1------------------------------------------------------------------------------
2--                                                                          --
3--                         GNAT COMPILER COMPONENTS                         --
4--                                                                          --
5--                    S Y S T E M . S T R I N G _ H A S H                   --
6--                                                                          --
7--                                 S p e c                                  --
8--                                                                          --
9--          Copyright (C) 2009-2018, Free Software Foundation, Inc.         --
10--                                                                          --
11-- GNAT is free software;  you can  redistribute it  and/or modify it under --
12-- terms of the  GNU General Public License as published  by the Free Soft- --
13-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
14-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
15-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
16-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
17--                                                                          --
18-- As a special exception under Section 7 of GPL version 3, you are granted --
19-- additional permissions described in the GCC Runtime Library Exception,   --
20-- version 3.1, as published by the Free Software Foundation.               --
21--                                                                          --
22-- You should have received a copy of the GNU General Public License and    --
23-- a copy of the GCC Runtime Library Exception along with this program;     --
24-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
25-- <http://www.gnu.org/licenses/>.                                          --
26--                                                                          --
27-- GNAT was originally developed  by the GNAT team at  New York University. --
28-- Extensive contributions were provided by Ada Core Technologies Inc.      --
29--                                                                          --
30------------------------------------------------------------------------------
31
32pragma Compiler_Unit_Warning;
33
34package body System.String_Hash is
35
36   --  Compute a hash value for a key. The approach here follows the algorithm
37   --  introduced in the ndbm substitute SDBM by Ozan Yigit and then reused in
38   --  GNU Awk (where they are implemented as a Duff's device).
39
40   ----------
41   -- Hash --
42   ----------
43
44   function Hash (Key : Key_Type) return Hash_Type is
45
46      pragma Compile_Time_Error
47        (Hash_Type'Modulus /= 2 ** 32
48          or else Hash_Type'First /= 0
49          or else Hash_Type'Last /= 2 ** 32 - 1,
50         "Hash_Type must be 32-bit modular with range 0 .. 2**32-1");
51
52      function Shift_Left
53        (Value  : Hash_Type;
54         Amount : Natural) return Hash_Type;
55      pragma Import (Intrinsic, Shift_Left);
56
57      H : Hash_Type;
58
59   begin
60      H := 0;
61      for J in Key'Range loop
62         H := Char_Type'Pos (Key (J))
63                + Shift_Left (H, 6) + Shift_Left (H, 16) - H;
64      end loop;
65
66      return H;
67   end Hash;
68
69end System.String_Hash;
70