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 conceivably this data structure could be used directly in Haskell applications as a read-only hash table, it is not recommened, as lookups are about 10x slower than HashMap.
This implementation was adapted from Steve Hanov's Blog.
Usage
The library is written generically to hash both strings and raw integers according to the FNV-1a algorithm. Integers are split by octets before hashing.
import Data.PerfectHash.Construction (createMinimalPerfectHash)
import qualified Data.Map as Map
tuples = [
(1000, 1)
, (5555, 2)
, (9876, 3)
]
lookup_table = createMinimalPerfectHash $ Map.fromList 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.
Demo
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
