use crate::runtime::blocking::BlockingPool;
use crate::runtime::scheduler::CurrentThread;
use crate::runtime::{context, EnterGuard, Handle};
use crate::task::JoinHandle;

use std::future::Future;
use std::time::Duration;

cfg_rt_multi_thread! {
    use crate::runtime::Builder;
    use crate::runtime::scheduler::MultiThread;
}

/// The Tokio runtime.
///
/// The runtime provides an I/O driver, task scheduler, [timer], and
/// blocking pool, necessary for running asynchronous tasks.
///
/// Instances of `Runtime` can be created using [`new`], or [`Builder`].
/// However, most users will use the `#[tokio::main]` annotation on their
/// entry point instead.
///
/// See [module level][mod] documentation for more details.
///
/// # Shutdown
///
/// Shutting down the runtime is done by dropping the value. The current
/// thread will block until the shut down operation has completed.
///
/// * Drain any scheduled work queues.
/// * Drop any futures that have not yet completed.
/// * Drop the reactor.
///
/// Once the reactor has dropped, any outstanding I/O resources bound to
/// that reactor will no longer function. Calling any method on them will
/// result in an error.
///
/// # Sharing
///
/// The Tokio runtime implements `Sync` and `Send` to allow you to wrap it
/// in a `Arc`. Most fn take `&self` to allow you to call them concurrently
/// across multiple threads.
///
/// Calls to `shutdown` and `shutdown_timeout` require exclusive ownership of
/// the runtime type and this can be achieved via `Arc::try_unwrap` when only
/// one strong count reference is left over.
///
/// [timer]: crate::time
/// [mod]: index.html
/// [`new`]: method@Self::new
/// [`Builder`]: struct@Builder
#[derive(Debug)]
pub struct Runtime {
    /// Task scheduler
    scheduler: Scheduler,

    /// Handle to runtime, also contains driver handles
    handle: Handle,

    /// Blocking pool handle, used to signal shutdown
    blocking_pool: BlockingPool,
}

/// The flavor of a `Runtime`.
///
/// This is the return type for [`Handle::runtime_flavor`](crate::runtime::Handle::runtime_flavor()).
#[derive(Debug, PartialEq, Eq)]
#[non_exhaustive]
pub enum RuntimeFlavor {
    /// The flavor that executes all tasks on the current thread.
    CurrentThread,
    /// The flavor that executes tasks across multiple threads.
    MultiThread,
}

/// The runtime scheduler is either a multi-thread or a current-thread executor.
#[derive(Debug)]
pub(super) enum Scheduler {
    /// Execute all tasks on the current-thread.
    CurrentThread(CurrentThread),

    /// Execute tasks across multiple threads.
    #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
    MultiThread(MultiThread),
}

impl Runtime {
    pub(super) fn from_parts(
        scheduler: Scheduler,
        handle: Handle,
        blocking_pool: BlockingPool,
    ) -> Runtime {
        Runtime {
            scheduler,
            handle,
            blocking_pool,
        }
    }

    cfg_not_wasi! {
        /// Creates a new runtime instance with default configuration values.
        ///
        /// This results in the multi threaded scheduler, I/O driver, and time driver being
        /// initialized.
        ///
        /// Most applications will not need to call this function directly. Instead,
        /// they will use the  [`#[tokio::main]` attribute][main]. When a more complex
        /// configuration is necessary, the [runtime builder] may be used.
        ///
        /// See [module level][mod] documentation for more details.
        ///
        /// # Examples
        ///
        /// Creating a new `Runtime` with default configuration values.
        ///
        /// ```
        /// use tokio::runtime::Runtime;
        ///
        /// let rt = Runtime::new()
        ///     .unwrap();
        ///
        /// // Use the runtime...
        /// ```
        ///
        /// [mod]: index.html
        /// [main]: ../attr.main.html
        /// [threaded scheduler]: index.html#threaded-scheduler
        /// [runtime builder]: crate::runtime::Builder
        #[cfg(feature = "rt-multi-thread")]
        #[cfg_attr(docsrs, doc(cfg(feature = "rt-multi-thread")))]
        pub fn new() -> std::io::Result<Runtime> {
            Builder::new_multi_thread().enable_all().build()
        }
    }

    /// Returns a handle to the runtime's spawner.
    ///
    /// The returned handle can be used to spawn tasks that run on this runtime, and can
    /// be cloned to allow moving the `Handle` to other threads.
    ///
    /// Calling [`Handle::block_on`] on a handle to a `current_thread` runtime is error-prone.
    /// Refer to the documentation of [`Handle::block_on`] for more.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::runtime::Runtime;
    ///
    /// let rt = Runtime::new()
    ///     .unwrap();
    ///
    /// let handle = rt.handle();
    ///
    /// // Use the handle...
    /// ```
    pub fn handle(&self) -> &Handle {
        &self.handle
    }

    /// Spawns a future onto the Tokio runtime.
    ///
    /// This spawns the given future onto the runtime's executor, usually a
    /// thread pool. The thread pool is then responsible for polling the future
    /// until it completes.
    ///
    /// The provided future will start running in the background immediately
    /// when `spawn` is called, even if you don't await the returned
    /// `JoinHandle`.
    ///
    /// See [module level][mod] documentation for more details.
    ///
    /// [mod]: index.html
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::runtime::Runtime;
    ///
    /// # fn dox() {
    /// // Create the runtime
    /// let rt = Runtime::new().unwrap();
    ///
    /// // Spawn a future onto the runtime
    /// rt.spawn(async {
    ///     println!("now running on a worker thread");
    /// });
    /// # }
    /// ```
    #[track_caller]
    pub fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
    where
        F: Future + Send + 'static,
        F::Output: Send + 'static,
    {
        self.handle.spawn(future)
    }

    /// Runs the provided function on an executor dedicated to blocking operations.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::runtime::Runtime;
    ///
    /// # fn dox() {
    /// // Create the runtime
    /// let rt = Runtime::new().unwrap();
    ///
    /// // Spawn a blocking function onto the runtime
    /// rt.spawn_blocking(|| {
    ///     println!("now running on a worker thread");
    /// });
    /// # }
    #[track_caller]
    pub fn spawn_blocking<F, R>(&self, func: F) -> JoinHandle<R>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        self.handle.spawn_blocking(func)
    }

    /// Runs a future to completion on the Tokio runtime. This is the
    /// runtime's entry point.
    ///
    /// This runs the given future on the current thread, blocking until it is
    /// complete, and yielding its resolved result. Any tasks or timers
    /// which the future spawns internally will be executed on the runtime.
    ///
    /// # Multi thread scheduler
    ///
    /// When the multi thread scheduler is used this will allow futures
    /// to run within the io driver and timer context of the overall runtime.
    ///
    /// Any spawned tasks will continue running after `block_on` returns.
    ///
    /// # Current thread scheduler
    ///
    /// When the current thread scheduler is enabled `block_on`
    /// can be called concurrently from multiple threads. The first call
    /// will take ownership of the io and timer drivers. This means
    /// other threads which do not own the drivers will hook into that one.
    /// When the first `block_on` completes, other threads will be able to
    /// "steal" the driver to allow continued execution of their futures.
    ///
    /// Any spawned tasks will be suspended after `block_on` returns. Calling
    /// `block_on` again will resume previously spawned tasks.
    ///
    /// # Panics
    ///
    /// This function panics if the provided future panics, or if called within an
    /// asynchronous execution context.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::runtime::Runtime;
    ///
    /// // Create the runtime
    /// let rt  = Runtime::new().unwrap();
    ///
    /// // Execute the future, blocking the current thread until completion
    /// rt.block_on(async {
    ///     println!("hello");
    /// });
    /// ```
    ///
    /// [handle]: fn@Handle::block_on
    #[track_caller]
    pub fn block_on<F: Future>(&self, future: F) -> F::Output {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let future = crate::util::trace::task(
            future,
            "block_on",
            None,
            crate::runtime::task::Id::next().as_u64(),
        );

        let _enter = self.enter();

        match &self.scheduler {
            Scheduler::CurrentThread(exec) => exec.block_on(&self.handle.inner, future),
            #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
            Scheduler::MultiThread(exec) => exec.block_on(&self.handle.inner, future),
        }
    }

    /// Enters the runtime context.
    ///
    /// This allows you to construct types that must have an executor
    /// available on creation such as [`Sleep`] or [`TcpStream`]. It will
    /// also allow you to call methods such as [`tokio::spawn`].
    ///
    /// [`Sleep`]: struct@crate::time::Sleep
    /// [`TcpStream`]: struct@crate::net::TcpStream
    /// [`tokio::spawn`]: fn@crate::spawn
    ///
    /// # Example
    ///
    /// ```
    /// use tokio::runtime::Runtime;
    ///
    /// fn function_that_spawns(msg: String) {
    ///     // Had we not used `rt.enter` below, this would panic.
    ///     tokio::spawn(async move {
    ///         println!("{}", msg);
    ///     });
    /// }
    ///
    /// fn main() {
    ///     let rt = Runtime::new().unwrap();
    ///
    ///     let s = "Hello World!".to_string();
    ///
    ///     // By entering the context, we tie `tokio::spawn` to this executor.
    ///     let _guard = rt.enter();
    ///     function_that_spawns(s);
    /// }
    /// ```
    pub fn enter(&self) -> EnterGuard<'_> {
        self.handle.enter()
    }

    /// Shuts down the runtime, waiting for at most `duration` for all spawned
    /// task to shutdown.
    ///
    /// Usually, dropping a `Runtime` handle is sufficient as tasks are able to
    /// shutdown in a timely fashion. However, dropping a `Runtime` will wait
    /// indefinitely for all tasks to terminate, and there are cases where a long
    /// blocking task has been spawned, which can block dropping `Runtime`.
    ///
    /// In this case, calling `shutdown_timeout` with an explicit wait timeout
    /// can work. The `shutdown_timeout` will signal all tasks to shutdown and
    /// will wait for at most `duration` for all spawned tasks to terminate. If
    /// `timeout` elapses before all tasks are dropped, the function returns and
    /// outstanding tasks are potentially leaked.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::runtime::Runtime;
    /// use tokio::task;
    ///
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// fn main() {
    ///    let runtime = Runtime::new().unwrap();
    ///
    ///    runtime.block_on(async move {
    ///        task::spawn_blocking(move || {
    ///            thread::sleep(Duration::from_secs(10_000));
    ///        });
    ///    });
    ///
    ///    runtime.shutdown_timeout(Duration::from_millis(100));
    /// }
    /// ```
    pub fn shutdown_timeout(mut self, duration: Duration) {
        // Wakeup and shutdown all the worker threads
        self.handle.inner.shutdown();
        self.blocking_pool.shutdown(Some(duration));
    }

    /// Shuts down the runtime, without waiting for any spawned tasks to shutdown.
    ///
    /// This can be useful if you want to drop a runtime from within another runtime.
    /// Normally, dropping a runtime will block indefinitely for spawned blocking tasks
    /// to complete, which would normally not be permitted within an asynchronous context.
    /// By calling `shutdown_background()`, you can drop the runtime from such a context.
    ///
    /// Note however, that because we do not wait for any blocking tasks to complete, this
    /// may result in a resource leak (in that any blocking tasks are still running until they
    /// return.
    ///
    /// This function is equivalent to calling `shutdown_timeout(Duration::from_nanos(0))`.
    ///
    /// ```
    /// use tokio::runtime::Runtime;
    ///
    /// fn main() {
    ///    let runtime = Runtime::new().unwrap();
    ///
    ///    runtime.block_on(async move {
    ///        let inner_runtime = Runtime::new().unwrap();
    ///        // ...
    ///        inner_runtime.shutdown_background();
    ///    });
    /// }
    /// ```
    pub fn shutdown_background(self) {
        self.shutdown_timeout(Duration::from_nanos(0))
    }
}

#[allow(clippy::single_match)] // there are comments in the error branch, so we don't want if-let
impl Drop for Runtime {
    fn drop(&mut self) {
        match &mut self.scheduler {
            Scheduler::CurrentThread(current_thread) => {
                // This ensures that tasks spawned on the current-thread
                // runtime are dropped inside the runtime's context.
                let _guard = context::try_set_current(&self.handle.inner);
                current_thread.shutdown(&self.handle.inner);
            }
            #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
            Scheduler::MultiThread(multi_thread) => {
                // The threaded scheduler drops its tasks on its worker threads, which is
                // already in the runtime's context.
                multi_thread.shutdown(&self.handle.inner);
            }
        }
    }
}

cfg_metrics! {
    impl Runtime {
        /// TODO
        pub fn metrics(&self) -> crate::runtime::RuntimeMetrics {
            self.handle.metrics()
        }
    }
}