2 hashing only differs for bytes >= 0x80).
fn tree_has_high_bit_paths(odb: &Odb, tree_oid: ObjectId) -> bool {
    let Ok(obj) = odb.read(&tree_oid) else {
        // Git treats an unreadable tree conservatively (no upgrade).
        return true;
    };
    let Ok(entries) = crate::objects::parse_tree(&obj.data) else {
        return true;
    };
    for e in &entries {
        if e.name.iter().any(|&b| b & 0x80 != 0) {
            return true;
        }
        if e.mode == 0o040000 && tree_has_high_bit_paths(odb, e.oid) {
            return true;
        }
    }
    false
}

/// Whether a commit's tree (recursively) has any high-bit path bytes
/// (`bloom.c:commit_tree_has_high_bit_paths`).
pub fn commit_tree_has_high_bit_paths(odb: &Odb, commit_oid: ObjectId) -> bool {
    match odb.commit_tree(&commit_oid) {
        Ok(tree) => tree_has_high_bit_paths(odb, tree),
        Err(_) => true,
    }
}

/// Parse all chunks for `test-tool read-graph` / debugging.
pub fn parse_graph_file(path: &Path) -> Option<ParsedGraphDump> {
    let raw = std::fs::read(path).ok()?;
    if raw.len() < 28 {
        return None;
    }
    let hash_len = commit_graph_hash_len(raw[5])?;
    let body = &raw[..raw.len() - hash_len];
    if body.len() < 8 || &body[0..4] != SIGNATURE {
        return None;
    }
    let header_word = u32::from_be_bytes(body[0..4].try_into().ok()?);
    let num_chunks = body[6] as usize;
    let toc_start = 8;
    let mut present: std::collections::HashSet<u32> = std::collections::HashSet::new();
    for i in 0..num_chunks {
        let e = toc_start + i * 12;
        let id = u32::from_be_bytes(body[e..e + 4].try_into().ok()?);
        present.insert(id);
    }
    // `git/t/helper/test-read-graph.c` prints a fixed set of recognized chunks in
    // a fixed order, omitting the BASE chunk and any unknown chunk.
    let mut chunk_names: Vec<String> = Vec::new();
    for (id, label) in [
        (CHUNK_OID_FANOUT, "oid_fanout"),
        (CHUNK_OID_LOOKUP, "oid_lookup"),
        (CHUNK_COMMIT_DATA, "commit_metadata"),
        (CHUNK_GENERATION_DATA, "generation_data"),
        (CHUNK_GENERATION_DATA_OVERFLOW, "generation_data_overflow"),
        (CHUNK_EXTRA_EDGES, "extra_edges"),
        (CHUNK_BLOOM_INDEXES, "bloom_indexes"),
        (CHUNK_BLOOM_DATA, "bloom_data"),
    ] {
        if present.contains(&id) {
            chunk_names.push(label.to_string());
        }
    }
    let layer = CommitGraphLayer::parse(path.to_path_buf(), raw.clone())?;
    let bloom_opt = layer.bloom_settings.map(|s| {
        format!(
            " bloom({},{},{})",
            s.hash_version, s.bits_per_entry, s.num_hashes
        )
    });
    let mut options = String::new();
    if let Some(b) = bloom_opt {
        options.push_str(&b);
    }
    if layer.read_generation_data {
        options.push_str(" read_generation_data");
    }
    Some(ParsedGraphDump {
        header_word,
        version: body[4],
        hash_ver: body[5],
        num_chunks: body[6],
        reserved: body[7],
        num_commits: layer.num_commits,
        chunks: chunk_names.join(" "),
        options,
    })
}

/// Parsed commit-graph header fields for debugging and `--dump` style output.
pub struct ParsedGraphDump {
    /// Commit-graph file signature word (`0x43475048`).
    pub header_word: u32,
    /// Commit-graph format version byte.
    pub version: u8,
    /// Object hash version used in the graph (`1` = SHA-1, `2` = SHA-256).
    pub hash_ver: u8,
    /// Number of chunk types present in the file.
    pub num_chunks: u8,
    /// Reserved header byte (must be zero in current formats).
    pub reserved: u8,
    /// Number of commits encoded in the graph.
    pub num_commits: u32,
    /// Space-separated chunk id names in file order.
    pub chunks: String,
    /// Human-readable graph option flags from the header.
    pub options: String,
}

/// Dump hex lines of Bloom filters (one per commit, empty line for empty filter).
pub fn dump_bloom_filters(path: &Path) -> Option<Vec<String>> {
    let raw = std::fs::read(path).ok()?;
    let layer = CommitGraphLayer::parse(path.to_path_buf(), raw)?;
    let mut out = Vec::new();
    for i in 0..layer.num_commits {
        let slice = layer.bloom_filter_slice(i).unwrap_or(&[]);
        if slice.is_empty() {
            out.push(String::new());
        } else {
            let hex: String = slice.iter().map(|b| format!("{b:02x}")).collect();
            out.push(hex);
        }
    }
    Some(out)
}

#[cfg(test)]
mod tests {
    use std::collections::HashMap;

    use crate::bloom::BloomFilterSettings;
    use crate::commit_graph_write::{build_commit_graph_bytes, load_commit_graph_commit_info};
    use crate::index::{Index, IndexEntry, MODE_REGULAR};
    use crate::objects::{serialize_commit, CommitData, ObjectId, ObjectKind};
    use crate::repo::{init_repository, Repository};
    use crate::write_tree::write_tree_from_index;

    use crate::commit_graph_write::CommitGraphCommitInfo;
    use crate::objects::OidHashMap;

    fn infos_std(
        infos: &OidHashMap<CommitGraphCommitInfo>,
    ) -> HashMap<ObjectId, CommitGraphCommitInfo> {
        infos.iter().map(|(k, v)| (*k, v.clone())).collect()
    }

    fn empty_byte_filters() -> HashMap<ObjectId, Vec<u8>> {
        HashMap::new()
    }

    fn one_commit_repo() -> (tempfile::TempDir, Repository, ObjectId) {
        let dir = tempfile::tempdir().expect("tempdir");
        let repo = init_repository(
            dir.path(),
            false,
            "main",
            None,
            crate::RefStorageFormat::Files,
        )
        .expect("init");
        let mut index = Index::new();
        index.hash_algo = repo.odb().hash_algo();
        let blob = repo.odb().write(ObjectKind::Blob, b"x\n").expect("blob");
        index.add_or_replace(IndexEntry {
            ctime_sec: 0,
            ctime_nsec: 0,
            mtime_sec: 0,
            mtime_nsec: 0,
            dev: 0,
            ino: 0,
            mode: MODE_REGULAR,
            uid: 0,
            gid: 0,
            size: 2,
            oid: blob,
            flags: 4,
            flags_extended: None,
            path: b"f.txt".to_vec(),
            base_index_pos: 0,
        });
        let tree = write_tree_from_index(&repo.odb(), &index, "").expect("tree");
        let raw = serialize_commit(&CommitData {
            tree,
            parents: vec![],
            author: "T <t@example.com> 100 +0000".into(),
            committer: "T <t@example.com> 100 +0000".into(),
            author_raw: Vec::new(),
            committer_raw: Vec::new(),
            encoding: None,
            message: "m\n".into(),
            raw_message: None,
            extra_headers: Vec::new(),
        });
        let commit = repo.odb().write(ObjectKind::Commit, &raw).expect("commit");
        crate::refs::write_ref(&repo.git_dir(), "refs/heads/main", &commit).expect("ref");
        (dir, repo, commit)
    }

    fn reseal_graph(bytes: &mut Vec<u8>, algo: crate::objects::HashAlgo) {
        let hash_len = algo.len();
        let body_len = bytes.len().saturating_sub(hash_len);
        let digest = algo.checksum_digest(&bytes[..body_len]);
        bytes.truncate(body_len);
        bytes.extend_from_slice(&digest);
    }

    fn memory_primary_over(
        dir: &tempfile::TempDir,
        files: &Repository,
        graph: Option<std::sync::Arc<super::CommitGraphChain>>,
    ) -> Repository {
        use crate::odb::store::{MemoryStore, ObjectStore, WritableObjectStore};
        let store = std::sync::Arc::new(MemoryStore::new(files.odb().hash_algo()));
        let source = files.odb().primary().expect("files primary");
        let mut oids = Vec::new();
        source
            .for_each_object(&mut |oid| {
                oids.push(*oid);
                std::ops::ControlFlow::Continue(())
            })
            .expect("list objects");
        for oid in oids {
            let obj = source.read(&oid).expect("read").expect("present");
            store
                .write(obj.kind, &obj.data, crate::odb::WriteOptions::default())
                .expect("copy");
        }
        let git_dir = dir.path().join(".git");
        let mut builder = crate::odb::OdbBuilder::files(git_dir.join("objects"))
            .primary(store)
            .alternates(false);
        if let Some(graph) = graph {
            builder = builder.commit_graph(graph);
        }
        crate::repo::RepositoryBuilder::new(crate::environment::RepositoryOptions::empty())
            .odb_builder(builder)
            .open(&git_dir, Some(dir.path()))
            .expect("open custom")
    }

    #[test]
    fn from_bytes_parses_a_written_graph_and_rejects_bad_files() {
        let (_dir, repo, commit) = one_commit_repo();
        repo.write_commit_graph(&crate::maintenance::CommitGraphOptions::default())
            .expect("write graph");
        let bytes =
            std::fs::read(repo.odb().objects_dir().join("info/commit-graph")).expect("graph file");

        let chain = super::CommitGraphChain::from_bytes(bytes.clone()).expect("parse");
        assert_eq!(chain.total_commits(), 1);
        assert_eq!(chain.hash_algo(), crate::objects::HashAlgo::Sha1);
        assert_eq!(chain.graph_commit(&commit).map(|(p, _)| p), Some(vec![]));

        let mut flipped = bytes.clone();
        let middle = flipped.len() / 2;
        flipped[middle] ^= 0xff;
        assert!(matches!(
            super::CommitGraphChain::from_bytes(flipped),
            Err(crate::error::Error::CorruptObject(_))
        ));
        assert!(super::CommitGraphChain::from_bytes(bytes[..20].to_vec()).is_err());
        assert!(super::CommitGraphChain::from_bytes(Vec::new()).is_err());

        let mut split_layer = bytes;
        split_layer[7] = 1;
        reseal_graph(&mut split_layer, crate::objects::HashAlgo::Sha1);
        assert!(matches!(
            super::CommitGraphChain::from_bytes(split_layer),
            Err(crate::error::Error::CorruptObject(_))
        ));
    }

    #[test]
    fn from_bytes_rejects_octopus_parents_without_an_edge_chunk() {
        let (_dir, repo, root) = one_commit_repo();
        let tree = repo.odb().read_commit(&root).expect("root commit").tree;
        let commit = |parents: Vec<ObjectId>, message: &str| {
            let raw = serialize_commit(&CommitData {
                tree,
                parents,
                author: "T <t@example.com> 200 +0000".into(),
                committer: "T <t@example.com> 200 +0000".into(),
                author_raw: Vec::new(),
                committer_raw: Vec::new(),
                encoding: None,
                message: message.into(),
                raw_message: None,
                extra_headers: Vec::new(),
            });
            repo.odb().write(ObjectKind::Commit, &raw).expect("commit")
        };
        let side_a = commit(vec![], "a\n");
        let side_b = commit(vec![], "b\n");
        let octopus = commit(vec![root, side_a, side_b], "octopus\n");
        crate::refs::write_ref(&repo.git_dir(), "refs/heads/main", &octopus).expect("ref");
        repo.write_commit_graph(&crate::maintenance::CommitGraphOptions::default())
            .expect("write graph");
        let bytes =
            std::fs::read(repo.odb().objects_dir().join("info/commit-graph")).expect("graph file");
        let chain = super::CommitGraphChain::from_bytes(bytes.clone()).expect("valid graph");
        assert_eq!(chain.total_commits(), 4);
        assert_eq!(
            chain
                .graph_commit(&octopus)
                .map(|(parents, _)| parents.len()),
            Some(3)
        );

        let mut no_edge_chunk = bytes;
        let edge_entry = (0..no_edge_chunk[6] as usize)
            .map(|i| 8 + i * 12)
            .find(|&e| no_edge_chunk[e..e + 4] == super::CHUNK_EXTRA_EDGES.to_be_bytes())
            .expect("octopus graph has an EDGE chunk");
        no_edge_chunk[edge_entry..edge_entry + 4].copy_from_slice(b"EDGX");
        reseal_graph(&mut no_edge_chunk, crate::objects::HashAlgo::Sha1);
        assert!(matches!(
            super::CommitGraphChain::from_bytes(no_edge_chunk),
            Err(crate::error::Error::CorruptObject(_))
        ));
    }

    #[test]
    fn custom_primary_ignores_graph_files_and_uses_the_attached_graph() {
        let (dir, files, commit) = one_commit_repo();
        files
            .write_commit_graph(&crate::maintenance::CommitGraphOptions::default())
            .expect("write graph");
        assert!(files.odb().commit_graph_chain().is_some());

        let custom = memory_primary_over(&dir, &files, None);
        assert!(
            custom.odb().commit_graph_chain().is_none(),
            "a graph file under objects/ does not describe a custom store"
        );

        let crate::maintenance::CommitGraphBuild::Built(built) = custom
            .build_commit_graph(&crate::maintenance::CommitGraphOptions::default())
            .expect("build")
        else {
            panic!("plain repository must not skip the graph");
        };
        assert_eq!(built.commit_count, 1);
        assert_eq!(built.bloom_stats, None);
        assert_eq!(
            built.bytes,
            std::fs::read(files.odb().objects_dir().join("info/commit-graph")).expect("file"),
            "the in-memory build matches the files writer byte for byte"
        );

        let chain = std::sync::Arc::new(built.to_chain().expect("chain"));
        let attached = memory_primary_over(&dir, &files, Some(std::sync::Arc::clone(&chain)));
        let used = attached.odb().commit_graph_chain().expect("attached graph");
        assert!(std::sync::Arc::ptr_eq(&used, &chain));
        assert!(used.graph_commit(&commit).is_some());
    }

    #[test]
    fn try_parse_rejects_bad_signature_and_truncated_toc() {
        let (_dir, repo, commit) = one_commit_repo();
        let info = load_commit_graph_commit_info(&repo.odb(), commit).expect("info");
        let mut infos = crate::objects::oid_hash_map();
        infos.insert(commit, info);
        let bloom = BloomFilterSettings::default();
        let (mut bytes, _) = build_commit_graph_bytes(
            &[commit],
            &infos_std(&infos),
            &repo.odb(),
            false,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        let algo = repo.odb().hash_algo();
        bytes[0] = 0;
        reseal_graph(&mut bytes, algo);
        let err = super::CommitGraphLayer::try_parse(std::path::PathBuf::from("g"), bytes.clone())
            .unwrap_err();
        assert!(matches!(err, crate::error::Error::CorruptObject(_)));
        bytes.truncate(40);
        let err =
            super::CommitGraphLayer::try_parse(std::path::PathBuf::from("g"), bytes).unwrap_err();
        assert!(matches!(err, crate::error::Error::CorruptObject(_)));
        let (mut good, _) = build_commit_graph_bytes(
            &[commit],
            &infos_std(&infos),
            &repo.odb(),
            false,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        good[4] = 2;
        reseal_graph(&mut good, algo);
        let err =
            super::CommitGraphLayer::try_parse(std::path::PathBuf::from("g"), good).unwrap_err();
        assert!(matches!(err, crate::error::Error::CorruptObject(_)));
        let (mut checksum_bad, _) = build_commit_graph_bytes(
            &[commit],
            &infos_std(&infos),
            &repo.odb(),
            false,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        let last = checksum_bad.len() - 1;
        checksum_bad[last] ^= 0xff;
        let err = super::CommitGraphLayer::try_parse(std::path::PathBuf::from("g"), checksum_bad)
            .unwrap_err();
        assert!(matches!(err, crate::error::Error::CorruptObject(_)));
    }

    #[test]
    fn parse_graph_file_returns_none_for_invalid_inputs() {
        let dir = tempfile::tempdir().expect("tempdir");
        let missing = dir.path().join("nope.graph");
        assert!(super::parse_graph_file(&missing).is_none());
        std::fs::write(dir.path().join("tiny.graph"), b"CGPH").expect("write");
        assert!(super::parse_graph_file(&dir.path().join("tiny.graph")).is_none());
        assert!(super::dump_bloom_filters(&missing).is_none());
    }

    #[test]
    fn try_load_missing_graph_is_none() {
        let dir = tempfile::tempdir().expect("tempdir");
        let objects = dir.path().join("objects");
        std::fs::create_dir_all(objects.join("info")).expect("info");
        assert!(super::CommitGraphChain::try_load(&objects)
            .expect("ok")
            .is_none());
    }

    #[test]
    fn try_load_across_finds_single_graph() {
        let (_dir, repo, _commit) = one_commit_repo();
        let objects = repo.odb().objects_dir();
        let sorted = vec![_commit];
        let info = load_commit_graph_commit_info(&repo.odb(), _commit).expect("info");
        let mut infos = crate::objects::oid_hash_map();
        infos.insert(_commit, info);
        let bloom = BloomFilterSettings::default();
        let (bytes, _) = build_commit_graph_bytes(
            &sorted,
            &infos_std(&infos),
            &repo.odb(),
            false,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        std::fs::write(objects.join("info/commit-graph"), bytes).expect("write");
        let chain = super::CommitGraphChain::try_load_across(objects, &[])
            .expect("across")
            .expect("some");
        assert_eq!(chain.total_commits(), 1);
    }

    #[test]
    fn try_parse_rejects_oidf_toc_offset_out_of_range() {
        let (_dir, repo, commit) = one_commit_repo();
        let info = load_commit_graph_commit_info(&repo.odb(), commit).expect("info");
        let mut infos = crate::objects::oid_hash_map();
        infos.insert(commit, info);
        let bloom = BloomFilterSettings::default();
        let (mut bytes, _) = build_commit_graph_bytes(
            &[commit],
            &infos_std(&infos),
            &repo.odb(),
            false,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        let algo = repo.odb().hash_algo();
        let num_chunks = bytes[6] as usize;
        for i in 0..num_chunks {
            let e = 8 + i * 12;
            let id = u32::from_be_bytes(bytes[e..e + 4].try_into().unwrap());
            if id == 0x4f49_4446 {
                bytes[e + 4..e + 12].copy_from_slice(&u64::MAX.to_be_bytes());
                break;
            }
        }
        reseal_graph(&mut bytes, algo);
        let err =
            super::CommitGraphLayer::try_parse(std::path::PathBuf::from("g"), bytes).unwrap_err();
        assert!(matches!(err, crate::error::Error::CorruptObject(_)));
    }

    #[test]
    fn generation_overflow_without_gdo2_fails_parse() {
        let (_dir, repo, commit) = one_commit_repo();
        let info = load_commit_graph_commit_info(&repo.odb(), commit).expect("info");
        let mut infos = crate::objects::oid_hash_map();
        infos.insert(commit, info);
        let bloom = BloomFilterSettings::default();
        let (mut bytes, _) = build_commit_graph_bytes(
            &[commit],
            &infos_std(&infos),
            &repo.odb(),
            false,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        if bytes.len() > 100 {
            for i in 80..120 {
                if i + 4 <= bytes.len() - 20 {
                    bytes[i] = 0x80;
                    bytes[i + 1] = 0x00;
                    bytes[i + 2] = 0x00;
                    bytes[i + 3] = 0x00;
                    break;
                }
            }
        }
        let err =
            super::CommitGraphLayer::try_parse(std::path::PathBuf::from("g"), bytes).unwrap_err();
        assert!(matches!(err, crate::error::Error::CorruptObject(_)));
    }

    #[test]
    fn bloom_precheck_not_in_graph_for_missing_oid() {
        let (_dir, repo, commit) = one_commit_repo();
        let info = load_commit_graph_commit_info(&repo.odb(), commit).expect("info");
        let mut infos = crate::objects::oid_hash_map();
        infos.insert(commit, info);
        let bloom = BloomFilterSettings::default();
        let (bytes, _) = build_commit_graph_bytes(
            &[commit],
            &infos_std(&infos),
            &repo.odb(),
            true,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        std::fs::write(repo.odb().objects_dir().join("info/commit-graph"), bytes).expect("write");
        let chain = super::CommitGraphChain::load(repo.odb().objects_dir()).expect("chain");
        let missing = ObjectId::from_hex("1234567890123456789012345678901234567890").expect("oid");
        let pre = chain
            .bloom_precheck_for_paths(&repo.odb(), missing, &[], None, -1, true)
            .expect("precheck");
        assert_eq!(pre, super::BloomPrecheck::NotInGraph);
    }

    #[test]
    fn bloom_precheck_respects_read_changed_paths_flag() {
        let (_dir, repo, commit) = one_commit_repo();
        let mut sorted = vec![commit];
        let info = load_commit_graph_commit_info(&repo.odb(), commit).expect("info");
        let mut infos = crate::objects::oid_hash_map();
        infos.insert(commit, info);
        let bloom = BloomFilterSettings::default();
        let (bytes, _) = build_commit_graph_bytes(
            &sorted,
            &infos_std(&infos),
            &repo.odb(),
            true,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("write");
        std::fs::write(repo.odb().objects_dir().join("info/commit-graph"), bytes).expect("write");
        let chain = super::CommitGraphChain::load(repo.odb().objects_dir()).expect("chain");
        let pre = chain
            .bloom_precheck_for_paths(&repo.odb(), commit, &["f.txt".to_owned()], None, -1, false)
            .expect("precheck");
        assert_eq!(pre, super::BloomPrecheck::Inapplicable);
        sorted.clear();
    }

    #[test]
    fn bloom_skips_most_steps_when_tracked_path_unchanged_in_linear_history() {
        use crate::bloom::bloom_keyvec_for_path;
        use crate::commit_graph_write::{build_commit_graph_bytes, load_commit_graph_commit_info};
        use crate::index::{Index, IndexEntry, MODE_REGULAR};
        use crate::objects::{serialize_commit, CommitData, ObjectKind};
        use crate::pathspec::bloom_lookup_prefix_with_cwd;
        use crate::repo::init_repository;
        use crate::write_tree::write_tree_from_index;
        use crate::RefStorageFormat;
        use std::fs;

        let dir = tempfile::tempdir().expect("tempdir");
        let repo = init_repository(dir.path(), false, "main", None, RefStorageFormat::Files)
            .expect("init");
        let path = "tracked.txt";
        let stable = b"stable payload\n";
        let blob = repo.odb().write(ObjectKind::Blob, stable).expect("blob");
        let mut parent: Option<ObjectId> = None;
        let mut oids = Vec::new();
        for i in 0..24usize {
            let mut index = Index::new();
            index.hash_algo = repo.odb().hash_algo();
            index.add_or_replace(IndexEntry {
                ctime_sec: 0,
                ctime_nsec: 0,
                mtime_sec: 0,
                mtime_nsec: 0,
                dev: 0,
                ino: 0,
                mode: MODE_REGULAR,
                uid: 0,
                gid: 0,
                size: stable.len() as u32,
                oid: blob,
                flags: 4,
                flags_extended: None,
                path: path.as_bytes().to_vec(),
                base_index_pos: 0,
            });
            if i % 5 == 3 {
                let noise = repo
                    .odb()
                    .write(ObjectKind::Blob, format!("noise {i}\n").as_bytes())
                    .expect("noise blob");
                index.add_or_replace(IndexEntry {
                    ctime_sec: 0,
                    ctime_nsec: 0,
                    mtime_sec: 0,
                    mtime_nsec: 0,
                    dev: 0,
                    ino: 0,
                    mode: MODE_REGULAR,
                    uid: 0,
                    gid: 0,
                    size: 8,
                    oid: noise,
                    flags: 4,
                    flags_extended: None,
                    path: format!("other{i}.txt").into_bytes(),
                    base_index_pos: 0,
                });
            }
            let tree = write_tree_from_index(&repo.odb(), &index, "").expect("tree");
            let mut parents = Vec::new();
            if let Some(p) = parent {
                parents.push(p);
            }
            let raw = serialize_commit(&CommitData {
                tree,
                parents,
                author: "T <t@example.com> 100 +0000".into(),
                committer: "T <t@example.com> 100 +0000".into(),
                author_raw: Vec::new(),
                committer_raw: Vec::new(),
                encoding: None,
                message: format!("c{i}\n"),
                raw_message: None,
                extra_headers: Vec::new(),
            });
            let commit = repo.odb().write(ObjectKind::Commit, &raw).expect("commit");
            oids.push(commit);
            parent = Some(commit);
        }
        let head = *oids.last().expect("head");
        let mut infos = crate::objects::oid_hash_map();
        for oid in &oids {
            infos.insert(
                *oid,
                load_commit_graph_commit_info(&repo.odb(), *oid).expect("info"),
            );
        }
        let bloom = BloomFilterSettings::default();
        let (bytes, _) = build_commit_graph_bytes(
            &oids,
            &infos_std(&infos),
            &repo.odb(),
            true,
            &bloom,
            None,
            &[],
            None,
            &empty_byte_filters(),
            &empty_byte_filters(),
            true,
        )
        .expect("graph bytes");
        fs::write(repo.odb().objects_dir().join("info/commit-graph"), bytes).expect("write graph");
        let chain = super::CommitGraphChain::load(repo.odb().objects_dir()).expect("chain");
        let settings = chain.top_layer_bloom_settings().expect("bloom");
        let norm = bloom_lookup_prefix_with_cwd(path, None).expect("prefix");
        let keys = bloom_keyvec_for_path(norm.as_str(), &settings);
        let mut current = head;
        let mut bloom_yes = 0usize;
        let mut bloom_no = 0usize;
        while bloom_yes + bloom_no < 10_000 {
            let Some((parents, _)) = chain.graph_commit(&current) else {
                break;
            };
            let Some(parent) = parents.first().copied() else {
                break;
            };
            if chain.bloom_definitely_not_for_keys(current, keys.as_slice()) {
                bloom_yes += 1;
            } else {
                bloom_no += 1;
            }
            current = parent;
        }
        assert!(
            bloom_yes > bloom_no,
            "expected bloom to skip unchanged tracked path (yes={bloom_yes} no={bloom_no})"
        );
    }

    #[test]
    #[ignore = "local git.git worktree; GRIT_BLAME_BENCH_REPO"]
    fn first_parent_makefile_bloom_skip_rate_on_git_dot_git() {
        use crate::bloom::bloom_keyvec_for_path;
        use crate::environment::RepositoryOptions;
        use crate::pathspec::bloom_lookup_prefix_with_cwd;
        use crate::repo::Repository;

        let repo_path = std::env::var("GRIT_BLAME_BENCH_REPO")
            .unwrap_or_else(|_| "/tmp/grit-bench/git-dot-git-worktree".to_string());
        if !std::path::Path::new(&repo_path).join(".git").exists() {
            return;
        }
        let git_dir = std::path::Path::new(&repo_path).join(".git");
        let repo =
            Repository::open(git_dir.as_path(), &RepositoryOptions::default()).expect("open");
        let chain = repo.odb().commit_graph_chain().expect("graph");
        let settings = chain.top_layer_bloom_settings().expect("bloom");
        let norm = bloom_lookup_prefix_with_cwd("Makefile", None).expect("prefix");
        let keys = bloom_keyvec_for_path(norm.as_str(), &settings);
        let head = *crate::state::resolve_head(repo.git_dir())
            .expect("head")
            .oid()
            .expect("commit");
        let mut current = head;
        let mut bloom_yes = 0usize;
        let mut bloom_no = 0usize;
        for _ in 0..10_000 {
            let Some((parents, _)) = chain.graph_commit(&current) else {
                break;
            };
            let Some(parent) = parents.first().copied() else {
                break;
            };
            if chain.bloom_definitely_not_for_keys(current, keys.as_slice()) {
                bloom_yes += 1;
            } else {
                bloom_no += 1;
            }
            current = parent;
        }
        assert!(
            bloom_yes > bloom_no,
            "expected mostly bloom skips, yes={bloom_yes} no={bloom_no}"
        );
    }

    #[test]
    fn bloom_definitely_not_when_directory_key_present_but_not_full_path() {
        use crate::bloom::{
            bloom_filter_contains, bloom_keyvec_for_path, build_bloom_filter_data,
            collect_changed_paths_for_bloom,
        };

        let bloom = BloomFilterSettings::default();
        let changed = collect_changed_paths_for_bloom(&["builtin/sibling.c".to_owned()]);
        let (filter, _) = build_bloom_filter_data(&changed, 1, &bloom);
        let path_keys = bloom_keyvec_for_path("builtin/commit.c", &bloom);
        assert!(path_keys.len() >= 2);
        assert_eq!(
            bloom_filter_contains(&path_keys[0], &filter, &bloom),
            Ok(false),
            "full path key must be absent when only a sibling changed"
        );
        assert_eq!(
            bloom_filter_contains(&path_keys[1], &filter, &bloom),
            Ok(true),
            "directory prefix key should be present"
        );
        assert!(super::bloom_definitely_not_from_filter(
            &filter, &bloom, &path_keys
        ));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn large_git_split_chain_layers_use_mapped_backing() {
        use std::process::Command;

        let git_ok = Command::new("git")
            .arg("--version")
            .output()
            .map(|o| o.status.success())
            .unwrap_or(false);
        if !git_ok {
            eprintln!("SKIP: git unavailable");
            return;
        }
        let dir = tempfile::tempdir().expect("tempdir");
        let run = |args: &[&str]| {
            let out = Command::new("git")
                .current_dir(dir.path())
                .args(args)
                .env("GIT_CONFIG_GLOBAL", "/dev/null")
                .env("GIT_CONFIG_SYSTEM", "/dev/null")
                .env("GIT_AUTHOR_NAME", "T")
                .env("GIT_AUTHOR_EMAIL", "t@example.com")
                .env("GIT_COMMITTER_NAME", "T")
                .env("GIT_COMMITTER_EMAIL", "t@example.com")
                .output()
                .expect("git");
            assert!(out.status.success(), "git {args:?}");
        };
        run(&["init", "-q", "-b", "main"]);
        run(&["commit", "--allow-empty", "-q", "-m", "bootstrap"]);
        for i in 0..150 {
            run(&[
                "commit",
                "--allow-empty",
                "-q",
                "-m",
                &format!("base-fill-{i}"),
            ]);
        }
        run(&[
            "commit-graph",
            "write",
            "--reachable",
            "--split",
            "--max-commits",
            "40",
        ]);
        for i in 150..190 {
            run(&[
                "commit",
                "--allow-empty",
                "-q",
                "-m",
                &format!("tip-fill-{i}"),
            ]);
        }
        run(&[
            "commit-graph",
            "write",
            "--reachable",
            "--split",
            "--max-commits",
            "40",
        ]);
        let objects = dir.path().join(".git/objects");
        let chain = super::CommitGraphChain::try_load(&objects)
            .expect("load")
            .expect("chain");
        assert!(
            chain.num_layers() >= 2,
            "expected multi-layer chain, got {}",
            chain.num_layers()
        );
        let mut saw_mapped_large_layer = false;
        for path in chain.layer_paths_oldest_first() {
            let len = std::fs::metadata(&path).expect("meta").len();
            if len <= 8192 {
                continue;
            }
            let storage = crate::pack_map::PackData::open_with_min_len(
                &path,
                super::COMMIT_GRAPH_MIN_FILE_LEN,
            )
            .expect("open layer");
            assert!(
                storage.is_mapped_backing(),
                "expected mmap backing for {} ({} bytes)",
                path.display(),
                len
            );
            saw_mapped_large_layer = true;
        }
        assert!(
            saw_mapped_large_layer,
            "expected at least one chain layer larger than 8192 bytes to use mmap"
        );
    }
}