open System
open System.Drawing
open System.Windows.Forms

let mainForm =
    let form = new Form(Text = "Ciao!", Width = 800, Height = 600)

    let pointsMasterList = ref []
    let pointsTempList = ref[]
    let mouseDown = ref 0
    let pen = ref(new Pen(Color.Black))

    let btnClear = new Button(Text = "Clear", Size = new Size(50, 20), Location = new Point(500, 500))
    let btnClose = new Button(Text = "Close", Size = new Size(50, 20), Location = new Point(500, 530))

    btnClear.Click
        |> Event.add(fun _ -> pointsTempList := []; pointsMasterList := []; form.Invalidate())

    btnClose.Click
        |> Event.add(fun _ -> exit 0)

    let mouseUp _ =
        printfn "Mouse uppin'"
        mouseDown := !mouseDown - 1
        if List.length !pointsTempList > 1 then
            let points = List.toArray !pointsTempList
            pointsMasterList :=
                (!pen, points) :: !pointsMasterList
        pointsTempList := []
        form.Invalidate()

    let mouseMove (e: MouseEventArgs) =
        pointsTempList := e.Location :: !pointsTempList
        form.Invalidate()

    let paint (e: PaintEventArgs) =
        if List.length !pointsTempList > 1 then
            e.Graphics.DrawLines
                (!pen, List.toArray !pointsTempList)

        !pointsMasterList
            |> List.iter (fun (pen, points) -> e.Graphics.DrawLines(pen, points))

    let (leftMouse, otherMouse) =
        form.MouseDown
            |> Event.partition(fun e -> e.Button = MouseButtons.Left)

    let (rightMouse, otherMouse) =
        otherMouse
            |> Event.partition(fun e -> e.Button = MouseButtons.Right)

    leftMouse
        |> Event.filter(fun _ -> !mouseDown = 0)
        |> Event.add(fun _ -> pen := new Pen(Color.Black))

    rightMouse
        |> Event.filter(fun _ -> !mouseDown = 0)
        |> Event.add(fun _ -> pen := new Pen(Color.Red))

    form.MouseDown
        |> Event.add(fun _ -> mouseDown := !mouseDown + 1; printfn "%d" !mouseDown)

    form.MouseUp
        |> Event.add mouseUp

    form.MouseMove
        |> Event.filter(fun _ -> !mouseDown > 0)
        |> Event.add mouseMove

    form.Paint
        |> Event.add paint

    form.Controls.Add(btnClear)
    form.Controls.Add(btnClose)

    form

[<STAThread>]
do Application.Run(mainForm)