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+/* zran.c -- example of zlib/gzip stream indexing and random access
+ * Copyright (C) 2005 Mark Adler
+ * For conditions of distribution and use, see copyright notice in zlib.h
+ Version 1.0 29 May 2005 Mark Adler */
+
+/* Illustrate the use of Z_BLOCK, inflatePrime(), and inflateSetDictionary()
+ for random access of a compressed file. A file containing a zlib or gzip
+ stream is provided on the command line. The compressed stream is decoded in
+ its entirety, and an index built with access points about every SPAN bytes
+ in the uncompressed output. The compressed file is left open, and can then
+ be read randomly, having to decompress on the average SPAN/2 uncompressed
+ bytes before getting to the desired block of data.
+
+ An access point can be created at the start of any deflate block, by saving
+ the starting file offset and bit of that block, and the 32K bytes of
+ uncompressed data that precede that block. Also the uncompressed offset of
+ that block is saved to provide a referece for locating a desired starting
+ point in the uncompressed stream. build_index() works by decompressing the
+ input zlib or gzip stream a block at a time, and at the end of each block
+ deciding if enough uncompressed data has gone by to justify the creation of
+ a new access point. If so, that point is saved in a data structure that
+ grows as needed to accommodate the points.
+
+ To use the index, an offset in the uncompressed data is provided, for which
+ the latest accees point at or preceding that offset is located in the index.
+ The input file is positioned to the specified location in the index, and if
+ necessary the first few bits of the compressed data is read from the file.
+ inflate is initialized with those bits and the 32K of uncompressed data, and
+ the decompression then proceeds until the desired offset in the file is
+ reached. Then the decompression continues to read the desired uncompressed
+ data from the file.
+
+ Another approach would be to generate the index on demand. In that case,
+ requests for random access reads from the compressed data would try to use
+ the index, but if a read far enough past the end of the index is required,
+ then further index entries would be generated and added.
+
+ There is some fair bit of overhead to starting inflation for the random
+ access, mainly copying the 32K byte dictionary. So if small pieces of the
+ file are being accessed, it would make sense to implement a cache to hold
+ some lookahead and avoid many calls to extract() for small lengths.
+
+ Another way to build an index would be to use inflateCopy(). That would
+ not be constrained to have access points at block boundaries, but requires
+ more memory per access point, and also cannot be saved to file due to the
+ use of pointers in the state. The approach here allows for storage of the
+ index in a file.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "zlib.h"
+
+#define local static
+
+#define SPAN 1048576L /* desired distance between access points */
+#define WINSIZE 32768U /* sliding window size */
+#define CHUNK 16384 /* file input buffer size */
+
+/* access point entry */
+struct point {
+ off_t out; /* corresponding offset in uncompressed data */
+ off_t in; /* offset in input file of first full byte */
+ int bits; /* number of bits (1-7) from byte at in - 1, or 0 */
+ unsigned char window[WINSIZE]; /* preceding 32K of uncompressed data */
+};
+
+/* access point list */
+struct access {
+ int have; /* number of list entries filled in */
+ int size; /* number of list entries allocated */
+ struct point *list; /* allocated list */
+};
+
+/* Deallocate an index built by build_index() */
+local void free_index(struct access *index)
+{
+ if (index != NULL) {
+ free(index->list);
+ free(index);
+ }
+}
+
+/* Add an entry to the access point list. If out of memory, deallocate the
+ existing list and return NULL. */
+local struct access *addpoint(struct access *index, int bits,
+ off_t in, off_t out, unsigned left, unsigned char *window)
+{
+ struct point *next;
+
+ /* if list is empty, create it (start with eight points) */
+ if (index == NULL) {
+ index = malloc(sizeof(struct access));
+ if (index == NULL) return NULL;
+ index->list = malloc(sizeof(struct point) << 3);
+ if (index->list == NULL) {
+ free(index);
+ return NULL;
+ }
+ index->size = 8;
+ index->have = 0;
+ }
+
+ /* if list is full, make it bigger */
+ else if (index->have == index->size) {
+ index->size <<= 1;
+ next = realloc(index->list, sizeof(struct point) * index->size);
+ if (next == NULL) {
+ free_index(index);
+ return NULL;
+ }
+ index->list = next;
+ }
+
+ /* fill in entry and increment how many we have */
+ next = index->list + index->have;
+ next->bits = bits;
+ next->in = in;
+ next->out = out;
+ if (left)
+ memcpy(next->window, window + WINSIZE - left, left);
+ if (left < WINSIZE)
+ memcpy(next->window + left, window, WINSIZE - left);
+ index->have++;
+
+ /* return list, possibly reallocated */
+ return index;
+}
+
+/* Make one entire pass through the compressed stream and build an index, with
+ access points about every span bytes of uncompressed output -- span is
+ chosen to balance the speed of random access against the memory requirements
+ of the list, about 32K bytes per access point. Note that data after the end
+ of the first zlib or gzip stream in the file is ignored. build_index()
+ returns the number of access points on success (>= 1), Z_MEM_ERROR for out
+ of memory, Z_DATA_ERROR for an error in the input file, or Z_ERRNO for a
+ file read error. On success, *built points to the resulting index. */
+local int build_index(FILE *in, off_t span, struct access **built)
+{
+ int ret;
+ off_t totin, totout; /* our own total counters to avoid 4GB limit */
+ off_t last; /* totout value of last access point */
+ struct access *index; /* access points being generated */
+ z_stream strm;
+ unsigned char input[CHUNK];
+ unsigned char window[WINSIZE];
+
+ /* initialize inflate */
+ strm.zalloc = Z_NULL;
+ strm.zfree = Z_NULL;
+ strm.opaque = Z_NULL;
+ strm.avail_in = 0;
+ strm.next_in = Z_NULL;
+ ret = inflateInit2(&strm, 47); /* automatic zlib or gzip decoding */
+ if (ret != Z_OK)
+ return ret;
+
+ /* inflate the input, maintain a sliding window, and build an index -- this
+ also validates the integrity of the compressed data using the check
+ information at the end of the gzip or zlib stream */
+ totin = totout = last = 0;
+ index = NULL; /* will be allocated by first addpoint() */
+ strm.avail_out = 0;
+ do {
+ /* get some compressed data from input file */
+ strm.avail_in = fread(input, 1, CHUNK, in);
+ if (ferror(in)) {
+ ret = Z_ERRNO;
+ goto build_index_error;
+ }
+ if (strm.avail_in == 0) {
+ ret = Z_DATA_ERROR;
+ goto build_index_error;
+ }
+ strm.next_in = input;
+
+ /* process all of that, or until end of stream */
+ do {
+ /* reset sliding window if necessary */
+ if (strm.avail_out == 0) {
+ strm.avail_out = WINSIZE;
+ strm.next_out = window;
+ }
+
+ /* inflate until out of input, output, or at end of block --
+ update the total input and output counters */
+ totin += strm.avail_in;
+ totout += strm.avail_out;
+ ret = inflate(&strm, Z_BLOCK); /* return at end of block */
+ totin -= strm.avail_in;
+ totout -= strm.avail_out;
+ if (ret == Z_NEED_DICT)
+ ret = Z_DATA_ERROR;
+ if (ret == Z_MEM_ERROR || ret == Z_DATA_ERROR)
+ goto build_index_error;
+ if (ret == Z_STREAM_END)
+ break;
+
+ /* if at end of block, consider adding an index entry (note that if
+ data_type indicates an end-of-block, then all of the
+ uncompressed data from that block has been delivered, and none
+ of the compressed data after that block has been consumed,
+ 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;
+}
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