//////////////////////////////////////////////////////////////////////
// CInView
function CInView(){
	// elements
	let el_control;
	// parameters
	let callback;
	// variables
	let b_run;

	// jQuery.offset
	function jqOffset(el){
		var rect=el.getBoundingClientRect();
		var el_body=document.documentElement;
		return {
			x:rect.left+el_body.scrollLeft,
			y:rect.top+el_body.scrollTop
		};
	};

	function getElementSize(el){
		const cs=window.getComputedStyle(el);
		return {
			w:parseInt(cs.getPropertyValue("width")),
			h:parseInt(cs.getPropertyValue("height"))
		};
	};

	function getElementRect(el){
		const pos=jqOffset(el);
		const size=getElementSize(el);
		return{
			top:pos.y,
			bottom:pos.y+size.h,
			left:pos.x,
			right:pos.x+size.w
		};
	};

	function getViewPort(){
		// scroll position
		var scrollpos={
			top:document.documentElement.scrollTop,
			left:document.documentElement.scrollLeft
		};

		// viewport
		var vp={};
		vp.top=scrollpos.top;
		vp.bottom=vp.top+window.innerHeight;
		vp.left=scrollpos.left;
		vp.right=vp.left+window.innerWidth;

		// return
		return vp;
	};

	function inZone(er,vp,hori,vert){
		if(hori===undefined){hori=0;}
		if(vert===undefined){vert=0;}
		const center={
			x:(er.left+er.right)/2,
			y:(er.top+er.bottom)/2
		};
		const ext_w=(vp.right-vp.left)*hori;
		const ext_h=(vp.bottom-vp.top)*vert;
		const zone={
			top:vp.top+ext_h,
			bottom:vp.bottom-ext_h,
			left:vp.left+ext_w,
			right:vp.right-ext_w
		};
		// is center of the element in strike zone?
		return(
			(center.y>=zone.top)&&
			(center.y<=zone.bottom)&&
			(center.x>=zone.left)&&
			(center.x<=zone.right)
		);
	};

	function action(){
		document.removeEventListener("scroll",checkVisibility);
		if(callback){callback();}
	};

	function checkVisibility(){
		if(!b_run){
			const er=getElementRect(el_control);
			const vp=getViewPort();
			if(inZone(er,vp)){
				b_run=true;
				action();
			}
		}
	};

	function run(px){
		// elements
		el_control=px.el_control;
		// parameters
		callback=px.callback;
		// variables
		b_run=false;
		// set scroll event handler
		document.addEventListener("scroll",checkVisibility);
		// check visibility
		checkVisibility();
	};

	return{
		run
	};
};