请问下面的IIR滤波代码如何提高效率

bool iir_DirectFormII(

            const DynamicArray< double > b,

            const DynamicArray< double > a,

            const DynamicArray< double > x,

            DynamicArray< double > &y)

{

    for(int i=0; i< y.Length; i++) y[i] =0;



    int na=a.Length;

    int nb=b.Length;

    for(int n=0; n<= nb-2; n++)

    {

        y[n] =0;

        for(int z=0; z<=nb-1; z++)

        { //分子

          if ( n>=z )

            y[n] += b[z] * x[n-z] ;

        }

        for(int z=1; z<=na-1; z++)

        { //分母

          if( n>=z )

            y[n] -= a[z] * y[n-z] ;

        }

    }

    for(int n=nb-1; n<= y.Length-1; n++)

    {

        for(int z=0; z<=nb-1; z++)

        {

          y[n] += b[z] * x[n-z] ;

        }

        for(int z=1; z<=na-1; z++)  //a[0] must be 1

        {

          y[n] -= a[z] * y[n-z] ;

        }

    }



    return 1;

}