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Description | |||||||||||||||||||||||

An implementation of extensible hash tables, as described in
Per-Ake Larson, Dynamic Hash Tables, CACM 31(4), April 1988,
pp. 446--457. The implementation is also derived from the one
in GHC's runtime system (ghc/rts/Hash.{c,h}).
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Synopsis | |||||||||||||||||||||||

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Basic hash table operations | |||||||||||||||||||||||

data HashTable key val | |||||||||||||||||||||||

new | |||||||||||||||||||||||

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insert :: HashTable key val -> key -> val -> IO () | |||||||||||||||||||||||

Inserts a key/value mapping into the hash table. Note that | |||||||||||||||||||||||

delete :: HashTable key val -> key -> IO () | |||||||||||||||||||||||

Remove an entry from the hash table. | |||||||||||||||||||||||

lookup :: HashTable key val -> key -> IO (Maybe val) | |||||||||||||||||||||||

Looks up the value of a key in the hash table. | |||||||||||||||||||||||

update :: HashTable key val -> key -> val -> IO Bool | |||||||||||||||||||||||

Updates an entry in the hash table, returning
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Converting to and from lists | |||||||||||||||||||||||

fromList :: Eq key => (key -> Int32) -> [(key, val)] -> IO (HashTable key val) | |||||||||||||||||||||||

Convert a list of key/value pairs into a hash table. Equality on keys is taken from the Eq instance for the key type. | |||||||||||||||||||||||

toList :: HashTable key val -> IO [(key, val)] | |||||||||||||||||||||||

Converts a hash table to a list of key/value pairs. | |||||||||||||||||||||||

Hash functions | |||||||||||||||||||||||

This implementation of hash tables uses the low-order If your keyspace is integrals such that the low-order bits between
keys are highly variable, then you could get away with using We provide some sample hash functions for | |||||||||||||||||||||||

hashInt :: Int -> Int32 | |||||||||||||||||||||||

A sample (and useful) hash function for Int and Int32, implemented by extracting the uppermost 32 bits of the 64-bit result of multiplying by a 33-bit constant. The constant is from Knuth, derived from the golden ratio: golden = round ((sqrt 5 - 1) * 2^32) We get good key uniqueness on small inputs (a problem with previous versions): (length $ group $ sort $ map hashInt [-32767..65536]) == 65536 + 32768 | |||||||||||||||||||||||

hashString :: String -> Int32 | |||||||||||||||||||||||

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. | |||||||||||||||||||||||

prime :: Int32 | |||||||||||||||||||||||

A prime larger than the maximum hash table size | |||||||||||||||||||||||

Diagnostics | |||||||||||||||||||||||

longestChain :: HashTable key val -> IO [(key, val)] | |||||||||||||||||||||||

This function is useful for determining whether your hash function is working well for your data set. It returns the longest chain of key/value pairs in the hash table for which all the keys hash to the same bucket. If this chain is particularly long (say, longer than 14 elements or so), then it might be a good idea to try a different hash function. | |||||||||||||||||||||||

Produced by Haddock version 0.8 |