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Add more robust hashing based on a paper

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The paper in question can be referenced [here](https://courses.cs.washington.edu/courses/cse521/15sp/refs/thorup1.pdf)

This will compute a hash that is almost just as performant as the
previous hashing, but handles the case where if lower numbered bits
are all 0 and the highest bit of the length of the container is less
than the lowest bit of the key, the hash will always be 0.
This commit is contained in:
Joey Yakimowich-Payne 2020-02-12 13:40:58 -07:00
commit 437461d809
2 changed files with 56 additions and 14 deletions
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10
lib/pure/collections/hashcommon.nim
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@ -50,8 +50,16 @@ template rawGetKnownHCImpl() {.dirty.} =
proc rawGetKnownHC[X, A](t: X, key: A, hc: Hash): int {.inline.} =
rawGetKnownHCImpl()
template bits(n): uint32 =
var temp = n
var bits = 0.uint32
while temp != 0:
temp = temp shr 1
bits += 1
bits
template genHashImpl(key, hc: typed) =
hc = hash(key)
hc = hash(key, targetBits=bits(maxHash(t)))
if hc == 0: # This almost never taken branch should be very predictable.
hc = 314159265 # Value doesn't matter; Any non-zero favorite is fine.