Rearrange equals and inserts for shorter diff scripts

Closes #7169.

Author: José Valim

lib/elixir/lib/list.ex

@@ -885,19 +885,24 @@ defmodule List do
     compact_reverse(rest, [{kind, [elem | result]} | acc])
   end
 
+  defp compact_reverse(rest, [{:eq, elem}, {:ins, elem}, {:eq, other} | acc]) do
+    compact_reverse(rest, [{:ins, elem}, {:eq, elem ++ other} | acc])
+  end
+
   defp compact_reverse([{kind, elem} | rest], acc) do
     compact_reverse(rest, [{kind, [elem]} | acc])
   end
 
   defp each_diagonal(diag, limit, _paths, next_paths) when diag > limit do
-    {:next, Enum.reverse(next_paths)}
+    {:next, :lists.reverse(next_paths)}
   end
 
   defp each_diagonal(diag, limit, paths, next_paths) do
     {path, rest} = proceed_path(diag, limit, paths)
 
-    with {:cont, path} <- follow_snake(path) do
-      each_diagonal(diag + 2, limit, rest, [path | next_paths])
+    case follow_snake(path) do
+      {:cont, path} -> each_diagonal(diag + 2, limit, rest, [path | next_paths])
+      {:done, edits} -> {:done, edits}
     end
   end
 

lib/elixir/test/elixir/list_test.exs

@@ -217,14 +217,30 @@ defmodule ListTest do
     end
   end
 
-  test "myers_difference/2" do
-    assert List.myers_difference([], []) == []
-    assert List.myers_difference([], [1, 2, 3]) == [ins: [1, 2, 3]]
-    assert List.myers_difference([1, 2, 3], []) == [del: [1, 2, 3]]
-    assert List.myers_difference([1, 2, 3], [1, 2, 3]) == [eq: [1, 2, 3]]
-    assert List.myers_difference([1, 2, 3], [1, 4, 2, 3]) == [eq: [1], ins: [4], eq: [2, 3]]
-    assert List.myers_difference([1, 4, 2, 3], [1, 2, 3]) == [eq: [1], del: [4], eq: [2, 3]]
-    assert List.myers_difference([1], [[1]]) == [del: [1], ins: [[1]]]
-    assert List.myers_difference([[1]], [1]) == [del: [[1]], ins: [1]]
+  describe "myers_difference/2" do
+    test "follows paper implementation" do
+      assert List.myers_difference([], []) == []
+      assert List.myers_difference([], [1, 2, 3]) == [ins: [1, 2, 3]]
+      assert List.myers_difference([1, 2, 3], []) == [del: [1, 2, 3]]
+      assert List.myers_difference([1, 2, 3], [1, 2, 3]) == [eq: [1, 2, 3]]
+      assert List.myers_difference([1, 2, 3], [1, 4, 2, 3]) == [eq: [1], ins: [4], eq: [2, 3]]
+      assert List.myers_difference([1, 4, 2, 3], [1, 2, 3]) == [eq: [1], del: [4], eq: [2, 3]]
+      assert List.myers_difference([1], [[1]]) == [del: [1], ins: [[1]]]
+      assert List.myers_difference([[1]], [1]) == [del: [[1]], ins: [1]]
+    end
+
+    test "rearranges inserts and equals for smaller diffs" do
+      assert List.myers_difference([3, 2, 0, 2], [2, 2, 0, 2]) ==
+               [del: [3], ins: [2], eq: [2, 0, 2]]
+
+      assert List.myers_difference([3, 2, 1, 0, 2], [2, 1, 2, 1, 0, 2]) ==
+               [del: [3], ins: [2, 1], eq: [2, 1, 0, 2]]
+
+      assert List.myers_difference([3, 2, 2, 1, 0, 2], [2, 2, 1, 2, 1, 0, 2]) ==
+               [del: [3], eq: [2, 2, 1], ins: [2, 1], eq: [0, 2]]
+
+      assert List.myers_difference([3, 2, 0, 2], [2, 2, 1, 0, 2]) ==
+               [del: [3], eq: [2], ins: [2, 1], eq: [0, 2]]
+    end
   end
 end