ac3enc: Remove bit allocation fallbacks.

They introduce too much complexity for very little benefit. The result is that
encoding will be more likely to fail at extremely low bitrates where quality
would be awful anyway.

libavcodec/ac3enc.c

@@ -1022,47 +1022,6 @@ static int cbr_bit_allocation(AC3EncodeContext *s)
 }
 
 
-/**
- * Downgrade exponent strategies to reduce the bits used by the exponents.
- * This is a fallback for when bit allocation fails with the normal exponent
- * strategies.  Each time this function is run it only downgrades the
- * strategy in 1 channel of 1 block.
- * @return non-zero if downgrade was unsuccessful
- */
-static int downgrade_exponents(AC3EncodeContext *s)
-{
-    int ch, blk;
-
-    for (blk = AC3_MAX_BLOCKS-1; blk >= 0; blk--) {
-        for (ch = !s->blocks[blk].cpl_in_use; ch <= s->fbw_channels; ch++) {
-            if (s->exp_strategy[ch][blk] == EXP_D15) {
-                s->exp_strategy[ch][blk] = EXP_D25;
-                return 0;
-            }
-        }
-    }
-    for (blk = AC3_MAX_BLOCKS-1; blk >= 0; blk--) {
-        for (ch = !s->blocks[blk].cpl_in_use; ch <= s->fbw_channels; ch++) {
-            if (s->exp_strategy[ch][blk] == EXP_D25) {
-                s->exp_strategy[ch][blk] = EXP_D45;
-                return 0;
-            }
-        }
-    }
-    /* block 0 cannot reuse exponents, so only downgrade D45 to REUSE if
-       the block number > 0 */
-    for (blk = AC3_MAX_BLOCKS-1; blk > 0; blk--) {
-        for (ch = !s->blocks[blk].cpl_in_use; ch <= s->fbw_channels; ch++) {
-            if (s->exp_strategy[ch][blk] > EXP_REUSE) {
-                s->exp_strategy[ch][blk] = EXP_REUSE;
-                return 0;
-            }
-        }
-    }
-    return -1;
-}
-
-
 /**
  * Perform bit allocation search.
  * Finds the SNR offset value that maximizes quality and fits in the specified
@@ -1071,39 +1030,11 @@ static int downgrade_exponents(AC3EncodeContext *s)
  */
 static int compute_bit_allocation(AC3EncodeContext *s)
 {
-    int ret;
-
     count_frame_bits(s);
 
     bit_alloc_masking(s);
 
-    ret = cbr_bit_allocation(s);
-    while (ret) {
-        /* fallback 1: disable channel coupling */
-        if (s->cpl_on) {
-            s->cpl_on = 0;
-            compute_coupling_strategy(s);
-            s->compute_rematrixing_strategy(s);
-            apply_rematrixing(s);
-            process_exponents(s);
-            ret = compute_bit_allocation(s);
-            continue;
-        }
-
-        /* fallback 2: downgrade exponents */
-        if (!downgrade_exponents(s)) {
-            extract_exponents(s);
-            encode_exponents(s);
-            group_exponents(s);
-            ret = compute_bit_allocation(s);
-            continue;
-        }
-
-        /* fallbacks were not enough... */
-        break;
-    }
-
-    return ret;
+    return cbr_bit_allocation(s);
 }