public class Jacobi {
  public double [][] a, abs;
  public int p, q;
  private static int n;

  public Jacobi(int in) {
    int i, j;

    n = in;

    a = new double[n][n];
    abs = new double[n][n];

    // initail value
    for (i = 0; i < n; i++) {
      for (j = 0; j <= i; j++) {
        a[i][j] = a[j][i] = 2.0 * (Math.random() - 0.5);

        abs[i][j] = abs[j][i] = Math.abs(a[i][j]);
      }
    }

    Max();
  }

  private double Max() {
    double max;

    p = 1;
    q = 0;
    max = abs[1][0];
    for (int i = 1; i < n; i++) {
      for (int j = 0; j < i; j++) {
        if (abs[i][j] > max) {
          p = i;
          q = j;
          max = abs[i][j];
        }
      }
    }

    return max;
  }

  private void Rotate(int p, int q, double c, double s) {
    double [][] ww;
    double cc, ss, cs;

    cc = c * c;
    ss = s * s;
    cs = c * s;

    ww = abs;

    for (int i = 0; i < n; i++) {
      ww[p][i] = ww[i][p] = a[p][i] * c - a[q][i] * s;
    }
    for (int i = 0; i < n; i++) {
      ww[q][i] = ww[i][q] = a[p][i] * s + a[q][i] * c;
    }

    ww[p][p] = a[p][p] * cc - 2 * a[p][q] * cs + a[q][q] * ss;
    ww[q][q] = a[p][p] * ss + 2 * a[p][q] * cs + a[q][q] * cc;
    ww[p][q] = ww[q][p] = (a[p][p] - a[q][q]) * cs + a[p][q] * (cc - ss);

    abs = a;
    a = ww;
  }

  public void Step(int loop) {
    double w, c, s;

    while (loop-- > 0) { 
      w = (a[q][q] - a[p][p]) / (2 * a[p][q]);
      if (w >= 0) {
        w = 1.0 / (w + Math.sqrt(w * w + 1));
      } else {
        w = 1.0 / (w - Math.sqrt(w * w + 1));
      }
      c = 1.0 / Math.sqrt(w * w + 1.0);
      s = w * c;
      Rotate(p, q, c, s);

      for (int i = 0; i < n; i++) {
        for (int j = 0; j <= i; j++) {
          abs[i][j] = abs[j][i] = Math.abs(a[i][j]);
        }
      }

      Max();
    }
  }

  public double SumOffDiag() {
    double sum = 0.0;

    for (int i = 1; i < n; i++) {
      for (int j = 0; j < i; j++) {
        sum += abs[i][j] * abs[i][j];
      }
    }

   return sum;
  }
}