Rescale Interpolation

interpolation.js

/*
	More interesting interpolation functions to make life easier.
	
	Rescale is a function which can interpolate a value between two different ranges.
	
	Say you have a number which represents the position of a slider.
	The slider has a start and end as well.
	You want the slider to control a box between two locations.
	
	So you could do the following:
		boxPosition = rescale(sliderStart, sliderEnd, boxStart, boxEnd, slideValue);
	
	Additionally there is a clamped variation, `rescale.clamped(…)` with the same
	function signature that clamps the output value in the `to` range
*/

function rescale(from_min, from_max, to_min, to_max, from) {
	return to_min + ((to_max-to_min) * ((from_min-from) / (from_min-from_max)));
}

rescale.clamped = function(from_min, from_max, to_min, to_max, from) {
	from = this(from_min, from_max, to_min, to_max, from);
	if(to_min < to_max) {
		if(from < to_min) return to_min;
		else if(from > to_max) return to_max;
	} else {
		if(from < to_max) return to_max;
		else if(from > to_min) return to_min;
	}
	return from;
}


/// An alternative API, works via two-element arrays: `from` and `to`
// E.g. retarget([0, 1], [0, 100], 0.23) => 23
function retarget(from, to, via) {
	if(!Array.isArray(from)) throw new Error('retarget: `from` is not an array');
	if(!Array.isArray(to)) throw new Error('retarget: `to` is not an array');
	if(from.length < 2) throw new Error('retarget: `from` needs at least 2 entries');
	if(to.length < 2) throw new Error('retarget: `to` needs at least 2 entries');
	
	if(via < from[0]) return rescale(from[0], from[1], to[0], to[1], via);
	if(via > from[from.length - 1]) return t = rescale(from[from.length - 2], from[from.length - 1], to[to.length - 2], to[to.length - 1], via);
	
	for(var iter = 0, total = from.length; iter < total; ++iter)
		if(via <= from[iter]) break;
	
	var a = from[iter - 1], b = from[iter];
	var t = (via - a) / (b-a);
	t = ((iter - 1 + t) / from.length) * to.length;
	var c = to[Math.floor(t)], d = to[Math.ceil(t)];
	return rescale(a, b, c, d, via);
}