avcodec/truemotion2: Simplify creating VLC table

ff_init_vlc_from_lengths() can be used to offload the computation
of the codes; it also allows to omit the check whether the codes
are already properly ordered (they are). In this case, this also allows
to avoid the allocation of the buffer for the codes.

This improves performance: The amount of decicycles for one call to
tm2_build_huff_tables() when decoding tm20.avi from the FATE-suite
decreased from 46239 to 40035. This test consisted of looping 50 times
over the file and iterating the test ten times.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/truemotion2.c

@@ -108,15 +108,14 @@ typedef struct TM2Huff {
     int num; ///< current number filled
     int max_num; ///< total number of codes
     int *nums; ///< literals
-    uint32_t *bits; ///< codes
-    int *lens; ///< codelengths
+    uint8_t *lens; ///< codelengths
 } TM2Huff;
 
 /**
  *
  * @returns the length of the longest code or an AVERROR code
  */
-static int tm2_read_tree(TM2Context *ctx, uint32_t prefix, int length, TM2Huff *huff)
+static int tm2_read_tree(TM2Context *ctx, int length, TM2Huff *huff)
 {
     int ret, ret2;
     if (length > huff->max_bits) {
@@ -134,14 +133,13 @@ static int tm2_read_tree(TM2Context *ctx, uint32_t prefix, int length, TM2Huff *
             return AVERROR_INVALIDDATA;
         }
         huff->nums[huff->num] = get_bits_long(&ctx->gb, huff->val_bits);
-        huff->bits[huff->num] = prefix;
         huff->lens[huff->num] = length;
         huff->num++;
         return length;
     } else { /* non-terminal node */
-        if ((ret2 = tm2_read_tree(ctx, prefix << 1, length + 1, huff)) < 0)
+        if ((ret2 = tm2_read_tree(ctx, length + 1, huff)) < 0)
             return ret2;
-        if ((ret = tm2_read_tree(ctx, (prefix << 1) | 1, length + 1, huff)) < 0)
+        if ((ret = tm2_read_tree(ctx, length + 1, huff)) < 0)
             return ret;
     }
     return FFMAX(ret, ret2);
@@ -177,15 +175,14 @@ static int tm2_build_huff_table(TM2Context *ctx, TM2Codes *code)
     /* allocate space for codes - it is exactly ceil(nodes / 2) entries */
     huff.max_num = (huff.nodes + 1) >> 1;
     huff.nums    = av_calloc(huff.max_num, sizeof(int));
-    huff.bits    = av_calloc(huff.max_num, sizeof(uint32_t));
-    huff.lens    = av_calloc(huff.max_num, sizeof(int));
+    huff.lens    = av_mallocz(huff.max_num);
 
-    if (!huff.nums || !huff.bits || !huff.lens) {
+    if (!huff.nums || !huff.lens) {
         res = AVERROR(ENOMEM);
         goto out;
     }
 
-    res = tm2_read_tree(ctx, 0, 0, &huff);
+    res = tm2_read_tree(ctx, 0, &huff);
 
     if (res >= 0 && res != huff.max_bits) {
         av_log(ctx->avctx, AV_LOG_ERROR, "Got less bits than expected: %i of %i\n",
@@ -200,9 +197,9 @@ static int tm2_build_huff_table(TM2Context *ctx, TM2Codes *code)
 
     /* convert codes to vlc_table */
     if (res >= 0) {
-        res = init_vlc(&code->vlc, huff.max_bits, huff.max_num,
-                       huff.lens, sizeof(int), sizeof(int),
-                       huff.bits, sizeof(uint32_t), sizeof(uint32_t), 0);
+        res = ff_init_vlc_from_lengths(&code->vlc, huff.max_bits, huff.max_num,
+                                       huff.lens, sizeof(huff.lens[0]),
+                                       NULL, 0, 0, 0, 0, ctx->avctx);
         if (res < 0)
             av_log(ctx->avctx, AV_LOG_ERROR, "Cannot build VLC table\n");
         else {
@@ -216,7 +213,6 @@ static int tm2_build_huff_table(TM2Context *ctx, TM2Codes *code)
 out:
     /* free allocated memory */
     av_free(huff.nums);
-    av_free(huff.bits);
     av_free(huff.lens);
 
     return res;