/*
    iir.c - IIR direct form II biquads implementation
    prototype: void iir(int *, int, int *, float *, int, float *);

    Entry: arg0: pointer to the input sample buffer
           arg1: size of the input sample buffer
           arg2: pointer to the output sample buffer
           arg3: pointer to the coefficients array
           arg4: number of second-order IIR sections
           arg5: pointer to the filter delay-line buffer

    Return: None
*/


#define CHECK_OVERFLOW 1
#if CHECK_OVERFLOW
static 	float w_max=-1.,w_min=1.;
#endif

void iir(int *x, int Nx, int *y, float *coef, int Ns, float *w)
{      
    int i,j,n,m,k,l,p;
    float temp, w_0;
        
    m=Ns*5;             /* Setup for circular buffer coef[] */
    k=Ns*2;             /* Setup for circular buffer d[] */

    for (j=0,l=0,n=0; n<Nx; n++)        /* IIR filter */ 
    {
        w_0 = (float)(x[n]/32767.0);    /* Q15 to float */ 
        for (i=0; i<Ns; i++)
        {
            w_0 -= *(w+l) * *(coef+j); j++; l=(l+Ns)%k;
            w_0 -= *(w+l) * *(coef+j); j++; 
	    temp = *(w+l);
	    *(w+l) = w_0;   
	    #if CHECK_OVERFLOW            
            for (p=0;p<k;p++)
            {
                if (w_max <= w_0) w_max = w_0;
                if (w_min > w_0)  w_min = w_0; 
            } 
            #endif   			                    
            w_0  =   temp * *(coef+j); j++; 
            w_0 += *(w+l) * *(coef+j); j++; l=(l+Ns)%k;
            w_0 += *(w+l) * *(coef+j); j=(j+1)%m; l=(l+1)%k;   
            #if CHECK_OVERFLOW            
            for (p=0;p<k;p++)
            {
                if (w_max <= w_0) w_max = w_0;
                if (w_min > w_0) w_min = w_0; 
            } 
            #endif           		
        }
        y[n] = (int)(w_0*32767);        /* Q15 format output */
    }
}