perfect-hash-generator

Perfect minimal hashing implementation in native Haskell https://github.com/kostmo/perfect-hash-generator#readme

Latest on Hackage:0.1.0.4

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow stackage.org to host generated Haddocks.

Apache-2.0 licensed and maintained by Karl Ostmo

A perfect hash function for a set S is a hash function that maps distinct elements in S to a set of integers, with no collisions. A minimal perfect hash function is a perfect hash function that maps n keys to n consecutive integers, e.g. the numbers from 0 to n-1.

In contrast with the PerfectHash package, which is a binding to a C-based library, this package is a fully-native Haskell implementation.

It is intended primarily for generating C code for embedded applications (compare to gperf). The output of this tool is a pair of arrays that can be included in generated C code for allocation-free hash tables.

Though lookups also perform reasonably well for Haskell applications, it hasn't been benchmarked thorougly with respect to other data structures.

This implementation was adapted from Steve Hanov's Blog.

Usage

The library is written generically to hash both strings and raw integers. Integers should be wrapped in the Atom newtype:

import Data.PerfectHash.Construction (createMinimalPerfectHash)

tuples = [
   (Atom 1000, 1)
 , (Atom 5555, 2)
 , (Atom 9876, 3)
 ]

lookup_table = createMinimalPerfectHash tuples

Generation of C code based on the arrays in lookup_table is left as an exercise to the reader. Algorithm documentation in the Data.PerfectHash.Hashing and Data.PerfectHash.Lookup modules will be helpful.

See the hash-perfectly-strings-demo and hash-perfectly-ints-demo, as well as the test suite, for working examples.

$ stack build
$ stack exec hash-perfectly-strings-demo

Caveats

Only integer keys of at most 32-bits have been demonstrated to work properly. Since the hash function masks to 32 bits, colliding 64-bit integers can hang the lookup table construction.

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