{-# OPTIONS_HADDOCK prune #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE Safe #-}
-- | Implements the specialized hash function for
-- this perfect hashing algorithm.
--
-- C code that makes use of the perfect hash table output must exactly
-- re-implement this 'hash' function.
module Data.PerfectHash.Hashing where
import Data.Binary (encode)
import Data.Bits (xor, (.&.))
import qualified Data.ByteString.Lazy as B (unpack)
import Data.Char (ord)
import Data.Foldable (foldl')
import Data.Text (Text)
import qualified Data.Text as T
import qualified Data.PerfectHash.Types.Nonces as Nonces
import Data.PerfectHash.Types.Nonces (Nonce)
-- Types
newtype SlotIndex = SlotIndex {getIndex :: Int}
newtype Hash = Hash {getHash :: Int}
deriving (Eq, Show)
newtype ArraySize = ArraySize Int
deriving Show
-- * Constants
-- | This choice of prime number @0x01000193@ was taken from the Python implementation
-- on <http://stevehanov.ca/blog/index.php?id=119 Steve Hanov's page>.
primeFNV :: Int
primeFNV = 0x01000193
mask32bits :: Int
mask32bits = 0xffffffff
-- * Class instances
-- | Mechanism for a key to be decomposed into units processable by the
-- <http://isthe.com/chongo/tech/comp/fnv/#FNV-1a FNV-1a> hashing algorithm.
class ToHashableChunks a where
toHashableChunks :: a -> [Hash]
instance ToHashableChunks Int where
toHashableChunks = map (Hash. fromIntegral) . B.unpack . encode
instance ToHashableChunks String where
toHashableChunks = map $ Hash . ord
instance ToHashableChunks Text where
toHashableChunks = toHashableChunks . T.unpack
-- Utilities
generateArrayIndices :: ArraySize -> [SlotIndex]
generateArrayIndices (ArraySize size) = map SlotIndex [0..(size - 1)]
-- * Main functions
hashToSlot :: ToHashableChunks a =>
Nonce
-> ArraySize
-> a -- ^ key
-> SlotIndex
hashToSlot nonce (ArraySize size) key =
SlotIndex $ getHash (hash nonce key) `mod` size
-- Used in the 'hash' function
getNonzeroNonceVal :: Nonce -> Int
getNonzeroNonceVal (Nonces.Nonce nonce) =
if nonce == 0
then primeFNV
else nonce
-- | The interface is comparable to the
-- <https://hackage.haskell.org/package/hashable-1.2.6.1/docs/Data-Hashable.html#v:hashWithSalt hashWithSalt>
-- function from the @hashable@ package.
--
-- Uses the \"FNV-1a\" algorithm from the
-- <http://isthe.com/chongo/tech/comp/fnv/#FNV-1a FNV website>:
--
-- > hash = offset_basis
-- > for each octet_of_data to be hashed
-- > hash = hash xor octet_of_data
-- > hash = hash * FNV_prime
-- > return hash
hash :: ToHashableChunks a =>
Nonce -- ^ nonce
-> a -- ^ key
-> Hash
hash nonce =
-- NOTE: This must be 'foldl', not 'foldr'
Hash . foldl' combine d . toHashableChunks
where
d = getNonzeroNonceVal nonce
combine acc = (.&. mask32bits) . (* primeFNV) . xor acc . getHash