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use core::cell::UnsafeCell;
use core::fmt;
use core::mem::MaybeUninit;
use core::ops::{Deref, DerefMut};
use core::sync::atomic::{AtomicBool, Ordering};

pub struct LazyInit<T> {
    inited: AtomicBool,
    data: UnsafeCell<MaybeUninit<T>>,
}

unsafe impl<T: Send + Sync> Sync for LazyInit<T> {}
unsafe impl<T: Send> Send for LazyInit<T> {}

#[allow(dead_code)]
impl<T> LazyInit<T> {
    pub const fn new() -> Self {
        Self {
            inited: AtomicBool::new(false),
            data: UnsafeCell::new(MaybeUninit::uninit()),
        }
    }

    pub fn init_by(&self, data: T) {
        assert!(!self.is_init());
        unsafe { (*self.data.get()).as_mut_ptr().write(data) };
        self.inited.store(true, Ordering::Release);
    }

    pub fn init_by_call<F: FnOnce(&mut T)>(&self, f: F) {
        assert!(!self.is_init());
        f(self.get_mut_unchecked());
        self.inited.store(true, Ordering::Release);
    }

    pub fn is_init(&self) -> bool {
        self.inited.load(Ordering::Acquire)
    }

    fn try_get(&self) -> Option<&T> {
        if self.is_init() {
            unsafe { Some(&*(*self.data.get()).as_ptr()) }
        } else {
            None
        }
    }

    fn get(&self) -> &T {
        assert!(self.is_init());
        unsafe { &*(*self.data.get()).as_ptr() }
    }

    fn get_mut(&mut self) -> &mut T {
        assert!(self.is_init());
        unsafe { &mut *(*self.data.get()).as_mut_ptr() }
    }

    #[allow(clippy::mut_from_ref)]
    fn get_mut_unchecked(&self) -> &mut T {
        unsafe { &mut *(*self.data.get()).as_mut_ptr() }
    }
}

impl<T: fmt::Debug> fmt::Debug for LazyInit<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.try_get() {
            Some(s) => write!(f, "LazyInit {{ data: ")
                .and_then(|()| s.fmt(f))
                .and_then(|()| write!(f, "}}")),
            None => write!(f, "LazyInit {{ <uninitialized> }}"),
        }
    }
}

impl<T> Deref for LazyInit<T> {
    type Target = T;
    fn deref(&self) -> &T {
        self.get()
    }
}

impl<T> DerefMut for LazyInit<T> {
    fn deref_mut(&mut self) -> &mut T {
        self.get_mut()
    }
}

impl<T> Drop for LazyInit<T> {
    fn drop(&mut self) {
        if self.is_init() {
            unsafe { core::ptr::drop_in_place((*self.data.get()).as_mut_ptr()) };
        }
    }
}