docs: public static void fft(boolean forward, long m, double[] x , double[] y)
      {
          int n, i, i1, j, k, i2, l, l1, l2;
          double c1, c2, tx, ty, t1, t2, u1, u2, z;
  
          /* Calculate the number of points */
          n = 1;
          for (i = 0; i < m; i++)
          {
              n *= 2;
          }
  
          /* Do the bit reversal */
          i2 = n >> 1;
          j = 0;
          for (i = 0; i < n - 1; i++)
          {
              if (i < j)
              {
                  tx = x[i];
                  ty = y[i];
                  x[i] = x[j];
                  y[i] = y[j];
                  x[j] = tx;
                  y[j] = ty;
              }
              k = i2;
              while (k <= j)
              {
                  j -= k;
                  k >>= 1;
              }
              j += k;
          }
  
          /* Compute the FFT */
          c1 = -1.0;
          c2 = 0.0;
          l2 = 1;
          for (l = 0; l < m; l++)
          {
              l1 = l2;
              l2 <<= 1;
              u1 = 1.0;
              u2 = 0.0;
              for (j = 0; j < l1; j++)
              {
                  for (i = j; i < n; i += l2)
                  {
                      i1 = i + l1;
                      t1 = u1 * x[i1] - u2 * y[i1];
                      t2 = u1 * y[i1] + u2 * x[i1];
                      x[i1] = x[i] - t1;
                      y[i1] = y[i] - t2;
                      x[i] += t1;
                      y[i] += t2;
                  }
                  z = u1 * c1 - u2 * c2;
                  u2 = u1 * c2 + u2 * c1;
                  u1 = z;
              }
              c2 = Math.sqrt((1.0 - c1) / 2.0);
              if (forward)
              {
                  c2 = -c2;
              }
              c1 = Math.sqrt((1.0 + c1) / 2.0);
          }
  
          /* Scaling for forward transform */
          if (forward)
          {
              for (i = 0; i < n; i++)
              {
                  x[i] /= n;
                  y[i] /= n;
              }
          }
      }