import js.dom

fn get_canvas(elem JS.HTMLElement) JS.HTMLCanvasElement {
    match elem {
        JS.HTMLCanvasElement {
            return elem
        }
        else {
            panic('Not a canvas')
        }
    }
}

fn draw_line(mut context JS.CanvasRenderingContext2D, x1 int, y1 int, x2 int, y2 int) {
    context.beginPath()
    context.strokeStyle = 'black'.str
    context.lineWidth = JS.Number(1)
    context.moveTo(x1, y1)
    context.lineTo(x2, y2)
    context.stroke()
    context.closePath()
}

struct DrawState {
mut:
    context JS.CanvasRenderingContext2D
    drawing bool
    x       int
    y       int
}

fn main() {
    window := dom.window()
    document := dom.document
    clear_btn := document.getElementById('clearButton'.str)?
    canvas_elem := document.getElementById('canvas'.str)?
    canvas := get_canvas(canvas_elem)
    ctx := canvas.getContext('2d'.str, js_undefined())?
    context := match ctx {
        JS.CanvasRenderingContext2D {
            ctx
        }
        else {
            panic('can not get 2d context')
        }
    }
    mut state := DrawState{context, false, 0, 0}

    canvas.addEventListener('mousedown'.str, fn [mut state] (event JS.Event) {
        state.drawing = true
        match event {
            JS.MouseEvent {
                state.x = int(event.offsetX)
                state.y = int(event.offsetY)
            }
            else {}
        }
    }, JS.EventListenerOptions{})
    canvas.addEventListener('mousemove'.str, fn [mut state] (event JS.Event) {
        if state.drawing {
            match event {
                JS.MouseEvent {
                    draw_line(mut state.context, state.x, state.y, int(event.offsetX),
                        int(event.offsetY))
                    state.x = int(event.offsetX)
                    state.y = int(event.offsetY)
                }
                else {}
            }
        }
    }, JS.EventListenerOptions{})

    window.addEventListener('mouseup'.str, fn [mut state] (event JS.Event) {
        if state.drawing {
            match event {
                JS.MouseEvent {
                    draw_line(mut state.context, state.x, state.y, int(event.offsetX),
                        int(event.offsetY))
                }
                else {}
            }
            state.x = 0
            state.y = 0
            state.drawing = false
        }
    }, JS.EventListenerOptions{})
    clear_btn.addEventListener('click'.str, fn [mut state, canvas] (_ JS.Event) {
        state.context.clearRect(0, 0, canvas.width, canvas.height)
    }, JS.EventListenerOptions{})
}