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Makes surround_analysis() work in fixed-point

--- a/src/opus_multistream_encoder.c

+++ b/src/opus_multistream_encoder.c

@@ -179,6 +179,45 @@

    }

 }

+#if 1

+/* Computes a rough approximation of log2(2^a + 2^b) */

+static opus_val16 logSum(opus_val16 a, opus_val16 b)

+{

+   opus_val16 max;

+   opus_val32 diff;

+   opus_val16 frac;

+   static const opus_val16 diff_table[17] = {

+         QCONST16(0.5000000f, DB_SHIFT), QCONST16(0.2924813f, DB_SHIFT), QCONST16(0.1609640f, DB_SHIFT), QCONST16(0.0849625f, DB_SHIFT),

+         QCONST16(0.0437314f, DB_SHIFT), QCONST16(0.0221971f, DB_SHIFT), QCONST16(0.0111839f, DB_SHIFT), QCONST16(0.0056136f, DB_SHIFT),

+         QCONST16(0.0028123f, DB_SHIFT)

+   };

+   int low;

+   if (a>b)

+   {

+      max = a;

+      diff = SUB32(EXTEND32(a),EXTEND32(b));

+   } else {

+      max = b;

+      diff = SUB32(EXTEND32(b),EXTEND32(a));

+   }

+   if (diff >= QCONST16(8.f, DB_SHIFT))

+      return max;

+#ifdef FIXED_POINT

+   low = SHR32(diff, DB_SHIFT-1);

+   frac = SHL16(diff - SHL16(low, DB_SHIFT-1), 16-DB_SHIFT);

+#else

+   low = floor(2*diff);

+   frac = 2*diff - low;

+#endif

+   return max + diff_table[low] + MULT16_16_Q15(frac, SUB16(diff_table[low+1], diff_table[low]));

+}

+#else

+opus_val16 logSum(opus_val16 a, opus_val16 b)

+{

+   return log2(pow(4, a)+ pow(4, b))/2;

+}

+#endif

+

 void surround_analysis(const CELTMode *celt_mode, const void *pcm, opus_val16 *bandLogE, opus_val32 *mem, opus_val32 *preemph_mem,

       int len, int overlap, int channels, int rate, opus_copy_channel_in_func copy_channel_in

 )

@@ -190,7 +229,6 @@

    int pos[8] = {0};

    int upsample;

    opus_val32 bandE[21];

-   opus_val32 maskE[3][21];

    opus_val16 maskLogE[3][21];

    VARDECL(opus_val32, in);

    VARDECL(opus_val16, x);

@@ -202,9 +240,9 @@

    channel_pos(channels, pos);

-   for (c=0;c<2;c++)

+   for (c=0;c<3;c++)

       for (i=0;i<21;i++)

-         maskE[c][i] = 0;

+         maskLogE[c][i] = -QCONST16(28.f, DB_SHIFT);

    upsample = resampling_factor(rate);

    for (c=0;c<channels;c++)

@@ -224,24 +262,23 @@

       }

       compute_band_energies(celt_mode, freq, bandE, 21, 1, 1<<LM);

-      /* FIXME: Figure out how to square bandE[] in fixed-point */

+      amp2Log2(celt_mode, 21, 21, bandE, bandLogE+21*c, 1);

       if (pos[c]==1)

       {

          for (i=0;i<21;i++)

-            maskE[0][i] += bandE[i]*bandE[i];

+            maskLogE[0][i] = logSum(maskLogE[0][i], bandLogE[21*c+i]);

       } else if (pos[c]==3)

       {

          for (i=0;i<21;i++)

-            maskE[1][i] += bandE[i]*bandE[i];

+            maskLogE[2][i] = logSum(maskLogE[2][i], bandLogE[21*c+i]);

       } else if (pos[c]==2)

       {

          for (i=0;i<21;i++)

          {

-            maskE[0][i] += HALF32(bandE[i]*bandE[i]);

-            maskE[1][i] += HALF32(bandE[i]*bandE[i]);

+            maskLogE[0][i] = logSum(maskLogE[0][i], bandLogE[21*c+i]-QCONST16(.5f, DB_SHIFT));

+            maskLogE[2][i] = logSum(maskLogE[2][i], bandLogE[21*c+i]-QCONST16(.5f, DB_SHIFT));

          }

       }

-      amp2Log2(celt_mode, 21, 21, bandE, bandLogE+21*c, 1);

 #if 0

       for (i=0;i<21;i++)

          printf("%f ", bandLogE[21*c+i]);

@@ -254,16 +291,10 @@

       OPUS_COPY(mem+c*overlap, in+len, overlap);

    }

    for (i=0;i<21;i++)

-      maskE[2][i] = MIN32(maskE[0][i],maskE[1][i]);

+      maskLogE[1][i] = MIN32(maskLogE[0][i],maskLogE[2][i]);

    for (c=0;c<3;c++)

       for (i=0;i<21;i++)

-         maskE[c][i] = sqrt(maskE[c][i]*2/(channels-1));

-   /* Left mask */

-   amp2Log2(celt_mode, 21, 21, &maskE[0][0], &maskLogE[0][0], 1);

-   /* Right mask */

-   amp2Log2(celt_mode, 21, 21, &maskE[1][0], &maskLogE[2][0], 1);

-   /* Centre mask */

-   amp2Log2(celt_mode, 21, 21, &maskE[2][0], &maskLogE[1][0], 1);

+         maskLogE[c][i] += QCONST16(.5f, DB_SHIFT)*log2(2.f/(channels-1));

 #if 0

    for (c=0;c<3;c++)

    {