except for up to seven bits) -- the totout == 0 provides an
               entry point after the zlib or gzip header, and assures that the
               index always has at least one access point; we avoid creating an
               access point after the last block by checking bit 6 of data_type
             */
            if ((strm.data_type & 128) && !(strm.data_type & 64) &&
                (totout == 0 || totout - last > span)) {
                index = addpoint(index, strm.data_type & 7, totin,
                                 totout, strm.avail_out, window);
                if (index == NULL) {
                    ret = Z_MEM_ERROR;
                    goto build_index_error;
                }
                last = totout;
            }
        } while (strm.avail_in != 0);
    } while (ret != Z_STREAM_END);

    /* clean up and return index (release unused entries in list) */
    (void)inflateEnd(&strm);
    index = realloc(index, sizeof(struct point) * index->have);
    index->size = index->have;
    *built = index;
    return index->size;

    /* return error */
  build_index_error:
    (void)inflateEnd(&strm);
    if (index != NULL)
        free_index(index);
    return ret;
}

/* Use the index to read len bytes from offset into buf, return bytes read or
   negative for error (Z_DATA_ERROR or Z_MEM_ERROR).  If data is requested past
   the end of the uncompressed data, then extract() will return a value less
   than len, indicating how much as actually read into buf.  This function
   should not return a data error unless the file was modified since the index
   was generated.  extract() may also return Z_ERRNO if there is an error on
   reading or seeking the input file. */
local int extract(FILE *in, struct access *index, off_t offset,
                  unsigned char *buf, int len)
{
    int ret, skip;
    z_stream strm;
    struct point *here;
    unsigned char input[CHUNK];
    unsigned char discard[WINSIZE];

    /* proceed only if something reasonable to do */
    if (len < 0)
        return 0;

    /* find where in stream to start */
    here = index->list;
    ret = index->have;
    while (--ret && here[1].out <= offset)
        here++;

    /* initialize file and inflate state to start there */
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;
    strm.avail_in = 0;
    strm.next_in = Z_NULL;
    ret = inflateInit2(&strm, -15);         /* raw inflate */
    if (ret != Z_OK)
        return ret;
    ret = fseeko(in, here->in - (here->bits ? 1 : 0), SEEK_SET);
    if (ret == -1)
        goto extract_ret;
    if (here->bits) {
        ret = getc(in);
        if (ret == -1) {
            ret = ferror(in) ? Z_ERRNO : Z_DATA_ERROR;
            goto extract_ret;
        }
        (void)inflatePrime(&strm, here->bits, ret >> (8 - here->bits));
    }
    (void)inflateSetDictionary(&strm, here->window, WINSIZE);

    /* skip uncompressed bytes until offset reached, then satisfy request */
    offset -= here->out;
    strm.avail_in = 0;
    skip = 1;                               /* while skipping to offset */
    do {
        /* define where to put uncompressed data, and how much */
        if (offset == 0 && skip) {          /* at offset now */
            strm.avail_out = len;
            strm.next_out = buf;
            skip = 0;                       /* only do this once */
        }
        if (offset > WINSIZE) {             /* skip WINSIZE bytes */
            strm.avail_out = WINSIZE;
            strm.next_out = discard;
            offset -= WINSIZE;
        }
        else if (offset != 0) {             /* last skip */
            strm.avail_out = (unsigned)offset;
            strm.next_out = discard;
            offset = 0;
        }

        /* uncompress until avail_out filled, or end of stream */
        do {
            if (strm.avail_in == 0) {
                strm.avail_in = fread(input, 1, CHUNK, in);
                if (ferror(in)) {
                    ret = Z_ERRNO;
                    goto extract_ret;
                }
                if (strm.avail_in == 0) {
                    ret = Z_DATA_ERROR;
                    goto extract_ret;
                }
                strm.next_in = input;
            }
            ret = inflate(&strm, Z_NO_FLUSH);       /* normal inflate */
            if (ret == Z_NEED_DICT)
                ret = Z_DATA_ERROR;
            if (ret == Z_MEM_ERROR || ret == Z_DATA_ERROR)
                goto extract_ret;
            if (ret == Z_STREAM_END)
                break;
        } while (strm.avail_out != 0);

        /* if reach end of stream, then don't keep trying to get more */
        if (ret == Z_STREAM_END)
            break;

        /* do until offset reached and requested data read, or stream ends */
    } while (skip);

    /* compute number of uncompressed bytes read after offset */
    ret = skip ? 0 : len - strm.avail_out;

    /* clean up and return bytes read or error */
  extract_ret:
    (void)inflateEnd(&strm);
    return ret;
}

/* Demonstrate the use of build_index() and extract() by processing the file
   provided on the command line, and the extracting 16K from about 2/3rds of
   the way through the uncompressed output, and writing that to stdout. */
int main(int argc, char **argv)
{
    int len;
    off_t offset;
    FILE *in;
    struct access *index;
    unsigned char buf[CHUNK];

    /* open input file */
    if (argc != 2) {
        fprintf(stderr, "usage: zran file.gz\n");
        return 1;
    }
    in = fopen(argv[1], "rb");
    if (in == NULL) {
        fprintf(stderr, "zran: could not open %s for reading\n", argv[1]);
        return 1;
    }

    /* build index */
    len = build_index(in, SPAN, &index);
    if (len < 0) {
        fclose(in);
        switch (len) {
        case Z_MEM_ERROR:
            fprintf(stderr, "zran: out of memory\n");
            break;
        case Z_DATA_ERROR:
            fprintf(stderr, "zran: compressed data error in %s\n", argv[1]);
            break;
        case Z_ERRNO:
            fprintf(stderr, "zran: read error on %s\n", argv[1]);
            break;
        default:
            fprintf(stderr, "zran: error %d while building index\n", len);
        }
        return 1;
    }
    fprintf(stderr, "zran: built index with %d access points\n", len);

    /* use index by reading some bytes from an arbitrary offset */
    offset = (index->list[index->have - 1].out << 1) / 3;
    len = extract(in, index, offset, buf, CHUNK);
    if (len < 0)
        fprintf(stderr, "zran: extraction failed: %s error\n",
                len == Z_MEM_ERROR ? "out of memory" : "input corrupted");
    else {
        fwrite(buf, 1, len, stdout);
        fprintf(stderr, "zran: extracted %d bytes at %llu\n", len, offset);
    }

    /* clean up and exit */
    free_index(index);
    fclose(in);
    return 0;
}