File : s-strhas.adb
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) 2009-2016, 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 -- --
19 -- --
20 -- --
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
32 pragma Compiler_Unit_Warning;
33
34 package 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
69 end System.String_Hash;