assert: make assertion_error user myers diff algorithm

[puskin94]

Fixes: #51733

Co-Authored-By: Pietro Marchini [email protected]

Why

As discussed in the linked issue, the diff algorithm used to display errors during assertions was a "quick, best-effort implementation." After researching, I found that there aren't many diff algorithms faster than Myers' algorithm, which I’ve implemented in this PR.

How

Following this article on Myers' diff algorithm: https://blog.jcoglan.com/2017/02/12/the-myers-diff-algorithm-part-1/, I implemented the algorithm. Myers' algorithm is relatively simple, based on a matrix of size N * M, where N is the length of the first string and M is the length of the second string. Although it’s primarily used for string comparison, I’ve adapted it here to handle the results of the assert function.

Differences with the old implementation

In addition to changing the diff algorithm, I also updated the way diffs are displayed:

1. Myers' algorithm excels at finding the minimal changes required to transform one string into another, making it ideal for diffing.

example:

const assert = require('assert');

const actual = {
  a: {
    b: {
      c: 1
    }
  }
};
const expected = {
  a: {
    b: {
      c: 1
    },
    d: {
      c: 1
    }
  }
};

assert.strictEqual(actual, expected);

Old implementation:

New implementation:

Another example:

const assert = require('assert');

const actual = {a: ['AAAA', '12345', '6789', 'BC!!!GHI']};
const expected = {a: ['12345', '6789', 'AAAA', 'BC!!!GHI']};

assert.deepStrictEqual(actual, expected);

Old implementation:

New implementation:

2. The ^ indicator is now only displayed for string diffs when there is sufficient space in the terminal and colors are not supported, otherwise the new Myers algorithm is used for single strings too:

example:

const assert = require('assert');

const actual = '123456789ABCDEFGHI';
const expected = '1!3!5!7!9!BC!!!GHI';

assert.strictEqual(actual, expected);

Old implementation:

New implementation:

OR

const assert = require('assert');

process.env.NODE_DISABLE_COLORS = '1';

const actual = '123456789ABCDEFGHI';
const expected = '12!!5!7!9!BC!!!GHI';

assert.strictEqual(actual, expected);

3. The new implementation is faster than the old one.
4. Standardized spacings, indendations and new lines in the diff output.

[codecov]

Codecov Report

Attention: Patch coverage is 88.52459% with 35 lines in your changes missing coverage. Please review.

Project coverage is 88.40%. Comparing base (231d5e4) to head (508d1fe).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/assert/assertion_error.js	86.95%	17 Missing and 1 partial ⚠️
lib/internal/assert/myers_diff.js	89.82%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #54862      +/-   ##
==========================================
- Coverage   88.41%   88.40%   -0.02%     
==========================================
  Files         652      653       +1     
  Lines      186889   186952      +63     
  Branches    36059    36075      +16     
==========================================
+ Hits       165239   165272      +33     
- Misses      14901    14926      +25     
- Partials     6749     6754       +5     

Files with missing lines	Coverage Δ
lib/internal/assert/myers_diff.js	89.82% <89.82%> (ø)
lib/internal/assert/assertion_error.js	95.14% <86.95%> (-4.86%)	⬇️

... and 38 files with indirect coverage changes

[targos]

@nodejs/assert

[RedYetiDev]

The new implementation is faster than the old one.

needs-benchmark-ci

[RedYetiDev]

IMO in the assert documentation, the Myers algorithm should be mentioned as the algorithm in use?

[puskin94]

IMO in the assert documentation, the Myers algorithm should be mentioned as the algorithm in use?

this is not changing what algorithm assert is using, but the algorithm to show the differences when an assertion fails 😄

[lpinca]

There is a typo in commit title: user -> use.

[puskin94]

There is a typo in commit title: user -> use.

oops, good catch! fixed 😄

[RedYetiDev]

nit: Commit mysers -> Myers

[puskin94]

@RedYetiDev updated the messages about skipped lines and the commit message

[RedYetiDev]

Per https://openjs-foundation.slack.com/archives/C019Y2T6STH/p1726429431411589:

Is this a breaking change? It modifies the output of an error, an programs may be relying on that output.

[RedYetiDev]

Are these prototypes needed?

+ one question

[puskin94]

Is there anything left on my side to push this forward?

[pmarchini]

Maybe @BridgeAR could be interested in this, considering this comment: #51733 (comment) 😁

[BridgeAR]

I think a few more persons should chime in to provide an opinion about the colored string comparison. The example above is a bit extreme and we might need a heuristic to limit that to e.g., a percentage of the input that may be different or a fixed number of changes before it falls back to the stacked version.

[puskin94]

More examples for the string colored Myers diff:

And this is a longer example with an example I found online filled with grammatical errors:

[BridgeAR]

I marked this as semver-minor due to the change how strings would be compared. I do believe the contribution can be considered a new feature.

[BridgeAR]

@MoLow @mcollina @jasnell @atlowChemi @aduh95 PTAL

You recently reviewed other assert PRs, so I thought it's good to give you a ping. A main focus would be the new string formatting. The other changes are just improvements that will limit the overall output for multiple cases.

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63124/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63129/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63134/

[pmarchini]

LGTM

[pmarchini]

nit: I've noticed that we now have several changes related to the trailing newline, was this introduced intentionally? 😁

[puskin94]

@pmarchini yessir, it is point 5 and it got approved by @BridgeAR as well :)

[mcollina]

lgtm

[nodejs-github-bot]

Landed in 4d6d7d6