function fibonacciIndex( F ) {
	var x;
	if (
		isnan( F ) ||
		isInteger( F ) === false ||
		F <= 1 ||
		F === PINF
	) {
		return NaN;
	}
	x = ( F*SQRT_5 ) + 0.5;
	return round( ln( x ) / LN_PHI );
}
Computes the Fibonacci number index.

## Notes

-   We use `round` instead of `floor` due to errors introduced by floating-point precision.

@param {NonNegativeInteger} F - Fibonacci number
@returns {NonNegativeInteger} Fibonacci number index

@example
var n = fibonacciIndex( 0 );
// returns NaN

@example
var n = fibonacciIndex( 1 );
// returns NaN

@example
var n = fibonacciIndex( 2 );
// returns 3

@example
var n = fibonacciIndex( 3 );
// returns 4

@example
var n = fibonacciIndex( 5 );
// returns 5

@example
var n = fibonacciIndex( 8 );
// returns 6

@example
var n = fibonacciIndex( NaN );
// returns NaN

@example
var n = fibonacciIndex( 3.14 );
// returns NaN

@example
var n = fibonacciIndex( -1 );
// returns NaN