Optimize Hash#transform_{keys,values}
#14502
Merged
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Preallocating the correct-size hash is faster for hash sizes > 4, but the empty hash is faster for sizes <= 4. It would be a lot closer, but
Hash#upsert
allocates an initial capacity of 4 when the hash is empty butHash#initialize
sets the floor forinitial_capacity
at 8 elements.This PR gives the best of both worlds by using the current approach when the source hash has <= 4 keys in it, or a fully preallocated hash for > 4.
Benchmark code