| ------------------------------------------------------------------------------ |
| -- -- |
| -- GNAT COMPILER COMPONENTS -- |
| -- -- |
| -- G N A T . P E R F E C T _ H A S H -- |
| -- -- |
| -- S p e c -- |
| -- -- |
| -- Copyright (C) 2002-2003 Ada Core Technologies, Inc. -- |
| -- -- |
| -- GNAT is free software; you can redistribute it and/or modify it under -- |
| -- terms of the GNU General Public License as published by the Free Soft- -- |
| -- ware Foundation; either version 2, or (at your option) any later ver- -- |
| -- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- |
| -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- |
| -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License -- |
| -- for more details. You should have received a copy of the GNU General -- |
| -- Public License distributed with GNAT; see file COPYING. If not, write -- |
| -- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, -- |
| -- MA 02111-1307, USA. -- |
| -- -- |
| -- As a special exception, if other files instantiate generics from this -- |
| -- unit, or you link this unit with other files to produce an executable, -- |
| -- this unit does not by itself cause the resulting executable to be -- |
| -- covered by the GNU General Public License. This exception does not -- |
| -- however invalidate any other reasons why the executable file might be -- |
| -- covered by the GNU Public License. -- |
| -- -- |
| -- GNAT was originally developed by the GNAT team at New York University. -- |
| -- Extensive contributions were provided by Ada Core Technologies Inc. -- |
| -- -- |
| ------------------------------------------------------------------------------ |
| |
| package GNAT.Perfect_Hash is |
| pragma Pure (Perfect_Hash); |
| |
| -- The packages in this hierarchy implement perfect hash |
| -- functions. To understand what a perfect hash function is, we |
| -- define several notions. These definitions are inspired from the |
| -- following paper: |
| -- |
| -- Zbigniew J. Czech, George Havas, and Bohdan S. Majewski ``An |
| -- Optimal Algorithm for Generating Minimal Perfect Hash Functions'', |
| -- Information Processing Letters, 43(1992) pp.257-264, Oct.1992 |
| -- |
| -- Let W be a set of m words. A hash function h is a function that |
| -- maps the set of words W into some given interval of integers |
| -- [0, k-1], where k is an integer, usually k >= m. h (w) where w |
| -- is a word computes an address or an integer from I for the |
| -- storage or the retrieval of that item. The storage area used to |
| -- store items is known as a hash table. Words for which the same |
| -- address is computed are called synonyms. Due to the existence |
| -- of synonyms a situation called collision may arise in which two |
| -- items w1 and w2 have the same address. Several schemes for |
| -- resolving known. A perfect hash function is an injection from |
| -- the word set W to the integer interval I with k >= m. If k = m, |
| -- then h is a minimal perfect hash function. A hash function is |
| -- order preserving if it puts entries into the hash table in a |
| -- prespecified order. |
| -- |
| -- A minimal perfect hash function is defined by two properties: |
| -- * Since no collisions occur each item can be retrieved from the |
| -- table in *one* probe. This represents the "perfect" property. |
| -- * The hash table size corresponds to the exact size of W and |
| -- *no larger*. This represents the "minimal" property. |
| |
| end GNAT.Perfect_Hash; |
