open System
open System.Threading
open System.Diagnostics

let lock (elem : int ref) value =
  Monitor.Enter(elem);
   try
    if value > elem.Value then elem := value
      finally
       Monitor.Exit(elem)

let searchMaxInRange (arr:int[]) l r :int =
  let mutable res = Int32.MinValue
  for x=l to r do
   if arr.[x]>res then res <- arr.[x]
  res

let searchMax (arr:int[], threadNumber:int) =
  let length = arr.Length
  let res = ref arr.[0]
  let step = length / threadNumber
  let threadArray = Array.init threadNumber (fun i ->
      new Thread(ThreadStart(fun _ ->
          let threadRes = searchMaxInRange arr (i * step) ((i+1) * step - 1)
          lock res threadRes
          )))
  for t in threadArray do
    t.Start()
  for t in threadArray do
    t.Join()
  if (threadNumber*step < length) then
   lock res (searchMaxInRange arr (threadNumber*step)(length-1))
  printfn "%A" res.Value


[<EntryPoint>]
let main argv =
 let timer = new System.Diagnostics.Stopwatch()
 timer.Start()
 let n = 10000000
 let arr = Array.init n (fun i -> System.Random(0).Next(0, n))
 searchMax (arr, 1)
 printfn "%i" timer.ElapsedMilliseconds
 0 // return an integer exit code