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Mutable hash tables for Haskell, in the ST monad

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This package provides a couple of different implementations of mutable hash
tables in the ST monad, as well as a typeclass abstracting their common
operations, and a set of wrappers to use the hash tables in the IO monad.

**Quick start**: documentation for the hash table operations is provided in the
`Data.HashTable.Class` module, and the IO wrappers are located in the
`Data.HashTable.IO` module.

This package currently contains three hash table implementations:

1. `Data.HashTable.ST.Basic` contains a basic open-addressing hash table
     using linear probing as the collision strategy. On a pure speed basis it
     should currently be the fastest available Haskell hash table
     implementation for lookups, although it has a higher memory overhead
     than the other tables and can suffer from long delays when the table is
     resized because all of the elements in the table need to be rehashed.

2. `Data.HashTable.ST.Cuckoo` contains an implementation of "cuckoo hashing"
     as introduced by Pagh and Rodler in 2001 (see
     [http://en.wikipedia.org/wiki/Cuckoo\_hashing](http://en.wikipedia.org/wiki/Cuckoo_hashing)).
     Cuckoo hashing has worst-case /O(1)/ lookups and can reach a high "load
     factor", in which the table can perform acceptably well even when more
     than 90% full. Randomized testing shows this implementation of cuckoo
     hashing to be slightly faster on insert and slightly slower on lookup than
     `Data.Hashtable.ST.Basic`, while being more space efficient by about a
     half-word per key-value mapping. Cuckoo hashing, like the basic hash table
     implementation using linear probing, can suffer from long delays when the
     table is resized.

3. `Data.HashTable.ST.Linear` contains a linear hash table (see
     [http://en.wikipedia.org/wiki/Linear\_hashing](http://en.wikipedia.org/wiki/Linear_hashing)),
     which trades some insert and lookup performance for higher space
     efficiency and much shorter delays when expanding the table. In most
     cases, benchmarks show this table to be currently slightly faster than
     `Data.HashTable` from the Haskell base library.

It is recommended to create a concrete type alias in your code when using this
package, i.e.:

import qualified Data.HashTable.IO as H
    
    type HashTable k v = H.BasicHashTable k v

foo :: IO (HashTable Int Int)
    foo = do
        ht <- H.new
        H.insert ht 1 1
        return ht

Firstly, this makes it easy to switch to a different hash table implementation,
and secondly, using a concrete type rather than leaving your functions abstract
in the HashTable class should allow GHC to optimize away the typeclass
dictionaries.

This package accepts a couple of different cabal flags:

* `unsafe-tricks`, default **on**. If this flag is enabled, we use some
    unsafe GHC-specific tricks to save indirections (namely `unsafeCoerce#` and
    `reallyUnsafePtrEquality#`. These techniques rely on assumptions about the
    behaviour of the GHC runtime system and, although they've been tested and
    should be safe under normal conditions, are slightly dangerous. Caveat
    emptor. In particular, these techniques are incompatible with HPC code
    coverage reports.

* `sse41`, default /off/. If this flag is enabled, we use some SSE 4.1
    instructions (see
    [http://en.wikipedia.org/wiki/SSE4](http://en.wikipedia.org/wiki/SSE4),
    first available on Intel Core 2 processors) to speed up cache-line searches
    for cuckoo hashing.

* `bounds-checking`, default /off/. If this flag is enabled, array accesses
    are bounds-checked.

* `debug`, default /off/. If turned on, we'll rudely spew debug output to
    stdout.

* `portable`, default /off/. If this flag is enabled, we use only pure
    Haskell code and try not to use unportable GHC extensions. Turning this
    flag on forces `unsafe-tricks` and `sse41` *OFF*.

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