aacenc: Fix window decision logic.

This keeps the encoder from switching away from an wight short sequence
prematurely when attacks are detected two frames in a row. It also allows for
the legal and useful LONG_STOP_SEQUENCE to LONG_START_SEQUENCE transition.

Originally committed as revision 23870 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aacpsy.c

@@ -185,6 +185,7 @@ static FFPsyWindowInfo psy_3gpp_window(FFPsyContext *ctx,
     Psy3gppContext *pctx = (Psy3gppContext*) ctx->model_priv_data;
     Psy3gppChannel *pch  = &pctx->ch[channel];
     uint8_t grouping     = 0;
+    int next_type        = pch->next_window_seq;
     FFPsyWindowInfo wi;
 
     memset(&wi, 0, sizeof(wi));
@@ -193,6 +194,7 @@ static FFPsyWindowInfo psy_3gpp_window(FFPsyContext *ctx,
         int switch_to_eight = 0;
         float sum = 0.0, sum2 = 0.0;
         int attack_n = 0;
+        int stay_short = 0;
         for (i = 0; i < 8; i++) {
             for (j = 0; j < 128; j++) {
                 v = iir_filter(la[(i*128+j)*ctx->avctx->channels], pch->iir_state);
@@ -214,20 +216,27 @@ static FFPsyWindowInfo psy_3gpp_window(FFPsyContext *ctx,
         switch (prev_type) {
         case ONLY_LONG_SEQUENCE:
             wi.window_type[0] = switch_to_eight ? LONG_START_SEQUENCE : ONLY_LONG_SEQUENCE;
+            next_type = switch_to_eight ? EIGHT_SHORT_SEQUENCE : ONLY_LONG_SEQUENCE;
             break;
         case LONG_START_SEQUENCE:
             wi.window_type[0] = EIGHT_SHORT_SEQUENCE;
             grouping = pch->next_grouping;
+            next_type = switch_to_eight ? EIGHT_SHORT_SEQUENCE : LONG_STOP_SEQUENCE;
             break;
         case LONG_STOP_SEQUENCE:
-            wi.window_type[0] = ONLY_LONG_SEQUENCE;
+            wi.window_type[0] = switch_to_eight ? LONG_START_SEQUENCE : ONLY_LONG_SEQUENCE;
+            next_type = switch_to_eight ? EIGHT_SHORT_SEQUENCE : ONLY_LONG_SEQUENCE;
             break;
         case EIGHT_SHORT_SEQUENCE:
-            wi.window_type[0] = switch_to_eight ? EIGHT_SHORT_SEQUENCE : LONG_STOP_SEQUENCE;
-            grouping = switch_to_eight ? pch->next_grouping : 0;
+            stay_short = next_type == EIGHT_SHORT_SEQUENCE || switch_to_eight;
+            wi.window_type[0] = stay_short ? EIGHT_SHORT_SEQUENCE : LONG_STOP_SEQUENCE;
+            grouping = next_type == EIGHT_SHORT_SEQUENCE ? pch->next_grouping : 0;
+            next_type = switch_to_eight ? EIGHT_SHORT_SEQUENCE : LONG_STOP_SEQUENCE;
             break;
         }
+
         pch->next_grouping = window_grouping[attack_n];
+        pch->next_window_seq = next_type;
     } else {
         for (i = 0; i < 3; i++)
             wi.window_type[i] = prev_type;