function getHTTPObject(){
    if (window.ActiveXObject)
    return new ActiveXObject("Microsoft.XMLHTTP");
    else if (window.XMLHttpRequest)
    return new XMLHttpRequest();
    else {
    alert("Your browser is ancient.  Get Chrome.");
    return null;
    }
}

function checkUsername(){
    if (document.getElementById('txtUsername').value.length == 0) {
    document.getElementById('tdUsername').innerHTML = '';
    }
    else if (document.getElementById('txtUsername').value.length < 4) {
    document.getElementById('tdUsername').innerHTML = '<img src="images/cross.png" alt="Invalid" />';
    } else {
    if (isAlphaNumeric(document.getElementById('txtUsername').value) == true) {
        httpObject = getHTTPObject();
        if (httpObject != null) {
        httpObject.open("GET", "backend/register.php?action=checkUsername&username=" + document.getElementById('txtUsername').value, true);
        httpObject.send(null);
        httpObject.onreadystatechange = setOutput;
        }
    } else {
        document.getElementById('tdUsername').innerHTML = '<img src="images/cross.png" alt="Invalid" />';
    }
    }
}

function checkPassword() {
    if (document.getElementById('txtPassword').value.length == 0) {
    document.getElementById('tdPassword').innerHTML = '';
    }
    else if (document.getElementById('txtPassword').value.length < 4) {
    document.getElementById('tdPassword').innerHTML = '<img src="images/cross.png" alt="Invalid" />';
    } else {
    document.getElementById('tdPassword').innerHTML = '<img src="images/tick.png" alt="Valid" />';
    }
}

function checkPasswordMatch() {
    if (document.getElementById('txtPasswordConfirm').value.length == 0) {
    document.getElementById('tdPasswordConfirm').innerHTML = '';
    }
    else if (document.getElementById('txtPasswordConfirm').value == document.getElementById('txtPassword').value) {
    document.getElementById('tdPasswordConfirm').innerHTML = '<img src="images/tick.png" alt="Valid" />';
    } else {
    document.getElementById('tdPasswordConfirm').innerHTML = '<img src="images/cross.png" alt="Invalid" />';
    }
}

function checkEmail() {
    if (document.getElementById('txtEmail').value.length == 0) {
    document.getElementById('tdEmail').innerHTML = '';
    }
    else if (isEmailAddress(document.getElementById('txtEmail').value) == true) {
    document.getElementById('tdEmail').innerHTML = '<img src="images/tick.png" alt="Valid" />';
    } else {
    document.getElementById('tdEmail').innerHTML = '<img src="images/cross.png" alt="Invalid" />';
    }
}

function registerUser() {
    httpObject = getHTTPObject();
    if (httpObject != null) {
    httpObject.open("GET", "backend/register.php?action=registerUser&username=" + document.getElementById('txtUsername').value + "&password=" + document.getElementById('txtPassword').value + "&emailAddress=" + document.getElementById('txtEmail').value, true);
    httpObject.send(null);
    httpObject.onreadystatechange = handleRegistration;
    }
}

function setOutput() {
    if(httpObject.readyState == 4) {
    if (httpObject.responseText == '1') {
        document.getElementById('tdUsername').innerHTML = '<img src="images/tick.png" alt="Valid" />';
    } else {
        document.getElementById('tdUsername').innerHTML = '<img src="images/cross.png" alt="Invalid" />';
    }

    }
}

function handleRegistration() {
    if(httpObject.readyState == 4) {
    if (httpObject.responseText == '1') {
        window.location = "/register2.php";
    } else if (httpObject.responseText == '0') {
        document.getElementById('tdUsername').innerHTML = 'Registration failed.';
    } else {
        document.getElementById('tdUsername').innerHTML = 'Server error.';
    }
    }
}

function isAlphaNumeric(val) {
    if (val.match(/^[a-zA-Z0-9]+$/)) {
    return true;
    } else {
    return false;
    }
}

function isEmailAddress(val) {
    if (val.match(/[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?/)) {
    return true;
    } else {
    return false;
    }
}