Document where git diff and gix-imara-diff diverge and potentially narrow the gap

[Christoph Rüßler (cruessler)]

This is a tracking issue related to what’s known as the “diff slider problem”. When calculating the diff between two versions of a file, a hunk can be ambiguous with respect to where it starts and ends. Consider the following example:

 fn test_a_different_thing() {
 }
 
+#[test]
+fn test_something() {
+}
+
 #[test]
 fn test_something_else() {
 } 

The following diff is, technically speaking, equally valid, even though human readers most likely would prefer the above diff.

 fn test_a_different_thing() {
 }
 
 #[test]
+fn test_something() {
+}
+
+#[test]
 fn test_something_else() {
 } 

gix-diff relies on gix-imara-diff (ported from imara-diff 0.2) under the hood for it’s diffing algorithm. There’s a certain number of cases, though, where gix-imara-diff and git diff yield different results as they employ different heuristics with respect to moving ambiguous diff hunks up or down. This number used to be bigger when gix-diff was still using imara-diff 0.1.

We assume that most people would expect gix-diff to yield the same results as git diff. Based on this assumption, we want to track progress towards closing the gap between gix-diff and git diff with respect to slider placement.

Tasks

• Extract a set of baseline tests for use in gix-diff/tests of gix-imara-diff/tests
• Document how closely gix-imara-diff matches git diff

Sources

https://github.com/mhagger/diff-slider-tools

Related

• Add feature flag blame-experimental to gix #2307
• Update to imara-diff 0.2 in diff slider test #2288
• Update gix-blame to imara-diff 0.2 #2287
• Add imara-diff v0.2 with slider heuristics support #2286
• Account for "\ No newline at end of file" in baseline diff parser #2285
• Report number of non-matching diffs in slider test #2276
• Exclude expectrl and imara-diff from grouped version updates #2269
• Add script to generate tests cases for gix-diff #2197

• Merged, but later reverted: Add Sink that implements git's diffing improvement heuristics #2011