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------------------------------------------------------------------------------
-- --
-- GNAT COMPILER COMPONENTS --
-- --
-- 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) 2002-2010, 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 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, 51 Franklin Street, Fifth Floor, --
-- Boston, MA 02110-1301, 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. --
-- --
------------------------------------------------------------------------------
-- 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.
package GNAT.Perfect_Hash_Generators is
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 (Memory_Space, CPU_Time);
-- 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 := False;
-- 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 Compute fails to find a perfect hash
-- function, invoke Initialize another time with other configuration
-- parameters (probably with a greater K_To_V ratio). Once successful,
-- invoke Produce and 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;
-- Raised after Tries unsuccessful runs
procedure Compute (Position : String := Default_Position);
-- Compute the hash function. Position allows to define 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 X.ads 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.
----------------------------------------------------------------
-- The routines and structures defined below allow producing the hash
-- function using a different way from the procedure above. The procedure
-- Define returns the lengths of an internal table and its item type size.
-- The function Value returns the value of each item in the table.
-- The hash function has the following form:
-- h (w) = (g (f1 (w)) + g (f2 (w))) mod m
-- G is a function based on a graph table [0,n-1] -> [0,m-1]. m is the
-- number of keys. n is an internally computed value and it can be obtained
-- as the length of vector G.
-- F1 and F2 are two functions based on two function tables T1 and T2.
-- Their definition depends on the chosen optimization mode.
-- Only some character positions are used in the words because they are
-- significant. They are listed in a character position table (P in the
-- pseudo-code below). For instance, in {"jan", "feb", "mar", "apr", "jun",
-- "jul", "aug", "sep", "oct", "nov", "dec"}, only positions 2 and 3 are
-- significant (the first character can be ignored). In this example, P =
-- {2, 3}
-- When Optimization is CPU_Time, the first dimension of T1 and T2
-- corresponds to the character position in the word and the second to the
-- character set. As all the character set is not used, we define a used
-- character table which associates a distinct index to each used character
-- (unused characters are mapped to zero). In this case, the second
-- dimension of T1 and T2 is reduced to the used character set (C in the
-- pseudo-code below). Therefore, the hash function has the following:
-- function Hash (S : String) return Natural is
-- F : constant Natural := S'First - 1;
-- L : constant Natural := S'Length;
-- F1, F2 : Natural := 0;
-- J : <t>;
-- begin
-- for K in P'Range loop
-- exit when L < P (K);
-- J := C (S (P (K) + F));
-- F1 := (F1 + Natural (T1 (K, J))) mod <n>;
-- F2 := (F2 + Natural (T2 (K, J))) mod <n>;
-- end loop;
-- return (Natural (G (F1)) + Natural (G (F2))) mod <m>;
-- end Hash;
-- When Optimization is Memory_Space, the first dimension of T1 and T2
-- corresponds to the character position in the word and the second
-- dimension is ignored. T1 and T2 are no longer matrices but vectors.
-- Therefore, the used character table is not available. The hash function
-- has the following form:
-- function Hash (S : String) return Natural is
-- F : constant Natural := S'First - 1;
-- L : constant Natural := S'Length;
-- F1, F2 : Natural := 0;
-- J : <t>;
-- begin
-- for K in P'Range loop
-- exit when L < P (K);
-- J := Character'Pos (S (P (K) + F));
-- F1 := (F1 + Natural (T1 (K) * J)) mod <n>;
-- F2 := (F2 + Natural (T2 (K) * J)) mod <n>;
-- end loop;
-- return (Natural (G (F1)) + Natural (G (F2))) mod <m>;
-- end Hash;
type Table_Name is
(Character_Position,
Used_Character_Set,
Function_Table_1,
Function_Table_2,
Graph_Table);
procedure Define
(Name : Table_Name;
Item_Size : out Natural;
Length_1 : out Natural;
Length_2 : out Natural);
-- Return the definition of the table Name. This includes the length of
-- dimensions 1 and 2 and the size of an unsigned integer item. When
-- Length_2 is zero, the table has only one dimension. All the ranges
-- start from zero.
function Value
(Name : Table_Name;
J : Natural;
K : Natural := 0) return Natural;
-- Return the value of the component (I, J) of the table Name. When the
-- table has only one dimension, J is ignored.
end GNAT.Perfect_Hash_Generators;
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