//! A region that stores options.

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

use crate::{IntoOwned, Push, Region, RegionPreference, ReserveItems};

impl<T: RegionPreference> RegionPreference for Option<T> {
    type Owned = Option<T::Owned>;
    type Region = OptionRegion<T::Region>;
}

/// A region to hold [`Option`]s.
///
/// # Examples
///
/// The region can hold options:
/// ```
/// # use flatcontainer::{RegionPreference, Push, OptionRegion, Region};
/// let mut r = <OptionRegion<<u8 as RegionPreference>::Region>>::default();
///
/// let some_index = r.push(Some(123));
/// // Type annotations required for `None`:
/// let none_index = r.push(Option::<u8>::None);
///
/// assert_eq!(Some(123), r.index(some_index));
/// assert_eq!(None, r.index(none_index));
/// ```
#[derive(Default, Debug)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct OptionRegion<R> {
    inner: R,
}

impl<R: Clone> Clone for OptionRegion<R> {
    fn clone(&self) -> Self {
        Self {
            inner: self.inner.clone(),
        }
    }

    fn clone_from(&mut self, source: &Self) {
        self.inner.clone_from(&source.inner);
    }
}

impl<R: Region> Region for OptionRegion<R> {
    type Owned = Option<R::Owned>;
    type ReadItem<'a> = Option<<R as Region>::ReadItem<'a>> where Self: 'a;
    type Index = Option<R::Index>;

    #[inline]
    fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
    where
        Self: 'a,
    {
        Self {
            inner: R::merge_regions(regions.map(|r| &r.inner)),
        }
    }

    #[inline]
    fn index(&self, index: Self::Index) -> Self::ReadItem<'_> {
        index.map(|t| self.inner.index(t))
    }

    #[inline]
    fn reserve_regions<'a, I>(&mut self, regions: I)
    where
        Self: 'a,
        I: Iterator<Item = &'a Self> + Clone,
    {
        self.inner.reserve_regions(regions.map(|r| &r.inner));
    }

    #[inline]
    fn clear(&mut self) {
        self.inner.clear();
    }

    #[inline]
    fn heap_size<F: FnMut(usize, usize)>(&self, callback: F) {
        self.inner.heap_size(callback);
    }

    #[inline]
    fn reborrow<'b, 'a: 'b>(item: Self::ReadItem<'a>) -> Self::ReadItem<'b>
    where
        Self: 'a,
    {
        item.map(R::reborrow)
    }
}

impl<'a, T> IntoOwned<'a> for Option<T>
where
    T: IntoOwned<'a>,
{
    type Owned = Option<T::Owned>;

    #[inline]
    fn into_owned(self) -> Self::Owned {
        self.map(IntoOwned::into_owned)
    }

    #[inline]
    fn clone_onto(self, other: &mut Self::Owned) {
        match (self, other) {
            (Some(item), Some(target)) => T::clone_onto(item, target),
            (Some(item), target) => *target = Some(T::into_owned(item)),
            (None, target) => *target = None,
        }
    }

    #[inline]
    fn borrow_as(owned: &'a Self::Owned) -> Self {
        owned.as_ref().map(T::borrow_as)
    }
}

impl<T, TR> Push<Option<T>> for OptionRegion<TR>
where
    TR: Region + Push<T>,
{
    #[inline]
    fn push(&mut self, item: Option<T>) -> <OptionRegion<TR> as Region>::Index {
        item.map(|t| self.inner.push(t))
    }
}

impl<'a, T: 'a, TR> Push<&'a Option<T>> for OptionRegion<TR>
where
    TR: Region + Push<&'a T>,
{
    #[inline]
    fn push(&mut self, item: &'a Option<T>) -> <OptionRegion<TR> as Region>::Index {
        item.as_ref().map(|t| self.inner.push(t))
    }
}

impl<T, TR> ReserveItems<Option<T>> for OptionRegion<TR>
where
    TR: Region + ReserveItems<T>,
{
    #[inline]
    fn reserve_items<I>(&mut self, items: I)
    where
        I: Iterator<Item = Option<T>> + Clone,
    {
        // Clippy is confused about using `flatten` here, which we cannot use because
        // the iterator isn't `Clone`.
        #[allow(clippy::filter_map_identity)]
        self.inner.reserve_items(items.filter_map(|r| r));
    }
}

impl<'a, T: 'a, TR> ReserveItems<&'a Option<T>> for OptionRegion<TR>
where
    TR: Region + ReserveItems<&'a T>,
{
    #[inline]
    fn reserve_items<I>(&mut self, items: I)
    where
        I: Iterator<Item = &'a Option<T>> + Clone,
    {
        self.inner.reserve_items(items.filter_map(|r| r.as_ref()));
    }
}

#[cfg(test)]
mod tests {
    use crate::{MirrorRegion, OwnedRegion, Region, ReserveItems};

    use super::*;

    #[test]
    fn test_reserve() {
        let mut r = <OptionRegion<MirrorRegion<u8>>>::default();
        ReserveItems::reserve_items(&mut r, [Some(0), None].iter());

        ReserveItems::reserve_items(&mut r, [Some(0), None].into_iter());
    }

    #[test]
    fn test_heap_size() {
        let mut r = <OptionRegion<OwnedRegion<u8>>>::default();
        ReserveItems::reserve_items(&mut r, [Some([1; 1]), None].iter());
        let mut cap = 0;
        r.heap_size(|_, ca| {
            cap += ca;
        });
        assert!(cap > 0);
    }
}