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Tuning the dynamic allocation probability and increment

Dynalloc becomes 2x more likely every time we use it, until it
reaches a probability of 1/4. Allocation increments now have
a floor of 1/8 bit/sample and a ceiling of 1 bit/sample.

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -753,6 +753,7 @@

    int dual_stereo=0;

    int effectiveBytes;

    celt_word16 pf_threshold;

+   int dynalloc_prob;

    SAVE_STACK;

    if (nbCompressedBytes<0 || pcm==NULL)

@@ -1012,17 +1013,25 @@

             offsets[i] += 1;

       }

    }

+   dynalloc_prob = 6;

    for (i=0;i<st->mode->nbEBands;i++)

    {

       int j;

-      ec_enc_bit_logp(enc, offsets[i]!=0, 6);

+      ec_enc_bit_logp(enc, offsets[i]!=0, dynalloc_prob);

       if (offsets[i]!=0)

       {

+         int width, quanta;

+         width = C*(st->mode->eBands[i+1]-st->mode->eBands[i])<<LM;

+         /* quanta is 6 bits, but no more than 1 bit/sample

+            and no less than 1/8 bit/sample */

+         quanta = IMIN(width<<BITRES, IMAX(6<<BITRES, width));

          for (j=0;j<offsets[i]-1;j++)

             ec_enc_bit_logp(enc, 1, 1);

          ec_enc_bit_logp(enc, 0, 1);

+         offsets[i] *= quanta;

+         /* Making dynalloc more likely */

+         dynalloc_prob = IMAX(2, dynalloc_prob-1);

       }

-      offsets[i] *= (6<<BITRES);

    }

    alloc_trim = alloc_trim_analysis(st->mode, X, bandLogE, st->mode->nbEBands, LM, C, N);

    ec_encode_bin(enc, trim_cdf[alloc_trim], trim_cdf[alloc_trim+1], 7);

@@ -1747,6 +1756,7 @@

    celt_word16 postfilter_gain;

    int intensity=0;

    int dual_stereo=0;

+   int dynalloc_prob;

    SAVE_STACK;

    if (pcm==NULL)

@@ -1852,14 +1862,22 @@

    for (i=0;i<st->mode->nbEBands;i++)

       offsets[i] = 0;

+   dynalloc_prob = 6;

    for (i=0;i<st->mode->nbEBands;i++)

    {

-      if (ec_dec_bit_logp(dec, 6))

+      if (ec_dec_bit_logp(dec, dynalloc_prob))

       {

+         int width, quanta;

+         width = C*(st->mode->eBands[i+1]-st->mode->eBands[i])<<LM;

+         /* quanta is 6 bits, but no more than 1 bit/sample

+            and no less than 1/8 bit/sample */

+         quanta = IMIN(width<<BITRES, IMAX(6<<BITRES, width));

          while (ec_dec_bit_logp(dec, 1))

             offsets[i]++;

          offsets[i]++;

-         offsets[i] *= (6<<BITRES);

+         offsets[i] *= quanta;

+         /* Making dynalloc more likely */

+         dynalloc_prob = IMAX(2, dynalloc_prob-1);

       }

    }