Make collapse-detection bitexact.

Jean-Marc's original anti-collapse patch used a threshold on the
 content of a decoded band to determine whether or not it should
 be filled with random noise.
Since this is highly sensitive to the accuracy of the
 implementation, it could lead to significant decoder output
 differences even if decoding error up to that point was relatively
 small.

This patch detects collapsed bands from the output of the vector
 quantizer, using exact integer arithmetic.
It makes two simplifying assumptions:
 a) If either input to haar1() is non-zero during TF resolution
     adjustments, then the output will be non-zero.
 b) If the content of a block is non-zero in any of the bands that
     are used for folding, then the folded output will be non-zero.
b) in particular is likely to be false when SPREAD_NONE is used.
It also ignores the case where mid and side are orthogonal in
 stereo_merge, but this is relatively unlikely.
This misses just over 3% of the cases that Jean-Marc's anti-collapse
 detection strategy would catch, but does not mis-classify any (all
 detected collapses are true collapses).

This patch overloads the "fill" parameter to mark which blocks have
 non-zero content for folding.
As a consequence, if a set of blocks on one side of a split has
 collapsed, _no_ folding is done: the result would be zero anyway,
 except for short blocks with SPREAD_AGGRESSIVE that are split down
 to a single block, but a) that means a lot of bits were available
 so a collapse is unlikely and b) anti-collapse can fill the block
 anyway, if it's used.
This also means that if itheta==0 or itheta==16384, we no longer
 fold at all on that side (even with long blocks), since we'd be
 multiplying the result by zero anyway.

5 files changed, 127 insertions, 41 deletions

diff --git a/libcelt/bands.c b/libcelt/bands.c
index 0951c54..04b1c90 100644
--- a/libcelt/bands.c
+++ b/libcelt/bands.c
@@ -212,7 +212,7 @@ void denormalise_bands(const CELTMode *m, const celt_norm * restrict X, celt_sig
 }
 
 /* This prevents energy collapse for transients with multiple short MDCTs */
-void anti_collapse(const CELTMode *m, celt_norm *_X, int LM, int C, int size,
+void anti_collapse(const CELTMode *m, celt_norm *_X, unsigned char *collapse_masks, int LM, int C, int size,
       int start, int end, celt_word16 *logE, celt_word16 *prev1logE,
       celt_word16 *prev2logE, int *pulses, celt_uint32 seed)
 {
@@ -257,11 +257,8 @@ void anti_collapse(const CELTMode *m, celt_norm *_X, int LM, int C, int size,
          X = _X+c*size+(m->eBands[i]<<LM);
          for (k=0;k<1<<LM;k++)
          {
-            celt_word32 sum=0;
             /* Detect collapse */
-            for (j=0;j<N0;j++)
-               sum += ABS16(X[(j<<LM)+k]);
-            if (sum<QCONST16(1e-4, 14))
+            if (!(collapse_masks[i*C+c]&1<<k))
             {
                /* Fill with noise */
                for (j=0;j<N0;j++)
@@ -605,7 +602,7 @@ static int compute_qn(int N, int b, int offset, int stereo)
    the mono and stereo case. Even in the mono case, it can split the band
    in two and transmit the energy difference with the two half-bands. It
    can be called recursively so bands can end up being split in 8 parts. */
-static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_norm *Y,
+static unsigned quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_norm *Y,
       int N, int b, int spread, int B, int intensity, int tf_change, celt_norm *lowband, int resynth, void *ec,
       celt_int32 *remaining_bits, int LM, celt_norm *lowband_out, const celt_ener *bandE, int level,
       celt_int32 *seed, celt_word16 gain, celt_norm *lowband_scratch, int fill)
@@ -623,6 +620,7 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
    int inv = 0;
    celt_word16 mid=0, side=0;
    int longBlocks;
+   unsigned cm;
 
    longBlocks = B0==1;
 
@@ -656,7 +654,7 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
       } while (++c<1+stereo);
       if (lowband_out)
          lowband_out[0] = SHR16(X[0],4);
-      return;
+      return 1;
    }
 
    if (!stereo && level == 0)
@@ -680,6 +678,7 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
             haar1(X, N>>k, 1<<k);
          if (lowband)
             haar1(lowband, N>>k, 1<<k);
+         fill |= fill<<(1<<k);
       }
       B>>=recombine;
       N_B<<=recombine;
@@ -691,6 +690,7 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
             haar1(X, N_B, B);
          if (lowband)
             haar1(lowband, N_B, B);
+         fill |= fill<<B;
          B <<= 1;
          N_B >>= 1;
          time_divide++;
@@ -718,6 +718,8 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
          Y = X+N;
          split = 1;
          LM -= 1;
+         if (B==1)
+            fill |= fill<<1;
          B = (B+1)>>1;
       }
    }
@@ -853,11 +855,13 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
       {
          imid = 32767;
          iside = 0;
+         fill &= (1<<B)-1;
          delta = -16384;
       } else if (itheta == 16384)
       {
          imid = 0;
          iside = 32767;
+         fill &= (1<<B)-1<<B;
          delta = 16384;
       } else {
          imid = bitexact_cos(itheta);
@@ -906,7 +910,9 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
             }
          }
          sign = 1-2*sign;
-         quant_band(encode, m, i, x2, NULL, N, mbits, spread, B, intensity, tf_change, lowband, resynth, ec, remaining_bits, LM, lowband_out, NULL, level, seed, gain, lowband_scratch, fill);
+         cm = quant_band(encode, m, i, x2, NULL, N, mbits, spread, B, intensity, tf_change, lowband, resynth, ec, remaining_bits, LM, lowband_out, NULL, level, seed, gain, lowband_scratch, fill);
+         /* We don't split N=2 bands, so cm is either 1 or 0 (for a fold-collapse),
+             and there's no need to worry about mixing with the other channel. */
          y2[0] = -sign*x2[1];
          y2[1] = sign*x2[0];
          if (resynth)
@@ -955,12 +961,14 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
 
          /* In stereo mode, we do not apply a scaling to the mid because we need the normalized
             mid for folding later */
-         quant_band(encode, m, i, X, NULL, N, mbits, spread, B, intensity, tf_change,
+         cm = quant_band(encode, m, i, X, NULL, N, mbits, spread, B, intensity, tf_change,
                lowband, resynth, ec, remaining_bits, LM, next_lowband_out1,
                NULL, next_level, seed, stereo ? Q15ONE : MULT16_16_P15(gain,mid), lowband_scratch, fill);
-         quant_band(encode, m, i, Y, NULL, N, sbits, spread, B, intensity, tf_change,
+         /* For a stereo split, the high bits of fill are always zero, so no
+             folding will be done to the side. */
+         cm |= quant_band(encode, m, i, Y, NULL, N, sbits, spread, B, intensity, tf_change,
                next_lowband2, resynth, ec, remaining_bits, LM, NULL,
-               NULL, next_level, seed, MULT16_16_P15(gain,side), NULL, fill && !stereo);
+               NULL, next_level, seed, MULT16_16_P15(gain,side), NULL, fill>>B)<<B;
       }
 
    } else {
@@ -984,9 +992,9 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
 
          /* Finally do the actual quantization */
          if (encode)
-            alg_quant(X, N, K, spread, B, lowband, resynth, (ec_enc*)ec, seed, gain);
+            cm = alg_quant(X, N, K, spread, B, lowband, resynth, (ec_enc*)ec, seed, gain);
          else
-            alg_unquant(X, N, K, spread, B, lowband, (ec_dec*)ec, seed, gain);
+            cm = alg_unquant(X, N, K, spread, B, lowband, (ec_dec*)ec, seed, gain);
       } else {
          /* If there's no pulse, fill the band anyway */
          int j;
@@ -996,6 +1004,7 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
             {
                for (j=0;j<N;j++)
                   X[j] = 0;
+               cm = 0;
             } else {
                if (lowband == NULL || (spread==SPREAD_AGGRESSIVE && B<=1))
                {
@@ -1005,10 +1014,12 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
                      *seed = lcg_rand(*seed);
                      X[j] = (celt_int32)(*seed)>>20;
                   }
+                  cm = (1<<B)-1;
                } else {
                   /* Folded spectrum */
                   for (j=0;j<N;j++)
                      X[j] = lowband[j];
+                  cm = fill;
                }
                renormalise_vector(X, N, gain);
             }
@@ -1022,7 +1033,10 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
       if (stereo)
       {
          if (N!=2)
+         {
+            cm |= cm>>B;
             stereo_merge(X, Y, mid, N);
+         }
          if (inv)
          {
             int j;
@@ -1044,11 +1058,15 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
          {
             B >>= 1;
             N_B <<= 1;
+            cm |= cm>>B;
             haar1(X, N_B, B);
          }
 
          for (k=0;k<recombine;k++)
+         {
+            cm |= cm<<(1<<k);
             haar1(X, N0>>k, 1<<k);
+         }
          B<<=recombine;
          N_B>>=recombine;
 
@@ -1063,10 +1081,11 @@ static void quant_band(int encode, const CELTMode *m, int i, celt_norm *X, celt_
          }
       }
    }
+   return cm;
 }
 
 void quant_all_bands(int encode, const CELTMode *m, int start, int end,
-      celt_norm *_X, celt_norm *_Y, const celt_ener *bandE, int *pulses,
+      celt_norm *_X, celt_norm *_Y, unsigned char *collapse_masks, const celt_ener *bandE, int *pulses,
       int shortBlocks, int spread, int dual_stereo, int intensity, int *tf_res, int resynth,
       int total_bits, void *ec, int LM, int codedBands)
 {
@@ -1097,7 +1116,7 @@ void quant_all_bands(int encode, const CELTMode *m, int start, int end,
    else
       seed = ((ec_dec*)ec)->rng;
    balance = 0;
-   lowband_offset = -1;
+   lowband_offset = 0;
    for (i=start;i<end;i++)
    {
       celt_int32 tell;
@@ -1107,7 +1126,9 @@ void quant_all_bands(int encode, const CELTMode *m, int start, int end,
       int effective_lowband=-1;
       celt_norm * restrict X, * restrict Y;
       int tf_change=0;
-      
+      unsigned x_cm;
+      unsigned y_cm;
+
       X = _X+M*eBands[i];
       if (_Y!=NULL)
          Y = _Y+M*eBands[i];
@@ -1131,8 +1152,8 @@ void quant_all_bands(int encode, const CELTMode *m, int start, int end,
          b = 0;
       }
 
-      if (M*eBands[i]-N >= M*eBands[start] && (update_lowband || lowband_offset==-1))
-            lowband_offset = M*eBands[i];
+      if (M*eBands[i]-N >= M*eBands[start] && (update_lowband || lowband_offset==0))
+            lowband_offset = i;
 
       tf_change = tf_res[i];
       if (i>=m->effEBands)
@@ -1143,8 +1164,30 @@ void quant_all_bands(int encode, const CELTMode *m, int start, int end,
       }
 
       /* This ensures we never repeat spectral content within one band */
-      if (lowband_offset != -1)
-         effective_lowband = IMAX(M*eBands[start], lowband_offset-N);
+      if (lowband_offset != 0)
+         effective_lowband = IMAX(M*eBands[start], M*eBands[lowband_offset]-N);
+
+      /* Get a conservative estimate of the collapse_mask's for the bands we're
+          going to be folding from. */
+      if (lowband_offset != 0 && (spread!=SPREAD_AGGRESSIVE || B>1))
+      {
+         int fold_start;
+         int fold_end;
+         int fold_i;
+         fold_start = lowband_offset;
+         while(M*eBands[--fold_start] > effective_lowband);
+         fold_end = lowband_offset-1;
+         while(M*eBands[++fold_end] < effective_lowband+N);
+         x_cm = y_cm = 0;
+         fold_i = fold_start; do {
+           x_cm |= collapse_masks[fold_i*C+0];
+           y_cm |= collapse_masks[fold_i*C+1];
+         } while (++fold_i<fold_end);
+      }
+      /* Otherwise, we'll be using the LCG to fold, so all blocks will (almost
+          always) be non-zero.*/
+      else
+         x_cm = y_cm = (1<<B)-1;
 
       if (dual_stereo && i==intensity)
       {
@@ -1157,16 +1200,19 @@ void quant_all_bands(int encode, const CELTMode *m, int start, int end,
       }
       if (dual_stereo)
       {
-         quant_band(encode, m, i, X, NULL, N, b/2, spread, B, intensity, tf_change,
+         x_cm = quant_band(encode, m, i, X, NULL, N, b/2, spread, B, intensity, tf_change,
                effective_lowband != -1 ? norm+effective_lowband : NULL, resynth, ec, &remaining_bits, LM,
-               norm+M*eBands[i], bandE, 0, &seed, Q15ONE, lowband_scratch, 1);
-         quant_band(encode, m, i, Y, NULL, N, b/2, spread, B, intensity, tf_change,
+               norm+M*eBands[i], bandE, 0, &seed, Q15ONE, lowband_scratch, x_cm);
+         y_cm = quant_band(encode, m, i, Y, NULL, N, b/2, spread, B, intensity, tf_change,
                effective_lowband != -1 ? norm2+effective_lowband : NULL, resynth, ec, &remaining_bits, LM,
-               norm2+M*eBands[i], bandE, 0, &seed, Q15ONE, lowband_scratch, 1);
+               norm2+M*eBands[i], bandE, 0, &seed, Q15ONE, lowband_scratch, y_cm);
+         collapse_masks[i*2+0] = (unsigned char)(x_cm&(1<<B)-1);
+         collapse_masks[i*2+1] = (unsigned char)(y_cm&(1<<B)-1);
       } else {
-         quant_band(encode, m, i, X, Y, N, b, spread, B, intensity, tf_change,
+         x_cm = quant_band(encode, m, i, X, Y, N, b, spread, B, intensity, tf_change,
                effective_lowband != -1 ? norm+effective_lowband : NULL, resynth, ec, &remaining_bits, LM,
-               norm+M*eBands[i], bandE, 0, &seed, Q15ONE, lowband_scratch, 1);
+               norm+M*eBands[i], bandE, 0, &seed, Q15ONE, lowband_scratch, x_cm|y_cm);
+         collapse_masks[i*C+1] = collapse_masks[i*C+0] = (unsigned char)(x_cm&(1<<B)-1);
       }
       balance += pulses[i] + tell;
 
diff --git a/libcelt/bands.h b/libcelt/bands.h
index 2bad608..bdf9f57 100644
--- a/libcelt/bands.h
+++ b/libcelt/bands.h
@@ -86,14 +86,14 @@ void haar1(celt_norm *X, int N0, int stride);
  * @param enc Entropy encoder
  */
 void quant_all_bands(int encode, const CELTMode *m, int start, int end,
-      celt_norm * X, celt_norm * Y, const celt_ener *bandE, int *pulses,
+      celt_norm * X, celt_norm * Y, unsigned char *collapse_masks, const celt_ener *bandE, int *pulses,
       int time_domain, int fold, int dual_stereo, int intensity, int *tf_res, int resynth,
       int total_bits, void *enc, int M, int codedBands);
 
 
 void stereo_decision(const CELTMode *m, celt_norm * restrict X, int *stereo_mode, int len, int M);
 
-void anti_collapse(const CELTMode *m, celt_norm *_X, int LM, int C, int size,
+void anti_collapse(const CELTMode *m, celt_norm *_X, unsigned char *collapse_masks, int LM, int C, int size,
       int start, int end, celt_word16 *logE, celt_word16 *prev1logE,
       celt_word16 *prev2logE, int *pulses, celt_uint32 seed);
 
diff --git a/libcelt/celt.c b/libcelt/celt.c
index 6ea83eb..e207ba8 100644
--- a/libcelt/celt.c
+++ b/libcelt/celt.c
@@ -792,6 +792,7 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
    VARDECL(int, offsets);
    VARDECL(int, fine_priority);
    VARDECL(int, tf_res);
+   VARDECL(unsigned char, collapse_masks);
    celt_sig *_overlap_mem;
    celt_sig *prefilter_mem;
    celt_word16 *oldBandE, *oldLogE2;
@@ -819,6 +820,7 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
    celt_int32 tell;
    int prefilter_tapset=0;
    int pf_on;
+   int anti_collapse_rsv;
    int anti_collapse_on=0;
    SAVE_STACK;
 
@@ -1265,8 +1267,10 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
    ALLOC(pulses, st->mode->nbEBands, int);
    ALLOC(fine_priority, st->mode->nbEBands, int);
 
-   /* bits =   packet size        -       where we are         - safety -  anti-collapse*/
-   bits = (nbCompressedBytes*8<<BITRES) - ec_enc_tell(enc, BITRES) - 1 - (isTransient&&LM>=2 ? (1<<BITRES) : 0);
+   /* bits =   packet size        -       where we are         - safety*/
+   bits = (nbCompressedBytes*8<<BITRES) - ec_enc_tell(enc, BITRES) - 1;
+   anti_collapse_rsv = isTransient&&LM>=2&&bits>=(LM+2<<BITRES) ? (1<<BITRES) : 0;
+   bits -= anti_collapse_rsv;
    codedBands = compute_allocation(st->mode, st->start, st->end, offsets,
          alloc_trim, &intensity, &dual_stereo, bits, pulses, fine_quant,
          fine_priority, C, LM, enc, 1, st->lastCodedBands);
@@ -1286,11 +1290,12 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
 #endif
 
    /* Residual quantisation */
-   quant_all_bands(1, st->mode, st->start, st->end, X, C==2 ? X+N : NULL,
+   ALLOC(collapse_masks, st->mode->nbEBands, unsigned char);
+   quant_all_bands(1, st->mode, st->start, st->end, X, C==2 ? X+N : NULL, collapse_masks,
          bandE, pulses, shortBlocks, st->spread_decision, dual_stereo, intensity, tf_res, resynth,
          nbCompressedBytes*8, enc, LM, codedBands);
 
-   if (isTransient && LM>=2)
+   if (anti_collapse_rsv > 0)
    {
       anti_collapse_on = st->consec_transient<2;
       ec_enc_bits(enc, anti_collapse_on, 1);
@@ -1311,7 +1316,7 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
 #endif
       if (anti_collapse_on)
       {
-         anti_collapse(st->mode, X, LM, C, N,
+         anti_collapse(st->mode, X, collapse_masks, LM, C, N,
                st->start, st->end, oldBandE, oldLogE, oldLogE2, pulses, enc->rng);
       }
 
@@ -1882,6 +1887,7 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
    VARDECL(int, offsets);
    VARDECL(int, fine_priority);
    VARDECL(int, tf_res);
+   VARDECL(unsigned char, collapse_masks);
    celt_sig *out_mem[2];
    celt_sig *decode_mem[2];
    celt_sig *overlap_mem[2];
@@ -1905,6 +1911,7 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
    celt_int32 tell;
    int dynalloc_logp;
    int postfilter_tapset;
+   int anti_collapse_rsv;
    int anti_collapse_on=0;
    SAVE_STACK;
 
@@ -2060,7 +2067,9 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
    alloc_trim = tell+(6<<BITRES) <= total_bits ?
          ec_dec_icdf(dec, trim_icdf, 7) : 5;
 
-   bits = (len*8<<BITRES) - ec_dec_tell(dec, BITRES) - 1 - (isTransient&&LM>=2 ? (1<<BITRES) : 0);
+   bits = (len*8<<BITRES) - ec_dec_tell(dec, BITRES) - 1;
+   anti_collapse_rsv = isTransient&&LM>=2&&bits>=(LM+2<<BITRES) ? (1<<BITRES) : 0;
+   bits -= anti_collapse_rsv;
    codedBands = compute_allocation(st->mode, st->start, st->end, offsets,
          alloc_trim, &intensity, &dual_stereo, bits, pulses, fine_quant,
          fine_priority, C, LM, dec, 0, 0);
@@ -2068,11 +2077,12 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
    unquant_fine_energy(st->mode, st->start, st->end, bandE, oldBandE, fine_quant, dec, C);
 
    /* Decode fixed codebook */
-   quant_all_bands(0, st->mode, st->start, st->end, X, C==2 ? X+N : NULL,
+   ALLOC(collapse_masks, st->mode->nbEBands, unsigned char);
+   quant_all_bands(0, st->mode, st->start, st->end, X, C==2 ? X+N : NULL, collapse_masks,
          NULL, pulses, shortBlocks, spread_decision, dual_stereo, intensity, tf_res, 1,
          len*8, dec, LM, codedBands);
 
-   if (isTransient && LM>=2)
+   if (anti_collapse_rsv > 0)
    {
       anti_collapse_on = ec_dec_bits(dec, 1);
    }
@@ -2081,7 +2091,7 @@ int celt_decode_with_ec_float(CELTDecoder * restrict st, const unsigned char *da
          fine_quant, fine_priority, len*8-ec_dec_tell(dec, 0), dec, C);
 
    if (anti_collapse_on)
-      anti_collapse(st->mode, X, LM, C, N,
+      anti_collapse(st->mode, X, collapse_masks, LM, C, N,
             st->start, st->end, oldBandE, oldLogE, oldLogE2, pulses, dec->rng);
 
    log2Amp(st->mode, st->start, st->end, bandE, oldBandE, C);
diff --git a/libcelt/vq.c b/libcelt/vq.c
index ed2c46a..e5f2b98 100644
--- a/libcelt/vq.c
+++ b/libcelt/vq.c
@@ -106,6 +106,8 @@ static void exp_rotation(celt_norm *X, int len, int dir, int stride, int K, int
       while ((stride2*stride2+stride2)*stride + (stride>>2) < len)
          stride2++;
    }
+   /*TODO: We should be passing around log2(B), not B, for both this and for
+      extract_collapse_mask().*/
    len /= stride;
    for (i=0;i<stride;i++)
    {
@@ -153,7 +155,27 @@ static void normalise_residual(int * restrict iy, celt_norm * restrict X,
    while (++i < N);
 }
 
-void alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband,
+static unsigned extract_collapse_mask(int *iy, int N, int B)
+{
+   unsigned collapse_mask;
+   int N0;
+   int i;
+   if (B<=1)
+      return 1;
+   /*TODO: We should be passing around log2(B), not B, for both this and for
+      exp_rotation().*/
+   N0 = N/B;
+   collapse_mask = 0;
+   i=0; do {
+      int j;
+      j=0; do {
+         collapse_mask |= (iy[i*N0+j]!=0)<<i;
+      } while (++j<N0);
+   } while (++i<B);
+   return collapse_mask;
+}
+
+unsigned alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband,
       int resynth, ec_enc *enc, celt_int32 *seed, celt_word16 gain)
 {
    VARDECL(celt_norm, y);
@@ -165,6 +187,7 @@ void alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband
    celt_word32 sum;
    celt_word32 xy;
    celt_word16 yy;
+   unsigned collapse_mask;
    SAVE_STACK;
 
    celt_assert2(K!=0, "alg_quant() needs at least one pulse");
@@ -308,17 +331,20 @@ void alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband
       normalise_residual(iy, X, N, K, yy, gain);
       exp_rotation(X, N, -1, B, K, spread);
    }
+   collapse_mask = extract_collapse_mask(iy, N, B);
    RESTORE_STACK;
+   return collapse_mask;
 }
 
 
 /** Decode pulse vector and combine the result with the pitch vector to produce
     the final normalised signal in the current band. */
-void alg_unquant(celt_norm *X, int N, int K, int spread, int B,
+unsigned alg_unquant(celt_norm *X, int N, int K, int spread, int B,
       celt_norm *lowband, ec_dec *dec, celt_int32 *seed, celt_word16 gain)
 {
    int i;
    celt_word32 Ryy;
+   unsigned collapse_mask;
    VARDECL(int, iy);
    SAVE_STACK;
 
@@ -332,7 +358,9 @@ void alg_unquant(celt_norm *X, int N, int K, int spread, int B,
    } while (++i < N);
    normalise_residual(iy, X, N, K, Ryy, gain);
    exp_rotation(X, N, -1, B, K, spread);
+   collapse_mask = extract_collapse_mask(iy, N, B);
    RESTORE_STACK;
+   return collapse_mask;
 }
 
 void renormalise_vector(celt_norm *X, int N, celt_word16 gain)
diff --git a/libcelt/vq.h b/libcelt/vq.h
index 9f2e9a0..6784e49 100644
--- a/libcelt/vq.h
+++ b/libcelt/vq.h
@@ -50,8 +50,9 @@
  * @param K Number of pulses to use
  * @param p Pitch vector (it is assumed that p+x is a unit vector)
  * @param enc Entropy encoder state
+ * @ret A mask indicating which blocks in the band received pulses
 */
-void alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband,
+unsigned alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband,
       int resynth, ec_enc *enc, celt_int32 *seed, celt_word16 gain);
 
 /** Algebraic pulse decoder
@@ -60,8 +61,9 @@ void alg_quant(celt_norm *X, int N, int K, int spread, int B, celt_norm *lowband
  * @param K Number of pulses to use
  * @param p Pitch vector (automatically added to x)
  * @param dec Entropy decoder state
+ * @ret A mask indicating which blocks in the band received pulses
  */
-void alg_unquant(celt_norm *X, int N, int K, int spread, int B,
+unsigned alg_unquant(celt_norm *X, int N, int K, int spread, int B,
       celt_norm *lowband, ec_dec *dec, celt_int32 *seed, celt_word16 gain);
 
 void renormalise_vector(celt_norm *X, int N, celt_word16 gain);