// FFTSpectrum.java
import java.awt.*;

public class FFTSpectrum extends Canvas
{
   private static final int MAX_HARMONICS = 64;
   private double[] data = null;
   private double min, max;
   private String errStr = null;

   public FFTSpectrum()
   {
      setBackground(Color.black);
   }

   public Dimension getPreferredSize()
   {
      return new Dimension(250, 100);
   }

   public void plot(double[] ar, double[] ai, int numOfPoints)
   {
      errStr = null;
      if (numOfPoints > MAX_HARMONICS)
         errStr = "Too many harmonics to display.";
      else if (numOfPoints > 0)
      {
         data = new double[numOfPoints];
         max = min = data[0] = Math.sqrt(ar[1] * ar[1] + ai[1] * ai[1]);
         for(int i = 1; i < numOfPoints; i++)
         {
            data[i] = Math.sqrt(ar[i+1] * ar[i+1] + ai[i+1] * ai[i+1]);
            if (data[i] > max)
               max = data[i];
            if (data[i] < min)
               min = data[i];
         }
      }
      else
         errStr = "All harmonics are filtered out.";
      repaint();
   }

   private int scalePoint(double point)
   {
      int heightOfMax = (int)(0.95 * this.getSize().height);
      return (int)(point / max * heightOfMax);
   }

   public void paint(Graphics g)
   {
      g.setColor(Color.black);
      g.fillRect(0, 0, this.getSize().width, this.getSize().height);
      if (errStr != null)
      {
         Font font = new Font("Monospaced", Font.PLAIN, 16);
         int length = getFontMetrics(font).stringWidth(errStr);
         g.setColor(Color.white);
         g.setFont(font);
         g.drawString(errStr, (this.getSize().width - length) / 2,
                               this.getSize().height / 2);
      }
      else if (data != null)
      {
         int width = this.getSize().width / data.length;
         for(int i = 0; i < data.length; i++)
         {
            int height = scalePoint(data[i]),
                x = i * width,
                y = this.getSize().height - height;
            g.setColor(Color.red);
            g.fillRect(x + 2, y, width - 4, height);
         }
      }
   }

}