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BuiltList won't compute hash as part of ==. #269

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BuiltList won't compute hash as part of ==. #269

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Commits on Jan 3, 2023

  1. BuiltList won't compute hash as part of ==. 

    The implementation of `BuiltList.operator==` used `this.hashCode != other.hashCode` as a short-cut in operator `==`.
However, computing `hashCode` can be as expensive as doing equality, since it has to visit all the same objects (otherwise the hash code won't be consistent with equality) and do a computation on each.

This change makes the `hashCode` only be used if it's already computed by both `this` and `other`, in which case it should be free.

It's a change in performance-behavior. A single `==` won't do double work, but repeated `==`s on the same objects might have been more efficient if the hash code was computed eagerly. For elements in a set or map, the hash code will be computed anyway, so it should not affect those. Having a list of `BuiltList`s and doing repeated `indexOf` with the same values on the list might suffer.
I believe the trade-off is worth it, because most lists are not compared for equality, and those that are, are either one-off comparisons, or used as map keys or set elements.
    @lrhn
    lrhn authored Jan 3, 2023
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