/* zran.c -- example of zlib/gzip stream indexing and random access

* Copyright (C) 2005, 2012 Mark Adler

* For conditions of distribution and use, see copyright notice in zlib.h

Version 1.1 29 Sep 2012 Mark Adler */

/* Version History:

1.0 29 May 2005 First version

1.1 29 Sep 2012 Fix memory reallocation error

*/

/* 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 {

};

/* access point list */

struct access {

};

/* 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->list = realloc(index->list, 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 = NULL;

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;

}