/*! SlideScroll.js v0.1.4 | (c) 2017-2020 Chan Young Park | MIT License */

(function(global, factory){
	'use strict';
	if(typeof module === 'object' && module.exports) {
		// CommonJS
		module.exports = factory();
	} else if(typeof define === 'function' && define.amd) {
		// AMD
		define(SlideScroll);
	} else {
		// Browser global
		global.SlideScroll = factory();
	}
})(typeof window !== 'undefined' ? window : this, function(){

	function SlideScroll(args){

		var root = this;

		args = args || {};
		root.duration = args.duration || 400;
		root.el = args.el || window;
		root.watchMode = args.watchMode || false;

		// Date.now() value when SlideScroll.to() is called
		root._initial_time;
		// window.pageYOffset or element.scrollTop value when SlideScroll.to() is called
		root._initial_top;

		// ONLY IN WATCH MODE
		// parameters recorded when SlideScroll.to() is called
		// _st = scroll_top, _cb = callback, _cba = callback_args
		root._st, root._cb, root._cba;

		// NOT USED IN WATCH MODE
		// turning true when SlideScroll.to() is called
		// turning false when callback() is called
		root._is_scrolling = false;
	};

	SlideScroll.prototype.to = function(scroll_top, callback, callback_args){
		var root = this;
		if (!root._is_scrolling) {
			root._initial_time = Date.now();
			root._initial_top = (root.el === window) ? window.pageYOffset : root.el.scrollTop;
			root._st = scroll_top;
			root._cb = callback;
			root._cba = callback_args;
			root._is_scrolling = true;
			if (!root.watchMode) engine.call(root);
		}
	};

	SlideScroll.prototype.watch = function(){
		if (this._is_scrolling) engine.call(this);
	};

	function engine(){
		var root = this;
		var target = Math.round(easeOutCubic(Date.now() - root._initial_time, root._initial_top, root._st - root._initial_top, root.duration));

		if (root._st - root._initial_top < 0) {
			// scroll up
			if (target < root._st) target = root._st;
		} else {
			// scroll down
			if (target > root._st) target = root._st;
		}

		if (root.el === window) window.scrollTo(0, target);
		else root.el.scrollTop = target;

		if (target === root._st) {
			root._is_scrolling = false;
			if (root._cb) {
				if (root._cba && root._cba.constructor !== Array) root._cba = [root._cba];
				root._cb.apply(null, root._cba);
			}
		} else {
			if (!root.watchMode) window.requestAnimationFrame(engine.bind(root));
		}
	}

	/**
	 * http://gizma.com/easing
	 * t: current time, b: start value, c: change in value, d: duration
	 */
	function easeOutCubic(t, b, c, d) {
		t /= d;
		t--;
		return c*(t*t*t + 1) + b;
	};

	return SlideScroll;
});