inv-chisquare-cdf/src/logGamma.js

Inverse chi-squared cumulative distribution function

module.exports = function logGamma(x) {
  if (x===1 || x===2) {
    return 0;
  } else if (x===0) {
    return Infinity;
  } else if (isNaN(x)) {
    // TODO fix: booleans and strings like '123' will not fall here.
    throw new Error(`The value is not a number.`);
  } else if (x<0) {
    throw new Error(`The value is a negative number.`);
  }

  // Lanczos approximation
  const cof = [
    76.18009172947146,
    -86.50532032941677,
    24.01409824083091,
    -1.231739572450155,
    0.1208650973866179e-2,
    -0.5395239384953e-5,
  ];
  let ser = 1.000000000190015;

  let xx;
  let y;
  let tmp;
  tmp = (y = xx = x) + 5.5;
  tmp -= (xx + 0.5) * Math.log(tmp);

  cof.map((approximation)=>{
    ser += approximation / ++y;
  });

  return Math.log(2.5066282746310005 * ser / xx) - tmp;
};