update README for patience diff

Author: myfreess

diff/README.mbt.md

@@ -1,6 +1,7 @@
-# Myers Diff
+# Diff
 
-Compute edit scripts between two sequences using the Myers diff algorithm.
+Compute edit scripts between two sequences using the Myers diff algorithm by
+default, or patience diff when you pass `patience=true`.
 
 `Diff` works with any element type that implements `Hash + Eq`. Constructing a
 `Diff[T]` bundles the source arrays with the edit script. Call `group` on the
@@ -33,6 +34,43 @@ test "Diff computes deletes inserts and equals" {
 }
 ```
 
+## Prefer Unique Anchors With Patience Diff
+
+Pass `patience=true` to `Diff(old~, new~, patience=true)` to enable patience
+diff. This first finds elements that appear exactly once in both inputs and
+uses them as anchors, then runs Myers diff on the unmatched ranges between
+those anchors. This can produce more stable result when repeated elements move
+around.
+
+```mbt check
+///|
+test "patience diff keeps unique anchors in place" {
+  let old = ["unique", "dup", "dup"][:]
+  let new = ["dup", "unique", "dup"][:]
+
+  let myers = @diff.Diff(old~, new~)
+  let patience = @diff.Diff(old~, new~, patience=true)
+
+  assert_true(
+    myers.edits[:]
+    is [
+      Delete(old_index=0, new_index=0, old_len=1),
+      Equal(old_index=1, new_index=0, len=1),
+      Insert(old_index=2, new_index=1, new_len=1),
+      Equal(old_index=2, new_index=2, len=1),
+    ],
+  )
+  assert_true(
+    patience.edits[:]
+    is [
+      Insert(old_index=0, new_index=0, new_len=1),
+      Equal(old_index=0, new_index=1, len=2),
+      Delete(old_index=2, new_index=3, old_len=1),
+    ],
+  )
+}
+```
+
 ## Group Into Hunks And Render
 
 `group` splits the edit script into `Hunk[T]` values, keeping `radius` lines