// SPDX-License-Identifier: Apache-2.0 OR MIT

// inspired by https://github.com/nbdd0121/pin-init/blob/trunk/examples/pthread_mutex.rs
#![allow(clippy::undocumented_unsafe_blocks)]
#![cfg_attr(feature = "alloc", feature(allocator_api))]
#[cfg(not(windows))]
mod pthread_mtx {
    #[cfg(feature = "alloc")]
    use core::alloc::AllocError;
    use core::{
        cell::UnsafeCell,
        marker::PhantomPinned,
        mem::MaybeUninit,
        ops::{Deref, DerefMut},
        pin::Pin,
    };
    use pin_init::*;
    use std::convert::Infallible;

    #[pin_data(PinnedDrop)]
    pub struct PThreadMutex<T> {
        #[pin]
        raw: UnsafeCell<libc::pthread_mutex_t>,
        data: UnsafeCell<T>,
        #[pin]
        pin: PhantomPinned,
    }

    unsafe impl<T: Send> Send for PThreadMutex<T> {}
    unsafe impl<T: Send> Sync for PThreadMutex<T> {}

    #[pinned_drop]
    impl<T> PinnedDrop for PThreadMutex<T> {
        fn drop(self: Pin<&mut Self>) {
            unsafe {
                libc::pthread_mutex_destroy(self.raw.get());
            }
        }
    }

    #[derive(Debug)]
    pub enum Error {
        #[expect(dead_code)]
        IO(std::io::Error),
        Alloc,
    }

    impl From<Infallible> for Error {
        fn from(e: Infallible) -> Self {
            match e {}
        }
    }

    #[cfg(feature = "alloc")]
    impl From<AllocError> for Error {
        fn from(_: AllocError) -> Self {
            Self::Alloc
        }
    }

    impl<T> PThreadMutex<T> {
        pub fn new(data: T) -> impl PinInit<Self, Error> {
            fn init_raw() -> impl PinInit<UnsafeCell<libc::pthread_mutex_t>, Error> {
                let init = |slot: *mut UnsafeCell<libc::pthread_mutex_t>| {
                    // we can cast, because `UnsafeCell` has the same layout as T.
                    let slot: *mut libc::pthread_mutex_t = slot.cast();
                    let mut attr = MaybeUninit::uninit();
                    let attr = attr.as_mut_ptr();
                    // SAFETY: ptr is valid
                    let ret = unsafe { libc::pthread_mutexattr_init(attr) };
                    if ret != 0 {
                        return Err(Error::IO(std::io::Error::from_raw_os_error(ret)));
                    }
                    // SAFETY: attr is initialized
                    let ret = unsafe {
                        libc::pthread_mutexattr_settype(attr, libc::PTHREAD_MUTEX_NORMAL)
                    };
                    if ret != 0 {
                        // SAFETY: attr is initialized
                        unsafe { libc::pthread_mutexattr_destroy(attr) };
                        return Err(Error::IO(std::io::Error::from_raw_os_error(ret)));
                    }
                    // SAFETY: slot is valid
                    unsafe { slot.write(libc::PTHREAD_MUTEX_INITIALIZER) };
                    // SAFETY: attr and slot are valid ptrs and attr is initialized
                    let ret = unsafe { libc::pthread_mutex_init(slot, attr) };
                    // SAFETY: attr was initialized
                    unsafe { libc::pthread_mutexattr_destroy(attr) };
                    if ret != 0 {
                        return Err(Error::IO(std::io::Error::from_raw_os_error(ret)));
                    }
                    Ok(())
                };
                // SAFETY: mutex has been initialized
                unsafe { pin_init_from_closure(init) }
            }
            try_pin_init!(Self {
            data: UnsafeCell::new(data),
            raw <- init_raw(),
            pin: PhantomPinned,
        }? Error)
        }

        pub fn lock(&self) -> PThreadMutexGuard<'_, T> {
            // SAFETY: raw is always initialized
            unsafe { libc::pthread_mutex_lock(self.raw.get()) };
            PThreadMutexGuard { mtx: self }
        }
    }

    pub struct PThreadMutexGuard<'a, T> {
        mtx: &'a PThreadMutex<T>,
    }

    impl<T> Drop for PThreadMutexGuard<'_, T> {
        fn drop(&mut self) {
            // SAFETY: raw is always initialized
            unsafe { libc::pthread_mutex_unlock(self.mtx.raw.get()) };
        }
    }

    impl<T> Deref for PThreadMutexGuard<'_, T> {
        type Target = T;

        fn deref(&self) -> &Self::Target {
            unsafe { &*self.mtx.data.get() }
        }
    }

    impl<T> DerefMut for PThreadMutexGuard<'_, T> {
        fn deref_mut(&mut self) -> &mut Self::Target {
            unsafe { &mut *self.mtx.data.get() }
        }
    }
}

#[cfg_attr(test, test)]
fn main() {
    #[cfg(all(any(feature = "std", feature = "alloc"), not(windows)))]
    {
        use core::pin::Pin;
        use pin_init::*;
        use pthread_mtx::*;
        use std::{
            sync::Arc,
            thread::{sleep, Builder},
            time::Duration,
        };
        let mtx: Pin<Arc<PThreadMutex<usize>>> = Arc::try_pin_init(PThreadMutex::new(0)).unwrap();
        let mut handles = vec![];
        let thread_count = 20;
        let workload = 1_000_000;
        for i in 0..thread_count {
            let mtx = mtx.clone();
            handles.push(
                Builder::new()
                    .name(format!("worker #{i}"))
                    .spawn(move || {
                        for _ in 0..workload {
                            *mtx.lock() += 1;
                        }
                        println!("{i} halfway");
                        sleep(Duration::from_millis((i as u64) * 10));
                        for _ in 0..workload {
                            *mtx.lock() += 1;
                        }
                        println!("{i} finished");
                    })
                    .expect("should not fail"),
            );
        }
        for h in handles {
            h.join().expect("thread panicked");
        }
        println!("{:?}", &*mtx.lock());
        assert_eq!(*mtx.lock(), workload * thread_count * 2);
    }
}