let n = 10000000
let global = Array.init n (fun i -> 0)
let cond = Condition.create ()
let mutex = Mutex.create ()

let rec check_consistent accum i =
  if i >= 0 then
    check_consistent (accum + global.(i)) (i-1)
  else if accum <> 0 then begin
    Printf.eprintf "Array sum inconsistent: %d\n" accum;
    flush stderr;
    exit 1
  end

let rec update_array x i =
  if i < n then begin
    if i mod 2 = 0 then
      global.(i) <- x
    else
      global.(i) <- -x;
    update_array x (i+1)
  end

let start = ref false

let barrier () =
  (* start all threads at same time *)
  Mutex.lock mutex;
  while !start = false do
    Condition.wait cond mutex
  done;
  Mutex.unlock mutex

let worker i =
  barrier ();
  while true; do
    (* the transaction *)
    update_array i 0;
    (* end of transaction *)
    Printf.printf "worker %d OK\n" i;
    flush stdout;
  done

let verifier () =
  barrier ();
  while true; do
    check_consistent 0 (n-1);
    Printf.printf "Check OK\n";
    flush stdout;
  done

let () =
  let a = Array.init 16 (fun i -> Thread.create worker i) in
  let t = Thread.create verifier () in
  start := true;
  Condition.broadcast cond;
  Printf.eprintf "started\n";
  flush stderr;
  Array.iter Thread.join a;
  Thread.join t