#
#        Asyncdispatch2 synchronization primitives
#
#           (c) Coprygith 2018 Eugene Kabanov
#  (c) Copyright 2018 Status Research & Development GmbH
#
#                Licensed under either of
#    Apache License, version 2.0, (LICENSE-APACHEv2)
#                MIT license (LICENSE-MIT)

## This module implements some core synchronization primitives
import asyncloop, deques

type
  AsyncLock* = ref object of RootRef
    ## A primitive lock is a synchronization primitive that is not owned by
    ## a particular coroutine when locked. A primitive lock is in one of two
    ## states, ``locked`` or ``unlocked``.
    ##
    ## When more than one coroutine is blocked in ``acquire()`` waiting for
    ## the state to turn to unlocked, only one coroutine proceeds when a
    ## ``release()`` call resets the state to unlocked; first coroutine which
    ## is blocked in ``acquire()`` is being processed.
    locked: bool
    waiters: Deque[Future[void]]

  AsyncEvent* = ref object of RootRef
    ## A primitive event object.
    ##
    ## An event manages a flag that can be set to `true` with the ``fire()``
    ## procedure and reset to `false` with the ``clear()`` procedure.
    ## The ``wait()`` coroutine blocks until the flag is `false`.
    ##
    ## If more than one coroutine blocked in ``wait()`` waiting for event
    ## state to be signaled, when event get fired, then all coroutines
    ## continue proceeds in order, they have entered waiting state.

    flag: bool
    waiters: Deque[Future[void]]

  AsyncQueue*[T] = ref object of RootRef
    ## A queue, useful for coordinating producer and consumer coroutines.
    ##
    ## If ``maxsize`` is less than or equal to zero, the queue size is
    ## infinite. If it is an integer greater than ``0``, then "await put()"
    ## will block when the queue reaches ``maxsize``, until an item is
    ## removed by "await get()".
    getters: Deque[Future[void]]
    putters: Deque[Future[void]]
    queue: Deque[T]
    maxsize: int

  AsyncQueueEmptyError* = object of Exception
    ## ``AsyncQueue`` is empty.
  AsyncQueueFullError* = object of Exception
    ## ``AsyncQueue`` is full.
  AsyncLockError* = object of Exception
    ## ``AsyncLock`` is either locked or unlocked.

proc newAsyncLock*(): AsyncLock =
  ## Creates new asynchronous lock ``AsyncLock``.
  ##
  ## Lock is created in the unlocked state. When the state is unlocked,
  ## ``acquire()`` changes the state to locked and returns immediately.
  ## When the state is locked, ``acquire()`` blocks until a call to
  ## ``release()`` in another coroutine changes it to unlocked.
  ##
  ## The ``release()`` procedure changes the state to unlocked and returns
  ## immediately.

  # Workaround for callSoon() not worked correctly before
  # getGlobalDispatcher() call.
  discard getGlobalDispatcher()
  result = new AsyncLock
  result.waiters = initDeque[Future[void]]()
  result.locked = false

proc acquire*(lock: AsyncLock) {.async.} =
  ## Acquire a lock ``lock``.
  ##
  ## This procedure blocks until the lock ``lock`` is unlocked, then sets it
  ## to locked and returns.
  if not lock.locked:
    lock.locked = true
  else:
    var w = newFuture[void]("asynclock.acquire")
    lock.waiters.addLast(w)
    yield w
    lock.locked = true

proc own*(lock: AsyncLock) =
  ## Acquire a lock ``lock``.
  ##
  ## This procedure not blocks, if ``lock`` is locked, then ``AsyncLockError``
  ## exception would be raised.
  if lock.locked:
    raise newException(AsyncLockError, "AsyncLock is already acquired!")
  lock.locked = true

proc locked*(lock: AsyncLock): bool =
  ## Return `true` if the lock ``lock`` is acquired, `false` otherwise.
  result = lock.locked

proc release*(lock: AsyncLock) =
  ## Release a lock ``lock``.
  ##
  ## When the ``lock`` is locked, reset it to unlocked, and return. If any
  ## other coroutines are blocked waiting for the lock to become unlocked,
  ## allow exactly one of them to proceed.
  var w: Future[void]
  if lock.locked:
    lock.locked = false
    while len(lock.waiters) > 0:
      w = lock.waiters.popFirst()
      if not w.finished:
        w.complete()
        break
  else:
    raise newException(AsyncLockError, "AsyncLock is not acquired!")

proc newAsyncEvent*(): AsyncEvent =
  ## Creates new asyncronous event ``AsyncEvent``.
  ##
  ## An event manages a flag that can be set to `true` with the `fire()`
  ## procedure and reset to `false` with the `clear()` procedure.
  ## The `wait()` procedure blocks until the flag is `true`. The flag is
  ## initially `false`.

  # Workaround for callSoon() not worked correctly before
  # getGlobalDispatcher() call.
  discard getGlobalDispatcher()
  result = new AsyncEvent
  result.waiters = initDeque[Future[void]]()
  result.flag = false

proc wait*(event: AsyncEvent) {.async.} =
  ## Block until the internal flag of ``event`` is `true`.
  ## If the internal flag is `true` on entry, return immediately. Otherwise,
  ## block until another task calls `fire()` to set the flag to `true`,
  ## then return.
  if event.flag:
    discard
  else:
    var w = newFuture[void]("asyncevent.wait")
    event.waiters.addLast(w)
    yield w

proc fire*(event: AsyncEvent) =
  ## Set the internal flag of ``event`` to `true`. All tasks waiting for it
  ## to become `true` are awakened. Task that call `wait()` once the flag is
  ## `true` will not block at all.
  var w: Future[void]
  if not event.flag:
    event.flag = true
    while len(event.waiters) > 0:
      w = event.waiters.popFirst()
      if not w.finished:
        w.complete()

proc clear*(event: AsyncEvent) =
  ## Reset the internal flag of ``event`` to `false`. Subsequently, tasks
  ## calling `wait()` will block until `fire()` is called to set the internal
  ## flag to `true` again.
  event.flag = false

proc isSet*(event: AsyncEvent): bool =
  ## Return `true` if and only if the internal flag of ``event`` is `true`.
  result = event.flag

proc newAsyncQueue*[T](maxsize: int = 0): AsyncQueue[T] =
  ## Creates a new asynchronous queue ``AsyncQueue``.

  # Workaround for callSoon() not worked correctly before
  # getGlobalDispatcher() call.
  discard getGlobalDispatcher()
  result = new AsyncQueue[T]
  result.getters = initDeque[Future[void]]()
  result.putters = initDeque[Future[void]]()
  result.queue = initDeque[T]()
  result.maxsize = maxsize

proc full*[T](aq: AsyncQueue[T]): bool {.inline.} =
  ## Return ``true`` if there are ``maxsize`` items in the queue.
  ##
  ## Note: If the ``aq`` was initialized with ``maxsize = 0`` (default),
  ## then ``full()`` is never ``true``.
  if aq.maxsize <= 0:
    result = false
  else:
    result = len(aq.queue) >= aq.maxsize

proc empty*[T](aq: AsyncQueue[T]): bool {.inline.} =
  ## Return ``true`` if the queue is empty, ``false`` otherwise.
  result = (len(aq.queue) == 0)

proc putNoWait*[T](aq: AsyncQueue[T], item: T) =
  ## Put an item into the queue ``aq`` immediately.
  ##
  ## If queue ``aq`` is full, then ``AsyncQueueFullError`` exception raised
  var w: Future[void]
  if aq.full():
    raise newException(AsyncQueueFullError, "AsyncQueue is full!")
  aq.queue.addLast(item)
  while len(aq.getters) > 0:
    w = aq.getters.popFirst()
    if not w.finished:
      w.complete()

proc getNoWait*[T](aq: AsyncQueue[T]): T =
  ## Remove and return ``item`` from the queue immediately.
  ##
  ## If queue ``aq`` is empty, then ``AsyncQueueEmptyError`` exception raised.
  var w: Future[void]
  if aq.empty():
    raise newException(AsyncQueueEmptyError, "AsyncQueue is empty!")
  result = aq.queue.popFirst()
  while len(aq.putters) > 0:
    w = aq.putters.popFirst()
    if not w.finished:
      w.complete()

proc put*[T](aq: AsyncQueue[T], item: T) {.async.} =
  ## Put an ``item`` into the queue ``aq``. If the queue is full, wait until
  ## a free slot is available before adding item.
  while aq.full():
    var putter = newFuture[void]("asyncqueue.putter")
    aq.putters.addLast(putter)
    yield putter
  aq.putNoWait(item)

proc get*[T](aq: AsyncQueue[T]): Future[T] {.async.} =
  ## Remove and return an item from the queue ``aq``.
  ##
  ## If queue is empty, wait until an item is available.
  while aq.empty():
    var getter = newFuture[void]("asyncqueue.getter")
    aq.getters.addLast(getter)
    yield getter
  result = aq.getNoWait()

proc len*[T](aq: AsyncQueue[T]): int {.inline.} =
  ## Return the number of elements in ``aq``.
  result = len(aq.queue)

proc size*[T](aq: AsyncQueue[T]): int {.inline.} =
  ## Return the maximum number of elements in ``aq``.
  result = len(aq.maxsize)