//! Utility functions for interacting with any runtime.

#[cfg(test)]
use crate::Runner;
use crate::{Metrics, Spawner};
#[cfg(test)]
use futures::stream::{FuturesUnordered, StreamExt};
use rayon::{ThreadPool as RThreadPool, ThreadPoolBuildError, ThreadPoolBuilder};
use std::{
    any::Any,
    future::Future,
    pin::Pin,
    sync::Arc,
    task::{Context, Poll},
};

pub mod buffer;
pub mod signal;

mod handle;
pub use handle::Handle;

/// Yield control back to the runtime.
pub async fn reschedule() {
    struct Reschedule {
        yielded: bool,
    }

    impl Future for Reschedule {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            if self.yielded {
                Poll::Ready(())
            } else {
                self.yielded = true;
                cx.waker().wake_by_ref();
                Poll::Pending
            }
        }
    }

    Reschedule { yielded: false }.await
}

fn extract_panic_message(err: &(dyn Any + Send)) -> String {
    if let Some(s) = err.downcast_ref::<&str>() {
        s.to_string()
    } else if let Some(s) = err.downcast_ref::<String>() {
        s.clone()
    } else {
        format!("{err:?}")
    }
}

/// A clone-able wrapper around a [rayon]-compatible thread pool.
pub type ThreadPool = Arc<RThreadPool>;

/// Creates a clone-able [rayon]-compatible thread pool with [Spawner::spawn_blocking].
///
/// # Arguments
/// - `context`: The runtime context implementing the [Spawner] trait.
/// - `concurrency`: The number of tasks to execute concurrently in the pool.
///
/// # Returns
/// A `Result` containing the configured [rayon::ThreadPool] or a [rayon::ThreadPoolBuildError] if the pool cannot be built.
pub fn create_pool<S: Spawner + Metrics>(
    context: S,
    concurrency: usize,
) -> Result<ThreadPool, ThreadPoolBuildError> {
    let pool = ThreadPoolBuilder::new()
        .num_threads(concurrency)
        .spawn_handler(move |thread| {
            // Tasks spawned in a thread pool are expected to run longer than any single
            // task and thus should be provisioned as a dedicated thread.
            context
                .with_label("rayon-thread")
                .spawn_blocking(true, move |_| thread.run());
            Ok(())
        })
        .build()?;

    Ok(Arc::new(pool))
}

/// Async reader–writer lock.
///
/// Powered by [async_lock::RwLock], `RwLock` provides both fair writer acquisition
/// and `try_read` / `try_write` without waiting (without any runtime-specific dependencies).
///
/// Usage:
/// ```rust
/// use commonware_runtime::{Spawner, Runner, deterministic, RwLock};
///
/// let executor = deterministic::Runner::default();
/// executor.start(|context| async move {
///     // Create a new RwLock
///     let lock = RwLock::new(2);
///
///     // many concurrent readers
///     let r1 = lock.read().await;
///     let r2 = lock.read().await;
///     assert_eq!(*r1 + *r2, 4);
///
///     // exclusive writer
///     drop((r1, r2));
///     let mut w = lock.write().await;
///     *w += 1;
/// });
/// ```
pub struct RwLock<T>(async_lock::RwLock<T>);

/// Shared guard returned by [RwLock::read].
pub type RwLockReadGuard<'a, T> = async_lock::RwLockReadGuard<'a, T>;

/// Exclusive guard returned by [RwLock::write].
pub type RwLockWriteGuard<'a, T> = async_lock::RwLockWriteGuard<'a, T>;

impl<T> RwLock<T> {
    /// Create a new lock.
    #[inline]
    pub const fn new(value: T) -> Self {
        Self(async_lock::RwLock::new(value))
    }

    /// Acquire a shared read guard.
    #[inline]
    pub async fn read(&self) -> RwLockReadGuard<'_, T> {
        self.0.read().await
    }

    /// Acquire an exclusive write guard.
    #[inline]
    pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
        self.0.write().await
    }

    /// Try to get a read guard without waiting.
    #[inline]
    pub fn try_read(&self) -> Option<RwLockReadGuard<'_, T>> {
        self.0.try_read()
    }

    /// Try to get a write guard without waiting.
    #[inline]
    pub fn try_write(&self) -> Option<RwLockWriteGuard<'_, T>> {
        self.0.try_write()
    }

    /// Get mutable access without locking (requires `&mut self`).
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        self.0.get_mut()
    }

    /// Consume the lock, returning the inner value.
    #[inline]
    pub fn into_inner(self) -> T {
        self.0.into_inner()
    }
}

#[cfg(test)]
async fn task(i: usize) -> usize {
    for _ in 0..5 {
        reschedule().await;
    }
    i
}

#[cfg(test)]
pub fn run_tasks(tasks: usize, runner: crate::deterministic::Runner) -> (String, Vec<usize>) {
    runner.start(|context| async move {
        // Randomly schedule tasks
        let mut handles = FuturesUnordered::new();
        for i in 0..=tasks - 1 {
            handles.push(context.clone().spawn(move |_| task(i)));
        }

        // Collect output order
        let mut outputs = Vec::new();
        while let Some(result) = handles.next().await {
            outputs.push(result.unwrap());
        }
        assert_eq!(outputs.len(), tasks);
        (context.auditor().state(), outputs)
    })
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{deterministic, tokio, Metrics};
    use commonware_macros::test_traced;
    use rayon::iter::{IntoParallelRefIterator, ParallelIterator};

    #[test_traced]
    fn test_create_pool() {
        let executor = tokio::Runner::default();
        executor.start(|context| async move {
            // Create a thread pool with 4 threads
            let pool = create_pool(context.with_label("pool"), 4).unwrap();

            // Create a vector of numbers
            let v: Vec<_> = (0..10000).collect();

            // Use the thread pool to sum the numbers
            pool.install(|| {
                assert_eq!(v.par_iter().sum::<i32>(), 10000 * 9999 / 2);
            });
        });
    }

    #[test_traced]
    fn test_rwlock() {
        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            // Create a new RwLock
            let lock = RwLock::new(100);

            // many concurrent readers
            let r1 = lock.read().await;
            let r2 = lock.read().await;
            assert_eq!(*r1 + *r2, 200);

            // exclusive writer
            drop((r1, r2)); // all readers must go away
            let mut w = lock.write().await;
            *w += 1;

            // Check the value
            assert_eq!(*w, 101);
        });
    }
}