function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a ) {
			return NINF;
		}
		// Case: x >= a
		if ( x < c ) {
			return LN2 + ln( x - a ) - denom1;
		}
		if ( x === c ) {
			return LN2 - denom2;
		}
		// Case: x > c
		if ( x <= b ) {
			return LN2 + ln( b - x ) - denom3;
		}
		// Case: x > b
		return NINF;
	}
Evaluates the natural logarithm of the probability density function (PDF) for a triangular distribution.

@private
@param {number} x - input value
@returns {number} evaluated logPDF

@example
var y = logpdf( 12.0 );
// returns <number>