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use iter_fixed::IntoIteratorFixed;

use crate::storage::Handle;

/// Trait for merging partial objects
///
/// Implemented for all partial objects themselves, and also some related types
/// that partial objects usually contain.
pub trait MergeWith: Sized {
    /// Merge this object with another
    ///
    /// # Panics
    ///
    /// Merging two objects that cannot be merged is considered a programmer
    /// error and will result in a panic.
    fn merge_with(self, other: impl Into<Self>) -> Self;
}

/// Wrapper struct that indicates that the contents can be merged
///
/// Used in connection with [`MergeWith`] to select one implementation over
/// another.
pub struct Mergeable<T>(pub T);

impl<T, const N: usize> MergeWith for [T; N]
where
    T: MergeWith,
{
    fn merge_with(self, other: impl Into<Self>) -> Self {
        self.into_iter_fixed()
            .zip(other.into())
            .collect::<[_; N]>()
            .map(|(a, b)| a.merge_with(b))
    }
}

impl<T> MergeWith for Option<T>
where
    T: PartialEq,
{
    fn merge_with(self, other: impl Into<Self>) -> Self {
        let other = other.into();

        if self == other {
            return self;
        }

        // We know that `self != other`, or we wouldn't have made it here.
        if self.is_some() && other.is_some() {
            panic!("Can't merge two `Option`s that are both `Some`")
        }

        self.xor(other)
    }
}

// We wouldn't need to use `Mergeable` here, if we had `specialization`:
// https://doc.rust-lang.org/nightly/unstable-book/language-features/specialization.html
//
// Or maybe `min_specialization`:
// https://doc.rust-lang.org/nightly/unstable-book/language-features/min-specialization.html
impl<T> MergeWith for Mergeable<Option<T>>
where
    T: MergeWith,
{
    fn merge_with(self, other: impl Into<Self>) -> Self {
        let merged = match (self.0, other.into().0) {
            (Some(a), Some(b)) => Some(a.merge_with(b)),
            (a, b) => a.xor(b),
        };

        Self(merged)
    }
}

impl<T> MergeWith for Vec<T> {
    fn merge_with(self, other: impl Into<Self>) -> Self {
        let other = other.into();

        match (self.is_empty(), other.is_empty()) {
            (true, true) => {
                panic!("Can't merge `PartialHalfEdge`, if both have half-edges")
            }
            (true, false) => other,
            (false, true) => self,
            (false, false) => self, // doesn't matter which we use
        }
    }
}

impl<T> MergeWith for Mergeable<Vec<T>> {
    fn merge_with(mut self, other: impl Into<Self>) -> Self {
        self.0.extend(other.into().0);
        self
    }
}

impl<T> MergeWith for Handle<T> {
    fn merge_with(self, other: impl Into<Self>) -> Self {
        if self.id() == other.into().id() {
            return self;
        }

        panic!("Can't merge two distinct objects")
    }
}