require "crystal/pointer_pairing_heap"

# List of `Pointer(T)` to `T` structs.
#
# Internally wraps a `PointerPairingHeap(T)` and thus requires that `T`
# implements `PointerPairingHeap::Node`.
#
# Thread unsafe: parallel accesses must be protected!
#
# NOTE: this is a struct because it only wraps a const pointer to an object
# allocated in the heap.
struct Crystal::EventLoop::Timers(T)
  def initialize
    @heap = PointerPairingHeap(T).new
  end

  def empty? : Bool
    @heap.empty?
  end

  # Returns the time of the next ready timer (if any).
  def next_ready? : Time::Instant?
    if event = @heap.first?
      event.value.wake_at
    end
  end

  # Dequeues and yields each ready timer (their `#wake_at` is lower than
  # `Crystal::System::Time.instant`) from the oldest to the most recent (i.e. time
  # ascending).
  def dequeue_ready(& : Pointer(T) -> Nil) : Nil
    now = Crystal::System::Time.instant

    while event = @heap.first?
      break if event.value.wake_at > now
      @heap.shift?
      yield event
    end
  end

  # Add a new timer into the list. Returns true if it is the next ready timer.
  def add(event : Pointer(T)) : Bool
    @heap.add(event)
    @heap.first? == event
  end

  # Remove a timer from the list. Returns a tuple(dequeued, was_next_ready) of
  # booleans. The first bool tells whether the event was dequeued, in which case
  # the second one tells if it was the next ready event.
  def delete(event : Pointer(T)) : {Bool, Bool}
    if @heap.first? == event
      @heap.shift?
      {true, true}
    elsif event.value.heap_previous?
      @heap.delete(event)
      {true, false}
    else
      {false, false}
    end
  end
end
