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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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