function is_empty(field_value) {
  if ((field_value.length == 0) || (field_value == null)){
    return true;
  }
  else {
    return false;
  }
}

function validate_email(email) {
  var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;

  if (!filter.test(email)) {
    return false;
  }

  return true;
}

function validate_form(check_name, check_email, check_cr, check_message) {
		if (check_name == 1 && is_empty(document.getElementById('name').value)) {
		  alert('Please enter your name.');
		  return false;
		}
		else if (check_email == 1 && is_empty(document.getElementById('email').value)) {
		  alert('Please enter your email address.');
		  return false;
		}
		else if (check_cr == 1 && is_empty(document.getElementById('cr').value)) {
		  alert('Please calculate and enter the correct amount.');
		  return false;
		}
		else if (check_message == 1 && is_empty(document.getElementById('message').value)) {
		  alert('Please enter your message.');
		  return false;
		}

		if (check_email == 1 && !validate_email(document.getElementById('email').value)) {
      alert('Please correct your email address.');
	    return false;
	  }
		else if (check_cr == 1 && !is_empty(document.getElementById('cr').value)) {
		  var result = parseInt(document.getElementById('cr1').innerHTML) + parseInt(document.getElementById('cr2').innerHTML);

		  if (result != parseInt(document.getElementById('cr').value)) {
		    alert('Please calculate and enter the correct amount.');
		    return false;
		  }
		}

		return true;
}