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Adding some hysteresis on the folding threshold frequency

This adds some side-information that can be used to change the
threshold freq arbitrarily.

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -80,6 +80,7 @@

    int fold_decision;

    int delayedIntra;

    int tonal_average;

+   int lastCodedBands;

    int prefilter_period;

    celt_word16 prefilter_gain;

@@ -740,6 +741,7 @@

    int intensity=0;

    int dual_stereo=0;

    int effectiveBytes;

+   int skip;

    SAVE_STACK;

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

@@ -1103,8 +1105,15 @@

    ALLOC(pulses, st->mode->nbEBands, int);

    ALLOC(fine_priority, st->mode->nbEBands, int);

-   bits = nbCompressedBytes*8 - ec_enc_tell(enc, 0) - 1;

-   codedBands = compute_allocation(st->mode, st->start, st->end, offsets, alloc_trim, bits, pulses, fine_quant, fine_priority, C, LM);

+   /* bits =   packet size    -    where we are   - safety - skip signalling */

+   bits = nbCompressedBytes*8 - ec_enc_tell(enc, 0) - 1    - (1<<BITRES);

+   skip=-1;

+   codedBands = compute_allocation(st->mode, st->start, st->end, offsets,

+         alloc_trim, bits, pulses, fine_quant, fine_priority, C, LM, &skip, st->lastCodedBands);

+   st->lastCodedBands = codedBands;

+   for (i=0;i<skip;i++)

+      ec_enc_bit_prob(enc, 0, 32768);

+   ec_enc_bit_prob(enc, 1, 32768);

    quant_fine_energy(st->mode, st->start, st->end, bandE, oldBandE, error, fine_quant, enc, C);

@@ -1692,6 +1701,7 @@

    celt_word16 postfilter_gain;

    int intensity=0;

    int dual_stereo=0;

+   int skip;

    SAVE_STACK;

    if (pcm==NULL)

@@ -1825,8 +1835,19 @@

       intensity = ec_dec_uint(dec, 1+st->end-st->start);

    }

-   bits = len*8 - ec_dec_tell(dec, 0) - 1;

-   codedBands = compute_allocation(st->mode, st->start, st->end, offsets, alloc_trim, bits, pulses, fine_quant, fine_priority, C, LM);

+   bits = len*8 - ec_dec_tell(dec, 0) - 1 - (1<<BITRES);

+   skip=0;

+   while (ec_dec_bit_prob(dec, 32768)==0)

+   {

+      skip++;

+      if (skip>21)

+      {

+         dec->error = 1;

+         break;

+      }

+   }

+   codedBands = compute_allocation(st->mode, st->start, st->end, offsets,

+         alloc_trim, bits, pulses, fine_quant, fine_priority, C, LM, &skip, 0);

    unquant_fine_energy(st->mode, st->start, st->end, bandE, oldBandE, fine_quant, dec, C);

--- a/libcelt/rate.c

+++ b/libcelt/rate.c

@@ -142,7 +142,7 @@

 static inline int interp_bits2pulses(const CELTMode *m, int start, int end,

       int *bits1, int *bits2, const int *thresh, int total, int *bits,

-      int *ebits, int *fine_priority, int len, int _C, int LM)

+      int *ebits, int *fine_priority, int len, int _C, int LM, int *skip, int prev)

 {

    int psum;

    int lo, hi;

@@ -192,6 +192,25 @@

       psum += bits[j];

    }

    codedBands++;

+   if (*skip==-1)

+   {

+      *skip=0;

+      for (j=codedBands-1;j>=0;j--)

+      {

+         if ((bits[j] > (7*(m->eBands[j+1]-m->eBands[j])<<LM<<BITRES)>>4 && j<prev)

+               || (bits[j] > (9*(m->eBands[j+1]-m->eBands[j])<<LM<<BITRES)>>4))

+            break;

+         else

+            (*skip)++;

+      }

+      *skip = IMIN(*skip, codedBands-start-1);

+   }

+   for (i=0;i<*skip;i++)

+   {

+      psum = psum - bits[codedBands-1] + ((C+1)<<BITRES);

+      bits[codedBands-1] = C<<BITRES;

+      codedBands--;

+   }

    /* Allocate the remaining bits */

    if (codedBands) {

       int left, perband;

@@ -267,7 +286,7 @@

 }

 int compute_allocation(const CELTMode *m, int start, int end, int *offsets, int alloc_trim,

-      int total, int *pulses, int *ebits, int *fine_priority, int _C, int LM)

+      int total, int *pulses, int *ebits, int *fine_priority, int _C, int LM, int *skip, int prev)

 {

    int lo, hi, len, j;

    const int C = CHANNELS(_C);

@@ -286,7 +305,7 @@

    /* Below this threshold, we don't allocate any PVQ bits */

    for (j=start;j<end;j++)

-      thresh[j] = (4*(m->eBands[j+1]-m->eBands[j])<<LM<<BITRES)>>3;

+      thresh[j] = ((C+1)<<BITRES) + (3*(m->eBands[j+1]-m->eBands[j])<<LM<<BITRES)>>4;

    /* Tilt of the allocation curve */

    for (j=start;j<end;j++)

       trim_offset[j] = C*(m->eBands[j+1]-m->eBands[j])*(alloc_trim-5-LM)*(m->nbEBands-j-1)

@@ -334,7 +353,7 @@

       bits1[j] += offsets[j];

    }

    codedBands = interp_bits2pulses(m, start, end, bits1, bits2, thresh,

-         total, pulses, ebits, fine_priority, len, C, LM);

+         total, pulses, ebits, fine_priority, len, C, LM, skip, prev);

    RESTORE_STACK;

    return codedBands;

 }

--- a/libcelt/rate.h

+++ b/libcelt/rate.h

@@ -103,7 +103,7 @@

  @return Total number of bits allocated

 */

 int compute_allocation(const CELTMode *m, int start, int end, int *offsets, int alloc_trim,

-      int total, int *pulses, int *ebits, int *fine_priority, int _C, int LM);

+      int total, int *pulses, int *ebits, int *fine_priority, int _C, int LM, int *skip, int prev);

 #endif