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optimisation: intra_prediction() uses a 16-bit numerator for the search

--- a/libcelt/vq.c

+++ b/libcelt/vq.c

@@ -273,7 +273,7 @@

 {

    int i,j,c;

    int best=0;

-   celt_word32_t best_num=-SHR32(VERY_LARGE32,4);

+   celt_word16_t best_num=-VERY_LARGE16;

    celt_word16_t best_den=0;

    celt_word16_t s = 1;

    int sign;

@@ -296,7 +296,7 @@

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

    {

       celt_word32_t xy=0, yy=0;

-      celt_word32_t num;

+      celt_word16_t num;

       celt_word16_t den;

       const celt_word16_t * restrict xp = Xr;

       const celt_word16_t * restrict yp = Y+B*i;

@@ -310,11 +310,11 @@

          yp++;

       } while (++j<B*N); /* Promises we loop at least once */

       /* Using xy^2/yy as the score but without having to do the division */

-      num = MULT16_16(ROUND16(xy,14),ROUND16(xy,14));

+      num = MULT16_16_Q15(ROUND16(xy,14),ROUND16(xy,14));

       den = ROUND16(yy,14);

       /* If you're really desperate for speed, just use xy as the score */

       /* OPT: Make sure to use a conditional move here */

-      if (MULT16_32_Q15(best_den, num) >  MULT16_32_Q15(den, best_num))

+      if (MULT16_16(best_den, num) >  MULT16_16(den, best_num))

       {

          best_num = num;

          best_den = den;