/**
 * Helper class that simulates some-of-what nativish
 * scrolling that continues after dragging (explicit calls to scroll())
 * have stopped
 */

// libraries
import $ from "./Zepto"

export default class Scroller {


	constructor($element) {
		this.$element = $element

		// the current scrolling speed (while animating, or average of last movement)
		this.speedY = 0

		// where the scrollable element is scrolled at currently
		this.lastY = 0

		// minimum distance to scroll, beyond lower we don't animate
		this.threshold = 0.01

		// the animation easing; could also use easing function, now just
		// exponentially multiply by this factor
		this.easing = 0.95

		// when storing the last moved distance, always use part of this.speedY
		// and the other part the current last distance, to somewhat ease
		// how an abrupt acceleration or deacceleration just before releasing
		// to animating should affect the movement
		this.decay = 0.5

		// store ref to requestAnimationFrame for cancelling
		this.animation = null

		// animation flag; new call to scroll() cancels automatic animation
		this.animate = false
	}


	/**
	 * Move to y
	 */
	scroll(y) {
		// store how much we moved since last recorded y
		let lastSpeed = this.lastY - y

		// perform the actual scroll
		this.$element.scrollTop(y)

		// cancel perpetual animation frame animation
		this.animate = false

		// on each manual scroll update the speed (distance) at which this manual
		// scroll happened, so that when this was the last manual scroll
		// we can continue and ease out this scroll
		// always balance this current speed with what was last recorded
		this.speedY = this.speedY * this.decay + lastSpeed * (1 - this.decay)

		// record current position
		this.lastY = y

		// trigger perpetual animation; only a new call to scroll will
		// cancel it, or having finished animating
		this.animation = window.requestAnimationFrame(this.continueScroll.bind(this))
	}


	/**
	 * Perpetual animation callback for scrolling after the last scroll() call
	 */
	continueScroll() {
		// check if we should still animate or if the animation got cancelled
		if (!this.animate) {
			window.cancelAnimationFrame(this.animation)
		}

		if (Math.abs(this.speedY) > this.threshold) {
			// scroll by speed, ease out current speed
			this.speedY *= this.easing
			this.lastY -= this.speedY

			// stop animating if we reached the end of the scrollable element
			if (this.lastY <= 0) {
				this.animate = false
			}

			// perform the actual scroll
			this.$element.scrollTop(this.lastY)

			window.requestAnimationFrame(this.continueScroll.bind(this))
		} else {
			// below threshold, stop animation by not issueing a new frame
			// also store status for reference
			this.animate = false
		}
	}


}