trees.c

给出了 zip 压缩算法的完整实现过程。

     * must be all ones.
     */
    Assert(code + bl_count[MAX_BITS]-1 == (1<< ((ush) MAX_BITS)) - 1,
            "inconsistent bit counts");
    Tracev((stderr,"\ngen_codes: max_code %d ", max_code));

    for (n = 0;  n <= max_code; n++) {
        int len = tree[n].Len;
        if (len == 0) continue;
        /* Now reverse the bits */
        tree[n].Code = (ush)bi_reverse(next_code[len]++, len);

        Tracec(tree != static_ltree, (stderr,"\nn %3d %c l %2d c %4x (%x) ",
             n, (isgraph(n) ? n : ' '), len, tree[n].Code, next_code[len]-1));
    }
}

/* ===========================================================================
 * Construct one Huffman tree and assigns the code bit strings and lengths.
 * Update the total bit length for the current block.
 * IN assertion: the field freq is set for all tree elements.
 * OUT assertions: the fields len and code are set to the optimal bit length
 *     and corresponding code. The length opt_len is updated; static_len is
 *     also updated if stree is not null. The field max_code is set.
 */
local void build_tree(desc)
    tree_desc near *desc; /* the tree descriptor */
{
    ct_data near *tree   = desc->dyn_tree;
    ct_data near *stree  = desc->static_tree;
    int elems            = desc->elems;
    int n, m;          /* iterate over heap elements */
    int max_code = -1; /* largest code with non zero frequency */
    int node = elems;  /* next internal node of the tree */

    /* Construct the initial heap, with least frequent element in
     * heap[SMALLEST]. The sons of heap[n] are heap[2*n] and heap[2*n+1].
     * heap[0] is not used.
     */
    heap_len = 0, heap_max = HEAP_SIZE;

    for (n = 0; n < elems; n++) {
        if (tree[n].Freq != 0) {
            heap[++heap_len] = max_code = n;
            depth[n] = 0;
        } else {
            tree[n].Len = 0;
        }
    }

    /* The pkzip format requires that at least one distance code exists,
     * and that at least one bit should be sent even if there is only one
     * possible code. So to avoid special checks later on we force at least
     * two codes of non zero frequency.
     */
    while (heap_len < 2) {
        int new = heap[++heap_len] = (max_code < 2 ? ++max_code : 0);
        tree[new].Freq = 1;
        depth[new] = 0;
        opt_len--; if (stree) static_len -= stree[new].Len;
        /* new is 0 or 1 so it does not have extra bits */
    }
    desc->max_code = max_code;

    /* The elements heap[heap_len/2+1 .. heap_len] are leaves of the tree,
     * establish sub-heaps of increasing lengths:
     */
    for (n = heap_len/2; n >= 1; n--) pqdownheap(tree, n);

    /* Construct the Huffman tree by repeatedly combining the least two
     * frequent nodes.
     */
    do {
        pqremove(tree, n);   /* n = node of least frequency */
        m = heap[SMALLEST];  /* m = node of next least frequency */

        heap[--heap_max] = n; /* keep the nodes sorted by frequency */
        heap[--heap_max] = m;

        /* Create a new node father of n and m */
        tree[node].Freq = (ush)(tree[n].Freq + tree[m].Freq);
        depth[node] = (uch) (Max(depth[n], depth[m]) + 1);
        tree[n].Dad = tree[m].Dad = (ush)node;
#ifdef DUMP_BL_TREE
        if (tree == bl_tree) {
            fprintf(stderr,"\nnode %d(%d), sons %d(%d) %d(%d)",
                    node, tree[node].Freq, n, tree[n].Freq, m, tree[m].Freq);
        }
#endif
        /* and insert the new node in the heap */
        heap[SMALLEST] = node++;
        pqdownheap(tree, SMALLEST);

    } while (heap_len >= 2);

    heap[--heap_max] = heap[SMALLEST];

    /* At this point, the fields freq and dad are set. We can now
     * generate the bit lengths.
     */
    gen_bitlen((tree_desc near *)desc);

    /* The field len is now set, we can generate the bit codes */
    gen_codes ((ct_data near *)tree, max_code);
}

/* ===========================================================================
 * Scan a literal or distance tree to determine the frequencies of the codes
 * in the bit length tree. Updates opt_len to take into account the repeat
 * counts. (The contribution of the bit length codes will be added later
 * during the construction of bl_tree.)
 */
local void scan_tree (tree, max_code)
    ct_data near *tree; /* the tree to be scanned */
    int max_code;       /* and its largest code of non zero frequency */
{
    int n;                     /* iterates over all tree elements */
    int prevlen = -1;          /* last emitted length */
    int curlen;                /* length of current code */
    int nextlen = tree[0].Len; /* length of next code */
    int count = 0;             /* repeat count of the current code */
    int max_count = 7;         /* max repeat count */
    int min_count = 4;         /* min repeat count */

    if (nextlen == 0) max_count = 138, min_count = 3;
    tree[max_code+1].Len = (ush)-1; /* guard */

    for (n = 0; n <= max_code; n++) {
        curlen = nextlen; nextlen = tree[n+1].Len;
        if (++count < max_count && curlen == nextlen) {
            continue;
        } else if (count < min_count) {
            bl_tree[curlen].Freq += (ush)count;
        } else if (curlen != 0) {
            if (curlen != prevlen) bl_tree[curlen].Freq++;
            bl_tree[REP_3_6].Freq++;
        } else if (count <= 10) {
            bl_tree[REPZ_3_10].Freq++;
        } else {
            bl_tree[REPZ_11_138].Freq++;
        }
        count = 0; prevlen = curlen;
        if (nextlen == 0) {
            max_count = 138, min_count = 3;
        } else if (curlen == nextlen) {
            max_count = 6, min_count = 3;
        } else {
            max_count = 7, min_count = 4;
        }
    }
}

/* ===========================================================================
 * Send a literal or distance tree in compressed form, using the codes in
 * bl_tree.
 */
local void send_tree (tree, max_code)
    ct_data near *tree; /* the tree to be scanned */
    int max_code;       /* and its largest code of non zero frequency */
{
    int n;                     /* iterates over all tree elements */
    int prevlen = -1;          /* last emitted length */
    int curlen;                /* length of current code */
    int nextlen = tree[0].Len; /* length of next code */
    int count = 0;             /* repeat count of the current code */
    int max_count = 7;         /* max repeat count */
    int min_count = 4;         /* min repeat count */

    /* tree[max_code+1].Len = -1; */  /* guard already set */
    if (nextlen == 0) max_count = 138, min_count = 3;

    for (n = 0; n <= max_code; n++) {
        curlen = nextlen; nextlen = tree[n+1].Len;
        if (++count < max_count && curlen == nextlen) {
            continue;
        } else if (count < min_count) {
            do { send_code(curlen, bl_tree); } while (--count != 0);

        } else if (curlen != 0) {
            if (curlen != prevlen) {
                send_code(curlen, bl_tree); count--;
            }
            Assert(count >= 3 && count <= 6, " 3_6?");
            send_code(REP_3_6, bl_tree); send_bits(count-3, 2);

        } else if (count <= 10) {
            send_code(REPZ_3_10, bl_tree); send_bits(count-3, 3);

        } else {
            send_code(REPZ_11_138, bl_tree); send_bits(count-11, 7);
        }
        count = 0; prevlen = curlen;
        if (nextlen == 0) {
            max_count = 138, min_count = 3;
        } else if (curlen == nextlen) {
            max_count = 6, min_count = 3;
        } else {
            max_count = 7, min_count = 4;
        }
    }
}

/* ===========================================================================
 * Construct the Huffman tree for the bit lengths and return the index in
 * bl_order of the last bit length code to send.
 */
local int build_bl_tree()
{
    int max_blindex;  /* index of last bit length code of non zero freq */

    /* Determine the bit length frequencies for literal and distance trees */
    scan_tree((ct_data near *)dyn_ltree, l_desc.max_code);
    scan_tree((ct_data near *)dyn_dtree, d_desc.max_code);

    /* Build the bit length tree: */
    build_tree((tree_desc near *)(&bl_desc));
    /* opt_len now includes the length of the tree representations, except
     * the lengths of the bit lengths codes and the 5+5+4 bits for the counts.
     */

    /* Determine the number of bit length codes to send. The pkzip format
     * requires that at least 4 bit length codes be sent. (appnote.txt says
     * 3 but the actual value used is 4.)
     */
    for (max_blindex = BL_CODES-1; max_blindex >= 3; max_blindex--) {
        if (bl_tree[bl_order[max_blindex]].Len != 0) break;
    }
    /* Update opt_len to include the bit length tree and counts */
    opt_len += 3*(max_blindex+1) + 5+5+4;
    Tracev((stderr, "\ndyn trees: dyn %ld, stat %ld", opt_len, static_len));

    return max_blindex;
}

/* ===========================================================================
 * Send the header for a block using dynamic Huffman trees: the counts, the
 * lengths of the bit length codes, the literal tree and the distance tree.
 * IN assertion: lcodes >= 257, dcodes >= 1, blcodes >= 4.
 */
local void send_all_trees(lcodes, dcodes, blcodes)
    int lcodes, dcodes, blcodes; /* number of codes for each tree */
{
    int rank;                    /* index in bl_order */

    Assert(lcodes >= 257 && dcodes >= 1 && blcodes >= 4, "not enough codes");
    Assert(lcodes <= L_CODES && dcodes <= D_CODES && blcodes <= BL_CODES,
            "too many codes");
    Tracev((stderr, "\nbl counts: "));
    send_bits(lcodes-257, 5);
    /* not +255 as stated in appnote.txt 1.93a or -256 in 2.04c */
    send_bits(dcodes-1,   5);
    send_bits(blcodes-4,  4); /* not -3 as stated in appnote.txt */
    for (rank = 0; rank < blcodes; rank++) {
        Tracev((stderr, "\nbl code %2d ", bl_order[rank]));
        send_bits(bl_tree[bl_order[rank]].Len, 3);
    }
    Tracev((stderr, "\nbl tree: sent %ld", bits_sent));

    send_tree((ct_data near *)dyn_ltree, lcodes-1); /* send the literal tree */
    Tracev((stderr, "\nlit tree: sent %ld", bits_sent));

    send_tree((ct_data near *)dyn_dtree, dcodes-1); /* send the distance tree */
    Tracev((stderr, "\ndist tree: sent %ld", bits_sent));
}

/* ===========================================================================
 * Determine the best encoding for the current block: dynamic trees, static
 * trees or store, and output the encoded block to the zip file. This function
 * returns the total compressed length (in bytes) for the file so far.
 */
ulg flush_block(buf, stored_len, eof)
    char *buf;        /* input block, or NULL if too old */
    ulg stored_len;   /* length of input block */
    int eof;          /* true if this is the last block for a file */
{
    ulg opt_lenb, static_lenb; /* opt_len and static_len in bytes */
    int max_blindex;  /* index of last bit length code of non zero freq */

    flag_buf[last_flags] = flags; /* Save the flags for the last 8 items */

     /* Check if the file is ascii or binary */
    if (*file_type == (ush)UNKNOWN) set_file_type();

    /* Construct the literal and distance trees */
    build_tree((tree_desc near *)(&l_desc));
    Tracev((stderr, "\nlit data: dyn %ld, stat %ld", opt_len, static_len));

    build_tree((tree_desc near *)(&d_desc));
    Tracev((stderr, "\ndist data: dyn %ld, stat %ld", opt_len, static_len));
    /* At this point, opt_len and static_len are the total bit lengths of
     * the compressed block data, excluding the tree representations.
     */

    /* Build the bit length tree for the above two trees, and get the index
     * in bl_order of the last bit length code to send.
     */
    max_blindex = build_bl_tree();

    /* Determine the best encoding. Compute first the block length in bytes */
    opt_lenb = (opt_len+3+7)>>3;
    static_lenb = (static_len+3+7)>>3;
#ifdef DEBUG
    input_len += stored_len; /* for debugging only */
#endif

    Trace((stderr, "\nopt %lu(%lu) stat %lu(%lu) stored %lu lit %u dist %u ",
            opt_lenb, opt_len, static_lenb, static_len, stored_len,
            last_lit, last_dist));

    if (static_lenb <= opt_lenb) opt_lenb = static_lenb;

#ifndef PGP /* PGP can't handle stored blocks */
    /* If compression failed and this is the first and last block,
     * the whole file is transformed into a stored file:
     */
#ifdef FORCE_METHOD
    if (level == 1 && eof && file_method != NULL &&
        cmpr_bytelen == 0L && cmpr_len_bits == 0L) { /* force stored file */
#else
    if (stored_len <= opt_lenb && eof && file_method != NULL &&
        cmpr_bytelen == 0L && cmpr_len_bits == 0L && seekable()) {
#endif
        /* Since LIT_BUFSIZE <= 2*WSIZE, the input data must be there: */
        if (buf == NULL) error ("block vanished");

        copy_block(buf, (unsigned)stored_len, 0); /* without header */
        cmpr_bytelen = stored_len;
        *file_method = STORE;
    } else
#endif /* PGP */

#ifdef FORCE_METHOD
    if (level <= 2 && buf != (char*)NULL) { /* force stored block */
#else
    if (stored_len+4 <= opt_lenb && buf != (char*)NULL) {
                       /* 4: two words for the lengths */
#endif
        /* The test buf != NULL is only necessary if LIT_BUFSIZE > WSIZE.
         * Otherwise we can't have processed more than WSIZE input bytes since
         * the last block flush, because compression would have been
         * successful. If LIT_BUFSIZE <= WSIZE, it is never too late to
         * transform a block into a stored block.
         */
        send_bits((STORED_BLOCK<<1)+eof, 3);  /* send block type */
        cmpr_bytelen += ((cmpr_len_bits + 3 + 7) >> 3) + stored_len + 4;
        cmpr_len_bits = 0L;

        copy_block(buf, (unsigned)stored_len, 1); /* with header */

#ifdef FORCE_METHOD
    } else if (level == 3) { /* force static trees */
#else
    } else if (static_lenb == opt_lenb) {
#endif
        send_bits((STATIC_TREES<<1)+eof, 3);
        compress_block((ct_data near *)static_ltree, (ct_data near *)static_dtree);
        cmpr_len_bits += 3 + static_len;
        cmpr_bytelen += cmpr_len_bits >> 3;
        cmpr_len_bits &= 7L;
    } else {
        send_bits((DYN_TREES<<1)+eof, 3);
        send_all_trees(l_desc.max_code+1, d_desc.max_code+1, max_blindex+1);
        compress_block((ct_data near *)dyn_ltree, (ct_data near *)dyn_dtree);
        cmpr_len_bits += 3 + opt_len;
        cmpr_bytelen += cmpr_len_bits >> 3;