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-- --
-- --
-- G N A T . P E R F E C T _ H A S H _ G E N E R A T O R S --
-- --
-- S p e c --
-- --
-- Copyright (C) 2002-2021, AdaCore --
-- --
-- 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 3, 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 --
-- --
-- As a special exception under Section 7 of GPL version 3, you are granted --
-- additional permissions described in the GCC Runtime Library Exception, --
-- version 3.1, as published by the Free Software Foundation. --
-- --
-- You should have received a copy of the GNU General Public License and --
-- a copy of the GCC Runtime Library Exception along with this program; --
-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
-- <>. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
-- This package provides a generator of static minimal 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 I of integers [0, k-1], where k is
-- an integer, usually k >= m. h (w) where w is a word in W 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
-- collisions are 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.
-- The functions generated by this package require the words to be known in
-- advance (they are "static" hash functions). The hash functions are also
-- order preserving. If w2 is inserted after w1 in the generator, then h (w1)
-- < h (w2). These hashing functions are convenient for use with realtime
-- applications.
with System.Perfect_Hash_Generators;
package GNAT.Perfect_Hash_Generators is
package SPHG renames System.Perfect_Hash_Generators;
Default_K_To_V : constant Float := 2.05;
-- Default ratio for the algorithm. When K is the number of keys, V =
-- (K_To_V) * K is the size of the main table of the hash function. To
-- converge, the algorithm requires K_To_V to be strictly greater than 2.0.
Default_Pkg_Name : constant String := "Perfect_Hash";
-- Default package name in which the hash function is defined
Default_Position : constant String := "";
-- The generator allows selection of the character positions used in the
-- hash function. By default, all positions are selected.
Default_Tries : constant Positive := 20;
-- This algorithm may not succeed to find a possible mapping on the first
-- try and may have to iterate a number of times. This constant bounds the
-- number of tries.
type Optimization is new SPHG.Optimization;
-- Optimize either the memory space or the execution time. Note: in
-- practice, the optimization mode has little effect on speed. The tables
-- are somewhat smaller with Memory_Space.
Verbose : Boolean renames SPHG.Verbose;
-- Output the status of the algorithm. For instance, the tables, the random
-- graph (edges, vertices) and selected char positions are output between
-- two iterations.
procedure Initialize
(Seed : Natural;
K_To_V : Float := Default_K_To_V;
Optim : Optimization := Memory_Space;
Tries : Positive := Default_Tries);
-- Initialize the generator and its internal structures. Set the ratio of
-- vertices over keys in the random graphs. This value has to be greater
-- than 2.0 in order for the algorithm to succeed. The word set is not
-- modified (in particular when it is already set). For instance, it is
-- possible to run several times the generator with different settings on
-- the same words.
-- A classical way of doing is to Insert all the words and then to invoke
-- Initialize and Compute. If this fails to find a perfect hash function,
-- invoke Initialize again with other configuration parameters (probably
-- with a greater K_To_V ratio). Once successful, invoke Produce and then
-- Finalize.
procedure Finalize;
-- Deallocate the internal structures and the words table
procedure Insert (Value : String);
-- Insert a new word into the table. ASCII.NUL characters are not allowed.
Too_Many_Tries : exception renames SPHG.Too_Many_Tries;
-- Raised after Tries unsuccessful runs
procedure Compute (Position : String := Default_Position);
-- Compute the hash function. Position allows the definition of selection
-- of character positions used in the word hash function. Positions can be
-- separated by commas and ranges like x-y may be used. Character '$'
-- represents the final character of a word. With an empty position, the
-- generator automatically produces positions to reduce the memory usage.
-- Raise Too_Many_Tries if the algorithm does not succeed within Tries
-- attempts (see Initialize).
procedure Produce
(Pkg_Name : String := Default_Pkg_Name;
Use_Stdout : Boolean := False);
-- Generate the hash function package Pkg_Name. This package includes the
-- minimal perfect Hash function. The output is normally placed in the
-- current directory, in files and X.adb, where X is the standard
-- GNAT file name for a package named Pkg_Name. If Use_Stdout is True, the
-- output goes to standard output, and no files are written.
end GNAT.Perfect_Hash_Generators;