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    use core::fmt;
    use core::mem::MaybeUninit;
    use core::ops::Deref;
    use core::ptr;
    use core::slice;

    /// A "history buffer", similar to a write-only ring buffer of fixed length.
    ///
    /// This buffer keeps a fixed number of elements.  On write, the oldest element
    /// is overwritten. Thus, the buffer is useful to keep a history of values with
    /// some desired depth, and for example calculate a rolling average.
    ///
    /// # Examples
    /// ```
    /// use heapless::HistoryBuffer;
    ///
    /// // Initialize a new buffer with 8 elements.
    /// let mut buf = HistoryBuffer::<_, 8>::new();
    ///
    /// // Starts with no data
    /// assert_eq!(buf.recent(), None);
    ///
    /// buf.write(3);
    /// buf.write(5);
    /// buf.extend(&[4, 4]);
    ///
    /// // The most recent written element is a four.
    /// assert_eq!(buf.recent(), Some(&4));
    ///
    /// // To access all elements in an unspecified order, use `as_slice()`.
    /// for el in buf.as_slice() { println!("{:?}", el); }
    ///
    /// // Now we can prepare an average of all values, which comes out to 4.
    /// let avg = buf.as_slice().iter().sum::<usize>() / buf.len();
    /// assert_eq!(avg, 4);
    /// ```
    pub struct HistoryBuffer<T, const N: usize> {
        data: [MaybeUninit<T>; N],
        write_at: usize,
        filled: bool,
    }

    impl<T, const N: usize> HistoryBuffer<T, N> {
        const INIT: MaybeUninit<T> = MaybeUninit::uninit();

        /// Constructs a new history buffer.
        ///
        /// The construction of a `HistoryBuffer` works in `const` contexts.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::HistoryBuffer;
        ///
        /// // Allocate a 16-element buffer on the stack
        /// let x: HistoryBuffer<u8, 16> = HistoryBuffer::new();
        /// assert_eq!(x.len(), 0);
        /// ```
        #[inline]
        pub const fn new() -> Self {
            // Const assert
            crate::sealed::greater_than_0::<N>();

            Self {
                data: [Self::INIT; N],
                write_at: 0,
                filled: false,
            }
        }

        /// Clears the buffer, replacing every element with the default value of
        /// type `T`.
        pub fn clear(&mut self) {
            *self = Self::new();
        }
    }

    impl<T, const N: usize> HistoryBuffer<T, N>
    where
        T: Copy + Clone,
    {
        /// Constructs a new history buffer, where every element is the given value.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::HistoryBuffer;
        ///
        /// // Allocate a 16-element buffer on the stack
        /// let mut x: HistoryBuffer<u8, 16> = HistoryBuffer::new_with(4);
        /// // All elements are four
        /// assert_eq!(x.as_slice(), [4; 16]);
        /// ```
        #[inline]
        pub fn new_with(t: T) -> Self {
            Self {
                data: [MaybeUninit::new(t); N],
                write_at: 0,
                filled: true,
            }
        }

        /// Clears the buffer, replacing every element with the given value.
        pub fn clear_with(&mut self, t: T) {
            *self = Self::new_with(t);
        }
    }

    impl<T, const N: usize> HistoryBuffer<T, N> {
        /// Returns the current fill level of the buffer.
        #[inline]
        pub fn len(&self) -> usize {
            if self.filled {
                N
            } else {
                self.write_at
            }
        }

        /// Returns the capacity of the buffer, which is the length of the
        /// underlying backing array.
        #[inline]
        pub fn capacity(&self) -> usize {
            N
        }

        /// Writes an element to the buffer, overwriting the oldest value.
        pub fn write(&mut self, t: T) {
            if self.filled {
                // Drop the old before we overwrite it.
                unsafe { ptr::drop_in_place(self.data[self.write_at].as_mut_ptr()) }
            }
            self.data[self.write_at] = MaybeUninit::new(t);

            self.write_at += 1;
            if self.write_at == self.capacity() {
                self.write_at = 0;
                self.filled = true;
            }
        }

        /// Clones and writes all elements in a slice to the buffer.
        ///
        /// If the slice is longer than the buffer, only the last `self.len()`
        /// elements will actually be stored.
        pub fn extend_from_slice(&mut self, other: &[T])
        where
            T: Clone,
        {
            for item in other {
                self.write(item.clone());
            }
        }

        /// Returns a reference to the most recently written value.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::HistoryBuffer;
        ///
        /// let mut x: HistoryBuffer<u8, 16> = HistoryBuffer::new();
        /// x.write(4);
        /// x.write(10);
        /// assert_eq!(x.recent(), Some(&10));
        /// ```
        pub fn recent(&self) -> Option<&T> {
            if self.write_at == 0 {
                if self.filled {
                    Some(unsafe { &*self.data[self.capacity() - 1].as_ptr() })
                } else {
                    None
                }
            } else {
                Some(unsafe { &*self.data[self.write_at - 1].as_ptr() })
            }
        }

        /// Returns the array slice backing the buffer, without keeping track
        /// of the write position. Therefore, the element order is unspecified.
        pub fn as_slice(&self) -> &[T] {
            unsafe { slice::from_raw_parts(self.data.as_ptr() as *const _, self.len()) }
        }

        /// Returns an iterator for iterating over the buffer from oldest to newest.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::HistoryBuffer;
        ///
        /// let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
        /// buffer.extend([0, 0, 0, 1, 2, 3, 4, 5, 6]);
        /// let expected = [1, 2, 3, 4, 5, 6];
        /// for (x, y) in buffer.oldest_ordered().zip(expected.iter()) {
        ///     assert_eq!(x, y)
        /// }
        ///
        /// ```
        pub fn oldest_ordered<'a>(&'a self) -> OldestOrdered<'a, T, N> {
            if self.filled {
                OldestOrdered {
                    buf: self,
                    cur: self.write_at,
                    wrapped: false,
                }
            } else {
                // special case: act like we wrapped already to handle empty buffer.
                OldestOrdered {
                    buf: self,
                    cur: 0,
                    wrapped: true,
                }
            }
        }
    }

    impl<T, const N: usize> Extend<T> for HistoryBuffer<T, N> {
        fn extend<I>(&mut self, iter: I)
        where
            I: IntoIterator<Item = T>,
        {
            for item in iter.into_iter() {
                self.write(item);
            }
        }
    }

    impl<'a, T, const N: usize> Extend<&'a T> for HistoryBuffer<T, N>
    where
        T: 'a + Clone,
    {
        fn extend<I>(&mut self, iter: I)
        where
            I: IntoIterator<Item = &'a T>,
        {
            self.extend(iter.into_iter().cloned())
        }
    }

    impl<T, const N: usize> Drop for HistoryBuffer<T, N> {
        fn drop(&mut self) {
            unsafe {
                ptr::drop_in_place(ptr::slice_from_raw_parts_mut(
                    self.data.as_mut_ptr() as *mut T,
                    self.len(),
                ))
            }
        }
    }

    impl<T, const N: usize> Deref for HistoryBuffer<T, N> {
        type Target = [T];

        fn deref(&self) -> &[T] {
            self.as_slice()
        }
    }

    impl<T, const N: usize> AsRef<[T]> for HistoryBuffer<T, N> {
        #[inline]
        fn as_ref(&self) -> &[T] {
            self
        }
    }

    impl<T, const N: usize> fmt::Debug for HistoryBuffer<T, N>
    where
        T: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            <[T] as fmt::Debug>::fmt(self, f)
        }
    }

    impl<T, const N: usize> Default for HistoryBuffer<T, N> {
        fn default() -> Self {
            Self::new()
        }
    }

    /// An iterator on the underlying buffer ordered from oldest data to newest
    #[derive(Clone)]
    pub struct OldestOrdered<'a, T, const N: usize> {
        buf: &'a HistoryBuffer<T, N>,
        cur: usize,
        wrapped: bool,
    }

    impl<'a, T, const N: usize> Iterator for OldestOrdered<'a, T, N> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            if self.cur == self.buf.len() && self.buf.filled {
                // roll-over
                self.cur = 0;
                self.wrapped = true;
            }

            if self.cur == self.buf.write_at && self.wrapped {
                return None;
            }

            let item = &self.buf[self.cur];
            self.cur += 1;
            Some(item)
        }
    }

    #[cfg(test)]
    mod tests {
        use crate::HistoryBuffer;
        use core::fmt::Debug;

        #[test]
        fn new() {
            let x: HistoryBuffer<u8, 4> = HistoryBuffer::new_with(1);
            assert_eq!(x.len(), 4);
            assert_eq!(x.as_slice(), [1; 4]);
            assert_eq!(*x, [1; 4]);

            let x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
            assert_eq!(x.as_slice(), []);
        }

        #[test]
        fn write() {
            let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
            x.write(1);
            x.write(4);
            assert_eq!(x.as_slice(), [1, 4]);

            x.write(5);
            x.write(6);
            x.write(10);
            assert_eq!(x.as_slice(), [10, 4, 5, 6]);

            x.extend([11, 12].iter());
            assert_eq!(x.as_slice(), [10, 11, 12, 6]);
        }

        #[test]
        fn clear() {
            let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new_with(1);
            x.clear();
            assert_eq!(x.as_slice(), []);

            let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
            x.clear_with(1);
            assert_eq!(x.as_slice(), [1; 4]);
        }

        #[test]
        fn recent() {
            let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
            assert_eq!(x.recent(), None);

            x.write(1);
            x.write(4);
            assert_eq!(x.recent(), Some(&4));

            x.write(5);
            x.write(6);
            x.write(10);
            assert_eq!(x.recent(), Some(&10));
        }

        #[test]
        fn as_slice() {
            let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();

            assert_eq!(x.as_slice(), []);

            x.extend([1, 2, 3, 4, 5].iter());

            assert_eq!(x.as_slice(), [5, 2, 3, 4]);
        }

        #[test]
        fn ordered() {
            // test on an empty buffer
            let buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
            let mut iter = buffer.oldest_ordered();
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);

            // test on a un-filled buffer
            let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
            buffer.extend([1, 2, 3]);
            assert_eq!(buffer.len(), 3);
            assert_eq_iter(buffer.oldest_ordered(), &[1, 2, 3]);

            // test on a filled buffer
            let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
            buffer.extend([0, 0, 0, 1, 2, 3, 4, 5, 6]);
            assert_eq!(buffer.len(), 6);
            assert_eq_iter(buffer.oldest_ordered(), &[1, 2, 3, 4, 5, 6]);

            // comprehensive test all cases
            for n in 0..50 {
                const N: usize = 7;
                let mut buffer: HistoryBuffer<u8, N> = HistoryBuffer::new();
                buffer.extend(0..n);
                assert_eq_iter(
                    buffer.oldest_ordered().copied(),
                    n.saturating_sub(N as u8)..n,
                );
            }
        }

        /// Compares two iterators item by item, making sure they stop at the same time.
        fn assert_eq_iter<I: Eq + Debug>(
            a: impl IntoIterator<Item = I>,
            b: impl IntoIterator<Item = I>,
        ) {
            let mut a = a.into_iter();
            let mut b = b.into_iter();

            let mut i = 0;
            loop {
                let a_item = a.next();
                let b_item = b.next();

                assert_eq!(a_item, b_item, "{}", i);

                i += 1;

                if b_item.is_none() {
                    break;
                }
            }
        }
    }