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multi-stream fix when setting bitrate to AUTO, MAX, or small values

OPUS_AUTO should now work fine, while using OPUS_BITRATE_MAX would still
be a bad idea for controlling CBR rate for multistream. The encoder should
now throw an error early on if max_data_bytes is just too small to do anything.

--- a/src/opus_multistream_encoder.c

+++ b/src/opus_multistream_encoder.c

@@ -591,7 +591,7 @@

    return st;

 }

-static void surround_rate_allocation(

+static opus_int32 surround_rate_allocation(

       OpusMSEncoder *st,

       opus_int32 *rate,

       int frame_size

@@ -605,6 +605,7 @@

    int lfe_offset;

    int coupled_ratio; /* Q8 */

    int lfe_ratio;     /* Q8 */

+   opus_int32 rate_sum=0;

    ptr = (char*)st + align(sizeof(OpusMSEncoder));

    opus_encoder_ctl((OpusEncoder*)ptr, OPUS_GET_SAMPLE_RATE(&Fs));

@@ -660,7 +661,10 @@

          rate[i] = stream_offset+channel_rate;

       else

          rate[i] = lfe_offset+(channel_rate*lfe_ratio>>8);

+      rate[i] = IMAX(rate[i], 500);

+      rate_sum += rate[i];

    }

+   return rate_sum;

 }

 /* Max size in case the encoder decides to return three frames */

@@ -695,6 +699,8 @@

    opus_val32 *mem = NULL;

    opus_val32 *preemph_mem=NULL;

    int frame_size;

+   opus_int32 rate_sum;

+   opus_int32 smallest_packet;

    ALLOC_STACK;

    if (st->surround)

@@ -738,6 +744,18 @@

       RESTORE_STACK;

       return OPUS_BAD_ARG;

    }

+   /* Estimate (slightly overestimating) of the smallest packet the encoder can produce. */

+   if (50*frame_size <= Fs)

+   {

+      smallest_packet = st->layout.nb_streams*4;

+   } else {

+      smallest_packet = st->layout.nb_streams*4*50*frame_size/Fs;

+   }

+   if (max_data_bytes < smallest_packet)

+   {

+      RESTORE_STACK;

+      return OPUS_BAD_ARG;

+   }

    ALLOC(buf, 2*frame_size, opus_val16);

    coupled_size = opus_encoder_get_size(2);

    mono_size = opus_encoder_get_size(1);

@@ -755,11 +773,19 @@

    }

    /* Compute bitrate allocation between streams (this could be a lot better) */

-   surround_rate_allocation(st, bitrates, frame_size);

+   rate_sum = surround_rate_allocation(st, bitrates, frame_size);

    if (!vbr)

-      max_data_bytes = IMIN(max_data_bytes, 3*st->bitrate_bps/(3*8*Fs/frame_size));

-

+   {

+      if (st->bitrate_bps == OPUS_AUTO)

+      {

+         max_data_bytes = IMIN(max_data_bytes, 3*rate_sum/(3*8*Fs/frame_size));

+      } else if (st->bitrate_bps != OPUS_BITRATE_MAX)

+      {

+         max_data_bytes = IMIN(max_data_bytes, IMAX(smallest_packet,

+                          3*st->bitrate_bps/(3*8*Fs/frame_size)));

+      }

+   }

    ptr = (char*)st + align(sizeof(OpusMSEncoder));

    for (s=0;s<st->layout.nb_streams;s++)

    {