gix_blame/file/

function.rs

use std::{num::NonZeroU32, ops::Range};

use gix_diff::{blob::intern::TokenSource, tree::Visit};
use gix_hash::ObjectId;
use gix_object::{
    bstr::{BStr, BString},
    FindExt,
};
use gix_traverse::commit::find as find_commit;
use smallvec::SmallVec;

use super::{process_changes, Change, UnblamedHunk};
use crate::{types::BlamePathEntry, BlameEntry, Error, Options, Outcome, Statistics};

/// Produce a list of consecutive [`BlameEntry`] instances to indicate in which commits the ranges of the file
/// at `suspect:<file_path>` originated in.
///
/// ## Parameters
///
/// * `odb`
///    - Access to database objects, also for used for diffing.
///    - Should have an object cache for good diff performance.
/// * `suspect`
///    - The first commit to be responsible for parts of `file_path`.
/// * `cache`
///    - Optionally, the commitgraph cache.
/// * `resource_cache`
///    - Used for diffing trees.
/// * `file_path`
///    - A *slash-separated* worktree-relative path to the file to blame.
/// * `options`
///    - An instance of [`Options`].
///
/// ## The algorithm
///
/// *For brevity, `HEAD` denotes the starting point of the blame operation. It could be any commit, or even commits that
/// represent the worktree state.
///
/// We begin with one or more *Unblamed Hunks* and a single suspect, usually the `HEAD` commit as the commit containing the
/// *Blamed File*, so that it contains the entire file, with the first commit being a candidate for the entire *Blamed File*.
/// We traverse the commit graph starting at the first suspect, and see if there have been changes to `file_path`.
/// If so, we have found a *Source File* and a *Suspect* commit, and have hunks that represent these changes.
/// Now the *Unblamed Hunk* is split at the boundaries of each matching change, creating a new *Unblamed Hunk* on each side,
/// along with a [`BlameEntry`] to represent the match.
/// This is repeated until there are no non-empty *Unblamed Hunk*s left.
///
/// At a high level, what we want to do is the following:
///
/// - get the commit
/// - walk through its parents
///   - for each parent, do a diff and mark lines that don’t have a suspect yet (this is the term
///     used in `libgit2`), but that have been changed in this commit
///
/// The algorithm in `libgit2` works by going through parents and keeping a linked list of blame
/// suspects. It can be visualized as follows:
///
/// <---------------------------------------->
/// <---------------><----------------------->
/// <---><----------><----------------------->
/// <---><----------><-------><-----><------->
/// <---><---><-----><-------><-----><------->
/// <---><---><-----><-------><-----><-><-><->
pub fn file(
    odb: impl gix_object::Find + gix_object::FindHeader,
    suspect: ObjectId,
    cache: Option<gix_commitgraph::Graph>,
    resource_cache: &mut gix_diff::blob::Platform,
    file_path: &BStr,
    options: Options,
) -> Result<Outcome, Error> {
    let _span = gix_trace::coarse!("gix_blame::file()", ?file_path, ?suspect);

    let mut stats = Statistics::default();
    let (mut buf, mut buf2, mut buf3) = (Vec::new(), Vec::new(), Vec::new());
    let blamed_file_entry_id = find_path_entry_in_commit(
        &odb,
        &suspect,
        file_path,
        cache.as_ref(),
        &mut buf,
        &mut buf2,
        &mut stats,
    )?
    .ok_or_else(|| Error::FileMissing {
        file_path: file_path.to_owned(),
        commit_id: suspect,
    })?;
    let blamed_file_blob = odb.find_blob(&blamed_file_entry_id, &mut buf)?.data.to_vec();
    let num_lines_in_blamed = tokens_for_diffing(&blamed_file_blob).tokenize().count() as u32;

    // Binary or otherwise empty?
    if num_lines_in_blamed == 0 {
        return Ok(Outcome::default());
    }

    let ranges_to_blame = options.ranges.to_zero_based_exclusive_ranges(num_lines_in_blamed);
    let mut hunks_to_blame = ranges_to_blame
        .into_iter()
        .map(|range| UnblamedHunk::new(range, suspect))
        .collect::<Vec<_>>();

    let (mut buf, mut buf2) = (Vec::new(), Vec::new());
    let commit = find_commit(cache.as_ref(), &odb, &suspect, &mut buf)?;
    let mut queue: gix_revwalk::PriorityQueue<gix_date::SecondsSinceUnixEpoch, ObjectId> =
        gix_revwalk::PriorityQueue::new();
    queue.insert(commit.commit_time()?, suspect);

    let mut out = Vec::new();
    let mut diff_state = gix_diff::tree::State::default();
    let mut previous_entry: Option<(ObjectId, ObjectId)> = None;
    let mut blame_path = if options.debug_track_path {
        Some(Vec::new())
    } else {
        None
    };

    'outer: while let Some(suspect) = queue.pop_value() {
        stats.commits_traversed += 1;
        if hunks_to_blame.is_empty() {
            break;
        }

        let first_hunk_for_suspect = hunks_to_blame.iter().find(|hunk| hunk.has_suspect(&suspect));
        let Some(first_hunk_for_suspect) = first_hunk_for_suspect else {
            // There are no `UnblamedHunk`s associated with this `suspect`, so we can continue with
            // the next one.
            continue 'outer;
        };

        let current_file_path = first_hunk_for_suspect
            .source_file_name
            .clone()
            .unwrap_or_else(|| file_path.to_owned());

        let commit = find_commit(cache.as_ref(), &odb, &suspect, &mut buf)?;
        let commit_time = commit.commit_time()?;

        if let Some(since) = options.since {
            if commit_time < since.seconds {
                if unblamed_to_out_is_done(&mut hunks_to_blame, &mut out, suspect) {
                    break 'outer;
                }

                continue;
            }
        }

        let parent_ids: ParentIds = collect_parents(commit, &odb, cache.as_ref(), &mut buf2)?;

        if parent_ids.is_empty() {
            if queue.is_empty() {
                // I’m not entirely sure if this is correct yet. `suspect`, at this point, is the
                // `id` of the last `item` that was yielded by `queue`, so it makes sense to assign
                // the remaining lines to it, even though we don’t explicitly check whether that is
                // true here. We could perhaps use diff-tree-to-tree to compare `suspect` against
                // an empty tree to validate this assumption.
                if unblamed_to_out_is_done(&mut hunks_to_blame, &mut out, suspect) {
                    if let Some(ref mut blame_path) = blame_path {
                        let entry = previous_entry
                            .take()
                            .filter(|(id, _)| *id == suspect)
                            .map(|(_, entry)| entry);

                        let blame_path_entry = BlamePathEntry {
                            source_file_path: current_file_path.clone(),
                            previous_source_file_path: None,
                            commit_id: suspect,
                            blob_id: entry.unwrap_or(ObjectId::null(gix_hash::Kind::Sha1)),
                            previous_blob_id: ObjectId::null(gix_hash::Kind::Sha1),
                            parent_index: 0,
                        };
                        blame_path.push(blame_path_entry);
                    }

                    break 'outer;
                }
            }
            // There is more, keep looking.
            continue;
        }

        let mut entry = previous_entry
            .take()
            .filter(|(id, _)| *id == suspect)
            .map(|(_, entry)| entry);
        if entry.is_none() {
            entry = find_path_entry_in_commit(
                &odb,
                &suspect,
                current_file_path.as_ref(),
                cache.as_ref(),
                &mut buf,
                &mut buf2,
                &mut stats,
            )?;
        }

        let Some(entry_id) = entry else {
            continue;
        };

        // This block asserts that, for every `UnblamedHunk`, all lines in the *Blamed File* are
        // identical to the corresponding lines in the *Source File*.
        #[cfg(debug_assertions)]
        {
            let source_blob = odb.find_blob(&entry_id, &mut buf)?.data.to_vec();
            let mut source_interner = gix_diff::blob::intern::Interner::new(source_blob.len() / 100);
            let source_lines_as_tokens: Vec<_> = tokens_for_diffing(&source_blob)
                .tokenize()
                .map(|token| source_interner.intern(token))
                .collect();

            let mut blamed_interner = gix_diff::blob::intern::Interner::new(blamed_file_blob.len() / 100);
            let blamed_lines_as_tokens: Vec<_> = tokens_for_diffing(&blamed_file_blob)
                .tokenize()
                .map(|token| blamed_interner.intern(token))
                .collect();

            for hunk in hunks_to_blame.iter() {
                if let Some(range_in_suspect) = hunk.get_range(&suspect) {
                    let range_in_blamed_file = hunk.range_in_blamed_file.clone();

                    let source_lines = range_in_suspect
                        .clone()
                        .map(|i| BString::new(source_interner[source_lines_as_tokens[i as usize]].into()))
                        .collect::<Vec<_>>();
                    let blamed_lines = range_in_blamed_file
                        .clone()
                        .map(|i| BString::new(blamed_interner[blamed_lines_as_tokens[i as usize]].into()))
                        .collect::<Vec<_>>();

                    assert_eq!(source_lines, blamed_lines);
                }
            }
        }

        for (pid, (parent_id, parent_commit_time)) in parent_ids.iter().enumerate() {
            if let Some(parent_entry_id) = find_path_entry_in_commit(
                &odb,
                parent_id,
                current_file_path.as_ref(),
                cache.as_ref(),
                &mut buf,
                &mut buf2,
                &mut stats,
            )? {
                let no_change_in_entry = entry_id == parent_entry_id;
                if pid == 0 {
                    previous_entry = Some((*parent_id, parent_entry_id));
                }
                if no_change_in_entry {
                    pass_blame_from_to(suspect, *parent_id, &mut hunks_to_blame);
                    queue.insert(*parent_commit_time, *parent_id);
                    continue 'outer;
                }
            }
        }

        let more_than_one_parent = parent_ids.len() > 1;
        for (index, (parent_id, parent_commit_time)) in parent_ids.iter().enumerate() {
            queue.insert(*parent_commit_time, *parent_id);
            let changes_for_file_path = tree_diff_at_file_path(
                &odb,
                current_file_path.as_ref(),
                suspect,
                *parent_id,
                cache.as_ref(),
                &mut stats,
                &mut diff_state,
                resource_cache,
                &mut buf,
                &mut buf2,
                &mut buf3,
                options.rewrites,
            )?;
            let Some(modification) = changes_for_file_path else {
                if more_than_one_parent {
                    // None of the changes affected the file we’re currently blaming.
                    // Copy blame to parent.
                    for unblamed_hunk in &mut hunks_to_blame {
                        unblamed_hunk.clone_blame(suspect, *parent_id);
                    }
                } else {
                    pass_blame_from_to(suspect, *parent_id, &mut hunks_to_blame);
                }
                continue;
            };

            match modification {
                TreeDiffChange::Addition { id } => {
                    if more_than_one_parent {
                        // Do nothing under the assumption that this always (or almost always)
                        // implies that the file comes from a different parent, compared to which
                        // it was modified, not added.
                    } else if unblamed_to_out_is_done(&mut hunks_to_blame, &mut out, suspect) {
                        if let Some(ref mut blame_path) = blame_path {
                            let blame_path_entry = BlamePathEntry {
                                source_file_path: current_file_path.clone(),
                                previous_source_file_path: None,
                                commit_id: suspect,
                                blob_id: id,
                                previous_blob_id: ObjectId::null(gix_hash::Kind::Sha1),
                                parent_index: index,
                            };
                            blame_path.push(blame_path_entry);
                        }

                        break 'outer;
                    }
                }
                TreeDiffChange::Deletion => {
                    unreachable!("We already found file_path in suspect^{{tree}}, so it can't be deleted")
                }
                TreeDiffChange::Modification { previous_id, id } => {
                    let changes = blob_changes(
                        &odb,
                        resource_cache,
                        id,
                        previous_id,
                        file_path,
                        file_path,
                        options.diff_algorithm,
                        &mut stats,
                    )?;
                    hunks_to_blame = process_changes(hunks_to_blame, changes.clone(), suspect, *parent_id);
                    if let Some(ref mut blame_path) = blame_path {
                        let has_blame_been_passed = hunks_to_blame.iter().any(|hunk| hunk.has_suspect(parent_id));

                        if has_blame_been_passed {
                            let blame_path_entry = BlamePathEntry {
                                source_file_path: current_file_path.clone(),
                                previous_source_file_path: Some(current_file_path.clone()),
                                commit_id: suspect,
                                blob_id: id,
                                previous_blob_id: previous_id,
                                parent_index: index,
                            };
                            blame_path.push(blame_path_entry);
                        }
                    }
                }
                TreeDiffChange::Rewrite {
                    source_location,
                    source_id,
                    id,
                } => {
                    let changes = blob_changes(
                        &odb,
                        resource_cache,
                        id,
                        source_id,
                        file_path,
                        source_location.as_ref(),
                        options.diff_algorithm,
                        &mut stats,
                    )?;
                    hunks_to_blame = process_changes(hunks_to_blame, changes, suspect, *parent_id);

                    let mut has_blame_been_passed = false;

                    for hunk in hunks_to_blame.iter_mut() {
                        if hunk.has_suspect(parent_id) {
                            hunk.source_file_name = Some(source_location.clone());

                            has_blame_been_passed = true;
                        }
                    }

                    if has_blame_been_passed {
                        if let Some(ref mut blame_path) = blame_path {
                            let blame_path_entry = BlamePathEntry {
                                source_file_path: current_file_path.clone(),
                                previous_source_file_path: Some(source_location.clone()),
                                commit_id: suspect,
                                blob_id: id,
                                previous_blob_id: source_id,
                                parent_index: index,
                            };
                            blame_path.push(blame_path_entry);
                        }
                    }
                }
            }
        }

        hunks_to_blame.retain_mut(|unblamed_hunk| {
            if unblamed_hunk.suspects.len() == 1 {
                if let Some(entry) = BlameEntry::from_unblamed_hunk(unblamed_hunk, suspect) {
                    // At this point, we have copied blame for every hunk to a parent. Hunks
                    // that have only `suspect` left in `suspects` have not passed blame to any
                    // parent, and so they can be converted to a `BlameEntry` and moved to
                    // `out`.
                    out.push(entry);
                    return false;
                }
            }
            unblamed_hunk.remove_blame(suspect);
            true
        });
    }

    debug_assert_eq!(
        hunks_to_blame,
        vec![],
        "only if there is no portion of the file left we have completed the blame"
    );

    // I don’t know yet whether it would make sense to use a data structure instead that preserves
    // order on insertion.
    out.sort_by(|a, b| a.start_in_blamed_file.cmp(&b.start_in_blamed_file));
    Ok(Outcome {
        entries: coalesce_blame_entries(out),
        blob: blamed_file_blob,
        statistics: stats,
        blame_path,
    })
}

/// Pass ownership of each unblamed hunk of `from` to `to`.
///
/// This happens when `from` didn't actually change anything in the blamed file.
fn pass_blame_from_to(from: ObjectId, to: ObjectId, hunks_to_blame: &mut Vec<UnblamedHunk>) {
    for unblamed_hunk in hunks_to_blame {
        unblamed_hunk.pass_blame(from, to);
    }
}

/// Convert each of the unblamed hunk in `hunks_to_blame` into a [`BlameEntry`], consuming them in the process.
///
/// Return `true` if we are done because `hunks_to_blame` is empty.
fn unblamed_to_out_is_done(
    hunks_to_blame: &mut Vec<UnblamedHunk>,
    out: &mut Vec<BlameEntry>,
    suspect: ObjectId,
) -> bool {
    let mut without_suspect = Vec::new();
    out.extend(hunks_to_blame.drain(..).filter_map(|hunk| {
        BlameEntry::from_unblamed_hunk(&hunk, suspect).or_else(|| {
            without_suspect.push(hunk);
            None
        })
    }));
    *hunks_to_blame = without_suspect;
    hunks_to_blame.is_empty()
}

/// This function merges adjacent blame entries. It merges entries that are adjacent both in the
/// blamed file and in the source file that introduced them. This follows `git`’s
/// behaviour. `libgit2`, as of 2024-09-19, only checks whether two entries are adjacent in the
/// blamed file which can result in different blames in certain edge cases. See [the commit][1]
/// that introduced the extra check into `git` for context. See [this commit][2] for a way to test
/// for this behaviour in `git`.
///
/// [1]: https://github.com/git/git/commit/c2ebaa27d63bfb7c50cbbdaba90aee4efdd45d0a
/// [2]: https://github.com/git/git/commit/6dbf0c7bebd1c71c44d786ebac0f2b3f226a0131
fn coalesce_blame_entries(lines_blamed: Vec<BlameEntry>) -> Vec<BlameEntry> {
    let len = lines_blamed.len();
    lines_blamed
        .into_iter()
        .fold(Vec::with_capacity(len), |mut acc, entry| {
            let previous_entry = acc.last();

            if let Some(previous_entry) = previous_entry {
                let previous_blamed_range = previous_entry.range_in_blamed_file();
                let current_blamed_range = entry.range_in_blamed_file();
                let previous_source_range = previous_entry.range_in_source_file();
                let current_source_range = entry.range_in_source_file();
                if previous_entry.commit_id == entry.commit_id
                    && previous_blamed_range.end == current_blamed_range.start
                    // As of 2024-09-19, the check below only is in `git`, but not in `libgit2`.
                    && previous_source_range.end == current_source_range.start
                {
                    let coalesced_entry = BlameEntry {
                        start_in_blamed_file: previous_blamed_range.start as u32,
                        start_in_source_file: previous_source_range.start as u32,
                        len: NonZeroU32::new((current_source_range.end - previous_source_range.start) as u32)
                            .expect("BUG: hunks are never zero-sized"),
                        commit_id: previous_entry.commit_id,
                        source_file_name: previous_entry.source_file_name.clone(),
                    };

                    acc.pop();
                    acc.push(coalesced_entry);
                } else {
                    acc.push(entry);
                }

                acc
            } else {
                acc.push(entry);

                acc
            }
        })
}

/// The union of [`gix_diff::tree::recorder::Change`] and [`gix_diff::tree_with_rewrites::Change`],
/// keeping only the blame-relevant information.
enum TreeDiffChange {
    Addition {
        id: ObjectId,
    },
    Deletion,
    Modification {
        previous_id: ObjectId,
        id: ObjectId,
    },
    Rewrite {
        source_location: BString,
        source_id: ObjectId,
        id: ObjectId,
    },
}

impl From<gix_diff::tree::recorder::Change> for TreeDiffChange {
    fn from(value: gix_diff::tree::recorder::Change) -> Self {
        use gix_diff::tree::recorder::Change;

        match value {
            Change::Addition { oid, .. } => Self::Addition { id: oid },
            Change::Deletion { .. } => Self::Deletion,
            Change::Modification { previous_oid, oid, .. } => Self::Modification {
                previous_id: previous_oid,
                id: oid,
            },
        }
    }
}

impl From<gix_diff::tree_with_rewrites::Change> for TreeDiffChange {
    fn from(value: gix_diff::tree_with_rewrites::Change) -> Self {
        use gix_diff::tree_with_rewrites::Change;

        match value {
            Change::Addition { id, .. } => Self::Addition { id },
            Change::Deletion { .. } => Self::Deletion,
            Change::Modification { previous_id, id, .. } => Self::Modification { previous_id, id },
            Change::Rewrite {
                source_location,
                source_id,
                id,
                ..
            } => Self::Rewrite {
                source_location,
                source_id,
                id,
            },
        }
    }
}

#[allow(clippy::too_many_arguments)]
fn tree_diff_at_file_path(
    odb: impl gix_object::Find + gix_object::FindHeader,
    file_path: &BStr,
    id: ObjectId,
    parent_id: ObjectId,
    cache: Option<&gix_commitgraph::Graph>,
    stats: &mut Statistics,
    state: &mut gix_diff::tree::State,
    resource_cache: &mut gix_diff::blob::Platform,
    commit_buf: &mut Vec<u8>,
    lhs_tree_buf: &mut Vec<u8>,
    rhs_tree_buf: &mut Vec<u8>,
    rewrites: Option<gix_diff::Rewrites>,
) -> Result<Option<TreeDiffChange>, Error> {
    let parent_tree_id = find_commit(cache, &odb, &parent_id, commit_buf)?.tree_id()?;

    let parent_tree_iter = odb.find_tree_iter(&parent_tree_id, lhs_tree_buf)?;
    stats.trees_decoded += 1;

    let tree_id = find_commit(cache, &odb, &id, commit_buf)?.tree_id()?;

    let tree_iter = odb.find_tree_iter(&tree_id, rhs_tree_buf)?;
    stats.trees_decoded += 1;

    let result = tree_diff_without_rewrites_at_file_path(&odb, file_path, stats, state, parent_tree_iter, tree_iter)?;

    // Here, we follow git’s behaviour. We return when we’ve found a `Modification`. We try a
    // second time with rename tracking when the change is either an `Addition` or a `Deletion`
    // because those can turn out to have been a `Rewrite`.
    // TODO(perf): renames are usually rare enough to not care about the work duplication done here.
    //             But in theory, a rename tracker could be used by us, on demand, and we could stuff the
    //             changes in there and have it find renames, without repeating the diff.
    if matches!(result, Some(TreeDiffChange::Modification { .. })) {
        return Ok(result);
    }
    let Some(rewrites) = rewrites else {
        return Ok(result);
    };

    let result = tree_diff_with_rewrites_at_file_path(
        &odb,
        file_path,
        stats,
        state,
        resource_cache,
        parent_tree_iter,
        tree_iter,
        rewrites,
    )?;

    Ok(result)
}

#[allow(clippy::too_many_arguments)]
fn tree_diff_without_rewrites_at_file_path(
    odb: impl gix_object::Find + gix_object::FindHeader,
    file_path: &BStr,
    stats: &mut Statistics,
    state: &mut gix_diff::tree::State,
    parent_tree_iter: gix_object::TreeRefIter<'_>,
    tree_iter: gix_object::TreeRefIter<'_>,
) -> Result<Option<TreeDiffChange>, Error> {
    struct FindChangeToPath {
        inner: gix_diff::tree::Recorder,
        interesting_path: BString,
        change: Option<gix_diff::tree::recorder::Change>,
    }

    impl FindChangeToPath {
        fn new(interesting_path: BString) -> Self {
            let inner =
                gix_diff::tree::Recorder::default().track_location(Some(gix_diff::tree::recorder::Location::Path));

            FindChangeToPath {
                inner,
                interesting_path,
                change: None,
            }
        }
    }

    impl Visit for FindChangeToPath {
        fn pop_front_tracked_path_and_set_current(&mut self) {
            self.inner.pop_front_tracked_path_and_set_current();
        }

        fn push_back_tracked_path_component(&mut self, component: &BStr) {
            self.inner.push_back_tracked_path_component(component);
        }

        fn push_path_component(&mut self, component: &BStr) {
            self.inner.push_path_component(component);
        }

        fn pop_path_component(&mut self) {
            self.inner.pop_path_component();
        }

        fn visit(&mut self, change: gix_diff::tree::visit::Change) -> gix_diff::tree::visit::Action {
            use gix_diff::tree::{visit, visit::Change::*};

            if self.inner.path() == self.interesting_path {
                self.change = Some(match change {
                    Deletion {
                        entry_mode,
                        oid,
                        relation,
                    } => gix_diff::tree::recorder::Change::Deletion {
                        entry_mode,
                        oid,
                        path: self.inner.path_clone(),
                        relation,
                    },
                    Addition {
                        entry_mode,
                        oid,
                        relation,
                    } => gix_diff::tree::recorder::Change::Addition {
                        entry_mode,
                        oid,
                        path: self.inner.path_clone(),
                        relation,
                    },
                    Modification {
                        previous_entry_mode,
                        previous_oid,
                        entry_mode,
                        oid,
                    } => gix_diff::tree::recorder::Change::Modification {
                        previous_entry_mode,
                        previous_oid,
                        entry_mode,
                        oid,
                        path: self.inner.path_clone(),
                    },
                });

                visit::Action::Cancel
            } else {
                visit::Action::Continue
            }
        }
    }

    let mut recorder = FindChangeToPath::new(file_path.into());
    let result = gix_diff::tree(parent_tree_iter, tree_iter, state, &odb, &mut recorder);
    stats.trees_diffed += 1;

    match result {
        Ok(_) | Err(gix_diff::tree::Error::Cancelled) => Ok(recorder.change.map(Into::into)),
        Err(error) => Err(Error::DiffTree(error)),
    }
}

#[allow(clippy::too_many_arguments)]
fn tree_diff_with_rewrites_at_file_path(
    odb: impl gix_object::Find + gix_object::FindHeader,
    file_path: &BStr,
    stats: &mut Statistics,
    state: &mut gix_diff::tree::State,
    resource_cache: &mut gix_diff::blob::Platform,
    parent_tree_iter: gix_object::TreeRefIter<'_>,
    tree_iter: gix_object::TreeRefIter<'_>,
    rewrites: gix_diff::Rewrites,
) -> Result<Option<TreeDiffChange>, Error> {
    let mut change: Option<gix_diff::tree_with_rewrites::Change> = None;

    let options: gix_diff::tree_with_rewrites::Options = gix_diff::tree_with_rewrites::Options {
        location: Some(gix_diff::tree::recorder::Location::Path),
        rewrites: Some(rewrites),
    };
    let result = gix_diff::tree_with_rewrites(
        parent_tree_iter,
        tree_iter,
        resource_cache,
        state,
        &odb,
        |change_ref| -> Result<_, std::convert::Infallible> {
            if change_ref.location() == file_path {
                change = Some(change_ref.into_owned());
                Ok(gix_diff::tree_with_rewrites::Action::Cancel)
            } else {
                Ok(gix_diff::tree_with_rewrites::Action::Continue)
            }
        },
        options,
    );
    stats.trees_diffed_with_rewrites += 1;

    match result {
        Ok(_) | Err(gix_diff::tree_with_rewrites::Error::Diff(gix_diff::tree::Error::Cancelled)) => {
            Ok(change.map(Into::into))
        }
        Err(error) => Err(Error::DiffTreeWithRewrites(error)),
    }
}

#[allow(clippy::too_many_arguments)]
fn blob_changes(
    odb: impl gix_object::Find + gix_object::FindHeader,
    resource_cache: &mut gix_diff::blob::Platform,
    oid: ObjectId,
    previous_oid: ObjectId,
    file_path: &BStr,
    previous_file_path: &BStr,
    diff_algorithm: gix_diff::blob::Algorithm,
    stats: &mut Statistics,
) -> Result<Vec<Change>, Error> {
    /// Record all [`Change`]s to learn about additions, deletions and unchanged portions of a *Source File*.
    struct ChangeRecorder {
        last_seen_after_end: u32,
        hunks: Vec<Change>,
        total_number_of_lines: u32,
    }

    impl ChangeRecorder {
        /// `total_number_of_lines` is used to fill in the last unchanged hunk if needed
        /// so that the entire file is represented by [`Change`].
        fn new(total_number_of_lines: u32) -> Self {
            ChangeRecorder {
                last_seen_after_end: 0,
                hunks: Vec::new(),
                total_number_of_lines,
            }
        }
    }

    impl gix_diff::blob::Sink for ChangeRecorder {
        type Out = Vec<Change>;

        fn process_change(&mut self, before: Range<u32>, after: Range<u32>) {
            // This checks for unchanged hunks.
            if after.start > self.last_seen_after_end {
                self.hunks
                    .push(Change::Unchanged(self.last_seen_after_end..after.start));
            }

            match (!before.is_empty(), !after.is_empty()) {
                (_, true) => {
                    self.hunks.push(Change::AddedOrReplaced(
                        after.start..after.end,
                        before.end - before.start,
                    ));
                }
                (true, false) => {
                    self.hunks.push(Change::Deleted(after.start, before.end - before.start));
                }
                (false, false) => unreachable!("BUG: imara-diff provided a non-change"),
            }
            self.last_seen_after_end = after.end;
        }

        fn finish(mut self) -> Self::Out {
            if self.total_number_of_lines > self.last_seen_after_end {
                self.hunks
                    .push(Change::Unchanged(self.last_seen_after_end..self.total_number_of_lines));
            }
            self.hunks
        }
    }

    resource_cache.set_resource(
        previous_oid,
        gix_object::tree::EntryKind::Blob,
        previous_file_path,
        gix_diff::blob::ResourceKind::OldOrSource,
        &odb,
    )?;
    resource_cache.set_resource(
        oid,
        gix_object::tree::EntryKind::Blob,
        file_path,
        gix_diff::blob::ResourceKind::NewOrDestination,
        &odb,
    )?;

    let outcome = resource_cache.prepare_diff()?;
    let input = gix_diff::blob::intern::InternedInput::new(
        tokens_for_diffing(outcome.old.data.as_slice().unwrap_or_default()),
        tokens_for_diffing(outcome.new.data.as_slice().unwrap_or_default()),
    );
    let number_of_lines_in_destination = input.after.len();
    let change_recorder = ChangeRecorder::new(number_of_lines_in_destination as u32);

    let res = gix_diff::blob::diff(diff_algorithm, &input, change_recorder);
    stats.blobs_diffed += 1;
    Ok(res)
}

fn find_path_entry_in_commit(
    odb: &impl gix_object::Find,
    commit: &gix_hash::oid,
    file_path: &BStr,
    cache: Option<&gix_commitgraph::Graph>,
    buf: &mut Vec<u8>,
    buf2: &mut Vec<u8>,
    stats: &mut Statistics,
) -> Result<Option<ObjectId>, Error> {
    let tree_id = find_commit(cache, odb, commit, buf)?.tree_id()?;
    let tree_iter = odb.find_tree_iter(&tree_id, buf)?;
    stats.trees_decoded += 1;

    let res = tree_iter.lookup_entry(
        odb,
        buf2,
        file_path.split(|b| *b == b'/').inspect(|_| stats.trees_decoded += 1),
    )?;
    stats.trees_decoded -= 1;
    Ok(res.map(|e| e.oid))
}

type ParentIds = SmallVec<[(gix_hash::ObjectId, i64); 2]>;

fn collect_parents(
    commit: gix_traverse::commit::Either<'_, '_>,
    odb: &impl gix_object::Find,
    cache: Option<&gix_commitgraph::Graph>,
    buf: &mut Vec<u8>,
) -> Result<ParentIds, Error> {
    let mut parent_ids: ParentIds = Default::default();
    match commit {
        gix_traverse::commit::Either::CachedCommit(commit) => {
            let cache = cache
                .as_ref()
                .expect("find returned a cached commit, so we expect cache to be present");
            for parent_pos in commit.iter_parents() {
                let parent = cache.commit_at(parent_pos?);
                parent_ids.push((parent.id().to_owned(), parent.committer_timestamp() as i64));
            }
        }
        gix_traverse::commit::Either::CommitRefIter(commit_ref_iter) => {
            for id in commit_ref_iter.parent_ids() {
                let parent = odb.find_commit_iter(id.as_ref(), buf).ok();
                let parent_commit_time = parent
                    .and_then(|parent| parent.committer().ok().map(|committer| committer.seconds()))
                    .unwrap_or_default();
                parent_ids.push((id, parent_commit_time));
            }
        }
    }
    Ok(parent_ids)
}

/// Return an iterator over tokens for use in diffing. These are usually lines, but it's important
/// to unify them so the later access shows the right thing.
pub(crate) fn tokens_for_diffing(data: &[u8]) -> impl TokenSource<Token = &[u8]> {
    gix_diff::blob::sources::byte_lines_with_terminator(data)
}