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Add a SET_LTP call to allow on the fly activation/deactivation of the long term
predictor even in high complexity mode (LTP off is more loss robust), fix the
case where turning on LTP on a running stream can cause prediction against
bogus data by delaying the usage of the LTP until enough data has been encoded.

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -77,6 +77,7 @@

    int channels;

    int pitch_enabled;

+   int pitch_available;

    celt_word16_t * restrict preemph_memE; /* Input is 16-bit, so why bother with 32 */

    celt_sig_t    * restrict preemph_memD;

@@ -109,6 +110,7 @@

    st->overlap = mode->overlap;

    st->pitch_enabled = 1;

+   st->pitch_available = 1;

    st->in_mem = celt_alloc(st->overlap*C*sizeof(celt_sig_t));

    st->out_mem = celt_alloc((MAX_PERIOD+st->overlap)*C*sizeof(celt_sig_t));

@@ -378,7 +380,6 @@

    int has_fold=1;

    ec_byte_buffer buf;

    ec_enc         enc;

-   celt_word32_t curr_power, pitch_power=0;

    VARDECL(celt_sig_t, in);

    VARDECL(celt_sig_t, freq);

    VARDECL(celt_norm_t, X);

@@ -477,6 +478,8 @@

    }

    /* Pitch analysis: we do it early to save on the peak stack space */

+   /* Don't use pitch if there isn't enough data available yet, or if we're using shortBlocks */

+   has_pitch = st->pitch_enabled && (st->pitch_available >= MAX_PERIOD) && (!shortBlocks);

 #ifdef EXP_PSY

    ALLOC(tonality, MAX_PERIOD/4, celt_word16_t);

    {

@@ -486,7 +489,7 @@

       compute_tonality(st->mode, X, st->psy_mem, MAX_PERIOD, tonality, MAX_PERIOD/4);

    }

 #else

-   if (st->pitch_enabled && !shortBlocks)

+   if (has_pitch)

    {

       find_spectral_pitch(st->mode, st->mode->fft, &st->mode->psy, in, st->out_mem, st->mode->window, NULL, 2*N-2*N4, MAX_PERIOD-(2*N-2*N4), &pitch_index);

    }

@@ -541,8 +544,9 @@

 #endif

    /* Compute MDCTs of the pitch part */

-   if (st->pitch_enabled && !shortBlocks)

+   if (has_pitch)

    {

+      celt_word32_t curr_power, pitch_power=0;

       /* Normalise the pitch vector as well (discard the energies) */

       VARDECL(celt_ener_t, bandEp);

@@ -551,19 +555,18 @@

       compute_band_energies(st->mode, freq, bandEp);

       normalise_bands(st->mode, freq, P, bandEp);

       pitch_power = bandEp[0]+bandEp[1]+bandEp[2];

+      /* Check if we can safely use the pitch (i.e. effective gain isn't too high) */

+      curr_power = bandE[0]+bandE[1]+bandE[2];

+      id=-1;

+      if ((MULT16_32_Q15(QCONST16(.1f, 15),curr_power) + QCONST32(10.f,ENER_SHIFT) < pitch_power))

+      {

+         /* Pitch prediction */

+         compute_pitch_gain(st->mode, X, P, gains);

+         id = quant_pitch(gains, st->mode->nbPBands);

+      } 

+      if (id == -1)

+         has_pitch = 0;

    }

-

-   /* Check if we can safely use the pitch (i.e. effective gain isn't too high) */

-   curr_power = bandE[0]+bandE[1]+bandE[2];

-   has_pitch = 0;

-   if (st->pitch_enabled && !shortBlocks && (MULT16_32_Q15(QCONST16(.1f, 15),curr_power) + QCONST32(10.f,ENER_SHIFT) < pitch_power))

-   {

-      /* Pitch prediction */

-      compute_pitch_gain(st->mode, X, P, gains);

-      id = quant_pitch(gains, st->mode->nbPBands);

-      if (id > -1)

-         has_pitch = 1;

-   }

    if (has_pitch) 

    {  

@@ -626,8 +629,11 @@

    quant_bands(st->mode, X, P, NULL, bandE, stereo_mode, pulses, shortBlocks, has_fold, nbCompressedBytes*8, &enc);

    /* Re-synthesis of the coded audio if required */

-   if (st->pitch_enabled || optional_synthesis!=NULL)

+   if (st->pitch_available>0 || optional_synthesis!=NULL)

    {

+      if (st->pitch_available>0 && st->pitch_available<MAX_PERIOD)

+        st->pitch_available+=st->frame_size;

+

       if (C==2)

          renormalise_bands(st->mode, X);

       /* Synthesis */

@@ -743,7 +749,22 @@

          int value = va_arg(ap, int);

          if (value<0 || value>10)

             goto bad_arg;

-         if (value<=2)

+         if (value<=2) {

+            st->pitch_enabled = 0; 

+            st->pitch_available = 0;

+         } else {

+              st->pitch_enabled = 1;

+              if (st->pitch_available<1)

+                st->pitch_available = 1;

+         }   

+      }

+      break;

+      case CELT_SET_LTP_REQUEST:

+      {

+         int value = va_arg(ap, int);

+         if (value<0 || value>1 || (value==1 && st->pitch_available==0))

+            goto bad_arg;

+         if (value==0)

             st->pitch_enabled = 0;

          else

             st->pitch_enabled = 1;

--- a/libcelt/celt.h

+++ b/libcelt/celt.h

@@ -71,6 +71,9 @@

 #define CELT_SET_COMPLEXITY_REQUEST    2

 /** Controls the complexity from 0-10 (int) */

 #define CELT_SET_COMPLEXITY(x) CELT_SET_COMPLEXITY_REQUEST, _celt_check_int(x)

+#define CELT_SET_LTP_REQUEST    3

+/** Activate or deactivate the use of the long term predictor (PITCH) from 0 or 1 (int) */

+#define CELT_SET_LTP(x) CELT_SET_LTP_REQUEST, _celt_check_int(x)

 /** GET the frame size used in the current mode */

 #define CELT_GET_FRAME_SIZE   1000