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Unit Adler;
{
adler32.c -- compute the Adler-32 checksum of a data stream
Copyright (C) 1995-1998 Mark Adler
Pascal tranlastion
Copyright (C) 1998 by Jacques Nomssi Nzali
For conditions of distribution and use, see copyright notice in readme.txt
}
interface
{$I zconf.inc}
uses
zutil;
function adler32(adler : uLong; buf : pBytef; len : uInt) : uLong;
{ Update a running Adler-32 checksum with the bytes buf[0..len-1] and
return the updated checksum. If buf is NIL, this function returns
the required initial value for the checksum.
An Adler-32 checksum is almost as reliable as a CRC32 but can be computed
much faster. Usage example:
var
adler : uLong;
begin
adler := adler32(0, Z_NULL, 0);
while (read_buffer(buffer, length) <> EOF) do
adler := adler32(adler, buffer, length);
if (adler <> original_adler) then
error();
end;
}
implementation
const
BASE = Long(65521); { largest prime smaller than 65536 }
{NMAX = 5552; original code with unsigned 32 bit integer }
{ NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 }
NMAX = 3854; { code with signed 32 bit integer }
{ NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^31-1 }
{ The penalty is the time loss in the extra MOD-calls. }
{ ========================================================================= }
function adler32(adler : uLong; buf : pBytef; len : uInt) : uLong;
var
s1, s2 : uLong;
k : int;
begin
s1 := adler and $ffff;
s2 := (adler shr 16) and $ffff;
if not Assigned(buf) then
begin
adler32 := uLong(1);
exit;
end;
while (len > 0) do
begin
if len < NMAX then
k := len
else
k := NMAX;
Dec(len, k);
{
while (k >= 16) do
begin
DO16(buf);
Inc(buf, 16);
Dec(k, 16);
end;
if (k <> 0) then
repeat
Inc(s1, buf^);
Inc(puf);
Inc(s2, s1);
Dec(k);
until (k = 0);
}
while (k > 0) do
begin
Inc(s1, buf^);
Inc(s2, s1);
Inc(buf);
Dec(k);
end;
s1 := s1 mod BASE;
s2 := s2 mod BASE;
end;
adler32 := (s2 shl 16) or s1;
end;
{
#define DO1(buf,i)
begin
Inc(s1, buf[i]);
Inc(s2, s1);
end;
#define DO2(buf,i) DO1(buf,i); DO1(buf,i+1);
#define DO4(buf,i) DO2(buf,i); DO2(buf,i+2);
#define DO8(buf,i) DO4(buf,i); DO4(buf,i+4);
#define DO16(buf) DO8(buf,0); DO8(buf,8);
}
end.
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