/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; and CC Bordeianu, Univ Bucharest, 2007.
   Support by National Science Foundation                              
*/
//DFT.java: Discrete Fourier Transform

import java.io.*;  

public class DFT {
  static final int N = 1000, Np = N;              // Global constants
  static double [] signal = new double[N + 1]; 
  static double twopi = 2.*Math.PI, sq2pi = 1./Math.sqrt(twopi); 
  static double h = twopi/N;
   
  public static void main(String[] argv){   
		
    double dftreal[] = new double[Np]; 
    double dftimag[] = new double[Np]; 
		
    f(signal); 
    fourier(dftreal, dftimag);       
  }
                   
  public static void fourier(double dftreal[], double dftimag[]){
    double real, imag;                            // Calc & plot Y(w)
    int n, k; 
    for ( n = 0;  n < Np;  n++ ) {               // Loop on frequency
      real = imag = 0. ;                           // Clear variables
			                                                  // Major loop
      for ( k = 0;  k < N;  k++ ) {                       
				real += signal[k] * Math.cos( twopi*k*n/N ); 
        imag += signal[k] * Math.sin( twopi*k*n/N ); 
      }
      dftreal[n] = real*sq2pi; 
      dftimag[n] = -imag*sq2pi; 
    }
  }
  
  public static void f(double [] signal) {         // Signal function
    int i; 
    double step = twopi/N, x = 0.; 
    for ( i=0;  i <= N;  i++ ){
      signal[i] = 5. + 10*Math.sin(x+2.);
      x += step; 
    }
  }
}