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+/* fitblk.c: example of fitting compressed output to a specified size
+ Not copyrighted -- provided to the public domain
+ Version 1.1 25 November 2004 Mark Adler */
+
+/* Version history:
+ 1.0 24 Nov 2004 First version
+ 1.1 25 Nov 2004 Change deflateInit2() to deflateInit()
+ Use fixed-size, stack-allocated raw buffers
+ Simplify code moving compression to subroutines
+ Use assert() for internal errors
+ Add detailed description of approach
+ */
+
+/* Approach to just fitting a requested compressed size:
+
+ fitblk performs three compression passes on a portion of the input
+ data in order to determine how much of that input will compress to
+ nearly the requested output block size. The first pass generates
+ enough deflate blocks to produce output to fill the requested
+ output size plus a specfied excess amount (see the EXCESS define
+ below). The last deflate block may go quite a bit past that, but
+ is discarded. The second pass decompresses and recompresses just
+ the compressed data that fit in the requested plus excess sized
+ buffer. The deflate process is terminated after that amount of
+ input, which is less than the amount consumed on the first pass.
+ The last deflate block of the result will be of a comparable size
+ to the final product, so that the header for that deflate block and
+ the compression ratio for that block will be about the same as in
+ the final product. The third compression pass decompresses the
+ result of the second step, but only the compressed data up to the
+ requested size minus an amount to allow the compressed stream to
+ complete (see the MARGIN define below). That will result in a
+ final compressed stream whose length is less than or equal to the
+ requested size. Assuming sufficient input and a requested size
+ greater than a few hundred bytes, the shortfall will typically be
+ less than ten bytes.
+
+ If the input is short enough that the first compression completes
+ before filling the requested output size, then that compressed
+ stream is return with no recompression.
+
+ EXCESS is chosen to be just greater than the shortfall seen in a
+ two pass approach similar to the above. That shortfall is due to
+ the last deflate block compressing more efficiently with a smaller
+ header on the second pass. EXCESS is set to be large enough so
+ that there is enough uncompressed data for the second pass to fill
+ out the requested size, and small enough so that the final deflate
+ block of the second pass will be close in size to the final deflate
+ block of the third and final pass. MARGIN is chosen to be just
+ large enough to assure that the final compression has enough room
+ to complete in all cases.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include "zlib.h"
+
+#define local static
+
+/* print nastygram and leave */
+local void quit(char *why)
+{
+ fprintf(stderr, "fitblk abort: %s\n", why);
+ exit(1);
+}
+
+#define RAWLEN 4096 /* intermediate uncompressed buffer size */
+
+/* compress from file to def until provided buffer is full or end of
+ input reached; return last deflate() return value, or Z_ERRNO if
+ there was read error on the file */
+local int partcompress(FILE *in, z_streamp def)
+{
+ int ret, flush;
+ unsigned char raw[RAWLEN];
+
+ flush = Z_NO_FLUSH;
+ do {
+ def->avail_in = fread(raw, 1, RAWLEN, in);
+ if (ferror(in))
+ return Z_ERRNO;
+ def->next_in = raw;
+ if (feof(in))
+ flush = Z_FINISH;
+ ret = deflate(def, flush);
+ assert(ret != Z_STREAM_ERROR);
+ } while (def->avail_out != 0 && flush == Z_NO_FLUSH);
+ return ret;
+}
+
+/* recompress from inf's input to def's output; the input for inf and
+ the output for def are set in those structures before calling;
+ return last deflate() return value, or Z_MEM_ERROR if inflate()
+ was not able to allocate enough memory when it needed to */
+local int recompress(z_streamp inf, z_streamp def)
+{
+ int ret, flush;
+ unsigned char raw[RAWLEN];
+
+ flush = Z_NO_FLUSH;
+ do {
+ /* decompress */
+ inf->avail_out = RAWLEN;
+ inf->next_out = raw;
+ ret = inflate(inf, Z_NO_FLUSH);
+ assert(ret != Z_STREAM_ERROR && ret != Z_DATA_ERROR &&
+ ret != Z_NEED_DICT);
+ if (ret == Z_MEM_ERROR)
+ return ret;
+
+ /* compress what was decompresed until done or no room */
+ def->avail_in = RAWLEN - inf->avail_out;
+ def->next_in = raw;
+ if (inf->avail_out != 0)
+ flush = Z_FINISH;
+ ret = deflate(def, flush);
+ assert(ret != Z_STREAM_ERROR);
+ } while (ret != Z_STREAM_END && def->avail_out != 0);
+ return ret;
+}
+
+#define EXCESS 256 /* empirically determined stream overage */
+#define MARGIN 8 /* amount to back off for completion */
+
+/* compress from stdin to fixed-size block on stdout */
+int main(int argc, char **argv)
+{
+ int ret; /* return code */
+ unsigned size; /* requested fixed output block size */
+ unsigned have; /* bytes written by deflate() call */
+ unsigned char *blk; /* intermediate and final stream */
+ unsigned char *tmp; /* close to desired size stream */
+ z_stream def, inf; /* zlib deflate and inflate states */
+
+ /* get requested output size */
+ if (argc != 2)
+ quit("need one argument: size of output block");
+ ret = strtol(argv[1], argv + 1, 10);
+ if (argv[1][0] != 0)
+ quit("argument must be a number");
+ if (ret < 8) /* 8 is minimum zlib stream size */
+ quit("need positive size of 8 or greater");
+ size = (unsigned)ret;
+
+ /* allocate memory for buffers and compression engine */
+ blk = malloc(size + EXCESS);
+ def.zalloc = Z_NULL;
+ def.zfree = Z_NULL;
+ def.opaque = Z_NULL;
+ ret = deflateInit(&def, Z_DEFAULT_COMPRESSION);
+ if (ret != Z_OK || blk == NULL)
+ quit("out of memory");
+
+ /* compress from stdin until output full, or no more input */
+ def.avail_out = size + EXCESS;
+ def.next_out = blk;
+ ret = partcompress(stdin, &def);
+ if (ret == Z_ERRNO)
+ quit("error reading input");
+
+ /* if it all fit, then size was undersubscribed -- done! */
+ if (ret == Z_STREAM_END && def.avail_out >= EXCESS) {
+ /* write block to stdout */
+ have = size + EXCESS - def.avail_out;
+ if (fwrite(blk, 1, have, stdout) != have || ferror(stdout))
+ quit("error writing output");
+
+ /* clean up and print results to stderr */
+ ret = deflateEnd(&def);
+ assert(ret != Z_STREAM_ERROR);
+ free(blk);
+ fprintf(stderr,
+ "%u bytes unused out of %u requested (all input)\n",
+ size - have, size);
+ return 0;
+ }
+
+ /* it didn't all fit -- set up for recompression */
+ inf.zalloc = Z_NULL;
+ inf.zfree = Z_NULL;
+ inf.opaque = Z_NULL;
+ inf.avail_in = 0;
+ inf.next_in = Z_NULL;
+ ret = inflateInit(&inf);
+ tmp = malloc(size + EXCESS);
+ if (ret != Z_OK || tmp == NULL)
+ quit("out of memory");
+ ret = deflateReset(&def);
+ assert(ret != Z_STREAM_ERROR);
+
+ /* do first recompression close to the right amount */
+ inf.avail_in = size + EXCESS;
+ inf.next_in = blk;
+ def.avail_out = size + EXCESS;
+ def.next_out = tmp;
+ ret = recompress(&inf, &def);
+ if (ret == Z_MEM_ERROR)
+ quit("out of memory");
+
+ /* set up for next reocmpression */
+ ret = inflateReset(&inf);
+ assert(ret != Z_STREAM_ERROR);
+ ret = deflateReset(&def);
+ assert(ret != Z_STREAM_ERROR);
+
+ /* do second and final recompression (third compression) */
+ inf.avail_in = size - MARGIN; /* assure stream will complete */
+ inf.next_in = tmp;
+ def.avail_out = size;
+ def.next_out = blk;
+ ret = recompress(&inf, &def);
+ if (ret == Z_MEM_ERROR)
+ quit("out of memory");
+ assert(ret == Z_STREAM_END); /* otherwise MARGIN too small */
+
+ /* done -- write block to stdout */
+ have = size - def.avail_out;
+ if (fwrite(blk, 1, have, stdout) != have || ferror(stdout))
+ quit("error writing output");
+
+ /* clean up and print results to stderr */
+ free(tmp);
+ ret = inflateEnd(&inf);
+ assert(ret != Z_STREAM_ERROR);
+ ret = deflateEnd(&def);
+ assert(ret != Z_STREAM_ERROR);
+ free(blk);
+ fprintf(stderr,
+ "%u bytes unused out of %u requested (%lu input)\n",
+ size - have, size, def.total_in);
+ return 0;
+}
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