avcodec/smacker: Disentangle two contexts

Smacker uses two types of Huffman trees: Those for eight bit values and
those for 16 bit values. Given that both return their values via arrays
and that both need to check not to overrun their array, the context for
parsing eight bit values (HuffContext) will necessarily exhibit certain
similarities with the context used for parsing 16 bit values (DBCtx).
These similarities led to using a HuffContext in addition a DBCtx for
parsing 16 bit trees.

This stands in the way of further developments for the HuffContext struct
(when parsing eight bit trees, the length of the arrays are always 256,
so that one can inline said value and move the currently heap-allocated
tables directly in the structure).

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/smacker.c

@@ -75,6 +75,8 @@ typedef struct HuffContext {
 
 /* common parameters used for decode_bigtree */
 typedef struct DBCtx {
+    int current, length;
+    int *values;
     VLC *v1, *v2;
     int *recode1, *recode2;
     int escapes[3];
@@ -138,8 +140,7 @@ static int smacker_decode_tree(GetBitContext *gb, HuffContext *hc, uint32_t pref
  *
  * Checks before the first read, can overread by 6 * SMKTREE_BITS on success.
  */
-static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc,
-                                  DBCtx *ctx, int length)
+static int smacker_decode_bigtree(GetBitContext *gb, DBCtx *ctx, int length)
 {
     // Larger length can cause segmentation faults due to too deep recursion.
     if (length > SMKTREE_DECODE_BIG_MAX_RECURSION) {
@@ -147,7 +148,7 @@ static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc,
         return AVERROR_INVALIDDATA;
     }
 
-    if (hc->current >= hc->length) {
+    if (ctx->current >= ctx->length) {
         av_log(NULL, AV_LOG_ERROR, "Tree size exceeded!\n");
         return AVERROR_INVALIDDATA;
     }
@@ -159,28 +160,28 @@ static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc,
         i2 = ctx->v2->table ? get_vlc2(gb, ctx->v2->table, SMKTREE_BITS, 3) : 0;
         val = ctx->recode1[i1] | (ctx->recode2[i2] << 8);
         if(val == ctx->escapes[0]) {
-            ctx->last[0] = hc->current;
+            ctx->last[0] = ctx->current;
             val = 0;
         } else if(val == ctx->escapes[1]) {
-            ctx->last[1] = hc->current;
+            ctx->last[1] = ctx->current;
             val = 0;
         } else if(val == ctx->escapes[2]) {
-            ctx->last[2] = hc->current;
+            ctx->last[2] = ctx->current;
             val = 0;
         }
 
-        hc->values[hc->current++] = val;
+        ctx->values[ctx->current++] = val;
         return 1;
     } else { //Node
         int r = 0, r_new, t;
 
-        t = hc->current++;
-        r = smacker_decode_bigtree(gb, hc, ctx, length + 1);
+        t = ctx->current++;
+        r = smacker_decode_bigtree(gb, ctx, length + 1);
         if(r < 0)
             return r;
-        hc->values[t] = SMK_NODE | r;
+        ctx->values[t] = SMK_NODE | r;
         r++;
-        r_new = smacker_decode_bigtree(gb, hc, ctx, length + 1);
+        r_new = smacker_decode_bigtree(gb, ctx, length + 1);
         if (r_new < 0)
             return r_new;
         return r + r_new;
@@ -195,7 +196,6 @@ static int smacker_decode_bigtree(GetBitContext *gb, HuffContext *hc,
  */
 static int smacker_decode_header_tree(SmackVContext *smk, GetBitContext *gb, int **recodes, int *last, int size)
 {
-    HuffContext huff;
     HuffContext h[2] = { 0 };
     VLC vlc[2] = { { 0 } };
     int escapes[3];
@@ -253,23 +253,22 @@ static int smacker_decode_header_tree(SmackVContext *smk, GetBitContext *gb, int
     ctx.recode1 = h[0].values;
     ctx.recode2 = h[1].values;
     ctx.last = last;
-
-    huff.length = (size + 3) >> 2;
-    huff.current = 0;
-    huff.values = av_malloc_array(huff.length + 3, sizeof(huff.values[0]));
-    if (!huff.values) {
+    ctx.length  = (size + 3) >> 2;
+    ctx.current = 0;
+    ctx.values  = av_malloc_array(ctx.length + 3, sizeof(ctx.values[0]));
+    if (!ctx.values) {
         err = AVERROR(ENOMEM);
         goto error;
     }
-    *recodes = huff.values;
+    *recodes = ctx.values;
 
-    err = smacker_decode_bigtree(gb, &huff, &ctx, 0);
+    err = smacker_decode_bigtree(gb, &ctx, 0);
     if (err < 0)
         goto error;
     skip_bits1(gb);
-    if(ctx.last[0] == -1) ctx.last[0] = huff.current++;
-    if(ctx.last[1] == -1) ctx.last[1] = huff.current++;
-    if(ctx.last[2] == -1) ctx.last[2] = huff.current++;
+    if (ctx.last[0] == -1) ctx.last[0] = ctx.current++;
+    if (ctx.last[1] == -1) ctx.last[1] = ctx.current++;
+    if (ctx.last[2] == -1) ctx.last[2] = ctx.current++;
 
     err = 0;
 error: