open System.Drawing
open System
open System.IO
open System.Threading
open System.Diagnostics

let transformImage trans (path: string) =
    let from = new Bitmap(path)
    let cvs = new Bitmap(from.Width, from.Height)

    for i in 0..cvs.Height - 1 do
        for j in 0..cvs.Width - 1 do
            let p: Color = trans from i j
            cvs.SetPixel(j, i, p)

    cvs.Save(path + ".cvs.jpg")

let rand = new ThreadLocal<Random>(fun () -> new Random())

let grayscaleTrans (cvs: Bitmap) i j =
    let p = cvs.GetPixel(j, i)
    let h = (0.2126 * float p.R + 0.7152 * float p.G + 0.0722 * float p.B) / 255.0

    if rand.Value.NextDouble() < h
    then Color.White
    else Color.Black

let grayscale = transformImage grayscaleTrans

let batchProcess conv dir =
    Directory.EnumerateFiles(dir) |> Seq.iter conv

let asyncProcess conv dir =
    Directory.EnumerateFiles(dir)
        |> Seq.map (fun path -> async { conv path })
        |> Async.Parallel
        |> Async.RunSynchronously
        |> ignore

Directory.EnumerateFiles("input", "*.cvs.jpg") |> Seq.iter File.Delete

let t = Stopwatch.StartNew()
asyncProcess grayscale "input"
printf "asyncProcess %A" t.Elapsed

Directory.EnumerateFiles("input", "*.cvs.jpg") |> Seq.iter File.Delete

t.Restart()
batchProcess grayscale "input"
printf "batchProcess %A" t.Elapsed