inv-chisquare-cdf/src/regLowGamma.js

Inverse chi-squared cumulative distribution function

const logGamma = require('./logGamma.js');

module.exports = function regLowGamma(a, x) {
  if (isNaN(a)) {
    // TODO fix: booleans and strings like '123' will not fall here.
    throw new Error('The value in param a is not a number.');
  } else if (isNaN(x)) {
    // TODO fix: booleans and strings like '123' will not fall here.
    throw new Error('The value in param x is not a number.');
  } else if (a <= 0) {
    throw new Error('The number in param a is equal or less tham 0.');
  } else if (x<0) {
    throw new Error('The number in param x is a negative number.');
  }

  const logGammaOfA = logGamma(a);
  let b = x + 1 - a;
  let c = 1 / 1.0e-30;
  let d = 1 / b;
  let h = d;
  let i = 1;
  const maxOfIterationsForA = -~(Math.log((a >= 1) ? a : 1 / a) * 8.5 + a * 0.4 + 17);

  if (x < a + 1) {
    let sum = 1 / a;
    let del = sum;
    for (let ap = a; i <= maxOfIterationsForA; i++) {
      sum += del *= x / ++ap;
    }
    return (sum * Math.exp(-x + a * Math.log(x) - (logGammaOfA)));
  }

  let an;
  for (; i <= maxOfIterationsForA; i++) {
    an = -i * (i - a);
    b += 2;
    d = an * d + b;
    c = b + an / c;
    d = 1 / d;
    h *= d * c;
  }

  return (1 - h * Math.exp(-x + a * Math.log(x) - (logGammaOfA)));
};