// Copyright 2023-Present Jonas Kruckenberg
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

use alloc::boxed::Box;
use core::ptr;
use core::sync::atomic::{AtomicPtr, Ordering};

pub(crate) struct AtomicCell<T> {
    data: AtomicPtr<T>,
}

// Safety: `AtomicCell` uses atomic swaps, so as long as the data is Send
// `AtomicCell` is Send
unsafe impl<T: Send> Send for AtomicCell<T> {}
// Safety: `AtomicCell` uses atomic swaps, so as long as the data is Send
// `AtomicCell` is Sync
unsafe impl<T: Send> Sync for AtomicCell<T> {}

impl<T> AtomicCell<T> {
    pub(crate) fn new(data: Option<Box<T>>) -> AtomicCell<T> {
        AtomicCell {
            data: AtomicPtr::new(to_raw(data)),
        }
    }

    pub(crate) fn swap(&self, val: Option<Box<T>>) -> Option<Box<T>> {
        let old = self.data.swap(to_raw(val), Ordering::AcqRel);
        from_raw(old)
    }

    pub(crate) fn set(&self, val: Box<T>) {
        let _ = self.swap(Some(val));
    }

    pub(crate) fn take(&self) -> Option<Box<T>> {
        self.swap(None)
    }
}

fn to_raw<T>(data: Option<Box<T>>) -> *mut T {
    data.map_or(ptr::null_mut(), Box::into_raw)
}

fn from_raw<T>(val: *mut T) -> Option<Box<T>> {
    if val.is_null() {
        None
    } else {
        // Safety: `from_raw` is only called on pointers created using `to_raw` which
        // are correctly boxed pointers
        Some(unsafe { Box::from_raw(val) })
    }
}

impl<T> Drop for AtomicCell<T> {
    fn drop(&mut self) {
        // Free any data still held by the cell
        let _ = self.take();
    }
}
