structure AppDraw =
struct
  type draw_object = {vertexBuffer: Word32.word, program: Word32.word}

  fun initDrawObject (vertexShaderString, fragmentShaderString) : draw_object =
    let
      val vertexBuffer = Gles3.createBuffer ()
      val vertexShader = Gles3.createShader (Gles3.VERTEX_SHADER ())
      val _ = Gles3.shaderSource (vertexShader, vertexShaderString)
      val _ = Gles3.compileShader vertexShader

      val fragmentBuffer = Gles3.createBuffer ()
      val fragmentShader = Gles3.createShader (Gles3.FRAGMENT_SHADER ())
      val _ = Gles3.shaderSource (fragmentShader, fragmentShaderString)
      val _ = Gles3.compileShader fragmentShader

      val program = Gles3.createProgram ()
      val _ = Gles3.attachShader (program, vertexShader)
      val _ = Gles3.attachShader (program, fragmentShader)
      val _ = Gles3.linkProgram program

      (* Flag shaders for deletion as we no longer need them 
       * once the program is linked. *)
    in
      {vertexBuffer = vertexBuffer, program = program}
    end

  fun initGraphLines () =
    let
      val graphDrawObject = initDrawObject
        (Constants.graphVertexShaderString, Constants.graphFragmentShaderString)
      val {vertexBuffer, program} = graphDrawObject

      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.bufferData (#[], 0, Gles3.STATIC_DRAW ())
      val _ = Gles3.vertexAttribPointer (0, 2, 2, 0)
      val _ = Gles3.enableVertexAttribArray 0
    in
      graphDrawObject
    end

  fun uploadGraphLines (graphDrawObject: draw_object, vec) =
    let
      val {vertexBuffer, ...} = graphDrawObject
      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.bufferData (vec, Vector.length vec, Gles3.STATIC_DRAW ())
    in
      ()
    end

  fun drawGraphLines (graphDrawObject: draw_object, graphDrawLength) =
    let
      val {vertexBuffer, program} = graphDrawObject
      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.vertexAttribPointer (0, 2, 2, 0)
      val _ = Gles3.enableVertexAttribArray 0
      val _ = Gles3.useProgram program
      val _ = Gles3.drawArrays (Gles3.TRIANGLES (), 0, graphDrawLength)
    in
      ()
    end

  fun initButton () =
    let
      val buttonDrawObject = initDrawObject
        ( Constants.colouredVertexShaderString
        , Constants.colouredFragmentShaderString
        )
      val {vertexBuffer, program} = buttonDrawObject

      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.bufferData (#[], 0, Gles3.STATIC_DRAW ())
      val _ = Gles3.vertexAttribPointer (0, 2, 5, 0)
      val _ = Gles3.enableVertexAttribArray 0

      val _ = Gles3.vertexAttribPointer (1, 3, 5, 8)
      val _ = Gles3.enableVertexAttribArray 1
    in
      buttonDrawObject
    end

  fun uploadButtonVector (buttonDrawObject: draw_object, vec) =
    let
      val {vertexBuffer, ...} = buttonDrawObject
      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.bufferData (vec, Vector.length vec, Gles3.STATIC_DRAW ())
    in
      ()
    end

  fun drawButton (buttonDrawObject: draw_object, buttonDrawLength) =
    if buttonDrawLength > 0 then
      let
        val {vertexBuffer, program} = buttonDrawObject
        val _ = Gles3.bindBuffer vertexBuffer
        val _ = Gles3.vertexAttribPointer (0, 2, 5, 0)
        val _ = Gles3.enableVertexAttribArray 0
        val _ = Gles3.vertexAttribPointer (1, 3, 5, 8)
        val _ = Gles3.enableVertexAttribArray 1
        val _ = Gles3.useProgram program
        val _ = Gles3.drawArrays (Gles3.TRIANGLES (), 0, buttonDrawLength)
      in
        ()
      end
    else
      ()

  fun initTriangles () =
    let
      val triangleDrawObject = initDrawObject
        (Constants.graphVertexShaderString, Constants.graphFragmentShaderString)
      val {vertexBuffer, program} = triangleDrawObject

      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.bufferData (#[], 0, Gles3.STATIC_DRAW ())
      val _ = Gles3.vertexAttribPointer (0, 2, 2, 0)
      val _ = Gles3.enableVertexAttribArray 0
    in
      triangleDrawObject
    end

  fun uploadTrianglesVector (triangleDrawObject: draw_object, vec) =
    let
      val {vertexBuffer, ...} = triangleDrawObject
      val _ = Gles3.bindBuffer vertexBuffer
      val _ = Gles3.bufferData (vec, Vector.length vec, Gles3.STATIC_DRAW ())
    in
      ()
    end

  fun drawTriangles (triangleDrawObject: draw_object, triangleDrawLength) =
    if triangleDrawLength > 0 then
      let
        val {vertexBuffer, program} = triangleDrawObject
        val _ = Gles3.bindBuffer vertexBuffer
        val _ = Gles3.vertexAttribPointer (0, 2, 2, 0)
        val _ = Gles3.enableVertexAttribArray 0
        val _ = Gles3.useProgram program
        val _ = Gles3.drawArrays (Gles3.TRIANGLES (), 0, triangleDrawLength)
      in
        ()
      end
    else
      ()
end