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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;
+    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;
+    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 */
+    char *blk;              /* intermediate and final stream */
+    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;
+        ret = fwrite(blk, 1, have, stdout);
+        if (ret != 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;
+    ret = fwrite(blk, 1, have, stdout);
+    if (ret != 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;
+}