1 ------------------------------------------------------------------------------
3 -- GNAT LIBRARY COMPONENTS --
5 -- S Y S T E M . C R C 3 2 --
11 -- Copyright (C) 2001 Ada Core Technologies, Inc. --
13 -- GNAT is free software; you can redistribute it and/or modify it under --
14 -- terms of the GNU General Public License as published by the Free Soft- --
15 -- ware Foundation; either version 2, or (at your option) any later ver- --
16 -- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
17 -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
18 -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
19 -- for more details. You should have received a copy of the GNU General --
20 -- Public License distributed with GNAT; see file COPYING. If not, write --
21 -- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, --
22 -- MA 02111-1307, USA. --
24 -- As a special exception, if other files instantiate generics from this --
25 -- unit, or you link this unit with other files to produce an executable, --
26 -- this unit does not by itself cause the resulting executable to be --
27 -- covered by the GNU General Public License. This exception does not --
28 -- however invalidate any other reasons why the executable file might be --
29 -- covered by the GNU Public License. --
31 -- GNAT is maintained by Ada Core Technologies Inc (http://www.gnat.com). --
33 ------------------------------------------------------------------------------
35 -- This package provides routines for computing a commonly used checksum
36 -- called CRC-32. This is a checksum based on treating the binary data
37 -- as a polynomial over a binary field, and the exact specifications of
38 -- the CRC-32 algorithm are as follows:
49 -- Note that this is the algorithm used by PKZip, Ethernet and FDDI.
51 -- For more information about this algorithm see:
53 -- ftp://ftp.rocksoft.com/papers/crc_v3.txt
55 -- "A Painless Guide to CRC Error Detection Algorithms", Ross N. Williams
57 -- "Computation of Cyclic Redundancy Checks via Table Look-Up", Communications
58 -- of the ACM, Vol. 31 No. 8, pp.1008-1013 Aug. 1988. Sarwate, D.V.
62 package System
.CRC32
is
64 type CRC32
is new Interfaces
.Unsigned_32
;
65 -- Used to represent CRC32 values, which are 32 bit bit-strings
67 procedure Initialize
(C
: out CRC32
);
68 pragma Inline
(Initialize
);
69 -- Initialize CRC value by assigning the standard Init value (16#FFFF_FFFF)
74 pragma Inline
(Update
);
75 -- Evolve CRC by including the contribution from Character'Pos (Value)
77 function Get_Value
(C
: CRC32
) return Interfaces
.Unsigned_32
;
78 pragma Inline
(Get_Value
);
79 -- Get_Value computes the CRC32 value by performing an XOR with the
80 -- standard XorOut value (16#FFFF_FFFF). Note that this does not
81 -- change the value of C, so it may be used to retrieve intermediate
82 -- values of the CRC32 value during a sequence of Update calls.