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More accurate sqrt approximation using MULT16_16_Q15() instead of Q14.

--- a/libcelt/mathops.h

+++ b/libcelt/mathops.h

@@ -67,14 +67,17 @@

 static inline celt_word32_t celt_sqrt(celt_word32_t x)

 {

    int k;

-   //printf ("%d ", x);

+   celt_word16_t n;

    celt_word32_t rt;

-   /* ((EC_ILOG(x)-1)>>1) is just the int log4(x) (EC_ILOG returns log2 + 1) */

-   k = ((EC_ILOG(x)-1)>>1)-6;

+   const celt_word16_t C[5] = {23174, 11584, -3011, 1570, -557};

+   if (x==0)

+      return 0;

+   k = (celt_ilog2(x)>>1)-7;

    x = VSHR32(x, (k<<1));

-   rt = ADD16(C0, MULT16_16_Q14(x, ADD16(C1, MULT16_16_Q14(x, ADD16(C2, MULT16_16_Q14(x, (C3)))))));

+   n = x-32768;

+   rt = ADD16(C[0], MULT16_16_Q15(n, ADD16(C[1], MULT16_16_Q15(n, ADD16(C[2], 

+              MULT16_16_Q15(n, ADD16(C[3], MULT16_16_Q15(n, (C[4])))))))));

    rt = VSHR32(rt,7-k);

-   //printf ("%d\n", rt);

    return rt;

 }