------------------------------------------------------------------------------ | |

-- -- | |

-- 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) 2002-2022, 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 -- | |

-- or FITNESS FOR A PARTICULAR PURPOSE. -- | |

-- -- | |

-- 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 -- | |

-- <http://www.gnu.org/licenses/>. -- | |

-- -- | |

-- GNAT was originally developed by the GNAT team at New York University. -- | |

-- Extensive contributions were provided by Ada Core Technologies Inc. -- | |

-- -- | |

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-- 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 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. | |

end GNAT.Perfect_Hash_Generators; |