// Simple generic form validator, based on Example 15-2 of O'Reilly's "Javascript - The
// Definitive Guide".
//
// Field properties understood here:
//   optional - field is optional
//   numeric  - field is numeric
//   min      - field is numeric and this is the minimum value
//   max      - field is numeric and this is the maximum value
//   email    - field should be a valid E-mail address
//
// A utility function that returns true if a string contains only 
// whitespace characters.
function isblank(s) {
    for(var i = 0; i < s.length; i++) {
        var c = s.charAt(i);
        if ((c != ' ') && (c != '\n') && (c != '\t')) return false;
    }
    return true;
}

// This is the function that performs form verification. It is invoked
// from the onsubmit event handler. The handler should return whatever
// value this function returns.
function form_validate(f) {
    var msg;
    var empty_fields = "";
    var errors = "";

    // Loop through the elements of the form, looking for all 
    // text and textarea elements that don't have an "optional" property
    // defined. Then, check for fields that are empty and make a list of them.
    // Also, if any of these elements have a "min" or a "max" property defined,
    // verify that they are numbers and in the right range.
    // If the element has a "numeric" property defined, verify that
    // it is a number, but don't check its range.
    // Put together error messages for fields that are wrong.
    for (var i = 0; i < f.length; i++) {
        var e = f.elements[i];
        if ((e.type == "text") || (e.type == "textarea")) {
			if (!e.optional) {
            	// first check if the field is empty
            	if ((e.value == null) || (e.value == "") || isblank(e.value)) {
                	empty_fields += "\n          " + e.name;
                	continue;
            	}

            	// Now check for fields that are supposed to be numeric.
            	if (e.numeric || (e.min != null) || (e.max != null)) { 
                	var v = parseFloat(e.value);
                	if (isNaN(v) || 
                    	((e.min != null) && (v < e.min)) || 
                    	((e.max != null) && (v > e.max))) {
                    	errors += "- The field " + e.name + " must be a number";
                    	if (e.min != null) 
                        	errors += " that is greater than " + e.min;
                    	if (e.max != null && e.min != null) 
                        	errors += " and less than " + e.max;
                    	else if (e.max != null)
                        	errors += " that is less than " + e.max;
                    	errors += ".\n";
                	}
					continue;
            	}
			}

			// If an E-mail address field is non-empty, it must be valid.
			if (e.email && (e.value != null) && ((e.value != "") || !isblank(e.value))) {
			    var at = 0;
				var dot = 0;
				var val = e.value;
			    for (var j = 0; j < val.length; j++) {
				    var c = val.charAt(j);
					if (c == '@') {
					    at++;
					}
					if (c == '.') {
						dot++;
					}
				}
			
				if ((at != 1) || (dot < 1)) {
					errors += "- The field " + e.name + " must be a valid E-mail address.\n";
				}
			}
        }
    }

    // Now, if there were any errors, display the messages, and
    // return false to prevent the form from being submitted. 
    // Otherwise return true.
    if (!empty_fields && !errors) return true;

    msg  = "______________________________________________________\n\n"
    msg += "The form was not submitted because of the following error(s).\n";
    msg += "Please correct these error(s) and re-submit.\n";
    msg += "______________________________________________________\n\n"

    if (empty_fields) {
        msg += "- The following required field(s) are empty:" 
                + empty_fields + "\n";
        if (errors) msg += "\n";
    }
    msg += errors;
    alert(msg);
    return false;
}