# perfect-hash-generator: Perfect minimal hashing implementation in native Haskell

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

## Modules

[Index] [Quick Jump]

*Data**PerfectHash*- Data.PerfectHash.Construction
- Data.PerfectHash.Hashing
- Data.PerfectHash.Lookup
*Types*- Data.PerfectHash.Types.Nonces

## Downloads

- perfect-hash-generator-1.0.0.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)

#### Maintainer's Corner

For package maintainers and hackage trustees

Candidates

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Versions [RSS] | 0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.1.0.4, 0.2.0.0, 0.2.0.1, 0.2.0.2, 0.2.0.3, 0.2.0.4, 0.2.0.5, 0.2.0.6, 1.0.0 (info) |
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Change log | changelog.md |

Dependencies | base (>=4.5 && <5), binary, bytestring, containers, data-default, data-ordlist, directory, filepath, hashable, optparse-applicative, perfect-hash-generator, random, sorted-list, text, unordered-containers, vector [details] |

License | Apache-2.0 |

Author | Karl Ostmo <kostmo@gmail.com> |

Maintainer | Karl Ostmo <kostmo@gmail.com> |

Category | Data Structures, Embedded |

Home page | https://github.com/kostmo/perfect-hash-generator#readme |

Bug tracker | https://github.com/kostmo/perfect-hash-generator/issues |

Source repo | head: git clone https://github.com/kostmo/perfect-hash-generator |

Uploaded | by kostmo at 2022-06-27T16:46:09Z |

Distributions | LTSHaskell:1.0.0, NixOS:1.0.0, Stackage:1.0.0 |

Executables | hash-perfectly-strings-demo, hash-perfectly-ints-demo |

Downloads | 4252 total (34 in the last 30 days) |

Rating | 2.25 (votes: 2) [estimated by Bayesian average] |

Your Rating | |

Status | Docs available [build log] Last success reported on 2022-06-27 [all 1 reports] |