Data.HashTable.hashInt seems somewhat sub-optimal
jmaessen at alum.mit.edu
Thu Aug 30 08:37:16 EDT 2007
Oh, yes, in case others are interested in playing with the Jenkins
hash, here's Haskell code for it. If you think I should have stuck
more "seq's" and "!"s into it, by all means go to town (I compile
with optimization and everything is strict).
-- | The burtleburtle.net hash function devised by Bob Jenkins and
-- used in perl et al. This is written gracefully in a very
-- imperative way, and looks quite ugly when functionalized.
mix :: Int32 -> Int32 -> Int32 -> (Int32 -> Int32 -> Int32 -> a) -> a
mix a0 b0 c0 k0 =
let mixR k a b c = (a-b-c) `xor` (c `shiftR` k)
mixL k b c a = (b-c-a) `xor` (a `shiftL` k)
mix3 k1 k2 k3 k a b c =
let a' = mixR k1 a b c
b' = mixL k2 b c a'
c' = mixR k3 c a' b'
in k a' b' c'
in (mix3 13 8 13 $ mix3 12 16 5 $ mix3 3 10 15 $ k0) a0 b0 c0
golden :: Int32
golden = -1640531527
hashInt :: Int -> Int32
hashInt x = mix golden 0 (fromIntegral x) $ \_ _ r -> r
-- | A hash function for Strings based on a slightly modified version
-- of the burtleburtle string hash. We use the same mix, but we mix
-- every 3 Chars (not 12) since Haskell Chars are unicode. That does
-- make this hash 4x more expensive in the common case.
hashString :: String -> Int32
hashString str = hs str golden 0 0
where hs (a':b':c':str) a b c = mix (a + orrd a') (b + orrd b') (c
+ orrd c') $
hs [b',c'] a b c = mix a (b + orrd b') (c + orrd c') $ \_ _
r -> r
hs [c'] a b c = mix a b (c + orrd c') $ \_ _ r -> r
hs  _ _ c = c
orrd :: Char -> Int32
orrd = fromIntegral . fromEnum
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