use generic_array::{ArrayLength, GenericArray, sequence::GenericSequence};

/// 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.
///
/// The buffer is always fully initialized; depending on the constructor, the
/// initial value is either the default value for the element type or a supplied
/// initial value. This simplifies the API and is mostly irrelevant for the
/// intended use case.
///
/// # Examples
/// ```
/// use heapless::HistoryBuffer;
/// use heapless::consts::*;
///
/// // Initialize a new buffer with 8 elements, all initially zero.
/// let mut buf = HistoryBuffer::<_, U8>::new();
///
/// buf.write(3);
/// buf.write(5);
/// buf.extend(&[4, 4]);
///
/// // The most recent written element is a four.
/// assert_eq!(buf.recent(), &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 2.
/// let avg = buf.as_slice().iter().sum::<usize>() / buf.len();
/// assert_eq!(avg, 2);
/// ```
#[derive(Clone)]
pub struct HistoryBuffer<T, N>
where
    N: ArrayLength<T>,
{
    data: GenericArray<T, N>,
    write_at: usize,
}


impl<T, N> HistoryBuffer<T, N>
where
    N: ArrayLength<T>,
    T: Default,
{
    /// Constructs a new history buffer, where every element is filled with the
    /// default value of the type `T`.
    ///
    /// `HistoryBuffer` currently cannot be constructed in `const` context.
    ///
    /// # Examples
    ///
    /// ```
    /// use heapless::HistoryBuffer;
    /// use heapless::consts::*;
    ///
    /// // Allocate a 16-element buffer on the stack
    /// let mut x: HistoryBuffer<u8, U16> = HistoryBuffer::new();
    /// // All elements are zero
    /// assert_eq!(x.as_slice(), [0; 16]);
    /// ```
    pub fn new() -> Self {
        Self {
            data: Default::default(),
            write_at: 0,
        }
    }

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

impl<T, N> HistoryBuffer<T, N>
where
    N: ArrayLength<T>,
    T: Clone,
{
    /// Constructs a new history buffer, where every element is the given value.
    ///
    /// # Examples
    ///
    /// ```
    /// use heapless::HistoryBuffer;
    /// use heapless::consts::*;
    ///
    /// // Allocate a 16-element buffer on the stack
    /// let mut x: HistoryBuffer<u8, U16> = HistoryBuffer::new_with(4);
    /// // All elements are four
    /// assert_eq!(x.as_slice(), [4; 16]);
    /// ```
    pub fn new_with(t: T) -> Self {
        Self {
            data: GenericArray::generate(|_| t.clone()),
            write_at: 0,
        }
    }

    /// 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, N> HistoryBuffer<T, N>
where
    N: ArrayLength<T>,
{
    /// Returns the capacity of the buffer, which is the length of the
    /// underlying backing array.
    pub fn len(&self) -> usize {
        self.data.len()
    }

    /// Writes an element to the buffer, overwriting the oldest value.
    pub fn write(&mut self, t: T) {
        self.data[self.write_at] = t;
        self.write_at = (self.write_at + 1) % self.len();
    }

    /// 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;
    /// use heapless::consts::*;
    ///
    /// let mut x: HistoryBuffer<u8, U16> = HistoryBuffer::new();
    /// x.write(4);
    /// x.write(10);
    /// assert_eq!(x.recent(), &10);
    /// ```
    pub fn recent(&self) -> &T {
        &self.data[(self.write_at + self.len() - 1) % self.len()]
    }

    /// 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] {
        &self.data
    }
}

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

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

#[cfg(test)]
mod tests {
    use crate::{consts::*, HistoryBuffer};

    #[test]
    fn new() {
        let x: HistoryBuffer<u8, U4> = HistoryBuffer::new_with(1);
        assert_eq!(x.len(), 4);
        assert_eq!(x.as_slice(), [1; 4]);

        let x: HistoryBuffer<u8, U4> = HistoryBuffer::new();
        assert_eq!(x.as_slice(), [0; 4]);
    }

    #[test]
    fn write() {
        let mut x: HistoryBuffer<u8, U4> = HistoryBuffer::new();
        x.write(1);
        x.write(4);
        assert_eq!(x.as_slice(), [1, 4, 0, 0]);

        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, U4> = HistoryBuffer::new_with(1);
        x.clear();
        assert_eq!(x.as_slice(), [0; 4]);

        let mut x: HistoryBuffer<u8, U4> = HistoryBuffer::new();
        x.clear_with(1);
        assert_eq!(x.as_slice(), [1; 4]);
    }

    #[test]
    fn recent() {
        let mut x: HistoryBuffer<u8, U4> = HistoryBuffer::new();
        assert_eq!(x.recent(), &0);

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

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

    #[test]
    fn as_slice() {
        let mut x: HistoryBuffer<u8, U4> = HistoryBuffer::new();

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

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