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