import threading
from types import SimpleNamespace

DEFAULT_INTERVAL = 0.05

gil_mutex = threading.RLock()
gil_condition = threading.Condition(lock=gil_mutex)
switch_condition = threading.Condition()

# dictionary-like object that supports dot (attribute) syntax
gil = SimpleNamespace(
    drop_request=False,
    locked=True,
    switch_number=0,
    last_holder=None,
    eval_breaker=True
)



def drop_gil(thread_id):
    if not gil.locked:
        raise Exception("GIL is not locked")

    gil_mutex.acquire()

    gil.last_holder = thread_id
    gil.locked = False

    # Signals that the GIL is now available for acquiring to the first awaiting thread
    gil_condition.notify()

    gil_mutex.release()

    # force switching
    # Lock current thread so it will not immediately reacquire the GIL
    # this ensures that another GIL-awaiting thread have a chance to get scheduled

    if gil.drop_request:
        switch_condition.acquire()
        if gil.last_holder == thread_id:
            gil.drop_request = False
            switch_condition.wait()

        switch_condition.release()


def take_gil(thread_id):
    gil_mutex.acquire()

    while gil.locked:
        saved_switchnum = gil.switch_number

        # Release the lock and wait for a signal from a GIL holding thread,
        # set drop_request=True if the wait is timed out

        timed_out = not gil_condition.wait(timeout=DEFAULT_INTERVAL)

        if timed_out and gil.locked and gil.switch_number == saved_switchnum:
            gil.drop_request = True

    # lock for force switching
    switch_condition.acquire()

    # Now we hold the GIL
    gil.locked = True

    if gil.last_holder != thread_id:
        gil.last_holder = thread_id
        gil.switch_number += 1

    # force switching, send signal to drop_gil
    switch_condition.notify()
    switch_condition.release()

    if gil.drop_request:
        gil.drop_request = False

    gil_mutex.release()


def execution_loop(target_function, thread_id):
    # Compile Python function down to bytecode and execute it in the while loop

    bytecode = compile(target_function)

    while True:

        # drop_request indicates that one or more threads are awaiting for the GIL
        if gil.drop_request:
            # release the gil from the current thread
            drop_gil(thread_id)

            # immediately request the GIL for the current thread
            # at this point the thread will be waiting for GIL and suspended until the function return
            take_gil(thread_id)

        # bytecode execution logic, executes one instruction at a time
        instruction = bytecode.next_instruction()
        if instruction is not None:
            execute_opcode(instruction)
        else:
            return

/*
   Notes about the implementation:

   - The GIL is just a boolean variable (locked) whose access is protected
     by a mutex (gil_mutex), and whose changes are signalled by a condition
     variable (gil_cond). gil_mutex is taken for short periods of time,
     and therefore mostly uncontended.

   - In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
     able to release the GIL on demand by another thread. A volatile boolean
     variable (gil_drop_request) is used for that purpose, which is checked
     at every turn of the eval loop. That variable is set after a wait of
     `interval` microseconds on `gil_cond` has timed out.

      [Actually, another volatile boolean variable (eval_breaker) is used
       which ORs several conditions into one. Volatile booleans are
       sufficient as inter-thread signalling means since Python is run
       on cache-coherent architectures only.]

   - A thread wanting to take the GIL will first let pass a given amount of
     time (`interval` microseconds) before setting gil_drop_request. This
     encourages a defined switching period, but doesn't enforce it since
     opcodes can take an arbitrary time to execute.

     The `interval` value is available for the user to read and modify
     using the Python API `sys.{get,set}switchinterval()`.

   - When a thread releases the GIL and gil_drop_request is set, that thread
     ensures that another GIL-awaiting thread gets scheduled.
     It does so by waiting on a condition variable (switch_cond) until
     the value of last_holder is changed to something else than its
     own thread state pointer, indicating that another thread was able to
     take the GIL.

     This is meant to prohibit the latency-adverse behaviour on multi-core
     machines where one thread would speculatively release the GIL, but still
     run and end up being the first to re-acquire it, making the "timeslices"
     much longer than expected.
     (Note: this mechanism is enabled with FORCE_SWITCHING above)
*/