function internal( x ) {
	var tmp;
	var N;
	var M;
	var i;
	var j;

	N = floor( x.length/2 );
	M = x.length - 1;
	for ( i = 0; i < N; i++ ) {
		j = M - i;
		tmp = x[ i ];
		x[ i ] = x[ j ];
		x[ j ] = tmp;
	}
	return x;
}
Reverses an array in-place.

@private
@param {Collection} x - input array
@returns {Collection} input array

@example
var x = [ 1, 2, 3, 4 ];

var out = internal( x );
// returns [ 4, 3, 2, 1 ]