chore: Add performance benchmark to diff-sequences package

[pedrottimark]

Summary

Compare improved diff-sequences package to baseline diff package.

• Added perf/index.js file
• Added perf/example.md file which contains copied result of a run
• Added a command to "scripts" in package.json file
• Added benchmark and diff as devDependencies

Because allocating and freeing of temporary objects is the root of the performance problem, the tests call global.gc() before every test cycle, so make sure to run node with --expose-gc option!

Above 2000 items, the benchmark package can’t keep the relative mean error below its target of 1%

For example, notice in next to last row of example.md that an outlier low 0.0083 ratio corresponds to outlier high 3.60% baseline rme: an inaccurate high denominator causes low ratio.

Your critique is always welcome and especially because perf benchmark is new for me :)

P.S. I added /* eslint import/no-extraneous-dependencies: "off" */ because the rule demanded that benchmark and diff become dependencies instead of devDependencies

Test plan

To achieve 1% relative mean error, run the benchmark:

• immediately after restart
• with 100% battery charge
• not connected to network

I ran it with node 10.15.0 not via yarn in subdirectory: node --expose-gc perf/index.js

See most values of ratio are between 0.05 and 0.15

The most relevant test for catastrophic performance problems is insert 80% that is, after every 2 expected items, insert 8 unexpected items:

• diff in report when test fails before implementation has been written Jest stalls after comparing to complex objects #1772
• diff in report when test fails because received object has unexpected properties If the received is different from the expected in some obvious way, reject it without full diff #6976
• diff of snapshot when test criterion changes Jest tests hanging forever with large snapshot changes #5392

[SimenB]

This is awesome, great job!

[thymikee]

Would be cool to include some tests that favor longer lines instead of more lines and see how that compares. Looks good:)

[SimenB]

@pedrottimark thoughts on @thymikee's comment?

[pedrottimark]

Yes, it is a great point to add more realistic tests.

@thymikee review comments always stretch my thinking: it hit me just now that an additional benchmark is compare diff-sequences to the amazing diff-match-patch for strings (array of characters) and see how much it cost to gain the flexibility of callback functions. Not realistic for me to write that test before Jest 24 though.

For y’all info, a long term goal to solve edge cases in report when assertion fails is for expect package to compute the differences as a recursive array tuple data structure (so type information isn’t lost by premature serialization) and therefore diff becomes more tactical (and catastrophic edge cases we solved will not occur). I have written some rough prototype code and the next step is for me to open an issue so we can discuss and develop some realistic tests of both clarity and performance.

[thymikee]

Not realistic for me to write that test before Jest 24 though

Don't bother it now, it's just a nice-to-have for somewhere later. If you have any personal to-do list, it's worth to put it there, or create an issue to not forget about a followup :)

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