function addToSelection(id_prn, o) {
	if (o.checked) {
		var num = 0;
		for ( var i = 0; i< selprn.length; i++) {
			if (selprn[i]) 
				num++;
		}
		if (num == 3) {
			alert(toomanyprods);
			o.checked = false;
			return;
		} else {
			selprn[id_prn] = 1;
		}
	} else {
		selprn[id_prn] = 0;
	}
}

var checkboxClick = false;

function updateSelection(id_prn) {
    el = document.getElementById('checkbox_'+id_prn);
	if(el.checked) {
		addToSelection(id_prn, el);
	}
}

function goEndCompare() {
	var str = "";var cnt = 0;
	for (var i = 0; i< selprn.length; i++) {
		if (selprn[i] == 1) {
			str += "&id_prn[]="+i;
			cnt++;
		}
	}
	if (cnt < 2 || cnt > 12) {
		alert (compareInvalid)
		return;
	}
	str = "?c=1"+str;
	window.location = "endcompare.php"+str;
}