Assert diff swaps actual and expected when comparing multiline strings

[marvinhagemeister]

Describe the bug

When assertEquals is used to compare a multiline string and the check fails, then the diff output confusingly swaps the order of actual and expected.

Steps to Reproduce

import { assertEquals } from "https://deno.land/std@0.203.0/assert/mod.ts";

Deno.test("multiline", () => {
  const actual = `<div>
  <div>
    <p>
    foo</p>
  </div>
  [object Object],barbaz</div>
</div>  
`;

  const expected = `<div>
  <div>
    <p>foo</p>
    barbaz
  </div>
</div>
`;
  assertEquals(actual, expected);
});

Expected behavior

It should not swap the order. It should mark actual as "actual" and expected as "expected".

Environment

• OS: [e.g. Ubuntu 20.04, MacOS 11]
• deno version: 1.37.1
• std version: 0.203.0

[marvinhagemeister]

Might be related #3039

[lucacasonato]

Duplicate of #3039

[kt3k]

It looks like this happens only when actual has more diff lines than expected. I'll look into more details.

import { assertEquals } from "https://deno.land/std@0.203.0/assert/mod.ts";
Deno.test("multiline", () => {
  assertEquals("a\na", "e"); // diff swaps
});
Deno.test("multiline", () => {
  assertEquals("a\na", "e\ne"); // diff doesn't swap
});
Deno.test("multiline", () => {
  assertEquals("a", "e\ne"); // diff doesn't swap
});

[marvinhagemeister]

@kt3k From what I can tell the code uses a variant of the myers algorithm. A common optmization there is to ensure that the grid is always in portrait mode, because the algorithm searches for the biggest x value. It might just be that the code forgets to swap the insertion/deletions back to the original landscape grid.