var Verlet = function(Width, Height)
{
    this.Masses = [];
    this.Constraints = [];

    this.Width = Width;
    this.Height = Height;

    this.AddMass = function(Mass)
    {
        this.Masses.push(Mass);
        return Mass;
    }

    this.AddConstraint = function(Constraint)
    {
        this.Constraints.push(Constraint);
    }

    this.Update = function(TimeDelta)
    {
        for (k in this.Constraints)
        {
            var c = this.Constraints[k];
            if (typeof(c) == "function")
                continue;

            c.Update(TimeDelta);
        }

        for (k in this.Masses)
        {
            var m = this.Masses[k];
            if (typeof(m) == "function")
                continue;

            m.Update(TimeDelta, {X: this.Width, Y: this.Height});
        }
    }
}
var Mass = function(X, Y, Weight)
{
    this.Weight = Weight;
    this.PX = this.X = X;
    this.PY = this.Y = Y;
    this.Active = true;
    this.Update = function(TimeDelta, Bounds)
    {
        if (!this.Active)
            return;
        // do a verlet step
        var VX = this.X - this.PX;
        var VY = this.Y - this.PY;
        this.PX = this.X;
        this.PY = this.Y;

        // shitty air friction
        if (VY > 0)
            VY = VY - TimeDelta
        else
            VY = VY + TimeDelta
            
        if (VX > 0)
            VX = VX - TimeDelta
        else
            VX = VX + TimeDelta


        // add gravity (mgt^2/2)
        VY = VY + (this.Weight * (9.98 * (TimeDelta*TimeDelta)) / 2) * 100;

        // add speed to X, Y
        this.X = this.X + VX;
        this.Y = this.Y + VY;
        // console.log(this);

        // limit to bounds
        if (this.Y < 0)
        {
            this.Y = 0;
            this.VY = 0;
        }
        if (this.X < 0)
        {
            this.X = 0;
            this.VX = 0;
        }
        if (this.Y > Bounds.Y)
        {
            this.Y = Bounds.Y;
            this.VY = 0;
        }
        if (this.X > Bounds.X)
        {
            this.X = Bounds.X;
            this.VX = 0;
        }
    }
}

var Constraint = function(Mass1, Mass2)
{
    this.Mass1 = Mass1;
    this.Mass2 = Mass2;
    this.Distance = Math.sqrt(Math.pow(Mass1.X - Mass2.X, 2) + Math.pow(Mass1.Y - Mass2.Y, 2));

    this.Update = function(TimeDelta)
    {
        var d1 = Math.sqrt(Math.pow(this.Mass1.X - this.Mass2.X, 2) + Math.pow(this.Mass1.Y - this.Mass2.Y, 2));
        var d2 = (d1 - this.Distance) / d1;

        var coefm1;
        var coefm2;
        if (!this.Mass1.Active)
        {
            coefm1 = 0;
            coefm2 = 1;
        }
        else if (!this.Mass2.Active)
        {
            coefm1 = 1;
            coefm2 = 0;
        }
        else
        {
            coefm1 = (this.Mass1.Weight / (this.Mass1.Weight + this.Mass2.Weight));
            coefm2 = (this.Mass2.Weight / (this.Mass1.Weight + this.Mass2.Weight));
        }

        this.Mass1.X = this.Mass1.X - (this.Mass1.X - this.Mass2.X) * d2 * coefm1;
        this.Mass2.X = this.Mass2.X + (this.Mass1.X - this.Mass2.X) * d2 * coefm2;
        this.Mass1.Y = this.Mass1.Y - (this.Mass1.Y - this.Mass2.Y) * d2 * coefm1;
        this.Mass2.Y = this.Mass2.Y + (this.Mass1.Y - this.Mass2.Y) * d2 * coefm2;
    }
}

var v = new Verlet(800, 600);
var m1 = v.AddMass(new Mass(5, 5, 1));
var m2 = v.AddMass(new Mass(55, 100, 1));
var m3 = v.AddMass(new Mass(105, 5, 1));
v.AddConstraint(new Constraint(m1, m2));
v.AddConstraint(new Constraint(m2, m3));
v.AddConstraint(new Constraint(m1, m3));

var c = document.getElementById("main");
var ctx = c.getContext('2d');
c.width = 800;
c.height = 600;

setInterval(function()
{
    ctx.fillStyle = '#000000';
    ctx.beginPath();
    ctx.rect(0, 0, 800, 600);
    ctx.closePath();
    ctx.fill();

    v.Update(1/50);

    ctx.fillStyle = '#FFFFFF';
    for (k in v.Masses)
    {
        var m = v.Masses[k];
        if (typeof(m) == "function")
            continue;

        ctx.beginPath();
        ctx.arc(m.X, m.Y, 5, 0, Math.PI*2, true);
        ctx.closePath();
        ctx.fill();
    }
    ctx.strokeStyle = '#FFFFFF';
    for (k in v.Constraints)
    {
        var c = v.Constraints[k];
        if (typeof(c) == "function")
            continue;

        ctx.beginPath();
        ctx.moveTo(c.Mass1.X, c.Mass1.Y);
        ctx.lineTo(c.Mass2.X, c.Mass2.Y);
        ctx.stroke();
    }
}, 1000/50);


var DragID = -1;
var DragOffsetX = 0;
var DragOffsetY = 0;
var DragMoveListener = function(e)
{
    var MouseX = e.layerX - c.offsetLeft;
    var MouseY = e.layerY - c.offsetTop;
    var m = v.Masses[DragID]
    m.PX = m.X;
    m.PY = m.Y;
    m.X = MouseX + DragOffsetX;
    m.Y = MouseY + DragOffsetY;
}
var DragEndListener = function(e)
{
    v.Masses[DragID].Active = true;
    DragID = -1;
    c.removeEventListener("mouseup", DragEndListener);
    c.removeEventListener("mousemove", DragMoveListener);
    c.addEventListener("mousedown", DragStartListener);
}
var DragStartListener = function(e)
{
    var MouseX = e.layerX - c.offsetLeft;
    var MouseY = e.layerY - c.offsetTop;
    for (k in v.Masses)
    {
        var m = v.Masses[k];
        if (typeof(m) == "function")
            continue;

        var DX = MouseX - m.X;
        var DY = MouseY - m.Y;
        if (Math.sqrt((DX*DX) + (DY*DY)) < 5)
        {
            DragID = k;
            DragOffsetX = DX;
            DragOffsetY = DY;
            m.Active = false;
            c.removeEventListener("mousedown", DragStartListener);
            c.addEventListener("mousemove", DragMoveListener);
            c.addEventListener("mouseup", DragEndListener);
        }
    }
}
c.addEventListener("mousedown", DragStartListener);