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------------------------------------------------------------------------------
-- --
-- GNAT COMPILER COMPONENTS --
-- --
-- S Y S T E M . S T R I N G _ H A S H --
-- --
-- S p e c --
-- --
-- Copyright (C) 2009-2021, Free Software Foundation, 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 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. --
-- --
------------------------------------------------------------------------------
pragma Compiler_Unit_Warning;
package body System.String_Hash is
-- Compute a hash value for a key. The approach here follows the algorithm
-- introduced in the ndbm substitute SDBM by Ozan Yigit and then reused in
-- GNU Awk (where they are implemented as a Duff's device).
----------
-- Hash --
----------
function Hash (Key : Key_Type) return Hash_Type is
pragma Compile_Time_Error
(Hash_Type'Modulus /= 2 ** 32
or else Hash_Type'First /= 0
or else Hash_Type'Last /= 2 ** 32 - 1,
"Hash_Type must be 32-bit modular with range 0 .. 2**32-1");
function Shift_Left
(Value : Hash_Type;
Amount : Natural) return Hash_Type;
pragma Import (Intrinsic, Shift_Left);
H : Hash_Type;
begin
H := 0;
for J in Key'Range loop
H := Char_Type'Pos (Key (J))
+ Shift_Left (H, 6) + Shift_Left (H, 16) - H;
end loop;
return H;
end Hash;
end System.String_Hash;
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