gcc/ada/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 -- 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
  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;