Avoid string allocation when git diffing

helix-vcs/Cargo.toml

@@ -13,6 +13,7 @@ homepage = "https://helix-editor.com"
 [dependencies]
 git2 = { version = "0.13", default-features = false }
 similar = "2.1"
+ropey = "1.3"
 
 [dev-dependencies]
 tempfile = "3.3"

helix-vcs/src/git.rs

@@ -5,9 +5,10 @@ use std::{
 };
 
 use git2::{Oid, Repository};
+use ropey::Rope;
 use similar::DiffTag;
 
-use crate::{LineDiff, LineDiffs, RepoRoot};
+use crate::{rope::RopeLine, LineDiff, LineDiffs, RepoRoot};
 
 pub struct Git {
     repo: Repository,
@@ -17,7 +18,7 @@ pub struct Git {
 
     /// A cache mapping absolute file paths to file contents
     /// in the HEAD commit.
-    head_cache: HashMap<PathBuf, String>,
+    head_cache: HashMap<PathBuf, Rope>,
 }
 
 impl std::fmt::Debug for Git {
@@ -54,7 +55,7 @@ impl Git {
         path.strip_prefix(&self.root).ok()
     }
 
-    pub fn read_file_from_head(&mut self, file: &Path) -> Option<&str> {
+    pub fn read_file_from_head(&mut self, file: &Path) -> Option<&Rope> {
         let current_head = Self::head_commit_id(&self.repo)?;
         // TODO: Check cache validity on events like WindowChange
         // instead of on every keypress ? Will require hooks.
@@ -70,30 +71,28 @@ impl Git {
             let blob = object.peel_to_blob().ok()?;
             let contents = std::str::from_utf8(blob.content()).ok()?;
             self.head_cache
-                .insert(file.to_path_buf(), contents.to_string());
+                .insert(file.to_path_buf(), Rope::from_str(contents));
         }
 
-        self.head_cache.get(file).map(|s| s.as_str())
+        self.head_cache.get(file)
     }
 
-    pub fn line_diff_with_head(&mut self, file: &Path, contents: &str) -> LineDiffs {
-        let base = match self.read_file_from_head(file) {
-            Some(b) => b,
+    pub fn line_diff_with_head(&mut self, file: &Path, contents: &Rope) -> LineDiffs {
+        let old = match self.read_file_from_head(file) {
+            Some(rope) => RopeLine::from_rope(rope),
             None => return LineDiffs::new(),
         };
-        let mut config = similar::TextDiff::configure();
-        config.timeout(std::time::Duration::from_millis(250));
+        let new = RopeLine::from_rope(contents);
 
         let mut line_diffs: LineDiffs = HashMap::new();
-
         let mut mark_lines = |range: Range<usize>, change: LineDiff| {
             for line in range {
                 line_diffs.insert(line, change);
             }
         };
 
-        let diff = config.diff_lines(base, contents);
-        for op in diff.ops() {
+        let diff = similar::capture_diff_slices(similar::Algorithm::Myers, &old, &new);
+        for op in diff {
             let (tag, _, line_range) = op.as_tag_tuple();
             let start = line_range.start;
             match tag {
@@ -173,15 +172,16 @@ mod test {
         exec_git_cmd("commit -m message", git_dir);
 
         let mut git = Git::discover_from_path(&file).unwrap();
+        let rope = Rope::from_str(contents);
         assert_eq!(
-            Some(contents),
+            Some(&rope),
             git.read_file_from_head(&file),
             "Wrong blob contents from HEAD on clean index"
         );
 
         fs::write(&file, "new text").expect("Could not write to file");
         assert_eq!(
-            Some(contents),
+            Some(&rope),
             git.read_file_from_head(&file),
             "Wrong blob contents from HEAD when index is dirty"
         );

helix-vcs/src/lib.rs

@@ -1,4 +1,5 @@
 mod git;
+mod rope;
 
 use std::{
     cell::RefCell,

helix-vcs/src/rope.rs

@@ -0,0 +1,20 @@
+use std::hash::Hash;
+
+use ropey::Rope;
+
+#[derive(PartialEq, Eq, PartialOrd, Ord, Clone)]
+pub struct RopeLine<'a>(pub ropey::RopeSlice<'a>);
+
+impl Hash for RopeLine<'_> {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        for chunk in self.0.chunks() {
+            chunk.hash(state);
+        }
+    }
+}
+
+impl<'a> RopeLine<'a> {
+    pub fn from_rope(rope: &'a Rope) -> Vec<Self> {
+        rope.lines().into_iter().map(RopeLine).collect()
+    }
+}

helix-view/src/document.rs

@@ -624,7 +624,7 @@ impl Document {
             .as_ref()
             .and_then(|v| v.try_borrow_mut().ok());
         if let Some((mut vcs, path)) = vcs.zip(self.path()) {
-            self.line_diffs = vcs.line_diff_with_head(path, &self.text().to_string());
+            self.line_diffs = vcs.line_diff_with_head(path, self.text());
         }
     }