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use core::sync::atomic::{Ordering, AtomicU8};
use core::mem::MaybeUninit;
use core::ptr;

use std::io;

use tokio::runtime;

//Not set yet
const UNINITIALIZED: u8 = 0;
//Being set
const INITIALIZING: u8 = 1;
//Set
const INITIALIZED: u8 = 2;

static GLOBAL_GUARD: AtomicU8 = AtomicU8::new(UNINITIALIZED);
static mut TOKIO: MaybeUninit<runtime::Runtime> = MaybeUninit::uninit();

const RUNTIME_NOT_AVAIL: &str = "Runtime is not set";
const RUNTIME_TWICE: &str = "Settings runtime twice";
const RUNTIME_RUN: &str = "Runtime is already running";

mod runner {
    pub static MUTEX: crate::guard::WakerVar = crate::guard::WakerVar::new();
}

///Tokio runtime guard
///
///Runtime gets terminated as soon as guard goes out of scope
pub struct Runtime {
}

impl Default for Runtime {
    #[inline]
    fn default() -> Self {
        Self::from_rt(tokio::runtime::Runtime::new().unwrap())
    }
}

impl Runtime {
    ///Initializes runtime from existing tokio's Runtime.
    pub fn from_rt(runtime: runtime::Runtime) -> Self {
        match GLOBAL_GUARD.compare_and_swap(UNINITIALIZED, INITIALIZING, Ordering::Release) {
            UNINITIALIZED => unsafe {
                ptr::write(TOKIO.as_mut_ptr(), runtime);
                GLOBAL_GUARD.store(INITIALIZED, Ordering::SeqCst);
            },
            _ => panic!(RUNTIME_TWICE),
        }

        Self {
        }
    }

    #[inline]
    ///Creates new instance from provided builder
    ///
    ///This function must be called prior to any usage of runtime related functionality
    ///
    ///## Panics
    ///
    ///If runtime is already initialized.
    pub fn init(builder: &mut runtime::Builder) -> io::Result<Self> {
        let runtime = builder.build()?;
        Ok(Self::from_rt(runtime))
    }

    ///Spins Runtime
    ///
    ///Once called, can be called again only after `stop`.
    ///
    ///Blocks current thread, until `stop` is called.
    pub fn run() {
        if runner::MUTEX.is_armed() {
            panic!(RUNTIME_RUN);
        }

        match GLOBAL_GUARD.load(Ordering::Acquire) {
            INITIALIZED => unsafe {
                let tokio = &mut *TOKIO.as_mut_ptr();
                tokio.block_on(runner::MUTEX.lock());
            },
            _ => panic!(RUNTIME_NOT_AVAIL)
        }
    }

    ///Signals Runtime to terminate
    pub fn stop() {
        runner::MUTEX.wake();
    }
}

fn with_runtime<R, FN: FnOnce(&runtime::Runtime) -> R>(cb: FN) -> R {
    match GLOBAL_GUARD.load(Ordering::Acquire) {
        INITIALIZED => unsafe {
            cb(&*TOKIO.as_ptr())
        },
        _ => panic!(RUNTIME_NOT_AVAIL)
    }
}

impl Drop for Runtime {
    fn drop(&mut self) {
        match GLOBAL_GUARD.compare_and_swap(INITIALIZED, INITIALIZING, Ordering::Release) {
            INITIALIZED => unsafe {
                ptr::drop_in_place(TOKIO.as_mut_ptr());
                GLOBAL_GUARD.store(UNINITIALIZED, Ordering::SeqCst);
            },
            _ => panic!("Runtime is not set, but dropping global guard!"),
        }
    }
}

struct Ptr<T>(*mut T);

impl<T> Ptr<T> {
    #[inline(always)]
    fn write(&self, val: T) {
        unsafe {
            ptr::write(self.0, val)
        }
    }
}

unsafe impl<T> Send for Ptr<T> {}
unsafe impl<T> Sync for Ptr<T> {}

impl<F: core::future::Future> super::AutoRuntime for F {
    fn wait(self) -> Self::Output where Self: Send + core::future::Future + 'static, Self::Output: Send + 'static {
        let mut result = MaybeUninit::uninit();

        let blocking = crate::blocking::Block::new();
        let (signal, lock) = blocking.split();

        let result_ptr = Ptr(result.as_mut_ptr());

        with_runtime(|rt| rt.spawn(async move {
            result_ptr.write(self.await);
            signal.signal();
        }));

        lock.wait();

        unsafe {
            result.assume_init()
        }
    }

    #[inline]
    fn spawn(self) -> tokio::task::JoinHandle<Self::Output> where Self: Send + core::future::Future + 'static, Self::Output: Send + 'static {
        with_runtime(|rt| rt.spawn(self))
    }
}