public static double incompleteBetaFraction2()

in samoa-api/src/main/java/org/apache/samoa/moa/core/Statistics.java [987:1065]

• 70 lines of code
• 8 McCabe index (conditional complexity)

  public static double incompleteBetaFraction2(double a, double b, double x) {

    double xk, pk, pkm1, pkm2, qk, qkm1, qkm2;
    double k1, k2, k3, k4, k5, k6, k7, k8;
    double r, t, ans, z, thresh;
    int n;

    k1 = a;
    k2 = b - 1.0;
    k3 = a;
    k4 = a + 1.0;
    k5 = 1.0;
    k6 = a + b;
    k7 = a + 1.0;
    ;
    k8 = a + 2.0;

    pkm2 = 0.0;
    qkm2 = 1.0;
    pkm1 = 1.0;
    qkm1 = 1.0;
    z = x / (1.0 - x);
    ans = 1.0;
    r = 1.0;
    n = 0;
    thresh = 3.0 * MACHEP;
    do {
      xk = -(z * k1 * k2) / (k3 * k4);
      pk = pkm1 + pkm2 * xk;
      qk = qkm1 + qkm2 * xk;
      pkm2 = pkm1;
      pkm1 = pk;
      qkm2 = qkm1;
      qkm1 = qk;

      xk = (z * k5 * k6) / (k7 * k8);
      pk = pkm1 + pkm2 * xk;
      qk = qkm1 + qkm2 * xk;
      pkm2 = pkm1;
      pkm1 = pk;
      qkm2 = qkm1;
      qkm1 = qk;

      if (qk != 0)
        r = pk / qk;
      if (r != 0) {
        t = Math.abs((ans - r) / r);
        ans = r;
      } else
        t = 1.0;

      if (t < thresh)
        return ans;

      k1 += 1.0;
      k2 -= 1.0;
      k3 += 2.0;
      k4 += 2.0;
      k5 += 1.0;
      k6 += 1.0;
      k7 += 2.0;
      k8 += 2.0;

      if ((Math.abs(qk) + Math.abs(pk)) > big) {
        pkm2 *= biginv;
        pkm1 *= biginv;
        qkm2 *= biginv;
        qkm1 *= biginv;
      }
      if ((Math.abs(qk) < biginv) || (Math.abs(pk) < biginv)) {
        pkm2 *= big;
        pkm1 *= big;
        qkm2 *= big;
        qkm1 *= big;
      }
    } while (++n < 300);

    return ans;
  }