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65 lines
1.9 KiB
C
65 lines
1.9 KiB
C
/* ------------------- htree.c -------------------- */
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#include "dflat.h"
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#include "htree.h"
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struct htree *ht;
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int root;
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int treect;
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/* ------ build a Huffman tree from a frequency array ------ */
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void buildtree(void)
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{
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int i;
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treect = 256;
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/* ---- preset node pointers to -1 ---- */
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for (i = 0; i < treect; i++) {
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ht[i].parent = -1;
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ht[i].right = -1;
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ht[i].left = -1;
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}
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/* ---- build the huffman tree ----- */
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while (1) {
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int h1 = -1, h2 = -1;
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/* ---- find the two lowest frequencies ---- */
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for (i = 0; i < treect; i++) {
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if (i != h1) {
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struct htree *htt = ht+i;
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/* --- find a node without a parent --- */
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if (htt->cnt > 0 && htt->parent == -1) {
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/* ---- h1 & h2 -> lowest nodes ---- */
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if (h1 == -1 || htt->cnt < ht[h1].cnt) {
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if (h2 == -1 || ht[h1].cnt < ht[h2].cnt)
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h2 = h1;
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h1 = i;
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}
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else if (h2 == -1 || htt->cnt < ht[h2].cnt)
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h2 = i;
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}
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}
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}
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/* --- if only h1 -> a node, that's the root --- */
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if (h2 == -1) {
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root = h1;
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break;
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}
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/* --- combine two nodes and add one --- */
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ht[h1].parent = treect;
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ht[h2].parent = treect;
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ht = realloc(ht, (treect+1) * sizeof(struct htree));
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if (ht == NULL)
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break;
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/* --- the new node's frequency is the sum of the two
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nodes with the lowest frequencies --- */
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ht[treect].cnt = ht[h1].cnt + ht[h2].cnt;
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/* - the new node points to the two that it combines */
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ht[treect].right = h1;
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ht[treect].left = h2;
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/* --- the new node has no parent (yet) --- */
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ht[treect].parent = -1;
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treect++;
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}
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}
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