# threads.ludic — Job.parallel_for on real OS threads, with Sync.* made safe to share.
# Each assertion that holds prints its number, so a full run prints:
#   1 2 3 4 5 6 7
# A worker is a top-level function taking (i: int, ctx: pointer-like) that computes on what ctx
# points at; `fn name` passes it. Workers never touch the world or a list another thread can see.
program Threads {
  const N: int = 20000

  state ThreadsState {
    calls: int = 0        # an atomic handle, made on the main thread before any work
    lock: int = 0         # a mutex handle
  }

  # out[i] = i * i: every index written exactly once, by whichever thread claimed it
  function square(i: int, out: words) -> void { out[i] = i * i }

  # count the call atomically, and add i to a shared total under the mutex. A worker may read a
  # state (the handles in it) but never change one - every thread runs it at once - so the total
  # lives in what the worker is handed
  function tally(threads_st: ThreadsState, i: int, total: words) -> void {
    Sync.add(threads_st.calls, 1)
    Sync.lock(threads_st.lock)
    total[0] = total[0] + i
    Sync.unlock(threads_st.lock)
  }

  # remember which calls ran on a pool thread - each with a little work in it, so on a busy machine
  # the calling thread cannot claim every index before a worker wakes
  function placed(i: int, out: words) -> void {
    var spin = 0
    for k in 0 .. 2000 { spin = spin + k % 7 }
    if Job.is_worker() { out[i] = 1 + spin * 0 } else { out[i] = spin * 0 }
  }

  entry (threads_st: mut ThreadsState) {
    if Sync.cpu_count() >= 1 { print(1) }
    if not Job.is_worker() { print(2) }

    let sq = words(N)
    Job.parallel_for(N, fn square, sq)
    var right = true
    for i in 0 .. N { if sq[i] != i * i { right = false } }
    if right { print(3) }

    threads_st.calls = Sync.atomic()
    threads_st.lock = Sync.mutex()
    let total = words(1)
    Job.parallel_for(N, fn tally, total)
    if Sync.get(threads_st.calls) == N { print(4) }
    let want = N * (N - 1) / 2
    if total[0] == want { print(5) }

    let ran = words(N)
    Job.parallel_for(N, fn placed, ran)
    var pooled = 0
    for i in 0 .. N { pooled += ran[i] }
    # with more than one core some calls ran on workers; with one, all ran here
    if (Sync.cpu_count() > 1 and pooled > 0) or (Sync.cpu_count() == 1 and pooled == 0) { print(6) }

    Job.parallel_for(0, fn square, sq)       # nothing to do: returns at once
    let fn = 7                                # `fn` is still an ordinary name, before `and` too
    if 0 < fn and fn < 8 { print(fn) }
  }
}