static double gammaPDerivative()

in commons-numbers-gamma/src/main/java/org/apache/commons/numbers/gamma/BoostGamma.java [1136:1167]

• 19 lines of code
• 9 McCabe index (conditional complexity)

    static double gammaPDerivative(double a, double x) {
        //
        // Usual error checks first:
        //
        if (Double.isNaN(a) || Double.isNaN(x) || a <= 0 || x < 0) {
            return Double.NaN;
        }
        //
        // Now special cases:
        //
        if (x == 0) {
            if (a > 1) {
                return 0;
            }
            return (a == 1) ? 1 : Double.POSITIVE_INFINITY;
        }
        //
        // Normal case:
        //
        double f1 = regularisedGammaPrefix(a, x);
        if (f1 == 0) {
            // Underflow in calculation, use logs instead:
            f1 = a * Math.log(x) - x - lgamma(a) - Math.log(x);
            f1 = Math.exp(f1);
        } else {
            // Will overflow when (x < 1) && (Double.MAX_VALUE * x < f1).
            // There is no exception for this case so just return the result.
            f1 /= x;
        }

        return f1;
    }