ginsu-0.8.1.1: OldHash.hs

module OldHash (hashInt, hashString) where

import Data.Bits (shiftR)
import Data.Char (ord)
import Data.Int (Int32, Int64)
import Data.List (foldl')

golden :: Int32
golden = 1013904242 -- = round ((sqrt 5 - 1) * 2^32) :: Int32
-- was -1640531527 = round ((sqrt 5 - 1) * 2^31) :: Int32
-- but that has bad mulHi properties (even adding 2^32 to get its inverse)
-- Whereas the above works well and contains no hash duplications for
-- [-32767..65536]

hashInt32 :: Int32 -> Int32
hashInt32 x = mulHi x golden + x

hashInt :: Int -> Int32
hashInt x = hashInt32 (fromIntegral x)

-- hi 32 bits of a x-bit * 32 bit -> 64-bit multiply
mulHi :: Int32 -> Int32 -> Int32
mulHi a b = fromIntegral (r `shiftR` 32)
   where r :: Int64
         r = fromIntegral a * fromIntegral b

-- | A sample hash function for Strings.  We keep multiplying by the
-- golden ratio and adding.  The implementation is:
--
-- > hashString = foldl' f golden
-- >   where f m c = fromIntegral (ord c) * magic + hashInt32 m
-- >         magic = 0xdeadbeef
--
-- Where hashInt32 works just as hashInt shown above.
--
-- Knuth argues that repeated multiplication by the golden ratio
-- will minimize gaps in the hash space, and thus it's a good choice
-- for combining together multiple keys to form one.
--
-- Here we know that individual characters c are often small, and this
-- produces frequent collisions if we use ord c alone.  A
-- particular problem are the shorter low ASCII and ISO-8859-1
-- character strings.  We pre-multiply by a magic twiddle factor to
-- obtain a good distribution.  In fact, given the following test:
--
-- > testp :: Int32 -> Int
-- > testp k = (n - ) . length . group . sort . map hs . take n $ ls
-- >   where ls = [] : [c : l | l <- ls, c <- ['\0'..'\xff']]
-- >         hs = foldl' f golden
-- >         f m c = fromIntegral (ord c) * k + hashInt32 m
-- >         n = 100000
--
-- We discover that testp magic = 0.

hashString :: String -> Int32
hashString = foldl' f golden
   where f m c = fromIntegral (ord c) * magic + hashInt32 m
         magic = 0xdeadbeef