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use core::{
    ops::{Deref, DerefMut},
    mem::{MaybeUninit, transmute},
    ptr::{drop_in_place, write},
};

/// Guard-struct used for correctly initialize uninitialized memory and `drop` it when guard goes out of scope.

/// Usually, you *should not* use this struct to handle your memory.

///

/// ### Safety

///

/// If you use this struct manually, remember: `&mut [MaybeUninit<T>]`'s content will be overwriten while initialization.

/// So it is *not safe* to apply this struct to already initialized data and it can lead to *memory leaks*.

///

/// ### Example

/// ```rust

/// use inplace_it::SliceMemoryGuard;

/// use std::mem::MaybeUninit;

///

/// // Placing uninitialized memory

/// let mut memory: [MaybeUninit<usize>; 100] = unsafe { MaybeUninit::uninit().assume_init() };

/// // Initializing guard

/// let mut memory_guard = unsafe {

///     SliceMemoryGuard::new(

///         // Borrowing memory

///         &mut memory,

///         // Forwarding initializer

///         |index| index * 2

///     )

/// };

///

/// // For now, memory contains content like [0, 2, 4, 6, ..., 196, 198]

///

/// // Using memory

/// // Sum of [0, 2, 4, 6, ..., 196, 198] = sum of [0, 1, 2, 3, ..., 98, 99] * 2 = ( 99 * (99+1) ) / 2 * 2

/// let sum: usize = memory_guard.iter().sum();

/// assert_eq!(sum, 99 * 100);

///

/// ```

pub struct SliceMemoryGuard<'a, T> {
    memory: &'a mut [MaybeUninit<T>],
}

impl<'a, T> SliceMemoryGuard<'a, T> {
    /// Initialize memory guard

    #[inline]
    pub unsafe fn new(memory: &'a mut [MaybeUninit<T>], mut init: impl FnMut(usize) -> T) -> Self {
        for (index, item) in memory.into_iter().enumerate() {
            write(item.as_mut_ptr(), init(index));
        }
        SliceMemoryGuard { memory }
    }
}

impl<'a, T> Deref for SliceMemoryGuard<'a, T> {
    type Target = [T];

    #[inline]
    fn deref(&self) -> &Self::Target {
        unsafe { transmute::<&[MaybeUninit<T>], &[T]>(&self.memory) }
    }
}

impl<'a, T> DerefMut for SliceMemoryGuard<'a, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut Self::Target {
        unsafe { transmute::<&mut [MaybeUninit<T>], &mut [T]>(&mut self.memory) }
    }
}

impl<'a, T> Drop for SliceMemoryGuard<'a, T> {
    #[inline]
    fn drop(&mut self) {
        for item in self.memory.into_iter() {
            unsafe { drop_in_place(item.as_mut_ptr()); }
        }
    }
}