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assert: improve deepEqual perf for large input #12849

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assert: improve deepEqual perf for large input 
Use a Map instead of an array for checking previously found
cyclic references.

This reduces complexity for an array-of-objects case from
O(n²) to O(n·log n).

Fixes: #12842
PR-URL: #12849
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
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@addaleax
addaleax committed May 7, 2017
commit bc62437fb6832e57b96f76b66307cc4332c241d1
21 changes: 15 additions & 6 deletions lib/assert.js
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Original file line number Diff line number Diff line change
Expand Up @@ -285,15 +285,24 @@ function _deepEqual(actual, expected, strict, memos) {
// Note: this accounts for both named and indexed properties on Arrays.

// Use memos to handle cycles.
memos = memos || { actual: [], expected: [] };
const actualIndex = memos.actual.indexOf(actual);
if (actualIndex !== -1) {
if (actualIndex === memos.expected.indexOf(expected)) {
if (!memos) {
memos = {
actual: { map: new Map(), position: 0 },

@joyeecheung joyeecheung May 5, 2017

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How about a WeakMap?

@addaleax addaleax May 5, 2017

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@joyeecheung That thought occurred to me and I couldn’t really find a difference … would there be one? Is that going to be more performant?

@joyeecheung joyeecheung May 5, 2017

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@addaleax I think the GC pressure would be smaller after we leave the function because the references don't have be traversed?

@addaleax addaleax May 5, 2017

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@joyeecheung ok, updated :)

expected: { map: new Map(), position: 0 }
};
}

const actualPosition = memos.actual.map.get(actual);
if (actualPosition !== undefined) {
if (actualPosition === memos.expected.map.get(expected)) {

@TimothyGu TimothyGu May 5, 2017

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Changing from Array.prototype.indexOf (which uses Strict Equality Comparison) to Map.prototype.get (which uses SameValueZero) can have subtle nuances that might result in incompatibilities, especially around NaN. Not sure if we need to care about it though.

@addaleax addaleax May 5, 2017

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I don’t think that actually affects the logic here – if it matters at all, it should only affect cases where non-primitives compare deep-equal to primitives, but that doesn’t seem to happen anyway (e.g. assert.deepEqual([0], 0) throws even though [0] == 0).

return true;
}
} else {
memos.actual.map.set(actual, memos.actual.position++);
}
if (!memos.expected.map.has(expected)) {
memos.expected.map.set(expected, memos.expected.position++);
}
memos.actual.push(actual);
memos.expected.push(expected);

return objEquiv(actual, expected, strict, memos);
}
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