inv-chisquare-cdf/src/invChiSquareCDF.js

Inverse chi-squared cumulative distribution function

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

module.exports = function invChiSquareCDF(probability, degreeOfFreedom) {
  if (isNaN(probability)) {
    // TODO fix: booleans and strings like '123' will not fall here.
    throw new Error('The value in param "probability" is not an number.');
  } else if (isNaN(degreeOfFreedom)) {
    // TODO fix: booleans and strings like '123' will not fall here.
    throw new Error('The value in param "degreeOfFreedom" is not an number.');
  } else if (probability >= 1 || probability <= 0) {
    throw new Error('The number in param "probability" must lie in the interval [0 1].');
  } else if (degreeOfFreedom <= 0) {
    throw new Error('The number in param "degreeOfFreedom" must be greater than 0.');
  }

  return 2 * invRegLowGamma(probability, 0.5 * degreeOfFreedom);
};