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Optimizes remove_doubling() by avoiding redundant calculations of yy

Using a sliding window to pre-compute all yy values.

--- a/celt/pitch.c

+++ b/celt/pitch.c

@@ -472,6 +472,9 @@

    opus_val32 best_xy, best_yy;

    int offset;

    int minperiod0;

+   int max

+   VARDECL(opus_val32, yy_lookup);

+   SAVE_STACK;

    minperiod0 = minperiod;

    maxperiod /= 2;

@@ -484,13 +487,21 @@

       *T0_=maxperiod-1;

    T = T0 = *T0_;

-   xx=xy=yy=0;

+   ALLOC(yy_lookup, maxperiod+1, opus_val32);

+   xy=xx=0;

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

    {

-      xy = MAC16_16(xy, x[i], x[i-T0]);

       xx = MAC16_16(xx, x[i], x[i]);

-      yy = MAC16_16(yy, x[i-T0],x[i-T0]);

+      xy = MAC16_16(xy, x[i], x[i-T0]);

    }

+   yy_lookup[0] = xx;

+   yy=xx;

+   for (i=1;i<=maxperiod;i++)

+   {

+      yy = yy+MULT16_16(x[-i],x[-i])-MULT16_16(x[N-i],x[N-i]);

+      yy_lookup[i] = MAX32(0, yy);

+   }

+   yy = yy_lookup[T0];

    best_xy = xy;

    best_yy = yy;

 #ifdef FIXED_POINT

@@ -526,15 +537,13 @@

       {

          T1b = (2*second_check[k]*T0+k)/(2*k);

       }

-      xy=yy=0;

+      xy=0;

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

       {

          xy = MAC16_16(xy, x[i], x[i-T1]);

-         yy = MAC16_16(yy, x[i-T1], x[i-T1]);

-

          xy = MAC16_16(xy, x[i], x[i-T1b]);

-         yy = MAC16_16(yy, x[i-T1b], x[i-T1b]);

       }

+      yy = yy_lookup[T1] + yy_lookup[T1b];

 #ifdef FIXED_POINT

       {

          opus_val32 x2y2;

@@ -594,5 +603,6 @@

    if (*T0_<minperiod0)

       *T0_=minperiod0;

+   RESTORE_STACK;

    return pg;

 }