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Bitstream change: prevent noise injection in hybrid mode

This makes it possible to use folding rather than LCG noise in the second CELT
band (9.6 to 12 kHz) in hybrid mode.

--- a/celt/bands.c

+++ b/celt/bands.c

@@ -1361,6 +1361,18 @@

    return cm;

 }

+static void special_hybrid_folding(const CELTMode *m, celt_norm *norm, celt_norm *norm2, int start, int M, int dual_stereo)

+{

+   int n1, n2;

+   const opus_int16 * OPUS_RESTRICT eBands = m->eBands;

+   n1 = M*(eBands[start+1]-eBands[start]);

+   n2 = M*(eBands[start+2]-eBands[start+1]);

+   /* Duplicate enough of the first band folding data to be able to fold the second band.

+      Copies no data for CELT-only mode. */

+   OPUS_COPY(&norm[n1], &norm[2*n1 - n2], n2-n1);

+   if (dual_stereo)

+      OPUS_COPY(&norm2[n1], &norm2[2*n1 - n2], n2-n1);

+}

 void quant_all_bands(int encode, const CELTMode *m, int start, int end,

       celt_norm *X_, celt_norm *Y_, unsigned char *collapse_masks,

@@ -1472,8 +1484,15 @@

          b = 0;

       }

+#ifdef ENABLE_UPDATE_DRAFT

+      if (resynth && (M*eBands[i]-N >= M*eBands[start] || i==start+1) && (update_lowband || lowband_offset==0))

+            lowband_offset = i;

+      if (i == start+1)

+         special_hybrid_folding(m, norm, norm2, start, M, dual_stereo);

+#else

       if (resynth && M*eBands[i]-N >= M*eBands[start] && (update_lowband || lowband_offset==0))

             lowband_offset = i;

+#endif

       tf_change = tf_res[i];

       ctx.tf_change = tf_change;

@@ -1499,7 +1518,11 @@

          fold_start = lowband_offset;

          while(M*eBands[--fold_start] > effective_lowband+norm_offset);

          fold_end = lowband_offset-1;

+#ifdef ENABLE_UPDATE_DRAFT

+         while(++fold_end < i && M*eBands[fold_end] < effective_lowband+norm_offset+N);

+#else

          while(M*eBands[++fold_end] < effective_lowband+norm_offset+N);

+#endif

          x_cm = y_cm = 0;

          fold_i = fold_start; do {

            x_cm |= collapse_masks[fold_i*C+0];

@@ -1575,6 +1598,8 @@

                ctx = ctx_save;

                OPUS_COPY(X, X_save, N);

                OPUS_COPY(Y, Y_save, N);

+               if (i == start+1)

+                  special_hybrid_folding(m, norm, norm2, start, M, dual_stereo);

                /* Encode and round up. */

                ctx.theta_round = 1;

                x_cm = quant_band_stereo(&ctx, X, Y, N, b, B,