annotate rust/hg-core/src/revlog/index.rs @ 51229:1b23aaf5eb7b

rust-index: optimize find_gca_candidates() on less than 8 revisions

This is expected to be by far the most common case, given that, e.g.,
merging involves using it on two revisions.

Using a `u8` as support for the bitset obviously divides the amount of
RAM needed by 8. To state the obvious, on a repository with 10 million
changesets, this spares 70MB. It is also possible that it'd be slightly
faster, because it is easier to allocate and provides better cache locality.

It is possible that some exhaustive listing of the traits implemented by
`u8` and `u64` would avoid the added duplication, but that can be done later
and would need a replacement for the `MAX` consts.

use std::collections::hash_map::RandomState;
use std::collections::{HashMap, HashSet};
use std::fmt::Debug;
use std::ops::Deref;
use std::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};

use byteorder::{BigEndian, ByteOrder};
use bytes_cast::{unaligned, BytesCast};

use super::REVIDX_KNOWN_FLAGS;
use crate::errors::HgError;
use crate::node::{NODE_BYTES_LENGTH, NULL_NODE, STORED_NODE_ID_BYTES};
use crate::revlog::node::Node;
use crate::revlog::{Revision, NULL_REVISION};
use crate::{
    dagops, BaseRevision, FastHashMap, Graph, GraphError, RevlogError,
    RevlogIndex, UncheckedRevision,
};

pub const INDEX_ENTRY_SIZE: usize = 64;
pub const COMPRESSION_MODE_INLINE: u8 = 2;

#[derive(Debug)]
pub struct IndexHeader {
    pub(super) header_bytes: [u8; 4],
}

#[derive(Copy, Clone)]
pub struct IndexHeaderFlags {
    flags: u16,
}

/// Corresponds to the high bits of `_format_flags` in python
impl IndexHeaderFlags {
    /// Corresponds to FLAG_INLINE_DATA in python
    pub fn is_inline(self) -> bool {
        self.flags & 1 != 0
    }
    /// Corresponds to FLAG_GENERALDELTA in python
    pub fn uses_generaldelta(self) -> bool {
        self.flags & 2 != 0
    }
}

/// Corresponds to the INDEX_HEADER structure,
/// which is parsed as a `header` variable in `_loadindex` in `revlog.py`
impl IndexHeader {
    fn format_flags(&self) -> IndexHeaderFlags {
        // No "unknown flags" check here, unlike in python. Maybe there should
        // be.
        IndexHeaderFlags {
            flags: BigEndian::read_u16(&self.header_bytes[0..2]),
        }
    }

    /// The only revlog version currently supported by rhg.
    const REVLOGV1: u16 = 1;

    /// Corresponds to `_format_version` in Python.
    fn format_version(&self) -> u16 {
        BigEndian::read_u16(&self.header_bytes[2..4])
    }

    pub fn parse(index_bytes: &[u8]) -> Result<Option<IndexHeader>, HgError> {
        if index_bytes.is_empty() {
            return Ok(None);
        }
        if index_bytes.len() < 4 {
            return Err(HgError::corrupted(
                "corrupted revlog: can't read the index format header",
            ));
        }
        Ok(Some(IndexHeader {
            header_bytes: {
                let bytes: [u8; 4] =
                    index_bytes[0..4].try_into().expect("impossible");
                bytes
            },
        }))
    }
}

/// Abstracts the access to the index bytes since they can be spread between
/// the immutable (bytes) part and the mutable (added) part if any appends
/// happened. This makes it transparent for the callers.
struct IndexData {
    /// Immutable bytes, most likely taken from disk
    bytes: Box<dyn Deref<Target = [u8]> + Send>,
    /// Used when stripping index contents, keeps track of the start of the
    /// first stripped revision, which is used to give a slice of the
    /// `bytes` field.
    truncation: Option<usize>,
    /// Bytes that were added after reading the index
    added: Vec<u8>,
}

impl IndexData {
    pub fn new(bytes: Box<dyn Deref<Target = [u8]> + Send>) -> Self {
        Self {
            bytes,
            truncation: None,
            added: vec![],
        }
    }

    pub fn len(&self) -> usize {
        match self.truncation {
            Some(truncation) => truncation + self.added.len(),
            None => self.bytes.len() + self.added.len(),
        }
    }

    fn remove(
        &mut self,
        rev: Revision,
        offsets: Option<&[usize]>,
    ) -> Result<(), RevlogError> {
        let rev = rev.0 as usize;
        let truncation = if let Some(offsets) = offsets {
            offsets[rev]
        } else {
            rev * INDEX_ENTRY_SIZE
        };
        if truncation < self.bytes.len() {
            self.truncation = Some(truncation);
            self.added.clear();
        } else {
            self.added.truncate(truncation - self.bytes.len());
        }
        Ok(())
    }

    fn is_new(&self) -> bool {
        self.bytes.is_empty()
    }
}

impl std::ops::Index<std::ops::Range<usize>> for IndexData {
    type Output = [u8];

    fn index(&self, index: std::ops::Range<usize>) -> &Self::Output {
        let start = index.start;
        let end = index.end;
        let immutable_len = match self.truncation {
            Some(truncation) => truncation,
            None => self.bytes.len(),
        };
        if start < immutable_len {
            if end > immutable_len {
                panic!("index data cannot span existing and added ranges");
            }
            &self.bytes[index]
        } else {
            &self.added[start - immutable_len..end - immutable_len]
        }
    }
}

#[derive(Debug, PartialEq, Eq)]
pub struct RevisionDataParams {
    pub flags: u16,
    pub data_offset: u64,
    pub data_compressed_length: i32,
    pub data_uncompressed_length: i32,
    pub data_delta_base: i32,
    pub link_rev: i32,
    pub parent_rev_1: i32,
    pub parent_rev_2: i32,
    pub node_id: [u8; NODE_BYTES_LENGTH],
    pub _sidedata_offset: u64,
    pub _sidedata_compressed_length: i32,
    pub data_compression_mode: u8,
    pub _sidedata_compression_mode: u8,
    pub _rank: i32,
}

impl Default for RevisionDataParams {
    fn default() -> Self {
        Self {
            flags: 0,
            data_offset: 0,
            data_compressed_length: 0,
            data_uncompressed_length: 0,
            data_delta_base: -1,
            link_rev: -1,
            parent_rev_1: -1,
            parent_rev_2: -1,
            node_id: [0; NODE_BYTES_LENGTH],
            _sidedata_offset: 0,
            _sidedata_compressed_length: 0,
            data_compression_mode: COMPRESSION_MODE_INLINE,
            _sidedata_compression_mode: COMPRESSION_MODE_INLINE,
            _rank: -1,
        }
    }
}

#[derive(BytesCast)]
#[repr(C)]
pub struct RevisionDataV1 {
    data_offset_or_flags: unaligned::U64Be,
    data_compressed_length: unaligned::I32Be,
    data_uncompressed_length: unaligned::I32Be,
    data_delta_base: unaligned::I32Be,
    link_rev: unaligned::I32Be,
    parent_rev_1: unaligned::I32Be,
    parent_rev_2: unaligned::I32Be,
    node_id: [u8; STORED_NODE_ID_BYTES],
}

fn _static_assert_size_of_revision_data_v1() {
    let _ = std::mem::transmute::<RevisionDataV1, [u8; 64]>;
}

impl RevisionDataParams {
    pub fn validate(&self) -> Result<(), RevlogError> {
        if self.flags & !REVIDX_KNOWN_FLAGS != 0 {
            return Err(RevlogError::corrupted(format!(
                "unknown revlog index flags: {}",
                self.flags
            )));
        }
        if self.data_compression_mode != COMPRESSION_MODE_INLINE {
            return Err(RevlogError::corrupted(format!(
                "invalid data compression mode: {}",
                self.data_compression_mode
            )));
        }
        // FIXME isn't this only for v2 or changelog v2?
        if self._sidedata_compression_mode != COMPRESSION_MODE_INLINE {
            return Err(RevlogError::corrupted(format!(
                "invalid sidedata compression mode: {}",
                self._sidedata_compression_mode
            )));
        }
        Ok(())
    }

    pub fn into_v1(self) -> RevisionDataV1 {
        let data_offset_or_flags = self.data_offset << 16 | self.flags as u64;
        let mut node_id = [0; STORED_NODE_ID_BYTES];
        node_id[..NODE_BYTES_LENGTH].copy_from_slice(&self.node_id);
        RevisionDataV1 {
            data_offset_or_flags: data_offset_or_flags.into(),
            data_compressed_length: self.data_compressed_length.into(),
            data_uncompressed_length: self.data_uncompressed_length.into(),
            data_delta_base: self.data_delta_base.into(),
            link_rev: self.link_rev.into(),
            parent_rev_1: self.parent_rev_1.into(),
            parent_rev_2: self.parent_rev_2.into(),
            node_id,
        }
    }
}

/// A Revlog index
pub struct Index {
    bytes: IndexData,
    /// Offsets of starts of index blocks.
    /// Only needed when the index is interleaved with data.
    offsets: RwLock<Option<Vec<usize>>>,
    uses_generaldelta: bool,
    is_inline: bool,
    /// Cache of the head revisions in this index, kept in sync. Should
    /// be accessed via the [`Self::head_revs`] method.
    head_revs: Vec<Revision>,
    /// Cache of the last filtered revisions in this index, used to make sure
    /// we haven't changed filters when returning the cached `head_revs`.
    filtered_revs: HashSet<Revision>,
}

impl Debug for Index {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Index")
            .field("offsets", &self.offsets)
            .field("uses_generaldelta", &self.uses_generaldelta)
            .finish()
    }
}

impl Graph for Index {
    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
        let err = || GraphError::ParentOutOfRange(rev);
        match self.get_entry(rev) {
            Some(entry) => {
                // The C implementation checks that the parents are valid
                // before returning
                Ok([
                    self.check_revision(entry.p1()).ok_or_else(err)?,
                    self.check_revision(entry.p2()).ok_or_else(err)?,
                ])
            }
            None => Ok([NULL_REVISION, NULL_REVISION]),
        }
    }
}

/// A cache suitable for find_snapshots
///
/// Logically equivalent to a mapping whose keys are [`BaseRevision`] and
/// values sets of [`BaseRevision`]
///
/// TODO the dubious part is insisting that errors must be RevlogError
/// we would probably need to sprinkle some magic here, such as an associated
/// type that would be Into<RevlogError> but even that would not be
/// satisfactory, as errors potentially have nothing to do with the revlog.
pub trait SnapshotsCache {
    fn insert_for(
        &mut self,
        rev: BaseRevision,
        value: BaseRevision,
    ) -> Result<(), RevlogError>;
}

impl SnapshotsCache for FastHashMap<BaseRevision, HashSet<BaseRevision>> {
    fn insert_for(
        &mut self,
        rev: BaseRevision,
        value: BaseRevision,
    ) -> Result<(), RevlogError> {
        let all_values = self.entry(rev).or_insert_with(HashSet::new);
        all_values.insert(value);
        Ok(())
    }
}

impl Index {
    /// Create an index from bytes.
    /// Calculate the start of each entry when is_inline is true.
    pub fn new(
        bytes: Box<dyn Deref<Target = [u8]> + Send>,
        default_header: IndexHeader,
    ) -> Result<Self, HgError> {
        let header =
            IndexHeader::parse(bytes.as_ref())?.unwrap_or(default_header);

        if header.format_version() != IndexHeader::REVLOGV1 {
            // A proper new version should have had a repo/store
            // requirement.
            return Err(HgError::corrupted("unsupported revlog version"));
        }

        // This is only correct because we know version is REVLOGV1.
        // In v2 we always use generaldelta, while in v0 we never use
        // generaldelta. Similar for [is_inline] (it's only used in v1).
        let uses_generaldelta = header.format_flags().uses_generaldelta();

        if header.format_flags().is_inline() {
            let mut offset: usize = 0;
            let mut offsets = Vec::new();

            while offset + INDEX_ENTRY_SIZE <= bytes.len() {
                offsets.push(offset);
                let end = offset + INDEX_ENTRY_SIZE;
                let entry = IndexEntry {
                    bytes: &bytes[offset..end],
                    offset_override: None,
                };

                offset += INDEX_ENTRY_SIZE + entry.compressed_len() as usize;
            }

            if offset == bytes.len() {
                Ok(Self {
                    bytes: IndexData::new(bytes),
                    offsets: RwLock::new(Some(offsets)),
                    uses_generaldelta,
                    is_inline: true,
                    head_revs: vec![],
                    filtered_revs: HashSet::new(),
                })
            } else {
                Err(HgError::corrupted("unexpected inline revlog length"))
            }
        } else {
            Ok(Self {
                bytes: IndexData::new(bytes),
                offsets: RwLock::new(None),
                uses_generaldelta,
                is_inline: false,
                head_revs: vec![],
                filtered_revs: HashSet::new(),
            })
        }
    }

    pub fn uses_generaldelta(&self) -> bool {
        self.uses_generaldelta
    }

    /// Value of the inline flag.
    pub fn is_inline(&self) -> bool {
        self.is_inline
    }

    /// Return a slice of bytes if `revlog` is inline. Panic if not.
    pub fn data(&self, start: usize, end: usize) -> &[u8] {
        if !self.is_inline() {
            panic!("tried to access data in the index of a revlog that is not inline");
        }
        &self.bytes[start..end]
    }

    /// Return number of entries of the revlog index.
    pub fn len(&self) -> usize {
        if let Some(offsets) = &*self.get_offsets() {
            offsets.len()
        } else {
            self.bytes.len() / INDEX_ENTRY_SIZE
        }
    }

    pub fn get_offsets(&self) -> RwLockReadGuard<Option<Vec<usize>>> {
        if self.is_inline() {
            {
                // Wrap in a block to drop the read guard
                // TODO perf?
                let mut offsets = self.offsets.write().unwrap();
                if offsets.is_none() {
                    offsets.replace(inline_scan(&self.bytes.bytes).1);
                }
            }
        }
        self.offsets.read().unwrap()
    }

    pub fn get_offsets_mut(&mut self) -> RwLockWriteGuard<Option<Vec<usize>>> {
        let mut offsets = self.offsets.write().unwrap();
        if self.is_inline() && offsets.is_none() {
            offsets.replace(inline_scan(&self.bytes.bytes).1);
        }
        offsets
    }

    /// Returns `true` if the `Index` has zero `entries`.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Return the index entry corresponding to the given revision or `None`
    /// for [`NULL_REVISION`]
    ///
    /// The specified revision being of the checked type, it always exists
    /// if it was validated by this index.
    pub fn get_entry(&self, rev: Revision) -> Option<IndexEntry> {
        if rev == NULL_REVISION {
            return None;
        }
        Some(if let Some(offsets) = &*self.get_offsets() {
            self.get_entry_inline(rev, offsets.as_ref())
        } else {
            self.get_entry_separated(rev)
        })
    }

    /// Return the binary content of the index entry for the given revision
    ///
    /// See [get_entry()](`Self::get_entry()`) for cases when `None` is
    /// returned.
    pub fn entry_binary(&self, rev: Revision) -> Option<&[u8]> {
        self.get_entry(rev).map(|e| {
            let bytes = e.as_bytes();
            if rev.0 == 0 {
                &bytes[4..]
            } else {
                bytes
            }
        })
    }

    pub fn entry_as_params(
        &self,
        rev: UncheckedRevision,
    ) -> Option<RevisionDataParams> {
        let rev = self.check_revision(rev)?;
        self.get_entry(rev).map(|e| RevisionDataParams {
            flags: e.flags(),
            data_offset: if rev.0 == 0 && !self.bytes.is_new() {
                e.flags() as u64
            } else {
                e.raw_offset()
            },
            data_compressed_length: e.compressed_len().try_into().unwrap(),
            data_uncompressed_length: e.uncompressed_len(),
            data_delta_base: e.base_revision_or_base_of_delta_chain().0,
            link_rev: e.link_revision().0,
            parent_rev_1: e.p1().0,
            parent_rev_2: e.p2().0,
            node_id: e.hash().as_bytes().try_into().unwrap(),
            ..Default::default()
        })
    }

    fn get_entry_inline(
        &self,
        rev: Revision,
        offsets: &[usize],
    ) -> IndexEntry {
        let start = offsets[rev.0 as usize];
        let end = start + INDEX_ENTRY_SIZE;
        let bytes = &self.bytes[start..end];

        // See IndexEntry for an explanation of this override.
        let offset_override = Some(end);

        IndexEntry {
            bytes,
            offset_override,
        }
    }

    fn get_entry_separated(&self, rev: Revision) -> IndexEntry {
        let start = rev.0 as usize * INDEX_ENTRY_SIZE;
        let end = start + INDEX_ENTRY_SIZE;
        let bytes = &self.bytes[start..end];

        // Override the offset of the first revision as its bytes are used
        // for the index's metadata (saving space because it is always 0)
        let offset_override = if rev == Revision(0) { Some(0) } else { None };

        IndexEntry {
            bytes,
            offset_override,
        }
    }

    fn null_entry(&self) -> IndexEntry {
        IndexEntry {
            bytes: &[0; INDEX_ENTRY_SIZE],
            offset_override: Some(0),
        }
    }

    /// Return the head revisions of this index
    pub fn head_revs(&mut self) -> Result<Vec<Revision>, GraphError> {
        self.head_revs_filtered(&HashSet::new())
    }

    /// Return the head revisions of this index
    pub fn head_revs_filtered(
        &mut self,
        filtered_revs: &HashSet<Revision>,
    ) -> Result<Vec<Revision>, GraphError> {
        if !self.head_revs.is_empty() && filtered_revs == &self.filtered_revs {
            return Ok(self.head_revs.to_owned());
        }
        let mut revs: HashSet<Revision, RandomState> =
            if filtered_revs.is_empty() {
                (0..self.len())
                    .into_iter()
                    .map(|i| Revision(i as BaseRevision))
                    .collect()
            } else {
                (0..self.len())
                    .into_iter()
                    .filter_map(|i| {
                        let r = Revision(i as BaseRevision);
                        if filtered_revs.contains(&r) {
                            None
                        } else {
                            Some(r)
                        }
                    })
                    .collect()
            };
        dagops::retain_heads(self, &mut revs)?;
        if self.is_empty() {
            revs.insert(NULL_REVISION);
        }
        let mut as_vec: Vec<Revision> =
            revs.into_iter().map(Into::into).collect();
        as_vec.sort_unstable();
        self.head_revs = as_vec.to_owned();
        self.filtered_revs = filtered_revs.to_owned();
        Ok(as_vec)
    }

    /// Obtain the delta chain for a revision.
    ///
    /// `stop_rev` specifies a revision to stop at. If not specified, we
    /// stop at the base of the chain.
    ///
    /// Returns a 2-tuple of (chain, stopped) where `chain` is a vec of
    /// revs in ascending order and `stopped` is a bool indicating whether
    /// `stoprev` was hit.
    pub fn delta_chain(
        &self,
        rev: Revision,
        stop_rev: Option<Revision>,
    ) -> Result<(Vec<Revision>, bool), HgError> {
        let mut current_rev = rev;
        let mut entry = self.get_entry(rev).unwrap();
        let mut chain = vec![];
        while current_rev.0 != entry.base_revision_or_base_of_delta_chain().0
            && stop_rev.map(|r| r != current_rev).unwrap_or(true)
        {
            chain.push(current_rev);
            let new_rev = if self.uses_generaldelta() {
                entry.base_revision_or_base_of_delta_chain()
            } else {
                UncheckedRevision(current_rev.0 - 1)
            };
            current_rev = self.check_revision(new_rev).ok_or_else(|| {
                HgError::corrupted(format!("Revision {new_rev} out of range"))
            })?;
            if current_rev.0 == NULL_REVISION.0 {
                break;
            }
            entry = self.get_entry(current_rev).unwrap()
        }

        let stopped = if stop_rev.map(|r| current_rev == r).unwrap_or(false) {
            true
        } else {
            chain.push(current_rev);
            false
        };
        chain.reverse();
        Ok((chain, stopped))
    }

    pub fn find_snapshots(
        &self,
        start_rev: UncheckedRevision,
        end_rev: UncheckedRevision,
        cache: &mut impl SnapshotsCache,
    ) -> Result<(), RevlogError> {
        let mut start_rev = start_rev.0;
        let mut end_rev = end_rev.0;
        end_rev += 1;
        let len = self.len().try_into().unwrap();
        if end_rev > len {
            end_rev = len;
        }
        if start_rev < 0 {
            start_rev = 0;
        }
        for rev in start_rev..end_rev {
            if !self.is_snapshot_unchecked(Revision(rev))? {
                continue;
            }
            let mut base = self
                .get_entry(Revision(rev))
                .unwrap()
                .base_revision_or_base_of_delta_chain();
            if base.0 == rev {
                base = NULL_REVISION.into();
            }
            cache.insert_for(base.0, rev)?;
        }
        Ok(())
    }

    /// TODO move this to the trait probably, along with other things
    pub fn append(
        &mut self,
        revision_data: RevisionDataParams,
    ) -> Result<(), RevlogError> {
        revision_data.validate()?;
        let new_offset = self.bytes.len();
        if let Some(offsets) = &mut *self.get_offsets_mut() {
            offsets.push(new_offset)
        }
        self.bytes.added.extend(revision_data.into_v1().as_bytes());
        self.head_revs.clear();
        Ok(())
    }

    pub fn pack_header(&self, header: i32) -> [u8; 4] {
        header.to_be_bytes()
    }

    pub fn remove(&mut self, rev: Revision) -> Result<(), RevlogError> {
        let offsets = self.get_offsets().clone();
        self.bytes.remove(rev, offsets.as_deref())?;
        if let Some(offsets) = &mut *self.get_offsets_mut() {
            offsets.truncate(rev.0 as usize)
        }
        self.head_revs.clear();
        Ok(())
    }

    pub fn clear_caches(&mut self) {
        // We need to get the 'inline' value from Python at init and use this
        // instead of offsets to determine whether we're inline since we might
        // clear caches. This implies re-populating the offsets on-demand.
        self.offsets = RwLock::new(None);
        self.head_revs.clear();
    }

    /// Unchecked version of `is_snapshot`.
    /// Assumes the caller checked that `rev` is within a valid revision range.
    pub fn is_snapshot_unchecked(
        &self,
        mut rev: Revision,
    ) -> Result<bool, RevlogError> {
        while rev.0 >= 0 {
            let entry = self.get_entry(rev).unwrap();
            let mut base = entry.base_revision_or_base_of_delta_chain().0;
            if base == rev.0 {
                base = NULL_REVISION.0;
            }
            if base == NULL_REVISION.0 {
                return Ok(true);
            }
            let [mut p1, mut p2] = self
                .parents(rev)
                .map_err(|_| RevlogError::InvalidRevision)?;
            while let Some(p1_entry) = self.get_entry(p1) {
                if p1_entry.compressed_len() != 0 || p1.0 == 0 {
                    break;
                }
                let parent_base =
                    p1_entry.base_revision_or_base_of_delta_chain();
                if parent_base.0 == p1.0 {
                    break;
                }
                p1 = self
                    .check_revision(parent_base)
                    .ok_or(RevlogError::InvalidRevision)?;
            }
            while let Some(p2_entry) = self.get_entry(p2) {
                if p2_entry.compressed_len() != 0 || p2.0 == 0 {
                    break;
                }
                let parent_base =
                    p2_entry.base_revision_or_base_of_delta_chain();
                if parent_base.0 == p2.0 {
                    break;
                }
                p2 = self
                    .check_revision(parent_base)
                    .ok_or(RevlogError::InvalidRevision)?;
            }
            if base == p1.0 || base == p2.0 {
                return Ok(false);
            }
            rev = self
                .check_revision(base.into())
                .ok_or(RevlogError::InvalidRevision)?;
        }
        Ok(rev == NULL_REVISION)
    }

    /// Return whether the given revision is a snapshot. Returns an error if
    /// `rev` is not within a valid revision range.
    pub fn is_snapshot(
        &self,
        rev: UncheckedRevision,
    ) -> Result<bool, RevlogError> {
        let rev = self
            .check_revision(rev)
            .ok_or_else(|| RevlogError::corrupted("test"))?;
        self.is_snapshot_unchecked(rev)
    }

    /// Slice revs to reduce the amount of unrelated data to be read from disk.
    ///
    /// The index is sliced into groups that should be read in one time.
    ///
    /// The initial chunk is sliced until the overall density
    /// (payload/chunks-span ratio) is above `target_density`.
    /// No gap smaller than `min_gap_size` is skipped.
    pub fn slice_chunk_to_density(
        &self,
        revs: &[Revision],
        target_density: f64,
        min_gap_size: usize,
    ) -> Vec<Vec<Revision>> {
        if revs.is_empty() {
            return vec![];
        }
        if revs.len() == 1 {
            return vec![revs.to_owned()];
        }
        let delta_chain_span = self.segment_span(revs);
        if delta_chain_span < min_gap_size {
            return vec![revs.to_owned()];
        }
        let entries: Vec<_> = revs
            .iter()
            .map(|r| {
                (*r, self.get_entry(*r).unwrap_or_else(|| self.null_entry()))
            })
            .collect();

        let mut read_data = delta_chain_span;
        let chain_payload: u32 =
            entries.iter().map(|(_r, e)| e.compressed_len()).sum();
        let mut density = if delta_chain_span > 0 {
            chain_payload as f64 / delta_chain_span as f64
        } else {
            1.0
        };

        if density >= target_density {
            return vec![revs.to_owned()];
        }

        // Store the gaps in a heap to have them sorted by decreasing size
        let mut gaps = Vec::new();
        let mut previous_end = None;

        for (i, (_rev, entry)) in entries.iter().enumerate() {
            let start = entry.c_start() as usize;
            let length = entry.compressed_len();

            // Skip empty revisions to form larger holes
            if length == 0 {
                continue;
            }

            if let Some(end) = previous_end {
                let gap_size = start - end;
                // Only consider holes that are large enough
                if gap_size > min_gap_size {
                    gaps.push((gap_size, i));
                }
            }
            previous_end = Some(start + length as usize);
        }
        if gaps.is_empty() {
            return vec![revs.to_owned()];
        }
        // sort the gaps to pop them from largest to small
        gaps.sort_unstable();

        // Collect the indices of the largest holes until
        // the density is acceptable
        let mut selected = vec![];
        while let Some((gap_size, gap_id)) = gaps.pop() {
            if density >= target_density {
                break;
            }
            selected.push(gap_id);

            // The gap sizes are stored as negatives to be sorted decreasingly
            // by the heap
            read_data -= gap_size;
            density = if read_data > 0 {
                chain_payload as f64 / read_data as f64
            } else {
                1.0
            };
            if density >= target_density {
                break;
            }
        }
        selected.sort_unstable();
        selected.push(revs.len());

        // Cut the revs at collected indices
        let mut previous_idx = 0;
        let mut chunks = vec![];
        for idx in selected {
            let chunk = self.trim_chunk(&entries, previous_idx, idx);
            if !chunk.is_empty() {
                chunks.push(chunk.iter().map(|(rev, _entry)| *rev).collect());
            }
            previous_idx = idx;
        }
        let chunk = self.trim_chunk(&entries, previous_idx, entries.len());
        if !chunk.is_empty() {
            chunks.push(chunk.iter().map(|(rev, _entry)| *rev).collect());
        }

        chunks
    }

    /// Get the byte span of a segment of sorted revisions.
    ///
    /// Occurrences of [`NULL_REVISION`] are ignored at the beginning of
    /// the `revs` segment.
    ///
    /// panics:
    ///  - if `revs` is empty or only made of `NULL_REVISION`
    ///  - if cannot retrieve entry for the last or first not null element of
    ///    `revs`.
    fn segment_span(&self, revs: &[Revision]) -> usize {
        if revs.is_empty() {
            return 0;
        }
        let last_entry = &self.get_entry(revs[revs.len() - 1]).unwrap();
        let end = last_entry.c_start() + last_entry.compressed_len() as u64;
        let first_rev = revs.iter().find(|r| r.0 != NULL_REVISION.0).unwrap();
        let start = if (*first_rev).0 == 0 {
            0
        } else {
            self.get_entry(*first_rev).unwrap().c_start()
        };
        (end - start) as usize
    }

    /// Returns `&revs[startidx..endidx]` without empty trailing revs
    fn trim_chunk<'a>(
        &'a self,
        revs: &'a [(Revision, IndexEntry)],
        start: usize,
        mut end: usize,
    ) -> &'a [(Revision, IndexEntry)] {
        // Trim empty revs at the end, except the very first rev of a chain
        let last_rev = revs[end - 1].0;
        if last_rev.0 < self.len() as BaseRevision {
            while end > 1
                && end > start
                && revs[end - 1].1.compressed_len() == 0
            {
                end -= 1
            }
        }
        &revs[start..end]
    }

    /// Computes the set of revisions for each non-public phase from `roots`,
    /// which are the last known roots for each non-public phase.
    pub fn compute_phases_map_sets(
        &self,
        roots: HashMap<Phase, Vec<Revision>>,
    ) -> Result<(usize, RootsPerPhase), GraphError> {
        let mut phases = HashMap::new();
        let mut min_phase_rev = NULL_REVISION;

        for phase in Phase::non_public_phases() {
            if let Some(phase_roots) = roots.get(phase) {
                let min_rev =
                    self.add_roots_get_min(phase_roots, &mut phases, *phase);
                if min_rev != NULL_REVISION
                    && (min_phase_rev == NULL_REVISION
                        || min_rev < min_phase_rev)
                {
                    min_phase_rev = min_rev;
                }
            } else {
                continue;
            };
        }
        let mut phase_sets: RootsPerPhase = Default::default();

        if min_phase_rev == NULL_REVISION {
            min_phase_rev = Revision(self.len() as BaseRevision);
        }

        for rev in min_phase_rev.0..self.len() as BaseRevision {
            let rev = Revision(rev);
            let [p1, p2] = self.parents(rev)?;

            const DEFAULT_PHASE: &Phase = &Phase::Public;
            if p1.0 >= 0
                && phases.get(&p1).unwrap_or(DEFAULT_PHASE)
                    > phases.get(&rev).unwrap_or(DEFAULT_PHASE)
            {
                phases.insert(rev, phases[&p1]);
            }
            if p2.0 >= 0
                && phases.get(&p2).unwrap_or(DEFAULT_PHASE)
                    > phases.get(&rev).unwrap_or(DEFAULT_PHASE)
            {
                phases.insert(rev, phases[&p2]);
            }
            let set = match phases.get(&rev).unwrap_or(DEFAULT_PHASE) {
                Phase::Public => continue,
                phase => &mut phase_sets[*phase as usize - 1],
            };
            set.insert(rev);
        }

        Ok((self.len(), phase_sets))
    }

    fn add_roots_get_min(
        &self,
        phase_roots: &[Revision],
        phases: &mut HashMap<Revision, Phase>,
        phase: Phase,
    ) -> Revision {
        let mut min_rev = NULL_REVISION;

        for root in phase_roots {
            phases.insert(*root, phase);
            if min_rev == NULL_REVISION || min_rev > *root {
                min_rev = *root;
            }
        }
        min_rev
    }

    /// Return `(heads(::(<roots> and <roots>::<heads>)))`
    /// If `include_path` is `true`, return `(<roots>::<heads>)`."""
    ///
    /// `min_root` and `roots` are unchecked since they are just used as
    /// a bound or for comparison and don't need to represent a valid revision.
    /// In practice, the only invalid revision passed is the working directory
    /// revision ([`i32::MAX`]).
    pub fn reachable_roots(
        &self,
        min_root: UncheckedRevision,
        mut heads: Vec<Revision>,
        roots: HashSet<UncheckedRevision>,
        include_path: bool,
    ) -> Result<HashSet<Revision>, GraphError> {
        if roots.is_empty() {
            return Ok(HashSet::new());
        }
        let mut reachable = HashSet::new();
        let mut seen = HashMap::new();

        while let Some(rev) = heads.pop() {
            if roots.contains(&rev.into()) {
                reachable.insert(rev);
                if !include_path {
                    continue;
                }
            }
            let parents = self.parents(rev)?;
            seen.insert(rev, parents);
            for parent in parents {
                if parent.0 >= min_root.0 && !seen.contains_key(&parent) {
                    heads.push(parent);
                }
            }
        }
        if !include_path {
            return Ok(reachable);
        }
        let mut revs: Vec<_> = seen.keys().collect();
        revs.sort_unstable();
        for rev in revs {
            for parent in seen[rev] {
                if reachable.contains(&parent) {
                    reachable.insert(*rev);
                }
            }
        }
        Ok(reachable)
    }

    /// Given a (possibly overlapping) set of revs, return all the
    /// common ancestors heads: `heads(::args[0] and ::a[1] and ...)`
    pub fn common_ancestor_heads(
        &self,
        revisions: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        // given that revisions is expected to be small, we find this shortcut
        // potentially acceptable, especially given that `hg-cpython` could
        // very much bypass this, constructing a vector of unique values from
        // the onset.
        let as_set: HashSet<Revision> = revisions.iter().copied().collect();
        // Besides deduplicating, the C version also implements the shortcut
        // for `NULL_REVISION`:
        if as_set.contains(&NULL_REVISION) {
            return Ok(vec![]);
        }

        let revisions: Vec<Revision> = as_set.into_iter().collect();

        if revisions.len() < 8 {
            self.find_gca_candidates::<u8>(&revisions)
        } else if revisions.len() < 64 {
            self.find_gca_candidates::<u64>(&revisions)
        } else {
            self.find_gca_candidates::<NonStaticPoisonableBitSet>(&revisions)
        }
    }

    pub fn ancestors(
        &self,
        revisions: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        self.find_deepest_revs(&self.common_ancestor_heads(revisions)?)
    }

    /// Given a disjoint set of revs, return all candidates for the
    /// greatest common ancestor. In revset notation, this is the set
    /// `heads(::a and ::b and ...)`
    fn find_gca_candidates<BS: PoisonableBitSet + Clone>(
        &self,
        revs: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        if revs.is_empty() {
            return Ok(vec![]);
        }
        let revcount = revs.len();
        let mut candidates = vec![];
        let max_rev = revs.iter().max().unwrap();

        let mut seen = BS::vec_of_empty(revs.len(), (max_rev.0 + 1) as usize);

        for (idx, rev) in revs.iter().enumerate() {
            seen[rev.0 as usize].add(idx);
        }
        let mut current_rev = *max_rev;
        // Number of revisions whose inspection in the main loop
        // will give a result or trigger inspection of other revisions
        let mut interesting = revcount;

        // The algorithm works on a vector of bit sets, indexed by revision
        // numbers and iterated on reverse order.
        // An entry in this vector is poisoned if and only if the corresponding
        // revision is a common, yet not maximal ancestor.

        // The principle of the algorithm is as follows:
        // For a revision `r`, when entering the loop, `seen[r]` is either
        // poisoned or the sub set of `revs` of which `r` is an ancestor.
        // In this sub set is full, then `r` is a solution and its parents
        // have to be poisoned.
        //
        // At each iteration, the bit sets of the parents are updated by
        // union with `seen[r]`.
        // As we walk the index from the end, we are sure we have encountered
        // all children of `r` before `r`, hence we know that `seen[r]` is
        // fully computed.
        //
        // On top of that there are several optimizations that make reading
        // less obvious than the comment above:
        // - The `interesting` counter allows to break early
        // - The loop starts from `max(revs)`
        // - Early return in case it is detected that one of the incoming revs
        //   is a common ancestor of all of them.
        while current_rev.0 >= 0 && interesting > 0 {
            let current_seen = seen[current_rev.0 as usize].clone();

            if current_seen.is_empty() {
                current_rev = Revision(current_rev.0 - 1);
                continue;
            }
            let mut poison = current_seen.is_poisoned();
            if !poison {
                interesting -= 1;
                if current_seen.is_full_range(revcount) {
                    candidates.push(current_rev);
                    poison = true;

                    // Being a common ancestor, if `current_rev` is among
                    // the input revisions, it is *the* answer.
                    for rev in revs {
                        if *rev == current_rev {
                            return Ok(candidates);
                        }
                    }
                }
            }
            for parent in self.parents(current_rev)? {
                if parent == NULL_REVISION {
                    continue;
                }
                let parent_seen = &mut seen[parent.0 as usize];
                if poison {
                    // this block is logically equivalent to poisoning parent
                    // and counting it as non interesting if it
                    // has been seen before (hence counted then as interesting)
                    if !parent_seen.is_empty() && !parent_seen.is_poisoned() {
                        interesting -= 1;
                    }
                    parent_seen.poison();
                } else {
                    if parent_seen.is_empty() {
                        interesting += 1;
                    }
                    parent_seen.union(&current_seen);
                }
            }

            current_rev = Revision(current_rev.0 - 1);
        }

        Ok(candidates)
    }

    /// Given a disjoint set of revs, return the subset with the longest path
    /// to the root.
    fn find_deepest_revs(
        &self,
        revs: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        // TODO replace this all with just comparing rank?
        // Also, the original implementations in C/Python are cryptic, not
        // even sure we actually need this?
        if revs.len() <= 1 {
            return Ok(revs.to_owned());
        }
        let max_rev = revs.iter().max().unwrap().0;
        let mut interesting = HashMap::new();
        let mut seen = vec![0; max_rev as usize + 1];
        let mut depth = vec![0; max_rev as usize + 1];
        let mut mapping = vec![];
        let mut revs = revs.to_owned();
        revs.sort_unstable();

        for (idx, rev) in revs.iter().enumerate() {
            depth[rev.0 as usize] = 1;
            let shift = 1 << idx;
            seen[rev.0 as usize] = shift;
            interesting.insert(shift, 1);
            mapping.push((shift, *rev));
        }

        let mut current_rev = Revision(max_rev);
        while current_rev.0 >= 0 && interesting.len() > 1 {
            let current_depth = depth[current_rev.0 as usize];
            if current_depth == 0 {
                current_rev = Revision(current_rev.0 - 1);
                continue;
            }

            let current_seen = seen[current_rev.0 as usize];
            for parent in self.parents(current_rev)? {
                if parent == NULL_REVISION {
                    continue;
                }
                let parent_seen = seen[parent.0 as usize];
                let parent_depth = depth[parent.0 as usize];
                if parent_depth <= current_depth {
                    depth[parent.0 as usize] = current_depth + 1;
                    if parent_seen != current_seen {
                        *interesting.get_mut(&current_seen).unwrap() += 1;
                        seen[parent.0 as usize] = current_seen;
                        if parent_seen != 0 {
                            let parent_interesting =
                                interesting.get_mut(&parent_seen).unwrap();
                            *parent_interesting -= 1;
                            if *parent_interesting == 0 {
                                interesting.remove(&parent_seen);
                            }
                        }
                    }
                } else if current_depth == parent_depth - 1 {
                    let either_seen = parent_seen | current_seen;
                    if either_seen == parent_seen {
                        continue;
                    }
                    seen[parent.0 as usize] = either_seen;
                    interesting
                        .entry(either_seen)
                        .and_modify(|v| *v += 1)
                        .or_insert(1);
                    *interesting.get_mut(&parent_seen).unwrap() -= 1;
                    if interesting[&parent_seen] == 0 {
                        interesting.remove(&parent_seen);
                    }
                }
            }
            *interesting.get_mut(&current_seen).unwrap() -= 1;
            if interesting[&current_seen] == 0 {
                interesting.remove(&current_seen);
            }

            current_rev = Revision(current_rev.0 - 1);
        }

        if interesting.len() != 1 {
            return Ok(vec![]);
        }
        let mask = interesting.keys().next().unwrap();

        Ok(mapping
            .into_iter()
            .filter_map(|(shift, rev)| {
                if (mask & shift) != 0 {
                    return Some(rev);
                }
                None
            })
            .collect())
    }
}

/// The kind of functionality needed by find_gca_candidates
///
/// This is a bit mask which can be declared to be "poisoned", which callers
/// interpret to break out of some loops.
///
/// The maximum capacity of the bit mask is up to the actual implementation
trait PoisonableBitSet: Sized + PartialEq {
    /// Return a vector of exactly n elements, initialized to be empty.
    ///
    /// Optimization can vastly depend on implementation. Those being `Copy`
    /// and having constant capacity typically can have a very simple
    /// implementation.
    fn vec_of_empty(sets_size: usize, vec_len: usize) -> Vec<Self>;

    /// The size of the bit mask in memory
    fn size(&self) -> usize;

    /// The number of elements that can be represented in the set.
    ///
    /// Another way to put it is that it is the highest integer `C` such that
    /// the set is guaranteed to always be a subset of the integer range
    /// `[0, C)`
    fn capacity(&self) -> usize;

    /// Declare `n` to belong to the set
    fn add(&mut self, n: usize);

    /// Declare `n` not to belong to the set
    fn discard(&mut self, n: usize);

    /// Replace this bit set by its union with other
    fn union(&mut self, other: &Self);

    /// Poison the bit set
    ///
    /// Interpretation up to the caller
    fn poison(&mut self);

    /// Is the bit set poisoned?
    ///
    /// Interpretation is up to the caller
    fn is_poisoned(&self) -> bool;

    /// Is the bit set empty?
    fn is_empty(&self) -> bool;

    /// return `true` if and only if the bit is the full range `[0, n)`
    /// of integers
    fn is_full_range(&self, n: usize) -> bool;
}

const U64_POISON: u64 = 1 << 63;
const U8_POISON: u8 = 1 << 7;

impl PoisonableBitSet for u64 {
    fn vec_of_empty(_sets_size: usize, vec_len: usize) -> Vec<Self> {
        vec![0u64; vec_len]
    }

    fn size(&self) -> usize {
        8
    }

    fn capacity(&self) -> usize {
        63
    }

    fn add(&mut self, n: usize) {
        (*self) |= 1u64 << n;
    }

    fn discard(&mut self, n: usize) {
        (*self) &= u64::MAX - (1u64 << n);
    }

    fn union(&mut self, other: &Self) {
        if *self != *other {
            (*self) |= *other;
        }
    }

    fn is_full_range(&self, n: usize) -> bool {
        *self + 1 == (1u64 << n)
    }

    fn is_empty(&self) -> bool {
        *self == 0
    }

    fn poison(&mut self) {
        *self = U64_POISON;
    }

    fn is_poisoned(&self) -> bool {
        // equality comparison would be tempting but would not resist
        // operations after poisoning (even if these should be bogus).
        *self >= U64_POISON
    }
}

impl PoisonableBitSet for u8 {
    fn vec_of_empty(_sets_size: usize, vec_len: usize) -> Vec<Self> {
        vec![0; vec_len]
    }

    fn size(&self) -> usize {
        1
    }

    fn capacity(&self) -> usize {
        7
    }

    fn add(&mut self, n: usize) {
        (*self) |= 1 << n;
    }

    fn discard(&mut self, n: usize) {
        (*self) &= u8::MAX - (1 << n);
    }

    fn union(&mut self, other: &Self) {
        if *self != *other {
            (*self) |= *other;
        }
    }

    fn is_full_range(&self, n: usize) -> bool {
        *self + 1 == (1 << n)
    }

    fn is_empty(&self) -> bool {
        *self == 0
    }

    fn poison(&mut self) {
        *self = U8_POISON;
    }

    fn is_poisoned(&self) -> bool {
        // equality comparison would be tempting but would not resist
        // operations after poisoning (even if these should be bogus).
        *self >= U8_POISON
    }
}

/// A poisonable bit set whose capacity is not known at compile time but
/// is constant after initial construction
///
/// This can be way further optimized if performance assessments (speed
/// and/or RAM) require it.
/// As far as RAM is concerned, for large vectors of these, the main problem
/// would be the repetition of set_size in each item. We would need a trait
/// to abstract over the idea of a vector of such bit sets to do better.
#[derive(Clone, PartialEq)]
struct NonStaticPoisonableBitSet {
    set_size: usize,
    bit_set: Vec<u64>,
}

/// Number of `u64` needed for a [`NonStaticPoisonableBitSet`] of given size
fn non_static_poisonable_inner_len(set_size: usize) -> usize {
    1 + (set_size + 1) / 64
}

impl NonStaticPoisonableBitSet {
    /// The index of the sub-bit set for the given n, and the index inside
    /// the latter
    fn index(&self, n: usize) -> (usize, usize) {
        (n / 64, n % 64)
    }
}

/// Mock implementation to ensure that the trait makes sense
impl PoisonableBitSet for NonStaticPoisonableBitSet {
    fn vec_of_empty(set_size: usize, vec_len: usize) -> Vec<Self> {
        let tmpl = Self {
            set_size,
            bit_set: vec![0u64; non_static_poisonable_inner_len(set_size)],
        };
        vec![tmpl; vec_len]
    }

    fn size(&self) -> usize {
        8 + self.bit_set.len() * 8
    }

    fn capacity(&self) -> usize {
        self.set_size
    }

    fn add(&mut self, n: usize) {
        let (sub_bs, bit_pos) = self.index(n);
        self.bit_set[sub_bs] |= 1 << bit_pos
    }

    fn discard(&mut self, n: usize) {
        let (sub_bs, bit_pos) = self.index(n);
        self.bit_set[sub_bs] |= u64::MAX - (1 << bit_pos)
    }

    fn union(&mut self, other: &Self) {
        assert!(
            self.set_size == other.set_size,
            "Binary operations on bit sets can only be done on same size"
        );
        for i in 0..self.bit_set.len() - 1 {
            self.bit_set[i] |= other.bit_set[i]
        }
    }

    fn is_full_range(&self, n: usize) -> bool {
        let (sub_bs, bit_pos) = self.index(n);
        self.bit_set[..sub_bs].iter().all(|bs| *bs == u64::MAX)
            && self.bit_set[sub_bs] == (1 << (bit_pos + 1)) - 1
    }

    fn is_empty(&self) -> bool {
        self.bit_set.iter().all(|bs| *bs == 0u64)
    }

    fn poison(&mut self) {
        let (sub_bs, bit_pos) = self.index(self.set_size);
        self.bit_set[sub_bs] = 1 << bit_pos;
    }

    fn is_poisoned(&self) -> bool {
        let (sub_bs, bit_pos) = self.index(self.set_size);
        self.bit_set[sub_bs] >= 1 << bit_pos
    }
}

/// Set of roots of all non-public phases
pub type RootsPerPhase = [HashSet<Revision>; Phase::non_public_phases().len()];

#[derive(Debug, Copy, Clone, PartialEq, Eq, Ord, PartialOrd, Hash)]
pub enum Phase {
    Public = 0,
    Draft = 1,
    Secret = 2,
    Archived = 3,
    Internal = 4,
}

impl TryFrom<usize> for Phase {
    type Error = RevlogError;

    fn try_from(value: usize) -> Result<Self, Self::Error> {
        Ok(match value {
            0 => Self::Public,
            1 => Self::Draft,
            2 => Self::Secret,
            32 => Self::Archived,
            96 => Self::Internal,
            v => {
                return Err(RevlogError::corrupted(format!(
                    "invalid phase value {}",
                    v
                )))
            }
        })
    }
}

impl Phase {
    pub const fn all_phases() -> &'static [Self] {
        &[
            Self::Public,
            Self::Draft,
            Self::Secret,
            Self::Archived,
            Self::Internal,
        ]
    }
    pub const fn non_public_phases() -> &'static [Self] {
        &[Self::Draft, Self::Secret, Self::Archived, Self::Internal]
    }
}

fn inline_scan(bytes: &[u8]) -> (usize, Vec<usize>) {
    let mut offset: usize = 0;
    let mut offsets = Vec::new();

    while offset + INDEX_ENTRY_SIZE <= bytes.len() {
        offsets.push(offset);
        let end = offset + INDEX_ENTRY_SIZE;
        let entry = IndexEntry {
            bytes: &bytes[offset..end],
            offset_override: None,
        };

        offset += INDEX_ENTRY_SIZE + entry.compressed_len() as usize;
    }
    (offset, offsets)
}

impl super::RevlogIndex for Index {
    fn len(&self) -> usize {
        self.len()
    }

    fn node(&self, rev: Revision) -> Option<&Node> {
        if rev == NULL_REVISION {
            return Some(&NULL_NODE);
        }
        self.get_entry(rev).map(|entry| entry.hash())
    }
}

#[derive(Debug)]
pub struct IndexEntry<'a> {
    bytes: &'a [u8],
    /// Allows to override the offset value of the entry.
    ///
    /// For interleaved index and data, the offset stored in the index
    /// corresponds to the separated data offset.
    /// It has to be overridden with the actual offset in the interleaved
    /// index which is just after the index block.
    ///
    /// For separated index and data, the offset stored in the first index
    /// entry is mixed with the index headers.
    /// It has to be overridden with 0.
    offset_override: Option<usize>,
}

impl<'a> IndexEntry<'a> {
    /// Return the offset of the data.
    pub fn offset(&self) -> usize {
        if let Some(offset_override) = self.offset_override {
            offset_override
        } else {
            let mut bytes = [0; 8];
            bytes[2..8].copy_from_slice(&self.bytes[0..=5]);
            BigEndian::read_u64(&bytes[..]) as usize
        }
    }
    pub fn raw_offset(&self) -> u64 {
        BigEndian::read_u64(&self.bytes[0..8])
    }

    /// Same result (except potentially for rev 0) as C `index_get_start()`
    fn c_start(&self) -> u64 {
        self.raw_offset() >> 16
    }

    pub fn flags(&self) -> u16 {
        BigEndian::read_u16(&self.bytes[6..=7])
    }

    /// Return the compressed length of the data.
    pub fn compressed_len(&self) -> u32 {
        BigEndian::read_u32(&self.bytes[8..=11])
    }

    /// Return the uncompressed length of the data.
    pub fn uncompressed_len(&self) -> i32 {
        BigEndian::read_i32(&self.bytes[12..=15])
    }

    /// Return the revision upon which the data has been derived.
    pub fn base_revision_or_base_of_delta_chain(&self) -> UncheckedRevision {
        // TODO Maybe return an Option when base_revision == rev?
        //      Requires to add rev to IndexEntry

        BigEndian::read_i32(&self.bytes[16..]).into()
    }

    pub fn link_revision(&self) -> UncheckedRevision {
        BigEndian::read_i32(&self.bytes[20..]).into()
    }

    pub fn p1(&self) -> UncheckedRevision {
        BigEndian::read_i32(&self.bytes[24..]).into()
    }

    pub fn p2(&self) -> UncheckedRevision {
        BigEndian::read_i32(&self.bytes[28..]).into()
    }

    /// Return the hash of revision's full text.
    ///
    /// Currently, SHA-1 is used and only the first 20 bytes of this field
    /// are used.
    pub fn hash(&self) -> &'a Node {
        (&self.bytes[32..52]).try_into().unwrap()
    }

    pub fn as_bytes(&self) -> &'a [u8] {
        self.bytes
    }
}

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

    #[cfg(test)]
    #[derive(Debug, Copy, Clone)]
    pub struct IndexEntryBuilder {
        is_first: bool,
        is_inline: bool,
        is_general_delta: bool,
        version: u16,
        offset: usize,
        compressed_len: usize,
        uncompressed_len: usize,
        base_revision_or_base_of_delta_chain: Revision,
        link_revision: Revision,
        p1: Revision,
        p2: Revision,
        node: Node,
    }

    #[cfg(test)]
    impl IndexEntryBuilder {
        #[allow(clippy::new_without_default)]
        pub fn new() -> Self {
            Self {
                is_first: false,
                is_inline: false,
                is_general_delta: true,
                version: 1,
                offset: 0,
                compressed_len: 0,
                uncompressed_len: 0,
                base_revision_or_base_of_delta_chain: Revision(0),
                link_revision: Revision(0),
                p1: NULL_REVISION,
                p2: NULL_REVISION,
                node: NULL_NODE,
            }
        }

        pub fn is_first(&mut self, value: bool) -> &mut Self {
            self.is_first = value;
            self
        }

        pub fn with_inline(&mut self, value: bool) -> &mut Self {
            self.is_inline = value;
            self
        }

        pub fn with_general_delta(&mut self, value: bool) -> &mut Self {
            self.is_general_delta = value;
            self
        }

        pub fn with_version(&mut self, value: u16) -> &mut Self {
            self.version = value;
            self
        }

        pub fn with_offset(&mut self, value: usize) -> &mut Self {
            self.offset = value;
            self
        }

        pub fn with_compressed_len(&mut self, value: usize) -> &mut Self {
            self.compressed_len = value;
            self
        }

        pub fn with_uncompressed_len(&mut self, value: usize) -> &mut Self {
            self.uncompressed_len = value;
            self
        }

        pub fn with_base_revision_or_base_of_delta_chain(
            &mut self,
            value: Revision,
        ) -> &mut Self {
            self.base_revision_or_base_of_delta_chain = value;
            self
        }

        pub fn with_link_revision(&mut self, value: Revision) -> &mut Self {
            self.link_revision = value;
            self
        }

        pub fn with_p1(&mut self, value: Revision) -> &mut Self {
            self.p1 = value;
            self
        }

        pub fn with_p2(&mut self, value: Revision) -> &mut Self {
            self.p2 = value;
            self
        }

        pub fn with_node(&mut self, value: Node) -> &mut Self {
            self.node = value;
            self
        }

        pub fn build(&self) -> Vec<u8> {
            let mut bytes = Vec::with_capacity(INDEX_ENTRY_SIZE);
            if self.is_first {
                bytes.extend(&match (self.is_general_delta, self.is_inline) {
                    (false, false) => [0u8, 0],
                    (false, true) => [0u8, 1],
                    (true, false) => [0u8, 2],
                    (true, true) => [0u8, 3],
                });
                bytes.extend(&self.version.to_be_bytes());
                // Remaining offset bytes.
                bytes.extend(&[0u8; 2]);
            } else {
                // Offset stored on 48 bits (6 bytes)
                bytes.extend(&(self.offset as u64).to_be_bytes()[2..]);
            }
            bytes.extend(&[0u8; 2]); // Revision flags.
            bytes.extend(&(self.compressed_len as u32).to_be_bytes());
            bytes.extend(&(self.uncompressed_len as u32).to_be_bytes());
            bytes.extend(
                &self.base_revision_or_base_of_delta_chain.0.to_be_bytes(),
            );
            bytes.extend(&self.link_revision.0.to_be_bytes());
            bytes.extend(&self.p1.0.to_be_bytes());
            bytes.extend(&self.p2.0.to_be_bytes());
            bytes.extend(self.node.as_bytes());
            bytes.extend(vec![0u8; 12]);
            bytes
        }
    }

    pub fn is_inline(index_bytes: &[u8]) -> bool {
        IndexHeader::parse(index_bytes)
            .expect("too short")
            .unwrap()
            .format_flags()
            .is_inline()
    }

    pub fn uses_generaldelta(index_bytes: &[u8]) -> bool {
        IndexHeader::parse(index_bytes)
            .expect("too short")
            .unwrap()
            .format_flags()
            .uses_generaldelta()
    }

    pub fn get_version(index_bytes: &[u8]) -> u16 {
        IndexHeader::parse(index_bytes)
            .expect("too short")
            .unwrap()
            .format_version()
    }

    #[test]
    fn flags_when_no_inline_flag_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_general_delta(false)
            .with_inline(false)
            .build();

        assert!(!is_inline(&bytes));
        assert!(!uses_generaldelta(&bytes));
    }

    #[test]
    fn flags_when_inline_flag_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_general_delta(false)
            .with_inline(true)
            .build();

        assert!(is_inline(&bytes));
        assert!(!uses_generaldelta(&bytes));
    }

    #[test]
    fn flags_when_inline_and_generaldelta_flags_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_general_delta(true)
            .with_inline(true)
            .build();

        assert!(is_inline(&bytes));
        assert!(uses_generaldelta(&bytes));
    }

    #[test]
    fn test_offset() {
        let bytes = IndexEntryBuilder::new().with_offset(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.offset(), 1)
    }

    #[test]
    fn test_with_overridden_offset() {
        let bytes = IndexEntryBuilder::new().with_offset(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: Some(2),
        };

        assert_eq!(entry.offset(), 2)
    }

    #[test]
    fn test_compressed_len() {
        let bytes = IndexEntryBuilder::new().with_compressed_len(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.compressed_len(), 1)
    }

    #[test]
    fn test_uncompressed_len() {
        let bytes = IndexEntryBuilder::new().with_uncompressed_len(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.uncompressed_len(), 1)
    }

    #[test]
    fn test_base_revision_or_base_of_delta_chain() {
        let bytes = IndexEntryBuilder::new()
            .with_base_revision_or_base_of_delta_chain(Revision(1))
            .build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.base_revision_or_base_of_delta_chain(), 1.into())
    }

    #[test]
    fn link_revision_test() {
        let bytes = IndexEntryBuilder::new()
            .with_link_revision(Revision(123))
            .build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.link_revision(), 123.into());
    }

    #[test]
    fn p1_test() {
        let bytes = IndexEntryBuilder::new().with_p1(Revision(123)).build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.p1(), 123.into());
    }

    #[test]
    fn p2_test() {
        let bytes = IndexEntryBuilder::new().with_p2(Revision(123)).build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.p2(), 123.into());
    }

    #[test]
    fn node_test() {
        let node = Node::from_hex("0123456789012345678901234567890123456789")
            .unwrap();
        let bytes = IndexEntryBuilder::new().with_node(node).build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(*entry.hash(), node);
    }

    #[test]
    fn version_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_version(2)
            .build();

        assert_eq!(get_version(&bytes), 2)
    }
}

#[cfg(test)]
pub use tests::IndexEntryBuilder;