use crate::data::arena::Index;

#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
#[derive(Clone, Debug)]
/// A container for data associated to item existing into another Arena.
pub struct Coarena<T> {
    data: Vec<(u64, T)>,
}

impl<T> Coarena<T> {
    /// A coarena with no element.
    pub fn new() -> Self {
        Self { data: Vec::new() }
    }

    /// Gets a specific element from the coarena, if it exists.
    pub fn get(&self, index: Index) -> Option<&T> {
        let (i, g) = index.into_raw_parts();
        self.data
            .get(i)
            .and_then(|(gg, t)| if g == *gg { Some(t) } else { None })
    }

    /// Gets a mutable reference to a specific element from the coarena, if it exists.
    pub fn get_mut(&mut self, index: Index) -> Option<&mut T> {
        let (i, g) = index.into_raw_parts();
        self.data
            .get_mut(i)
            .and_then(|(gg, t)| if g == *gg { Some(t) } else { None })
    }

    /// Ensure that elements at the two given indices exist in this coarena, and return their reference.
    ///
    /// Missing elements are created automatically and initialized with the `default` value.
    pub fn ensure_pair_exists(&mut self, a: Index, b: Index, default: T) -> (&mut T, &mut T)
    where
        T: Clone,
    {
        let (i1, g1) = a.into_raw_parts();
        let (i2, g2) = b.into_raw_parts();

        assert_ne!(i1, i2, "Cannot index the same object twice.");

        let (elt1, elt2) = if i1 > i2 {
            if self.data.len() <= i1 {
                self.data.resize(i1 + 1, (u32::MAX as u64, default.clone()));
            }

            let (left, right) = self.data.split_at_mut(i1);
            (&mut right[0], &mut left[i2])
        } else {
            // i2 > i1
            if self.data.len() <= i2 {
                self.data.resize(i2 + 1, (u32::MAX as u64, default.clone()));
            }

            let (left, right) = self.data.split_at_mut(i2);
            (&mut left[i1], &mut right[0])
        };

        if elt1.0 != g1 {
            *elt1 = (g1, default.clone());
        }

        if elt2.0 != g2 {
            *elt2 = (g2, default);
        }

        (&mut elt1.1, &mut elt2.1)
    }
}