------------------------------------------------------------------------------ -- -- -- 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;