function gamma(x) {
    var p = [0.99999999999980993, 676.5203681218851, -1259.1392167224028,
        771.32342877765313, -176.61502916214059, 12.507343278686905,
        -0.13857109526572012, 9.9843695780195716e-6, 1.5056327351493116e-7
    ];

    var g = 7;
    if (x < 0.5) {
        return Math.PI / (Math.sin(Math.PI * x) * gamma(1 - x));
    }

    x -= 1;
    var a = p[0];
    var t = x + g + 0.5;
    for (var i = 1; i < p.length; i++) {
        a += p[i] / (x + i);
    }

    return Math.sqrt(2 * Math.PI) * Math.pow(t, x + 0.5) * Math.exp(-t) * a;
}