var secondsPerFrame;
var unit = "px";

var start = function () {
    //Remove previous errors
    $('.error').remove();
    //Grab input
    var startUnit, endUnit, startTime, endTime, accelMod;
    startUnit = Number($('.start-unit').val());
    endUnit = Number($('.end-unit').val());
    startTime = Number($('.start-time').val());
    endTime = Number($('.end-time').val());
    accelMod = Number($('.accel-mod').val());
    secondsPerFrame = Math.pow($('.fps').val(), -1);
    unit = $('.unit:selected').val();
    //Check constraints
	if (meetsConstraints(startUnit, endUnit, startTime, endTime, accelMod)){
	    $('button').attr('disabled', 'disabled');
		animate(startUnit, endUnit, startTime, endTime, accelMod)
    }
}

var meetsConstraints = function (startUnit, endUnit, startTime, endTime, accelMod) {
    var isUndefined = (startUnit === "" || endUnit === "" || startTime === "" || endTime === "" || accelMod === "");
    var isNonNumber = (isNaN(secondsPerFrame) || isNaN(startUnit) || isNaN(endUnit) || isNaN(startTime) || isNaN(endTime) || isNaN(accelMod));
    var variableConstraints = (startUnit < 0 || startTime < 0 || startTime > endTime || startUnit > endUnit || startTime == endTime || secondsPerFrame > endTime || secondsPerFrame < 0.01);
    if (isUndefined || isNonNumber || variableConstraints) {
        error("Your input does not meet the constraints");
        return false;
    }
    return true;
}

var animate = function (a, b, c, d, n) {
    //Starts at 1 to ensure that x doesnt equal zero when n < 1 (constraint)
    var secondTracker = 1;
    c += 1;
    d += 1;
    //Start loop
    animation = setInterval(function () {
        secondTracker += secondsPerFrame;
        //When we hit the end time... stop
        if (secondTracker >= d) {
            stopAnimation();
            return;
        }
        //When we hit the start time... start
        if (secondTracker >= c) {
            $('.box').css("left", animationFunction(a, b, c, d, n, secondTracker) + unit);
        }
    }, secondsPerFrame * 1000);
};

/*
    a = Starting Unit
    b = Ending Unit
    c = Animation Start (in seconds)
    d = Animation End (in seconds)
    n = Rate at which the element moves
    x is the current time in the animation 
*/
var animationFunction = function (a, b, c, d, n, x) {
    var x = Math.pow(x, n);
    var m = (b - a) / (Math.pow(d, n) - Math.pow(c, n));
    var k = (a * Math.pow(d, n) - b * Math.pow(c, n)) / (Math.pow(d, n) - Math.pow(c, n));
    return m * x + k
};

var stopAnimation = function () {
    clearInterval(animation);
    $('button').removeAttr('disabled');
};

var error = function (msg) {
    $('body').append("<p class='error'>" + msg + "</p>")
}