open Js_of_ocaml

let (pixelRatio : float) =
  2 * Js.Unsafe.get Dom_html.window (Js.string "devicePixelRatio")
  |> float_of_int

class type xmlSerializer = object
  method serializeToString : Dom.element Js.t -> Js.js_string Js.t Js.meth
end

let (xmlSerializer : xmlSerializer Js.t Js.constr) =
  Js.Unsafe.global##._XMLSerializer

(* Extract an image from a svg element *)
let generate_png (svg_image : Dom_svg.svgElement Js.t) callback =
  let image = Dom_html.createImg Dom_html.document in

  image##.onload :=
    Dom_html.handler (fun _ev ->
        let canvas = Dom_html.createCanvas Dom_html.document in
        let context = canvas##getContext Dom_html._2d_ in
        let width = svg_image##.width##.baseVal##.value |> Js.to_float
        and height = svg_image##.height##.baseVal##.value |> Js.to_float in

        image##.width := Float.to_int @@ (pixelRatio *. width);
        image##.height := Float.to_int @@ (pixelRatio *. height);

        canvas##.width := Float.to_int @@ (pixelRatio *. width);
        canvas##.height := Float.to_int @@ (pixelRatio *. height);

        context##drawImage_withSize
          image (Js.number_of_float 0.0) (Js.number_of_float 0.0)
          (Js.number_of_float (pixelRatio *. width))
          (Js.number_of_float (pixelRatio *. height));

        callback @@ canvas##toDataURL_type (Js.string "image/png");
        Js._false);

  let xml = new%js xmlSerializer in
  let svg_xml = xml##serializeToString (svg_image :> Dom.element Js.t) in
  let svg_src =
    (Js.string "data:image/svg+xml;base64,")##concat
      (Dom_html.window##btoa (Js.unescape (Js.encodeURIComponent svg_xml)))
  in
  image##.src := svg_src