feat: provider new rename-tracking faciliites.

They generalize reneame tracking to the point where it can work for different
kinds of changes.

There is still some way to go until it is truly correct though, as it still
lacks worktree conversions and diff filters.

Author: Byron

Cargo.lock

@@ -12,7 +12,7 @@ autotests = false
 
 [features]
 default = ["blob"]
-## Enable diffing of blobs using imara-diff.
+## Enable diffing of blobs using imara-diff, which also allows for a generic rewrite tracking implementation.
 blob = ["dep:imara-diff"]
 ## Data structures implement `serde::Serialize` and `serde::Deserialize`.
 serde = ["dep:serde", "gix-hash/serde", "gix-object/serde"]
@@ -25,10 +25,12 @@ doctest = false
 [dependencies]
 gix-hash = { version = "^0.13.1", path = "../gix-hash" }
 gix-object = { version = "^0.38.0", path = "../gix-object" }
+
 thiserror = "1.0.32"
 imara-diff = { version = "0.1.3", optional = true }
 serde = { version = "1.0.114", optional = true, default-features = false, features = ["derive"]}
 getrandom = { version = "0.2.8", optional = true, default-features = false, features = ["js"] }
+bstr = { version = "1.5.0", default-features = false }
 
 document-features = { version = "0.2.0", optional = true }
 

gix-diff/Cargo.toml

@@ -1,3 +1,18 @@
 //! For using text diffs, please have a look at the [`imara-diff` documentation](https://docs.rs/imara-diff),
 //! maintained by [Pascal Kuthe](https://github.com/pascalkuthe).
+//!
+//!
+/// Information about the diff performed to detect similarity.
+#[derive(Debug, Default, Clone, Copy, Eq, PartialEq)]
+pub struct DiffLineStats {
+    /// The amount of lines to remove from the source to get to the destination.
+    pub removals: u32,
+    /// The amount of lines to add to the source to get to the destination.
+    pub insertions: u32,
+    /// The amount of lines of the previous state, in the source.
+    pub before: u32,
+    /// The amount of lines of the new state, in the destination.
+    pub after: u32,
+}
+
 pub use imara_diff::*;

gix-diff/src/blob.rs

@@ -8,6 +8,34 @@ cfg_attr(doc, doc = ::document_features::document_features!())
 #![deny(missing_docs, rust_2018_idioms)]
 #![forbid(unsafe_code)]
 
+/// A structure to capture how to perform rename and copy tracking, used by the [rewrites::Tracker].
+#[derive(Debug, Copy, Clone, PartialEq)]
+#[cfg(feature = "blob")]
+pub struct Rewrites {
+    /// If `Some(…)`, also find copies. `None` is the default which does not try to detect copies at all.
+    ///
+    /// Note that this is an even more expensive operation than detecting renames stemming from additions and deletions
+    /// as the resulting set to search through is usually larger.
+    pub copies: Option<rewrites::Copies>,
+    /// The percentage of similarity needed for files to be considered renamed, defaulting to `Some(0.5)`.
+    /// This field is similar to `git diff -M50%`.
+    ///
+    /// If `None`, files are only considered equal if their content matches 100%.
+    /// Note that values greater than 1.0 have no different effect than 1.0.
+    pub percentage: Option<f32>,
+    /// The amount of files to consider for fuzzy rename or copy tracking. Defaults to 1000, meaning that only 1000*1000
+    /// combinations can be tested for fuzzy matches, i.e. the ones that try to find matches by comparing similarity.
+    /// If 0, there is no limit.
+    ///
+    /// If the limit would not be enough to test the entire set of combinations, the algorithm will trade in precision and not
+    /// run the fuzzy version of identity tests at all. That way results are never partial.
+    pub limit: usize,
+}
+
+/// Contains a [Tracker](rewrites::Tracker) to detect rewrites.
+#[cfg(feature = "blob")]
+pub mod rewrites;
+
 ///
 pub mod tree;
 

gix-diff/src/lib.rs

@@ -0,0 +1,77 @@
+use crate::Rewrites;
+
+/// Types related to the rename tracker for renames, rewrites and copies.
+pub mod tracker;
+
+/// A type to retain state related to an ongoing tracking operation to retain sets of interesting changes
+/// of which some are retained to at a later stage compute the ones that seem to be renames or copies.
+pub struct Tracker<T> {
+    /// The tracked items thus far, which will be used to determine renames/copies and rewrites later.
+    items: Vec<tracker::Item<T>>,
+    /// A place to store all paths in to reduce amount of allocations.
+    path_backing: Vec<u8>,
+    /// A buffer for use when fetching objects for similarity tests.
+    buf1: Vec<u8>,
+    /// Another buffer for use when fetching objects for similarity tests.
+    buf2: Vec<u8>,
+    /// How to track copies and/or rewrites.
+    rewrites: Rewrites,
+    /// The diff algorithm to use when checking for similarity.
+    diff_algo: crate::blob::Algorithm,
+}
+
+/// Determine in which set of files to search for copies.
+#[derive(Default, Debug, Copy, Clone, Eq, PartialEq)]
+pub enum CopySource {
+    /// Find copies from the set of modified files only.
+    #[default]
+    FromSetOfModifiedFiles,
+    /// Find copies from the set of modified files, as well as all files known to the source (i.e. previous state of the tree).
+    ///
+    /// This can be an expensive operation as it scales exponentially with the total amount of files in the set.
+    FromSetOfModifiedFilesAndAllSources,
+}
+
+/// Under which circumstances we consider a file to be a copy.
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub struct Copies {
+    /// The set of files to search when finding the source of copies.
+    pub source: CopySource,
+    /// Equivalent to [`Rewrites::percentage`], but used for copy tracking.
+    ///
+    /// Useful to have similarity-based rename tracking and cheaper copy tracking.
+    pub percentage: Option<f32>,
+}
+
+impl Default for Copies {
+    fn default() -> Self {
+        Copies {
+            source: CopySource::default(),
+            percentage: Some(0.5),
+        }
+    }
+}
+
+/// Information collected while handling rewrites of files which may be tracked.
+#[derive(Default, Clone, Copy, Debug, PartialEq)]
+pub struct Outcome {
+    /// The options used to guide the rewrite tracking. Either fully provided by the caller or retrieved from git configuration.
+    pub options: Rewrites,
+    /// The amount of similarity checks that have been conducted to find renamed files and potentially copies.
+    pub num_similarity_checks: usize,
+    /// Set to the amount of worst-case rename permutations we didn't search as our limit didn't allow it.
+    pub num_similarity_checks_skipped_for_rename_tracking_due_to_limit: usize,
+    /// Set to the amount of worst-case copy permutations we didn't search as our limit didn't allow it.
+    pub num_similarity_checks_skipped_for_copy_tracking_due_to_limit: usize,
+}
+
+/// The default settings for rewrites according to the git configuration defaults.
+impl Default for Rewrites {
+    fn default() -> Self {
+        Rewrites {
+            copies: None,
+            percentage: Some(0.5),
+            limit: 1000,
+        }
+    }
+}