using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

/* @author Geir K. Nilsen (geir.kjetil.nilsen@gmail.com) 2017 */

namespace ParametricEqualizer
{
    public class Filter
    {

        private List<double> a;
        private List<double> b;

        private List<double> x_past;
        private List<double> y_past;

        public Filter(List<double> a, List<double> b)
        {
            this.a = a;
            this.b = b;
        }

        public void apply(List<double> x, out List<double> y)
        {
            int ord = a.Count - 1;
            int np = x.Count - 1;

            if (np < ord)
            {
                for (int k = 0; k < ord - np; k++)
                    x.Add(0.0);
                np = ord;
            }

            y = new List<double>();
            for (int k = 0; k < np + 1; k++)
            {
                y.Add(0.0);
            }

            if (x_past == null)
            {
                x_past = new List<double>();

                for (int s = 0; s < x.Count; s++)
                    x_past.Add(0);
            }

            if (y_past == null)
            {
                y_past = new List<double>();

                for (int s = 0; s < y.Count; s++)
                    y_past.Add(0);
            }

            for (int i = 0; i < np + 1; i++)
            {
                y[i] = 0.0;

                for (int j = 0; j < ord + 1; j++)
                {
                    if (i - j < 0)
                        y[i] = y[i] + b[j] * x_past[x_past.Count - j];
                    else
                        y[i] = y[i] + b[j] * x[i - j];
                }

                for (int j = 0; j < ord; j++)
                {
                    if (i - j - 1 < 0)
                        y[i] = y[i] - a[j + 1] * y_past[y_past.Count - j - 1];
                    else
                        y[i] = y[i] - a[j + 1] * y[i - j - 1];
                }

                y[i] = y[i] / a[0];
            }

            x_past = x;
            y_past = y;

        }
    }
}