//! AVL Platform async runtime - ZERO external dependencies
//!
//! Custom async runtime using:
//! - std::thread for thread pool
//! - epoll/kqueue/IOCP for async I/O
//! - Hierarchical timer wheel for timeouts

use std::collections::VecDeque;
use std::future::Future;
use std::pin::Pin;
use std::sync::{Arc, Mutex, Condvar};
use std::task::{Context, Poll, Waker};
use std::time::{Duration, Instant};
use std::thread;

use crate::reactor::Reactor;
use crate::timer::GlobalTimerWheel;

/// Global runtime instance
thread_local! {
    static RUNTIME: Runtime = Runtime::new();
}

/// AVL Platform async runtime
pub struct Runtime {
    thread_pool: Arc<ThreadPool>,
    reactor: Arc<Mutex<Reactor>>,
    timer_wheel: Arc<GlobalTimerWheel>,
}

impl Runtime {
    /// Create new runtime
    pub fn new() -> Self {
        let cpu_count = thread::available_parallelism()
            .map(|n| n.get())
            .unwrap_or(4);

        let reactor = Arc::new(Mutex::new(Reactor::new().expect("Failed to create reactor")));
        let timer_wheel = Arc::new(GlobalTimerWheel::new());

        // Spawn reactor thread
        let reactor_clone = Arc::clone(&reactor);
        let timer_clone = Arc::clone(&timer_wheel);
        thread::spawn(move || {
            let mut events = Vec::with_capacity(1024);
            loop {
                // Poll reactor for I/O events
                {
                    events.clear();
                    let mut r = reactor_clone.lock().unwrap();
                    match r.wait(&mut events, Some(Duration::from_millis(1))) {
                        Ok(_) => {
                            r.wake_events(&events);
                        }
                        Err(e) => {
                            eprintln!("Reactor wait error: {:?}", e);
                        }
                    }
                }

                // Tick timer wheel
                timer_clone.tick();

                // Small sleep to avoid busy loop
                thread::sleep(Duration::from_micros(100));
            }
        });

        Self {
            thread_pool: Arc::new(ThreadPool::new(cpu_count)),
            reactor,
            timer_wheel,
        }
    }

    /// Get reactor instance
    pub fn reactor(&self) -> Arc<Mutex<Reactor>> {
        Arc::clone(&self.reactor)
    }

    /// Get timer wheel instance
    pub fn timer_wheel(&self) -> Arc<GlobalTimerWheel> {
        Arc::clone(&self.timer_wheel)
    }

    /// Block on a future until completion
    pub fn block_on<F: Future>(&self, future: F) -> F::Output {
        let mut pinned = Box::pin(future);

        loop {
            // Create a dummy waker for now
            let waker = noop_waker();
            let mut cx = Context::from_waker(&waker);

            match pinned.as_mut().poll(&mut cx) {
                Poll::Ready(output) => return output,
                Poll::Pending => {
                    // Yield to other tasks
                    thread::yield_now();
                }
            }
        }
    }

    /// Spawn a task on the thread pool
    pub fn spawn<F>(&self, future: F)
    where
        F: Future<Output = ()> + Send + 'static,
    {
        self.thread_pool.spawn(future);
    }
}

impl Default for Runtime {
    fn default() -> Self {
        Self::new()
    }
}

/// Thread pool for executing async tasks
struct ThreadPool {
    workers: Vec<Worker>,
    sender: Arc<Mutex<VecDeque<Task>>>,
    condvar: Arc<Condvar>,
}

type Task = Pin<Box<dyn Future<Output = ()> + Send>>;

impl ThreadPool {
    fn new(size: usize) -> Self {
        let sender = Arc::new(Mutex::new(VecDeque::new()));
        let condvar = Arc::new(Condvar::new());

        let mut workers = Vec::with_capacity(size);

        for id in 0..size {
            workers.push(Worker::new(
                id,
                Arc::clone(&sender),
                Arc::clone(&condvar),
            ));
        }

        Self {
            workers,
            sender,
            condvar,
        }
    }

    fn spawn<F>(&self, future: F)
    where
        F: Future<Output = ()> + Send + 'static,
    {
        let task = Box::pin(future);

        {
            let mut queue = self.sender.lock().unwrap();
            queue.push_back(task);
        }

        self.condvar.notify_one();
    }
}

/// Worker thread
struct Worker {
    _thread: thread::JoinHandle<()>,
}

impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<VecDeque<Task>>>,
        condvar: Arc<Condvar>,
    ) -> Self {
        let thread = thread::spawn(move || {
            loop {
                let task = {
                    let mut queue = receiver.lock().unwrap();

                    while queue.is_empty() {
                        queue = condvar.wait(queue).unwrap();
                    }

                    queue.pop_front()
                };

                if let Some(mut task) = task {
                    // Poll the task
                    let waker = noop_waker();
                    let mut cx = Context::from_waker(&waker);

                    let _ = task.as_mut().poll(&mut cx);
                }
            }
        });

        Worker { _thread: thread }
    }
}

/// Create a no-op waker (temporary implementation)
fn noop_waker() -> Waker {
    use std::task::{RawWaker, RawWakerVTable};

    unsafe fn clone(_: *const ()) -> RawWaker {
        RawWaker::new(std::ptr::null(), &VTABLE)
    }

    unsafe fn wake(_: *const ()) {}
    unsafe fn wake_by_ref(_: *const ()) {}
    unsafe fn drop(_: *const ()) {}

    static VTABLE: RawWakerVTable = RawWakerVTable::new(clone, wake, wake_by_ref, drop);

    unsafe {
        Waker::from_raw(RawWaker::new(std::ptr::null(), &VTABLE))
    }
}

/// Sleep for a duration using timer wheel
pub fn sleep(duration: Duration) -> crate::timer::Sleep {
    crate::timer::sleep(duration)
}

/// Execute future on global runtime
pub fn block_on<F: Future>(future: F) -> F::Output {
    RUNTIME.with(|rt| rt.block_on(future))
}

/// Spawn task on global runtime
pub fn spawn<F>(future: F)
where
    F: Future<Output = ()> + Send + 'static,
{
    RUNTIME.with(|rt| rt.spawn(future));
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_runtime_creation() {
        let runtime = Runtime::new();
        assert!(runtime.thread_pool.workers.len() > 0);
    }

    #[test]
    fn test_block_on() {
        let runtime = Runtime::new();

        let result = runtime.block_on(async {
            42
        });

        assert_eq!(result, 42);
    }
}
