blast.c

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/* blast.c
 * Copyright (C) 2003 Mark Adler
 * For conditions of distribution and use, see copyright notice in blast.h
 * version 1.1, 16 Feb 2003
 *
 * blast.c decompresses data compressed by the PKWare Compression Library.
 * This function provides functionality similar to the explode() function of
 * the PKWare library, hence the name "blast".
 *
 * This decompressor is based on the excellent format description provided by
 * Ben Rudiak-Gould in comp.compression on August 13, 2001.  Interestingly, the
 * example Ben provided in the post is incorrect.  The distance 110001 should
 * instead be 111000.  When corrected, the example byte stream becomes:
 *
 *    00 04 82 24 25 8f 80 7f
 *
 * which decompresses to "AIAIAIAIAIAIA" (without the quotes).
 */

/*
 * Change history:
 *
 * 1.0  12 Feb 2003     - First version
 * 1.1  16 Feb 2003     - Fixed distance check for > 4 GB uncompressed data
 */

#include <setjmp.h>             /* for setjmp(), longjmp(), and jmp_buf */
#include "blast.h"              /* prototype for blast() */

#define local static            /* for local function definitions */
#define MAXBITS 13              /* maximum code length */
#define MAXWIN 4096             /* maximum window size */

/* input and output state */
struct state {
    /* input state */
    blast_in infun;             /* input function provided by user */
    void *inhow;                /* opaque information passed to infun() */
    unsigned char *in;          /* next input location */
    unsigned left;              /* available input at in */
    int bitbuf;                 /* bit buffer */
    int bitcnt;                 /* number of bits in bit buffer */

    /* input limit error return state for bits() and decode() */
    jmp_buf env;

    /* output state */
    blast_out outfun;           /* output function provided by user */
    void *outhow;               /* opaque information passed to outfun() */
    unsigned next;              /* index of next write location in out[] */
    int first;                  /* true to check distances (for first 4K) */
    unsigned char out[MAXWIN];  /* output buffer and sliding window */
};

/*
 * Return need bits from the input stream.  This always leaves less than
 * eight bits in the buffer.  bits() works properly for need == 0.
 *
 * Format notes:
 *
 * - Bits are stored in bytes from the least significant bit to the most
 *   significant bit.  Therefore bits are dropped from the bottom of the bit
 *   buffer, using shift right, and new bytes are appended to the top of the
 *   bit buffer, using shift left.
 */
local int bits(struct state *s, int need)
{
    int val;            /* bit accumulator */

    /* load at least need bits into val */
    val = s->bitbuf;
    while (s->bitcnt < need) {
        if (s->left == 0) {
            s->left = s->infun(s->inhow, &(s->in));
            if (s->left == 0) longjmp(s->env, 1);       /* out of input */
        }
        val |= (int)(*(s->in)++) << s->bitcnt;          /* load eight bits */
        s->left--;
        s->bitcnt += 8;
    }

    /* drop need bits and update buffer, always zero to seven bits left */
    s->bitbuf = val >> need;
    s->bitcnt -= need;

    /* return need bits, zeroing the bits above that */
    return val & ((1 << need) - 1);
}

/*
 * Huffman code decoding tables.  count[1..MAXBITS] is the number of symbols of
 * each length, which for a canonical code are stepped through in order.
 * symbol[] are the symbol values in canonical order, where the number of
 * entries is the sum of the counts in count[].  The decoding process can be
 * seen in the function decode() below.
 */
struct huffman {
    short *count;       /* number of symbols of each length */
    short *symbol;      /* canonically ordered symbols */
};

/*
 * Decode a code from the stream s using huffman table h.  Return the symbol or
 * a negative value if there is an error.  If all of the lengths are zero, i.e.
 * an empty code, or if the code is incomplete and an invalid code is received,
 * then -9 is returned after reading MAXBITS bits.
 *
 * Format notes:
 *
 * - The codes as stored in the compressed data are bit-reversed relative to
 *   a simple integer ordering of codes of the same lengths.  Hence below the
 *   bits are pulled from the compressed data one at a time and used to
 *   build the code value reversed from what is in the stream in order to
 *   permit simple integer comparisons for decoding.
 *
 * - The first code for the shortest length is all ones.  Subsequent codes of
 *   the same length are simply integer decrements of the previous code.  When
 *   moving up a length, a one bit is appended to the code.  For a complete
 *   code, the last code of the longest length will be all zeros.  To support
 *   this ordering, the bits pulled during decoding are inverted to apply the
 *   more "natural" ordering starting with all zeros and incrementing.
 */
local int decode(struct state *s, struct huffman *h)
{
    int len;            /* current number of bits in code */
    int code;           /* len bits being decoded */
    int first;          /* first code of length len */
    int count;          /* number of codes of length len */
    int index;          /* index of first code of length len in symbol table */
    int bitbuf;         /* bits from stream */
    int left;           /* bits left in next or left to process */
    short *next;        /* next number of codes */

    bitbuf = s->bitbuf;
    left = s->bitcnt;
    code = first = index = 0;
    len = 1;
    next = h->count + 1;
    while (1) {
        while (left--) {
            code |= (bitbuf & 1) ^ 1;   /* invert code */
            bitbuf >>= 1;
            count = *next++;
            if (code < first + count) { /* if length len, return symbol */
                s->bitbuf = bitbuf;
                s->bitcnt = (s->bitcnt - len) & 7;
                return h->symbol[index + (code - first)];
            }
            index += count;             /* else update for next length */
            first += count;
            first <<= 1;
            code <<= 1;
            len++;
        }
        left = (MAXBITS+1) - len;
        if (left == 0) break;
        if (s->left == 0) {
            s->left = s->infun(s->inhow, &(s->in));
            if (s->left == 0) longjmp(s->env, 1);       /* out of input */
        }
        bitbuf = *(s->in)++;
        s->left--;
        if (left > 8) left = 8;
    }
    return -9;                          /* ran out of codes */
}

/*
 * Given a list of repeated code lengths rep[0..n-1], where each byte is a
 * count (high four bits + 1) and a code length (low four bits), generate the
 * list of code lengths.  This compaction reduces the size of the object code.
 * Then given the list of code lengths length[0..n-1] representing a canonical
 * Huffman code for n symbols, construct the tables required to decode those
 * codes.  Those tables are the number of codes of each length, and the symbols
 * sorted by length, retaining their original order within each length.  The
 * return value is zero for a complete code set, negative for an over-
 * subscribed code set, and positive for an incomplete code set.  The tables
 * can be used if the return value is zero or positive, but they cannot be used
 * if the return value is negative.  If the return value is zero, it is not
 * possible for decode() using that table to return an error--any stream of
 * enough bits will resolve to a symbol.  If the return value is positive, then
 * it is possible for decode() using that table to return an error for received
 * codes past the end of the incomplete lengths.
 */
local int construct(struct huffman *h, const unsigned char *rep, int n)
{
    int symbol;         /* current symbol when stepping through length[] */
    int len;            /* current length when stepping through h->count[] */
    int left;           /* number of possible codes left of current length */
    short offs[MAXBITS+1];      /* offsets in symbol table for each length */
    short length[256];  /* code lengths */

    /* convert compact repeat counts into symbol bit length list */
    symbol = 0;
    do {
        len = *rep++;
        left = (len >> 4) + 1;
        len &= 15;
        do {
            length[symbol++] = len;
        } while (--left);
    } while (--n);
    n = symbol;

    /* count number of codes of each length */
    for (len = 0; len <= MAXBITS; len++)
        h->count[len] = 0;
    for (symbol = 0; symbol < n; symbol++)
        (h->count[length[symbol]])++;   /* assumes lengths are within bounds */
    if (h->count[0] == n)               /* no codes! */
        return 0;                       /* complete, but decode() will fail */

    /* check for an over-subscribed or incomplete set of lengths */
    left = 1;                           /* one possible code of zero length */
    for (len = 1; len <= MAXBITS; len++) {
        left <<= 1;                     /* one more bit, double codes left */
        left -= h->count[len];          /* deduct count from possible codes */
        if (left < 0) return left;      /* over-subscribed--return negative */
    }                                   /* left > 0 means incomplete */

    /* generate offsets into symbol table for each length for sorting */
    offs[1] = 0;
    for (len = 1; len < MAXBITS; len++)