/* zran.c -- example of deflate stream indexing and random access * Copyright (C) 2005, 2012, 2018, 2023 Mark Adler * For conditions of distribution and use, see copyright notice in zlib.h
* Version 1.4 13 Apr 2023 Mark Adler */
/* Version History: 1.0 29 May 2005 First version 1.1 29 Sep 2012 Fix memory reallocation error 1.2 14 Oct 2018 Handle gzip streams with multiple members Add a header file to facilitate usage in applications 1.3 18 Feb 2023 Permit raw deflate streams as well as zlib and gzip Permit crossing gzip member boundaries when extracting Support a size_t size when extracting (was an int) Do a binary search over the index for an access point Expose the access point type to enable save and load 1.4 13 Apr 2023 Add a NOPRIME define to not use inflatePrime()
*/
// Illustrate the use of Z_BLOCK, inflatePrime(), and inflateSetDictionary() // for random access of a compressed file. A file containing a raw deflate // 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 reference for locating a desired starting // point in the uncompressed stream. deflate_index_build() decompresses the // input raw deflate stream a block at a time, and at the end of each block // decides 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 access 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 // decompression then proceeds until the desired offset in the file is reached. // Then decompression continues to read the requested uncompressed data from // the file. // // There is some fair bit of overhead to starting inflation for the random // access, mainly copying the 32K byte dictionary. If small pieces of the file // are being accessed, it would make sense to implement a cache to hold some // lookahead to avoid many calls to deflate_index_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 would require // more memory per access point, and could not be saved to a file due to the // use of pointers in the state. The approach here allows for storage of the // index in a file.
// See comments in zran.h. void deflate_index_free(struct deflate_index *index) { if (index != NULL) {
free(index->list);
free(index);
}
}
// Add an access point to the list. If out of memory, deallocate the existing // list and return NULL. index->mode is temporarily the allocated number of // access points, until it is time for deflate_index_build() to return. Then // index->mode is set to the mode of inflation. staticstruct deflate_index *add_point(struct deflate_index *index, int bits,
off_t in, off_t out, unsigned left, unsignedchar *window) { if (index == NULL) { // The list is empty. Create it, starting with eight access points.
index = malloc(sizeof(struct deflate_index)); if (index == NULL) return NULL;
index->have = 0;
index->mode = 8;
index->list = malloc(sizeof(point_t) * index->mode); if (index->list == NULL) {
free(index); return NULL;
}
}
elseif (index->have == index->mode) { // The list is full. Make it bigger.
index->mode <<= 1;
point_t *next = realloc(index->list, sizeof(point_t) * index->mode); if (next == NULL) {
deflate_index_free(index); return NULL;
}
index->list = next;
}
// Fill in the access point and increment how many we have.
point_t *next = (point_t *)(index->list) + index->have++; if (index->have < 0) { // Overflowed the int!
deflate_index_free(index); return NULL;
}
next->out = out;
next->in = in;
next->bits = bits; if (left)
memcpy(next->window, window + WINSIZE - left, left); if (left < WINSIZE)
memcpy(next->window + left, window, WINSIZE - left);
// Return the index, which may have been newly allocated or destroyed. return index;
}
// Decompression modes. These are the inflateInit2() windowBits parameter. #define RAW -15 #define ZLIB 15 #define GZIP 31
// See comments in zran.h. int deflate_index_build(FILE *in, off_t span, struct deflate_index **built) { // Set up inflation state.
z_stream strm = {0}; // inflate engine (gets fired up later) unsignedchar buf[CHUNK]; // input buffer unsignedchar win[WINSIZE] = {0}; // output sliding window
off_t totin = 0; // total bytes read from input
off_t totout = 0; // total bytes uncompressed int mode = 0; // mode: RAW, ZLIB, or GZIP (0 => not set yet)
// Decompress from in, generating access points along the way. int ret; // the return value from zlib, or Z_ERRNO
off_t last; // last access point uncompressed offset struct deflate_index *index = NULL; // list of access points do { // Assure available input, at least until reaching EOF. if (strm.avail_in == 0) {
strm.avail_in = fread(buf, 1, sizeof(buf), in);
totin += strm.avail_in;
strm.next_in = buf; if (strm.avail_in < sizeof(buf) && ferror(in)) {
ret = Z_ERRNO; break;
}
if (mode == 0) { // At the start of the input -- determine the type. Assume raw // if it is neither zlib nor gzip. This could in theory result // in a false positive for zlib, but in practice the fill bits // after a stored block are always zeros, so a raw stream won't // start with an 8 in the low nybble.
mode = strm.avail_in == 0 ? RAW : // empty -- will fail
(strm.next_in[0] & 0xf) == 8 ? ZLIB :
strm.next_in[0] == 0x1f ? GZIP : /* else */ RAW;
ret = inflateInit2(&strm, mode); if (ret != Z_OK) break;
}
}
// Assure available output. This rotates the output through, for use as // a sliding window on the uncompressed data. if (strm.avail_out == 0) {
strm.avail_out = sizeof(win);
strm.next_out = win;
}
if (mode == RAW && index == NULL) // We skip the inflate() call at the start of raw deflate data in // order generate an access point there. Set data_type to imitate // the end of a header.
strm.data_type = 0x80; else { // Inflate and update the number of uncompressed bytes. unsigned before = strm.avail_out;
ret = inflate(&strm, Z_BLOCK);
totout += before - strm.avail_out;
}
if ((strm.data_type & 0xc0) == 0x80 &&
(index == NULL || totout - last >= span)) { // We are at the end of a header or a non-last deflate block, so we // can add an access point here. Furthermore, we are either at the // very start for the first access point, or there has been span or // more uncompressed bytes since the last access point, so we want // to add an access point here.
index = add_point(index, strm.data_type & 7, totin - strm.avail_in,
totout, strm.avail_out, win); if (index == NULL) {
ret = Z_MEM_ERROR; break;
}
last = totout;
}
if (ret == Z_STREAM_END && mode == GZIP &&
(strm.avail_in || ungetc(getc(in), in) != EOF)) // There is more input after the end of a gzip member. Reset the // inflate state to read another gzip member. On success, this will // set ret to Z_OK to continue decompressing.
ret = inflateReset2(&strm, GZIP);
// Keep going until Z_STREAM_END or error. If the compressed data ends // prematurely without a file read error, Z_BUF_ERROR is returned.
} while (ret == Z_OK);
inflateEnd(&strm);
if (ret != Z_STREAM_END) { // An error was encountered. Discard the index and return a negative // error code.
deflate_index_free(index); return ret == Z_NEED_DICT ? Z_DATA_ERROR : ret;
}
// Shrink the index to only the occupied access points and return it.
index->mode = mode;
index->length = totout;
point_t *list = realloc(index->list, sizeof(point_t) * index->have); if (list == NULL) { // Seems like a realloc() to make something smaller should always work, // but just in case.
deflate_index_free(index); return Z_MEM_ERROR;
}
index->list = list;
*built = index; return index->have;
}
#ifdef NOPRIME // Support zlib versions before 1.2.3 (July 2005), or incomplete zlib clones // that do not have inflatePrime().
# define INFLATEPRIME inflatePreface
// Append the low bits bits of value to in[] at bit position *have, updating // *have. value must be zero above its low bits bits. bits must be positive. // This assumes that any bits above the *have bits in the last byte are zeros. // That assumption is preserved on return, as any bits above *have + bits in // the last byte written will be set to zeros. staticinlinevoid append_bits(unsigned value, int bits, unsignedchar *in, int *have) {
in += *have >> 3; // where the first bits from value will go int k = *have & 7; // the number of bits already there
*have += bits; if (k)
*in |= value << k; // write value above the low k bits else
*in = value;
k = 8 - k; // the number of bits just appended while (bits > k) {
value >>= k; // drop the bits appended
bits -= k;
k = 8; // now at a byte boundary
*++in = value;
}
}
// Insert enough bits in the form of empty deflate blocks in front of the the // low bits bits of value, in order to bring the sequence to a byte boundary. // Then feed that to inflate(). This does what inflatePrime() does, except that // a negative value of bits is not supported. bits must be in 0..16. If the // arguments are invalid, Z_STREAM_ERROR is returned. Otherwise the return // value from inflate() is returned. staticint inflatePreface(z_stream *strm, int bits, int value) { // Check input. if (strm == Z_NULL || bits < 0 || bits > 16) return Z_STREAM_ERROR; if (bits == 0) return Z_OK;
value &= (2 << (bits - 1)) - 1;
// An empty dynamic block with an odd number of bits (95). The high bit of // the last byte is unused. staticconstunsignedchar dyn[] = {
4, 0xe0, 0x81, 8, 0, 0, 0, 0, 0x20, 0xa8, 0xab, 0x1f
}; constint dynlen = 95; // number of bits in the block
// Build an input buffer for inflate that is a multiple of eight bits in // length, and that ends with the low bits bits of value. unsignedchar in[(dynlen + 3 * 10 + 16 + 7) / 8]; int have = 0; if (bits & 1) { // Insert an empty dynamic block to get to an odd number of bits, so // when bits bits from value are appended, we are at an even number of // bits.
memcpy(in, dyn, sizeof(dyn));
have = dynlen;
} while ((have + bits) & 7) // Insert empty fixed blocks until appending bits bits would put us on // a byte boundary. This will insert at most three fixed blocks.
append_bits(2, 10, in, &have);
// Append the bits bits from value, which takes us to a byte boundary.
append_bits(value, bits, in, &have);
// Deliver the input to inflate(). There is no output space provided, but // inflate() can't get stuck waiting on output not ingesting all of the // provided input. The reason is that there will be at most 16 bits of // input from value after the empty deflate blocks (which themselves // generate no output). At least ten bits are needed to generate the first // output byte from a fixed block. The last two bytes of the buffer have to // be ingested in order to get ten bits, which is the most that value can // occupy.
strm->avail_in = have >> 3;
strm->next_in = in;
strm->avail_out = 0;
strm->next_out = in; // not used, but can't be NULL return inflate(strm, Z_NO_FLUSH);
}
#else # define INFLATEPRIME inflatePrime #endif
// See comments in zran.h.
ptrdiff_t deflate_index_extract(FILE *in, struct deflate_index *index,
off_t offset, unsignedchar *buf, size_t len) { // Do a quick sanity check on the index. if (index == NULL || index->have < 1 || index->list[0].out != 0) return Z_STREAM_ERROR;
// If nothing to extract, return zero bytes extracted. if (len == 0 || offset < 0 || offset >= index->length) return 0;
// Find the access point closest to but not after offset. int lo = -1, hi = index->have;
point_t *point = index->list; while (hi - lo > 1) { int mid = (lo + hi) >> 1; if (offset < point[mid].out)
hi = mid; else
lo = mid;
}
point += lo;
// Initialize the input file and prime the inflate engine to start there. int ret = fseeko(in, point->in - (point->bits ? 1 : 0), SEEK_SET); if (ret == -1) return Z_ERRNO; int ch = 0; if (point->bits && (ch = getc(in)) == EOF) return ferror(in) ? Z_ERRNO : Z_BUF_ERROR;
z_stream strm = {0};
ret = inflateInit2(&strm, RAW); if (ret != Z_OK) return ret; if (point->bits)
INFLATEPRIME(&strm, point->bits, ch >> (8 - point->bits));
inflateSetDictionary(&strm, point->window, WINSIZE);
// Skip uncompressed bytes until offset reached, then satisfy request. unsignedchar input[CHUNK]; unsignedchar discard[WINSIZE];
offset -= point->out; // number of bytes to skip to get to offset
size_t left = len; // number of bytes left to read after offset do { if (offset) { // Discard up to offset uncompressed bytes.
strm.avail_out = offset < WINSIZE ? (unsigned)offset : WINSIZE;
strm.next_out = discard;
} else { // Uncompress up to left bytes into buf.
strm.avail_out = left < UINT_MAX ? (unsigned)left : UINT_MAX;
strm.next_out = buf + len - left;
}
// Uncompress, setting got to the number of bytes uncompressed. if (strm.avail_in == 0) { // Assure available input.
strm.avail_in = fread(input, 1, CHUNK, in); if (strm.avail_in < CHUNK && ferror(in)) {
ret = Z_ERRNO; break;
}
strm.next_in = input;
} unsigned got = strm.avail_out;
ret = inflate(&strm, Z_NO_FLUSH);
got -= strm.avail_out;
// Update the appropriate count. if (offset)
offset -= got; else
left -= got;
// If we're at the end of a gzip member and there's more to read, // continue to the next gzip member. if (ret == Z_STREAM_END && index->mode == GZIP) { // Discard the gzip trailer. unsigned drop = 8; // length of gzip trailer if (strm.avail_in >= drop) {
strm.avail_in -= drop;
strm.next_in += drop;
} else { // Read and discard the remainder of the gzip trailer.
drop -= strm.avail_in;
strm.avail_in = 0; do { if (getc(in) == EOF) // The input does not have a complete trailer. return ferror(in) ? Z_ERRNO : Z_BUF_ERROR;
} while (--drop);
}
if (strm.avail_in || ungetc(getc(in), in) != EOF) { // There's more after the gzip trailer. Use inflate to skip the // gzip header and resume the raw inflate there.
inflateReset2(&strm, GZIP); do { if (strm.avail_in == 0) {
strm.avail_in = fread(input, 1, CHUNK, in); if (strm.avail_in < CHUNK && ferror(in)) {
ret = Z_ERRNO; break;
}
strm.next_in = input;
}
strm.avail_out = WINSIZE;
strm.next_out = discard;
ret = inflate(&strm, Z_BLOCK); // stop at end of header
} while (ret == Z_OK && (strm.data_type & 0x80) == 0); if (ret != Z_OK) break;
inflateReset2(&strm, RAW);
}
}
// Continue until we have the requested data, the deflate data has // ended, or an error is encountered.
} while (ret == Z_OK && left);
inflateEnd(&strm);
// Return the number of uncompressed bytes read into buf, or the error. return ret == Z_OK || ret == Z_STREAM_END ? len - left : ret;
}
#ifdef TEST
#define SPAN 1048576L // desired distance between access points #define LEN 16384 // number of bytes to extract
// Demonstrate the use of deflate_index_build() and deflate_index_extract() by // processing the file provided on the command line, and extracting LEN bytes // from 2/3rds of the way through the uncompressed output, writing that to // stdout. An offset can be provided as the second argument, in which case the // data is extracted from there instead. int main(int argc, char **argv) { // Open the input file. if (argc < 2 || argc > 3) {
fprintf(stderr, "usage: zran file.raw [offset]\n"); return 1;
}
FILE *in = fopen(argv[1], "rb"); if (in == NULL) {
fprintf(stderr, "zran: could not open %s for reading\n", argv[1]); return 1;
}
// Get optional offset.
off_t offset = -1; if (argc == 3) { char *end;
offset = strtoll(argv[2], &end, 10); if (*end || offset < 0) {
fprintf(stderr, "zran: %s is not a valid offset\n", argv[2]); return 1;
}
}
// Build index. struct deflate_index *index = NULL; int len = deflate_index_build(in, SPAN, &index); if (len < 0) {
fclose(in); switch (len) { case Z_MEM_ERROR:
fprintf(stderr, "zran: out of memory\n"); break; case Z_BUF_ERROR:
fprintf(stderr, "zran: %s ended prematurely\n", argv[1]); 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. unsignedchar buf[LEN]; if (offset == -1)
offset = ((index->length + 1) << 1) / 3;
ptrdiff_t got = deflate_index_extract(in, index, offset, buf, LEN); if (got < 0)
fprintf(stderr, "zran: extraction failed: %s error\n",
got == Z_MEM_ERROR ? "out of memory" : "input corrupted"); else {
fwrite(buf, 1, got, stdout);
fprintf(stderr, "zran: extracted %ld bytes at %lld\n", got, offset);
}
// Clean up and exit.
deflate_index_free(index);
fclose(in); return 0;
}
#endif
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