use crate::partitioning::{
    BVTNodeId, BestFirstVisitStatus, BestFirstVisitor, DBVTNodeId, SimultaneousVisitor,
    VisitStatus, Visitor, BVT, DBVT,
};
use na::RealField;
use std::cmp::Ordering;
use std::collections::BinaryHeap;

/// Trait implemented by Bounding Volume Hierarchy.
pub trait BVH<T, BV> {
    /// Type of a node identifiers on this BVH.
    type Node: Copy;

    /// The root of the BVH.
    fn root(&self) -> Option<Self::Node>;
    /// The number of children of the given node.
    fn num_children(&self, node: Self::Node) -> usize;
    /// The i-th child of the given node.
    fn child(&self, i: usize, node: Self::Node) -> Self::Node;
    /// The bounding volume and data contained by the given node.
    fn content(&self, node: Self::Node) -> (&BV, Option<&T>);

    /// Traverses this BVH using a visitor.
    fn visit(&self, visitor: &mut impl Visitor<T, BV>) {
        // FIXME: find a way to avoid the allocation.
        let mut stack = Vec::new();

        if let Some(root) = self.root() {
            stack.push(root);

            while let Some(node) = stack.pop() {
                let content = self.content(node);

                match visitor.visit(content.0, content.1) {
                    VisitStatus::Continue => {
                        for i in 0..self.num_children(node) {
                            stack.push(self.child(i, node))
                        }
                    }
                    VisitStatus::ExitEarly => return,
                    VisitStatus::Stop => {}
                }
            }
        }
    }

    /// Visits the bounding volume test tree implicitly formed with `other`.
    fn visit_bvtt(&self, other: &impl BVH<T, BV>, visitor: &mut impl SimultaneousVisitor<T, BV>) {
        // FIXME: find a way to avoid the allocation.
        let mut stack = Vec::new();

        if let (Some(root1), Some(root2)) = (self.root(), other.root()) {
            stack.push((root1, root2));

            while let Some((node1, node2)) = stack.pop() {
                let content1 = self.content(node1);
                let content2 = other.content(node2);

                match visitor.visit(content1.0, content1.1, content2.0, content2.1) {
                    VisitStatus::Continue => {
                        let nchild1 = self.num_children(node1);
                        let nchild2 = other.num_children(node2);

                        match (nchild1, nchild2) {
                            (0, 0) => {}
                            (0, _) => {
                                for j in 0..nchild2 {
                                    let n2 = other.child(j, node2);
                                    stack.push((node1, n2))
                                }
                            }
                            (_, 0) => {
                                for i in 0..nchild1 {
                                    let n1 = self.child(i, node1);
                                    stack.push((n1, node2))
                                }
                            }
                            (_, _) => {
                                for i in 0..nchild1 {
                                    let n1 = self.child(i, node1);

                                    for j in 0..nchild2 {
                                        let n2 = other.child(j, node2);
                                        stack.push((n1, n2))
                                    }
                                }
                            }
                        }
                    }
                    VisitStatus::ExitEarly => return,
                    VisitStatus::Stop => {}
                }
            }
        }
    }

    /// Performs a best-first-search on the BVH.
    ///
    /// Returns the content of the leaf with the smallest associated cost, and a result of
    /// user-defined type.
    fn best_first_search<N, BFS>(&self, visitor: &mut BFS) -> Option<(Self::Node, BFS::Result)>
    where
        N: RealField,
        BFS: BestFirstVisitor<N, T, BV>,
    {
        let mut queue: BinaryHeap<WeightedValue<N, Self::Node>> = BinaryHeap::new();
        // The lowest cost collision with actual scene geometry.
        let mut best_cost = N::max_value();
        let mut best_result = None;

        if let Some(root) = self.root() {
            let (root_bv, root_data) = self.content(root);

            match visitor.visit(best_cost, root_bv, root_data) {
                BestFirstVisitStatus::Continue { cost, result } => {
                    // Root may be a leaf node
                    if let Some(res) = result {
                        best_cost = cost;
                        best_result = Some((root, res));
                    }

                    queue.push(WeightedValue::new(root, -cost))
                }
                BestFirstVisitStatus::Stop => return None,
                BestFirstVisitStatus::ExitEarly(result) => return result.map(|res| (root, res)),
            }

            while let Some(entry) = queue.pop() {
                if -entry.cost >= best_cost {
                    // No BV left in the tree that has a lower cost than best_result
                    break; // Solution found.
                }

                for i in 0..self.num_children(entry.value) {
                    let child = self.child(i, entry.value);
                    let (child_bv, child_data) = self.content(child);

                    match visitor.visit(best_cost, child_bv, child_data) {
                        BestFirstVisitStatus::Continue { cost, result } => {
                            if cost < best_cost {
                                if result.is_some() {
                                    // This is the nearest collision so far
                                    best_cost = cost;
                                    best_result = result.map(|res| (child, res));
                                }
                                // BV may have a child with lower cost, evaluate it next.
                                queue.push(WeightedValue::new(child, -cost))
                            }
                        }
                        BestFirstVisitStatus::ExitEarly(result) => {
                            return result.map(|res| (child, res)).or(best_result)
                        }
                        BestFirstVisitStatus::Stop => {}
                    }
                }
            }
        }

        best_result
    }
}

/// An enum grouping references to all the BVH implementations on ncollide.
#[derive(Copy, Clone)]
pub enum BVHImpl<'a, N: 'a + RealField, T: 'a, BV: 'a> {
    /// A static binary bounding volume tree.
    BVT(&'a BVT<T, BV>),
    /// A dynamic binary bounding volume tree.
    DBVT(&'a DBVT<N, T, BV>),
}

/// The Id of a node of a BVH.
pub enum BVHNodeId {
    // The Id of a BVT.
    BVTNodeId(BVTNodeId),
    // The Id of a DBVT.
    DBVTNodeId(DBVTNodeId),
}

impl<'a, N: RealField, T, BV> BVHImpl<'a, N, T, BV> {
    /// Gets the underlying reference to a BVT, or panics if this is not a `BVTImpl::BVT`.
    #[inline]
    pub fn unwrap_bvt(self) -> &'a BVT<T, BV> {
        match self {
            BVHImpl::BVT(bvt) => bvt,
            _ => panic!("This BVTImpl is not a BVT."),
        }
    }

    /// Gets the underlying reference to a DBVT, or panics if this is not a `BVTImpl::DBVT`.
    #[inline]
    pub fn unwrap_dbvt(self) -> &'a DBVT<N, T, BV> {
        match self {
            BVHImpl::DBVT(dbvt) => dbvt,
            _ => panic!("This BVTImpl is not a DBVT."),
        }
    }

    /// Traverses this tree using a visitor.
    pub fn visit(self, visitor: &mut impl Visitor<T, BV>) {
        match self {
            BVHImpl::BVT(bvt) => bvt.visit(visitor),
            BVHImpl::DBVT(dbvt) => dbvt.visit(visitor),
        }
    }

    /// Visits the bounding volume traversal tree implicitly formed with `other`.
    pub fn visit_bvtt(
        self,
        other: BVHImpl<N, T, BV>,
        visitor: &mut impl SimultaneousVisitor<T, BV>,
    ) {
        // Note: the dispatch on each pair is split into two method to avoid
        // having to write a manually a match over each possible pair.
        match other {
            BVHImpl::BVT(bvh2) => self.visit_bvtt_dispatch(bvh2, visitor),
            BVHImpl::DBVT(bvh2) => self.visit_bvtt_dispatch(bvh2, visitor),
        }
    }

    fn visit_bvtt_dispatch(
        self,
        bvh2: &impl BVH<T, BV>,
        visitor: &mut impl SimultaneousVisitor<T, BV>,
    ) {
        match self {
            BVHImpl::BVT(bvh1) => bvh1.visit_bvtt(bvh2, visitor),
            BVHImpl::DBVT(bvh1) => bvh1.visit_bvtt(bvh2, visitor),
        }
    }

    /// Performs a best-fist-search on the tree.
    ///
    /// Returns the content of the leaf with the smallest associated cost, and a result of
    /// user-defined type.
    pub fn best_first_search<BFS>(self, visitor: &mut BFS) -> Option<(BVHNodeId, BFS::Result)>
    where
        BFS: BestFirstVisitor<N, T, BV>,
    {
        match self {
            BVHImpl::BVT(bvt) => bvt
                .best_first_search(visitor)
                .map(|res| (BVHNodeId::BVTNodeId(res.0), res.1)),
            BVHImpl::DBVT(dbvt) => dbvt
                .best_first_search(visitor)
                .map(|res| (BVHNodeId::DBVTNodeId(res.0), res.1)),
        }
    }
}

struct WeightedValue<N, T> {
    pub value: T,
    pub cost: N,
}

impl<N, T> WeightedValue<N, T> {
    /// Creates a new reference packed with a cost value.
    #[inline]
    pub fn new(value: T, cost: N) -> WeightedValue<N, T> {
        WeightedValue {
            value: value,
            cost: cost,
        }
    }
}

impl<N: PartialEq, T> PartialEq for WeightedValue<N, T> {
    #[inline]
    fn eq(&self, other: &WeightedValue<N, T>) -> bool {
        self.cost.eq(&other.cost)
    }
}

impl<N: PartialEq, T> Eq for WeightedValue<N, T> {}

impl<N: PartialOrd, T> PartialOrd for WeightedValue<N, T> {
    #[inline]
    fn partial_cmp(&self, other: &WeightedValue<N, T>) -> Option<Ordering> {
        self.cost.partial_cmp(&other.cost)
    }
}

impl<N: PartialOrd, T> Ord for WeightedValue<N, T> {
    #[inline]
    fn cmp(&self, other: &WeightedValue<N, T>) -> Ordering {
        if self.cost < other.cost {
            Ordering::Less
        } else if self.cost > other.cost {
            Ordering::Greater
        } else {
            Ordering::Equal
        }
    }
}