open System.Drawing
open System.Drawing.Imaging
open System.IO
open System.Diagnostics


let applyTransform file (trans: Bitmap -> Bitmap) =
    let outFile = Path.GetFileNameWithoutExtension(file) + "-out.jpg";
    use inputImage = new Bitmap(file)
    use outputImage = trans inputImage
    outputImage.Save(outFile, ImageFormat.Jpeg)


let batchTransform directory (trans: Bitmap -> Bitmap) =
    let asyncApply file = async { applyTransform file trans }

    Directory.EnumerateFiles(directory, "*.jpg")
        |> Seq.map asyncApply
        |> Async.Parallel
        |> Async.RunSynchronously
        |> ignore

type ColorTransform = Color -> Color
type UVTransform = int * int -> int * int

let applyPixelTransform (colorTrans: ColorTransform) (uvTrans: UVTransform) (width: int) (height: int) (image: Bitmap) =
    let output = new Bitmap(width, height)

    for i in 0..(width - 1) do
        for j in 0..(height - 1) do
            output.SetPixel(i, j, image.GetPixel(i, j) |> colorTrans) |> ignore

    output


let var1_grayscalePixel (color: Color) =
    let x = (int color.R + int color.G + int color.B) / 3
    Color.FromArgb(x, x, x)

let var2_invertPixel (color: Color) =
    let inv x = 255 - int x
    Color.FromArgb(inv color.R, inv color.G, inv color.B)



let timer = Stopwatch.StartNew()
batchTransform "input" (applyPixelTransform var1_grayscalePixel)
printf "%A" timer.Elapsed