puff.c

minix3的源代码

 *
 * - The maximum number of bits in a code is 15, so the possible lengths for
 *   any code are 1..15.
 *
 * - The fact that a length of zero is not permitted for a code has an
 *   interesting consequence.  Normally if only one symbol is used for a given
 *   code, then in fact that code could be represented with zero bits.  However
 *   in deflate, that code has to be at least one bit.  So for example, if
 *   only a single distance base symbol appears in a block, then it will be
 *   represented by a single code of length one, in particular one 0 bit.  This
 *   is an incomplete code, since if a 1 bit is received, it has no meaning,
 *   and should result in an error.  So incomplete distance codes of one symbol
 *   should be permitted, and the receipt of invalid codes should be handled.
 *
 * - It is also possible to have a single literal/length code, but that code
 *   must be the end-of-block code, since every dynamic block has one.  This
 *   is not the most efficient way to create an empty block (an empty fixed
 *   block is fewer bits), but it is allowed by the format.  So incomplete
 *   literal/length codes of one symbol should also be permitted.
 *
 * - If there are only literal codes and no lengths, then there are no distance
 *   codes.  This is represented by one distance code with zero bits.
 *
 * - The list of up to 286 length/literal lengths and up to 30 distance lengths
 *   are themselves compressed using Huffman codes and run-length encoding.  In
 *   the list of code lengths, a 0 symbol means no code, a 1..15 symbol means
 *   that length, and the symbols 16, 17, and 18 are run-length instructions.
 *   Each of 16, 17, and 18 are follwed by extra bits to define the length of
 *   the run.  16 copies the last length 3 to 6 times.  17 represents 3 to 10
 *   zero lengths, and 18 represents 11 to 138 zero lengths.  Unused symbols
 *   are common, hence the special coding for zero lengths.
 *
 * - The symbols for 0..18 are Huffman coded, and so that code must be
 *   described first.  This is simply a sequence of up to 19 three-bit values
 *   representing no code (0) or the code length for that symbol (1..7).
 *
 * - A dynamic block starts with three fixed-size counts from which is computed
 *   the number of literal/length code lengths, the number of distance code
 *   lengths, and the number of code length code lengths (ok, you come up with
 *   a better name!) in the code descriptions.  For the literal/length and
 *   distance codes, lengths after those provided are considered zero, i.e. no
 *   code.  The code length code lengths are received in a permuted order (see
 *   the order[] array below) to make a short code length code length list more
 *   likely.  As it turns out, very short and very long codes are less likely
 *   to be seen in a dynamic code description, hence what may appear initially
 *   to be a peculiar ordering.
 *
 * - Given the number of literal/length code lengths (nlen) and distance code
 *   lengths (ndist), then they are treated as one long list of nlen + ndist
 *   code lengths.  Therefore run-length coding can and often does cross the
 *   boundary between the two sets of lengths.
 *
 * - So to summarize, the code description at the start of a dynamic block is
 *   three counts for the number of code lengths for the literal/length codes,
 *   the distance codes, and the code length codes.  This is followed by the
 *   code length code lengths, three bits each.  This is used to construct the
 *   code length code which is used to read the remainder of the lengths.  Then
 *   the literal/length code lengths and distance lengths are read as a single
 *   set of lengths using the code length codes.  Codes are constructed from
 *   the resulting two sets of lengths, and then finally you can start
 *   decoding actual compressed data in the block.
 *
 * - For reference, a "typical" size for the code description in a dynamic
 *   block is around 80 bytes.
 */
local int dynamic(struct state *s)
{
    int nlen, ndist, ncode;             /* number of lengths in descriptor */
    int index;                          /* index of lengths[] */
    int err;                            /* construct() return value */
    short lengths[MAXCODES];            /* descriptor code lengths */
    short lencnt[MAXBITS+1], lensym[MAXLCODES];         /* lencode memory */
    short distcnt[MAXBITS+1], distsym[MAXDCODES];       /* distcode memory */
    struct huffman lencode = {lencnt, lensym};          /* length code */
    struct huffman distcode = {distcnt, distsym};       /* distance code */
    static const short order[19] =      /* permutation of code length codes */
        {16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15};

    /* get number of lengths in each table, check lengths */
    nlen = bits(s, 5) + 257;
    ndist = bits(s, 5) + 1;
    ncode = bits(s, 4) + 4;
    if (nlen > MAXLCODES || ndist > MAXDCODES)
        return -3;                      /* bad counts */

    /* read code length code lengths (really), missing lengths are zero */
    for (index = 0; index < ncode; index++)
        lengths[order[index]] = bits(s, 3);
    for (; index < 19; index++)
        lengths[order[index]] = 0;

    /* build huffman table for code lengths codes (use lencode temporarily) */
    err = construct(&lencode, lengths, 19);
    if (err != 0) return -4;            /* require complete code set here */

    /* read length/literal and distance code length tables */
    index = 0;
    while (index < nlen + ndist) {
        int symbol;             /* decoded value */
        int len;                /* last length to repeat */

        symbol = decode(s, &lencode);
        if (symbol < 16)                /* length in 0..15 */
            lengths[index++] = symbol;
        else {                          /* repeat instruction */
            len = 0;                    /* assume repeating zeros */
            if (symbol == 16) {         /* repeat last length 3..6 times */
                if (index == 0) return -5;      /* no last length! */
                len = lengths[index - 1];       /* last length */
                symbol = 3 + bits(s, 2);
            }
            else if (symbol == 17)      /* repeat zero 3..10 times */
                symbol = 3 + bits(s, 3);
            else                        /* == 18, repeat zero 11..138 times */
                symbol = 11 + bits(s, 7);
            if (index + symbol > nlen + ndist)
                return -6;              /* too many lengths! */
            while (symbol--)            /* repeat last or zero symbol times */
                lengths[index++] = len;
        }
    }

    /* build huffman table for literal/length codes */
    err = construct(&lencode, lengths, nlen);
    if (err < 0 || (err > 0 && nlen - lencode.count[0] != 1))
        return -7;      /* only allow incomplete codes if just one code */

    /* build huffman table for distance codes */
    err = construct(&distcode, lengths + nlen, ndist);
    if (err < 0 || (err > 0 && ndist - distcode.count[0] != 1))
        return -8;      /* only allow incomplete codes if just one code */

    /* decode data until end-of-block code */
    return codes(s, &lencode, &distcode);
}

/*
 * Inflate source to dest.  On return, destlen and sourcelen are updated to the
 * size of the uncompressed data and the size of the deflate data respectively.
 * On success, the return value of puff() is zero.  If there is an error in the
 * source data, i.e. it is not in the deflate format, then a negative value is
 * returned.  If there is not enough input available or there is not enough
 * output space, then a positive error is returned.  In that case, destlen and
 * sourcelen are not updated to facilitate retrying from the beginning with the
 * provision of more input data or more output space.  In the case of invalid
 * inflate data (a negative error), the dest and source pointers are updated to
 * facilitate the debugging of deflators.
 *
 * puff() also has a mode to determine the size of the uncompressed output with
 * no output written.  For this dest must be (unsigned char *)0.  In this case,
 * the input value of *destlen is ignored, and on return *destlen is set to the
 * size of the uncompressed output.
 *
 * The return codes are:
 *
 *   2:  available inflate data did not terminate
 *   1:  output space exhausted before completing inflate
 *   0:  successful inflate
 *  -1:  invalid block type (type == 3)
 *  -2:  stored block length did not match one's complement
 *  -3:  dynamic block code description: too many length or distance codes
 *  -4:  dynamic block code description: code lengths codes incomplete
 *  -5:  dynamic block code description: repeat lengths with no first length
 *  -6:  dynamic block code description: repeat more than specified lengths
 *  -7:  dynamic block code description: invalid literal/length code lengths
 *  -8:  dynamic block code description: invalid distance code lengths
 *  -9:  invalid literal/length or distance code in fixed or dynamic block
 * -10:  distance is too far back in fixed or dynamic block
 *
 * Format notes:
 *
 * - Three bits are read for each block to determine the kind of block and
 *   whether or not it is the last block.  Then the block is decoded and the
 *   process repeated if it was not the last block.
 *
 * - The leftover bits in the last byte of the deflate data after the last
 *   block (if it was a fixed or dynamic block) are undefined and have no
 *   expected values to check.
 */
int puff(unsigned char *dest,           /* pointer to destination pointer */
         unsigned long *destlen,        /* amount of output space */
         unsigned char *source,         /* pointer to source data pointer */
         unsigned long *sourcelen)      /* amount of input available */
{
    struct state s;             /* input/output state */
    int last, type;             /* block information */
    int err;                    /* return value */

    /* initialize output state */
    s.out = dest;
    s.outlen = *destlen;                /* ignored if dest is NIL */
    s.outcnt = 0;

    /* initialize input state */
    s.in = source;
    s.inlen = *sourcelen;
    s.incnt = 0;
    s.bitbuf = 0;
    s.bitcnt = 0;

    /* return if bits() or decode() tries to read past available input */
    if (setjmp(s.env) != 0)             /* if came back here via longjmp() */
        err = 2;                        /* then skip do-loop, return error */
    else {
        /* process blocks until last block or error */
        do {
            last = bits(&s, 1);         /* one if last block */
            type = bits(&s, 2);         /* block type 0..3 */
            err = type == 0 ? stored(&s) :
                  (type == 1 ? fixed(&s) :
                   (type == 2 ? dynamic(&s) :
                    -1));               /* type == 3, invalid */
            if (err != 0) break;        /* return with error */
        } while (!last);
    }

    /* update the lengths and return */
    if (err <= 0) {
        *destlen = s.outcnt;
        *sourcelen = s.incnt;
    }
    return err;
}

#ifdef TEST
/* Example of how to use puff() */
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>

local unsigned char *yank(char *name, unsigned long *len)
{
    unsigned long size;
    unsigned char *buf;
    FILE *in;
    struct stat s;

    *len = 0;
    if (stat(name, &s)) return NULL;
    if ((s.st_mode & S_IFMT) != S_IFREG) return NULL;
    size = (unsigned long)(s.st_size);
    if (size == 0 || (off_t)size != s.st_size) return NULL;
    in = fopen(name, "r");
    if (in == NULL) return NULL;
    buf = malloc(size);
    if (buf != NULL && fread(buf, 1, size, in) != size) {
        free(buf);
        buf = NULL;
    }
    fclose(in);
    *len = size;
    return buf;
}

int main(int argc, char **argv)
{
    int ret;
    unsigned char *source;
    unsigned long len, sourcelen, destlen;

    if (argc < 2) return 2;
    source = yank(argv[1], &len);
    if (source == NULL) return 2;
    sourcelen = len;
    ret = puff(NIL, &destlen, source, &sourcelen);
    if (ret)
        printf("puff() failed with return code %d\n", ret);
    else {
        printf("puff() succeeded uncompressing %lu bytes\n", destlen);
        if (sourcelen < len) printf("%lu compressed bytes unused\n",
                                    len - sourcelen);
    }
    free(source);
    return ret;
}
#endif