function isEmpty(theField)	{
	if (theField.value=="" || theField.value=="@")		return true;
	else	return false;
}

function isMail(theField) {
// check if e-mail contains mandatory characters "@" & "."
	var delka=theField.value.length;
	check_1 = false;
	check_2 = false;
	for ( var i=0 ; i < delka; i++ )	{
		var CH = theField.value.substring (i, i+1 );
		if ( CH == "@")	{ check_1 = true;}
		if ( CH == ".")	{ check_2 = true;}
	}
	if (check_1==true && check_2==true) return true;
	else return false;
}

function DateInterval(FromDay, FromMonth, FromYear, ToDay, ToMonth, ToYear) {
	From = FromYear.value + FromMonth.value + FromDay.value;
	To = ToYear.value + ToMonth.value + ToDay.value;
	// alert (From + ' - ' + To); // DEBUG
	if (From > To) return false;
	else return true;
}

function isRequired(X)	{
// check if fields are not empty 
	var Empty = false; 
	var Mail = true;
	var CorrectDateInterval = true;
	var Text_1 = 'Please, fill in box \"';
	var Text_2 = '\". \nThank you.';

	if (isEmpty(X.name)==true)		{
		alert( Text_1 + "Name" + Text_2);
		Empty=true; 
	}
	if (Empty == false && isEmpty(X.surname)==true)		{
		alert( Text_1 + "Surname" + Text_2);
		Empty=true; 
	}
	if (Empty == false && (isEmpty(X.phone)==true && isEmpty(X.mobile)==true))		{
		alert("Please, fill in box \"Phone\" or \"Mobile\", so we can contact you. Thank you.");
		Empty=true; 
	}
	
	if (Empty == false && isEmpty(X.number_of_persons)==true)		{
		alert( Text_1 + "Number of guests" + Text_2);
		Empty=true; 
	}
	
	if (Empty == false && (isMail(X.email)==false && !isEmpty(X.email)) ) {
		alert("The field \"E-mail\" does not contain an e-mail address. Please check this field.\nThank you.");
		Mail=false; 
	}
	
	CorrectDateInterval = DateInterval(X.from_day, X.from_month, X.from_year, X.to_day, X.to_month, X.to_year);
	if (Empty == false && CorrectDateInterval==false) {
		alert ("The date of departure must be later then the date of arrival.\nPlease, check the contents of \"Departure date\" field.\nThank you.");
	}
	
	if ( Empty == false && Mail == true && CorrectDateInterval==true) return true;
	else return false;
}