fix(parser/diff): pyMod

internal/parser/diff/myers.go

@@ -119,11 +119,13 @@ func diffInternal(e, f []any, equals func(any, any) bool, i, j int) []Op {
  * e.g -1%3 = 2.
  * In golang it matches the sign of the numerator.
  * See https://en.wikipedia.org/wiki/Modulo_operation#Variants_of_the_definition
- * Since we always have a positive denominator here, we can emulate the
- * pyMod x%y as (x+y) % y
  */
 func pyMod(x, y int) int {
-	return (x + y) % y
+	mod := x % y
+	if mod < 0 {
+		mod += y
+	}
+	return mod
 }
 
 // Let us map element in same way as in

internal/parser/diff/myers_test.go

@@ -18,6 +18,7 @@ func TestDiff(t *t.T) {
 	testCases := []TestCase{
 		{[]string{}, []string{}, []Op{}},
 		{[]string{}, []string{"foo"}, []Op{{OpInsert, 0, 0, "foo"}}},
+		{[]string{"foo"}, []string{}, []Op{{OpDelete, 0, -1, "foo"}}},
 		{[]string{"foo", "bar", "baz"}, []string{"foo", "bar", "baz"}, []Op{}},
 		{[]string{"foo", "bar", "baz"}, []string{"foo", "baz"}, []Op{{OpDelete, 1, -1, "bar"}}},
 		{[]string{"baz"}, []string{"foo", "baz"}, []Op{{OpInsert, 0, 0, "foo"}}},