public static double betainv(double x, double p, double q) {
// ALGORITHM AS 63 APPL. STATIST. VOL.32, NO.1
// Computes P(Beta>x)
double beta = Maths.logBeta(p, q), acu = 1E-14;
double cx, psq, pp, qq, x2, term, ai, betain, ns, rx, temp;
boolean indx;
if (p <= 0 || q <= 0) return (-1.0);
if (x <= 0 || x >= 1) return (-1.0);
psq = p + q;
cx = 1 - x;
if (p < psq * x) {
x2 = cx;
cx = x;
pp = q;
qq = p;
indx = true;
} else {
x2 = x;
pp = p;
qq = q;
indx = false;
}
term = 1;
ai = 1;
betain = 1;
ns = qq + cx * psq;
rx = x2 / cx;
temp = qq - ai;
if (ns == 0) rx = x2;
while (temp > acu && temp > acu * betain) {
term = term * temp * rx / (pp + ai);
betain = betain + term;
temp = Math.abs(term);
if (temp > acu && temp > acu * betain) {
ai++;
ns--;
if (ns >= 0) {
temp = qq - ai;
if (ns == 0) rx = x2;
} else {
temp = psq;
psq += 1;
}
}
}
betain *= Math.exp(pp * Math.log(x2) + (qq - 1) * Math.log(cx) - beta) / pp;
if (indx) betain = 1 - betain;
return (betain);
}
