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More allocation function updates.

This moves more of the decisions about when to stop skipping bands into the
 encoder-specific branch, so they are not forced in the decoder (because there
 is currently no bit-savings from forcing them).
It also no longer requires an extra bit to code the fine energy in a skipped
 band: that was meant to account for the skip flag, but we already subtracted
 that.

--- a/libcelt/rate.c

+++ b/libcelt/rate.c

@@ -227,17 +227,20 @@

       force_skipping = force_skipping && band_bits < thresh[j];

       if (!force_skipping)

       {

-         /*If we have enough for the fine energy, but not more than a full bit

-            beyond that, or no more than one bit total, then don't bother

-            skipping this band: there's no extra bits to redistribute.*/

-         if ((band_bits >= alloc_floor && band_bits <= alloc_floor + (1<<BITRES))

-               || band_bits < (1<<BITRES))

-            break;

-         /*Never skip the first band: we'd be coding a bit to signal that we're

-            going to waste all of the other bits.*/

-         if (j==start)break;

          if (unforced_skips == -1)

          {

+            /*This if() block is the only part of the allocation function that

+               is not a mandatory part of the bitstream: any bands we choose to

+               skip here must be explicitly signaled.*/

+            /*If we have enough for the fine energy, but not more than a full

+               bit beyond that, or no more than one bit total, then don't bother

+               skipping this band: there's no extra bits to redistribute.*/

+            if ((band_bits>=alloc_floor && band_bits<=alloc_floor+(1<<BITRES))

+                  || band_bits<(1<<BITRES))

+               break;

+            /*Never skip the first band: we'd be coding a bit to signal that

+               we're going to waste all of the other bits.*/

+            if (j==start)break;

             /*Choose a threshold with some hysteresis to keep bands from

                fluctuating in and out.*/

             if (band_bits > ((j<prev?7:9)*band_width<<LM<<BITRES)>>4)

@@ -251,14 +254,14 @@

       }

       /*Reclaim the bits originally allocated to this band.*/

       psum -= bits[j];

-      if (band_bits >= alloc_floor + (1<<BITRES))

+      if (band_bits >= alloc_floor)

       {

          /*If we have enough for a fine energy bit per channel, use it.*/

          psum += alloc_floor;

-         bits[codedBands-1] = alloc_floor;

+         bits[j] = alloc_floor;

       } else {

          /*Otherwise this band gets nothing at all.*/

-         bits[codedBands-1] = 0;

+         bits[j] = 0;

       }

       codedBands--;

    }