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Simplifying celt_fir5() since x==y and mem[]={0}

Addresses issue https://github.com/xiph/opus/issues/58

--- a/celt/pitch.c

+++ b/celt/pitch.c

@@ -102,11 +102,9 @@

    }

 }

-static void celt_fir5(const opus_val16 *x,

+static void celt_fir5(opus_val16 *x,

          const opus_val16 *num,

-         opus_val16 *y,

-         int N,

-         opus_val16 *mem)

+         int N)

 {

    int i;

    opus_val16 num0, num1, num2, num3, num4;

@@ -116,11 +114,11 @@

    num2=num[2];

    num3=num[3];

    num4=num[4];

-   mem0=mem[0];

-   mem1=mem[1];

-   mem2=mem[2];

-   mem3=mem[3];

-   mem4=mem[4];

+   mem0=0;

+   mem1=0;

+   mem2=0;

+   mem3=0;

+   mem4=0;

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

    {

       opus_val32 sum = SHL32(EXTEND32(x[i]), SIG_SHIFT);

@@ -134,13 +132,8 @@

       mem2 = mem1;

       mem1 = mem0;

       mem0 = x[i];

-      y[i] = ROUND16(sum, SIG_SHIFT);

+      x[i] = ROUND16(sum, SIG_SHIFT);

    }

-   mem[0]=mem0;

-   mem[1]=mem1;

-   mem[2]=mem2;

-   mem[3]=mem3;

-   mem[4]=mem4;

 }

@@ -150,7 +143,7 @@

    int i;

    opus_val32 ac[5];

    opus_val16 tmp=Q15ONE;

-   opus_val16 lpc[4], mem[5]={0,0,0,0,0};

+   opus_val16 lpc[4];

    opus_val16 lpc2[5];

    opus_val16 c1 = QCONST16(.8f,15);

 #ifdef FIXED_POINT

@@ -211,7 +204,7 @@

    lpc2[2] = lpc[2] + MULT16_16_Q15(c1,lpc[1]);

    lpc2[3] = lpc[3] + MULT16_16_Q15(c1,lpc[2]);

    lpc2[4] = MULT16_16_Q15(c1,lpc[3]);

-   celt_fir5(x_lp, lpc2, x_lp, len>>1, mem);

+   celt_fir5(x_lp, lpc2, len>>1);

 }

 /* Pure C implementation. */