huffman: add ff_huff_gen_len_table

The function will be used by utvideo as well.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

1 files changed, 58 insertions, 0 deletions

diff --git a/libavcodec/huffman.c b/libavcodec/huffman.c
index 676fffe..aef4929 100644
--- a/libavcodec/huffman.c
+++ b/libavcodec/huffman.c
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2006 Konstantin Shishkov
+ * Copyright (c) 2007 Loren Merritt
  *
  * This file is part of Libav.
  *
@@ -30,6 +31,63 @@
 /* symbol for Huffman tree node */
 #define HNODE -1
 
+typedef struct {
+    uint64_t val;
+    int name;
+} HeapElem;
+
+static void heap_sift(HeapElem *h, int root, int size)
+{
+    while (root * 2 + 1 < size) {
+        int child = root * 2 + 1;
+        if (child < size - 1 && h[child].val > h[child+1].val)
+            child++;
+        if (h[root].val > h[child].val) {
+            FFSWAP(HeapElem, h[root], h[child]);
+            root = child;
+        } else
+            break;
+    }
+}
+
+void ff_huff_gen_len_table(uint8_t *dst, const uint64_t *stats)
+{
+    HeapElem h[256];
+    int up[2*256];
+    int len[2*256];
+    int offset, i, next;
+    int size = 256;
+
+    for (offset = 1; ; offset <<= 1) {
+        for (i=0; i < size; i++) {
+            h[i].name = i;
+            h[i].val = (stats[i] << 8) + offset;
+        }
+        for (i = size / 2 - 1; i >= 0; i--)
+            heap_sift(h, i, size);
+
+        for (next = size; next < size * 2 - 1; next++) {
+            // merge the two smallest entries, and put it back in the heap
+            uint64_t min1v = h[0].val;
+            up[h[0].name] = next;
+            h[0].val = INT64_MAX;
+            heap_sift(h, 0, size);
+            up[h[0].name] = next;
+            h[0].name = next;
+            h[0].val += min1v;
+            heap_sift(h, 0, size);
+        }
+
+        len[2 * size - 2] = 0;
+        for (i = 2 * size - 3; i >= size; i--)
+            len[i] = len[up[i]] + 1;
+        for (i = 0; i < size; i++) {
+            dst[i] = len[up[i]] + 1;
+            if (dst[i] >= 32) break;
+        }
+        if (i==size) break;
+    }
+}
 
 static void get_tree_codes(uint32_t *bits, int16_t *lens, uint8_t *xlat,
                            Node *nodes, int node,