code: plan9front

ref: d376fac66cbd5efbe2f9f61a1e8b31af03a84a92
dir: /sys/src/libflate/deflatezlibblock.c/

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#include <u.h>
#include <libc.h>
#include <flate.h>
#include "zlib.h"

int
deflatezlibblock(uchar *dst, int dsize, uchar *src, int ssize, int level, int debug)
{
	ulong adler;
	int n;

	if(dsize < 6)
		return FlateOutputFail;

	n = deflateblock(dst + 2, dsize - 6, src, ssize, level, debug);
	if(n < 0)
		return n;

	dst[0] = ZlibDeflate | ZlibWin32k;

	/* bogus zlib encoding of compression level */
	dst[1] = ((level > 2) + (level > 5) + (level > 8)) << 6;

	/* header check field */
	dst[1] |= 31 - ((dst[0] << 8) | dst[1]) % 31;

	adler = adler32(1, src, ssize);
	dst[n + 2] = adler >> 24;
	dst[n + 3] = adler >> 16;
	dst[n + 4] = adler >> 8;
	dst[n + 5] = adler;

	return n + 6;
}