diff options
Diffstat (limited to 'gcc/ada/g-perhas.ads')
| -rw-r--r-- | gcc/ada/g-perhas.ads | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/gcc/ada/g-perhas.ads b/gcc/ada/g-perhas.ads new file mode 100644 index 00000000000..92a899cf600 --- /dev/null +++ b/gcc/ada/g-perhas.ads @@ -0,0 +1,67 @@ +------------------------------------------------------------------------------ +-- -- +-- 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; |