module.exports = function(xs, ys) {
	var i, length = xs.length;

	// Deal with length issues
	if (length != ys.length) { throw 'Need an equal count of xs and ys.'; }
	if (length === 0) { return function(x) { return 0; }; }
	if (length === 1) {
		// Impl: Precomputing the result prevents problems if ys is mutated later and allows garbage collection of ys
		// Impl: Unary plus properly converts values to numbers
		var result = +ys[0];
		return function(x) { return result; };
	}

	// Rearrange xs and ys so that xs is sorted
	var indexes = [];
	for (i = 0; i < length; i++) { indexes.push(i); }
	indexes.sort(function(a, b) { return xs[a] < xs[b] ? -1 : 1; });
	var oldXs = xs, oldYs = ys;
	// Impl: Creating new arrays also prevents problems if the input arrays are mutated later
	xs = []; ys = [];
	// Impl: Unary plus properly converts values to numbers
	for (i = 0; i < length; i++) { xs.push(+oldXs[indexes[i]]); ys.push(+oldYs[indexes[i]]); }

	// Get consecutive differences and slopes
	var dys = [], dxs = [], ms = [];
	for (i = 0; i < length - 1; i++) {
		var dx = xs[i + 1] - xs[i], dy = ys[i + 1] - ys[i];
		dxs.push(dx); dys.push(dy); ms.push(dy / dx);
	}

	// Get degree-1 coefficients
	var c1s = [ms[0]];
	for (i = 0; i < dxs.length - 1; i++) {
		var m = ms[i], mNext = ms[i + 1];
		if (m * mNext <= 0) {
			c1s.push(0);
		} else {
			var dx = dxs[i], dxNext = dxs[i + 1], common = dx + dxNext;
			c1s.push(3 * common / ((common + dxNext) / m + (common + dx) / mNext));
		}
	}
	c1s.push(ms[ms.length - 1]);

	// Get degree-2 and degree-3 coefficients
	var c2s = [], c3s = [];
	for (i = 0; i < c1s.length - 1; i++) {
		var c1 = c1s[i], m = ms[i], invDx = 1 / dxs[i], common = c1 + c1s[i + 1] - m - m;
		c2s.push((m - c1 - common) * invDx); c3s.push(common * invDx * invDx);
	}

	// Return interpolant function
	return function(x) {
		// The rightmost point in the dataset should give an exact result
		var i = xs.length - 1;
		if (x == xs[i]) { return ys[i]; }

		// Search for the interval x is in, returning the corresponding y if x is one of the original xs
		var low = 0, mid, high = c3s.length - 1;
		while (low <= high) {
			mid = Math.floor(0.5 * (low + high));
			var xHere = xs[mid];
			if (xHere < x) { low = mid + 1; }
			else if (xHere > x) { high = mid - 1; }
			else { return ys[mid]; }
		}
		i = Math.max(0, high);

		// Interpolate
		var diff = x - xs[i], diffSq = diff * diff;
		return ys[i] + c1s[i] * diff + c2s[i] * diffSq + c3s[i] * diff * diffSq;
	};
};