"""A generally useful event scheduler class.

Each instance of this class manages its own queue.

No multi-threading is implied; you are supposed to hack that

yourself, or use a single instance per application.

Each instance is parametrized with two functions, one that is

supposed to return the current time, one that is supposed to

implement a delay.  You can implement real-time scheduling by

substituting time and sleep from built-in module time, or you can

implement simulated time by writing your own functions.  This can

also be used to integrate scheduling with STDWIN events; the delay

function is allowed to modify the queue.  Time can be expressed as

integers or floating point numbers, as long as it is consistent.

Events are specified by tuples (time, priority, action, argument).

As in UNIX, lower priority numbers mean higher priority; in this

way the queue can be maintained as a priority queue.  Execution of the

event means calling the action function, passing it the argument.

Remember that in Python, multiple function arguments can be packed

in a tuple.   The action function may be an instance method so it

has another way to reference private data (besides global variables).

Parameterless functions or methods cannot be used, however.

"""

# XXX The timefunc and delayfunc should have been defined as methods

# XXX so you can define new kinds of schedulers using subclassing

# XXX instead of having to define a module or class just to hold

# XXX the global state of your particular time and delay functions.

import heapq

__all__ = ["scheduler"]

class scheduler:

    def __init__(self, timefunc, delayfunc):

        """Initialize a new instance, passing the time and delay

        functions"""

        self.queue = []

        self.timefunc = timefunc

        self.delayfunc = delayfunc

    def enterabs(self, time, priority, action, argument):

        """Enter a new event in the queue at an absolute time.

        Returns an ID for the event which can be used to remove it,

        if necessary.

        """

        event = time, priority, action, argument

        heapq.heappush(self.queue, event)

        return event # The ID

    def enter(self, delay, priority, action, argument):

        """A variant that specifies the time as a relative time.

        This is actually the more commonly used interface.

        """

        time = self.timefunc() + delay

        return self.enterabs(time, priority, action, argument)

    def cancel(self, event):

        """Remove an event from the queue.

        This must be presented the ID as returned by enter().

        If the event is not in the queue, this raises RuntimeError.

        """

        self.queue.remove(event)

        heapq.heapify(self.queue)

    def empty(self):

        """Check whether the queue is empty."""

        return not self.queue

    def run(self):

        """Execute events until the queue is empty.

        When there is a positive delay until the first event, the

        delay function is called and the event is left in the queue;

        otherwise, the event is removed from the queue and executed

        (its action function is called, passing it the argument).  If

        the delay function returns prematurely, it is simply

        restarted.

        It is legal for both the delay function and the action

        function to to modify the queue or to raise an exception;

        exceptions are not caught but the scheduler's state remains

        well-defined so run() may be called again.

        A questionably hack is added to allow other threads to run:

        just after an event is executed, a delay of 0 is executed, to

        avoid monopolizing the CPU when other threads are also

        runnable.

        """

        # localize variable access to minimize overhead

        # and to improve thread safety

        q = self.queue

        delayfunc = self.delayfunc

        timefunc = self.timefunc

        pop = heapq.heappop

        while q:

            time, priority, action, argument = checked_event = q[0]

            now = timefunc()

            if now < time:

                delayfunc(time - now)

            else:

                event = pop(q)

                # Verify that the event was not removed or altered

                # by another thread after we last looked at q[0].

                if event is checked_event:

                    void = action(*argument)

                    delayfunc(0)   # Let other threads run

                else:

                    heapq.heappush(event)