膨胀算法的zlib实现

时间:2011-12-18 16:41:38

标签: algorithm deflate

inftrees.c中,这是从规范的霍夫曼表示构造查找表的代码,作者写道:

 /* replicate for those indices with low len bits equal to huff */
    incr = 1U << (len - drop);
    fill = 1U << curr;
    min = fill;                 /* save offset to next table */
    do {
        fill -= incr;
        next[(huff >> drop) + fill] = here;
    } while (fill != 0);

    /* backwards increment the len-bit code huff */
    incr = 1U << (len - 1);
    while (huff & incr)
        incr >>= 1;
    if (incr != 0) {
        huff &= incr - 1;
        huff += incr;
    }
    else
        huff = 0

虽然我多次阅读评论,但我能弄清楚drop的含义是什么。另一个问题是作者使用什么方法来构建霍夫曼代码?什么是向后增量?

你能为我解释一下,谢谢。

1 个答案:

答案 0 :(得分:2)

“向后增量huff”表示huff = rev(rev(huff) + 1),其中rev反转位0, ..., len - 1

假设len == 7huff是二进制的1110100。我们寻找第一个清除位,清除所有低位(含义)/更高位(位模式),然后设置该位。

1110100
^
1000000 == incr: loop continues
 ^
0100000 == incr: loop continues
  ^
0010000 == incr: loop continues
   ^
0001000 == incr: loop breaks

1110100: huff
0000111: incr - 1
0000100: huff &= (incr - 1)
0001100: huff += incr