use smallvec::SmallVec;

use super::dcel_operations;
use super::dcel_operations::IsolateVertexResult;
use super::handles::*;
use super::math;

use crate::HintGenerator;
use crate::Point2;
use crate::{HasPosition, InsertionError, PositionInTriangulation, Triangulation};

use alloc::{collections::VecDeque, vec::Vec};

impl<T> TriangulationExt for T where T: Triangulation + ?Sized {}

pub enum PositionWhenAllVerticesOnLine {
    OnEdge(FixedDirectedEdgeHandle),
    OnVertex(FixedVertexHandle),
    NotOnLine(FixedDirectedEdgeHandle),
    ExtendingLine(FixedVertexHandle),
}

pub enum InsertionResult {
    NewlyInserted(FixedVertexHandle),
    Updated(FixedVertexHandle),
}

pub struct RemovalResult<V> {
    pub removed_vertex: V,
    pub swapped_in_vertex: Option<FixedVertexHandle>,
}

pub trait TriangulationExt: Triangulation {
    fn insert_with_hint_option(
        &mut self,
        t: Self::Vertex,
        hint: Option<FixedVertexHandle>,
    ) -> Result<FixedVertexHandle, InsertionError> {
        math::validate_vertex(&t)?;
        let position = t.position();
        let result = self.insert_with_hint_option_impl(t, hint);

        self.hint_generator_mut()
            .notify_vertex_inserted(result, position);
        Ok(result)
    }

    fn insert_with_hint_option_impl(
        &mut self,
        t: Self::Vertex,
        hint: Option<FixedVertexHandle>,
    ) -> FixedVertexHandle {
        use PositionInTriangulation::*;

        let insertion_result = match self.num_vertices() {
            0 => return dcel_operations::insert_first_vertex(self.s_mut(), t),
            1 => self.insert_second_vertex(t),
            _ => {
                let pos = t.position();

                if self.all_vertices_on_line() {
                    let location = self.locate_when_all_vertices_on_line(pos);
                    self.insert_when_all_vertices_on_line(location, t)
                } else {
                    let position_in_triangulation = self.locate_with_hint_option_core(pos, hint);
                    match position_in_triangulation {
                        OutsideOfConvexHull(edge) => InsertionResult::NewlyInserted(
                            self.insert_outside_of_convex_hull(edge, t),
                        ),
                        OnFace(face) => {
                            InsertionResult::NewlyInserted(self.insert_into_face(face, t))
                        }
                        OnEdge(edge) => {
                            let (new_handle, split_parts) = self.insert_on_edge(edge, t);

                            if self.is_defined_legal(edge.as_undirected()) {
                                // If the edge is defined as legal the resulting edges must
                                // be redefined as legal
                                self.handle_legal_edge_split(split_parts);
                            }
                            self.legalize_vertex(new_handle);

                            InsertionResult::NewlyInserted(new_handle)
                        }
                        OnVertex(vertex) => {
                            self.s_mut().update_vertex(vertex, t);
                            InsertionResult::Updated(vertex)
                        }
                        NoTriangulation => panic!("Error during vertex lookup. This is a bug."),
                    }
                }
            }
        };

        match insertion_result {
            InsertionResult::NewlyInserted(new_handle) => new_handle,
            InsertionResult::Updated(update_handle) => update_handle,
        }
    }

    fn locate_when_all_vertices_on_line(
        &mut self,
        position: Point2<<Self::Vertex as HasPosition>::Scalar>,
    ) -> PositionWhenAllVerticesOnLine {
        use PositionWhenAllVerticesOnLine::*;

        let edge = self
            .directed_edges()
            .next()
            .expect("Must not be called on empty triangulations");
        let query = edge.side_query(position);
        if query.is_on_left_side() {
            return NotOnLine(edge.fix());
        }
        if query.is_on_right_side() {
            return NotOnLine(edge.fix().rev());
        }

        let mut vertices: Vec<_> = self.vertices().collect();
        vertices.sort_by(|left, right| left.position().partial_cmp(&right.position()).unwrap());

        let index_to_insert =
            match vertices.binary_search_by(|v| v.position().partial_cmp(&position).unwrap()) {
                Ok(index) => return OnVertex(vertices[index].fix()),
                Err(index) => index,
            };

        if index_to_insert == 0 {
            return ExtendingLine(vertices.first().unwrap().fix());
        }
        if index_to_insert == vertices.len() {
            return ExtendingLine(vertices.last().unwrap().fix());
        }

        let v1 = vertices[index_to_insert];
        let v2 = vertices[index_to_insert - 1];

        let edge = self
            .get_edge_from_neighbors(v1.fix(), v2.fix())
            .expect("Expected edge between sorted neighbors. This is a bug in spade.");
        OnEdge(edge.fix())
    }

    fn insert_second_vertex(&mut self, vertex: Self::Vertex) -> InsertionResult {
        assert_eq!(self.num_vertices(), 1);

        let first_vertex = FixedVertexHandle::new(0);
        if self.vertex(first_vertex).position() == vertex.position() {
            self.s_mut().update_vertex(first_vertex, vertex);
            return InsertionResult::Updated(first_vertex);
        }

        let second_vertex = dcel_operations::insert_second_vertex(self.s_mut(), vertex);

        InsertionResult::NewlyInserted(second_vertex)
    }

    fn insert_when_all_vertices_on_line(
        &mut self,
        location: PositionWhenAllVerticesOnLine,
        new_vertex: Self::Vertex,
    ) -> InsertionResult {
        match location {
            PositionWhenAllVerticesOnLine::OnEdge(edge) => {
                let is_constraint_edge = self.is_defined_legal(edge.as_undirected());
                let (new_edges, new_vertex) = dcel_operations::split_edge_when_all_vertices_on_line(
                    self.s_mut(),
                    edge,
                    new_vertex,
                );

                if is_constraint_edge {
                    self.handle_legal_edge_split(new_edges);
                }
                InsertionResult::NewlyInserted(new_vertex)
            }
            PositionWhenAllVerticesOnLine::OnVertex(vertex) => InsertionResult::Updated(vertex),
            PositionWhenAllVerticesOnLine::NotOnLine(edge) => {
                let result = self.insert_outside_of_convex_hull(edge, new_vertex);
                InsertionResult::NewlyInserted(result)
            }
            PositionWhenAllVerticesOnLine::ExtendingLine(end_vertex) => {
                let result = dcel_operations::extend_line(self.s_mut(), end_vertex, new_vertex);
                InsertionResult::NewlyInserted(result)
            }
        }
    }

    fn locate_with_hint_option_core(
        &self,
        point: Point2<<Self::Vertex as HasPosition>::Scalar>,
        hint: Option<FixedVertexHandle>,
    ) -> PositionInTriangulation {
        let start = hint.unwrap_or_else(|| self.hint_generator().get_hint(point));
        self.locate_with_hint_fixed_core(point, start)
    }

    fn insert_outside_of_convex_hull(
        &mut self,
        convex_hull_edge: FixedDirectedEdgeHandle,
        new_vertex: Self::Vertex,
    ) -> FixedVertexHandle {
        let position = new_vertex.position();

        assert!(self
            .directed_edge(convex_hull_edge)
            .side_query(position)
            .is_on_left_side());

        let result = dcel_operations::create_new_face_adjacent_to_edge(
            self.s_mut(),
            convex_hull_edge,
            new_vertex,
        );

        let ccw_walk_start = self.directed_edge(convex_hull_edge).prev().rev().fix();
        let cw_walk_start = self.directed_edge(convex_hull_edge).next().rev().fix();

        self.legalize_edge(convex_hull_edge);

        let mut current_edge = ccw_walk_start;
        loop {
            let handle = self.directed_edge(current_edge);
            let prev = handle.prev();
            current_edge = prev.fix();
            if prev.side_query(position).is_on_left_side() {
                let new_edge = dcel_operations::create_single_face_between_edge_and_next(
                    self.s_mut(),
                    current_edge,
                );
                self.legalize_edge(current_edge);
                current_edge = new_edge;
            } else {
                break;
            }
        }

        let mut current_edge = cw_walk_start;
        loop {
            let handle = self.directed_edge(current_edge);
            let next = handle.next();
            let next_fix = next.fix();
            if next.side_query(position).is_on_left_side() {
                let new_edge = dcel_operations::create_single_face_between_edge_and_next(
                    self.s_mut(),
                    current_edge,
                );
                self.legalize_edge(next_fix);
                current_edge = new_edge;
            } else {
                break;
            }
        }

        result
    }

    fn get_vertex_facing_edges<ForwardPredicate, BackwardPredicate>(
        &self,
        start_edge: FixedDirectedEdgeHandle,
        position: Point2<<Self::Vertex as HasPosition>::Scalar>,
        forward_predicate: ForwardPredicate,
        backward_predicate: BackwardPredicate,
    ) -> VecDeque<FixedDirectedEdgeHandle>
    where
        ForwardPredicate: Fn(
            DirectedEdgeHandle<Self::Vertex, Self::DirectedEdge, Self::UndirectedEdge, Self::Face>,
        ) -> bool,
        BackwardPredicate: Fn(
            DirectedEdgeHandle<Self::Vertex, Self::DirectedEdge, Self::UndirectedEdge, Self::Face>,
        ) -> bool,
    {
        let mut result = VecDeque::with_capacity(8);
        let mut current_edge_forward = self.directed_edge(start_edge);

        debug_assert!(current_edge_forward.side_query(position).is_on_left_side());

        result.push_back(current_edge_forward.fix());

        loop {
            current_edge_forward = current_edge_forward.next();

            if forward_predicate(current_edge_forward) {
                result.push_back(current_edge_forward.fix());
            } else {
                break;
            }
        }

        let mut current_edge_backward = self.directed_edge(start_edge);
        loop {
            current_edge_backward = current_edge_backward.prev();

            if backward_predicate(current_edge_backward) {
                result.push_front(current_edge_backward.fix());
            } else {
                break;
            }
        }
        result
    }

    fn insert_into_face(
        &mut self,
        face: FixedFaceHandle<InnerTag>,
        t: Self::Vertex,
    ) -> FixedVertexHandle {
        let new_handle = dcel_operations::insert_into_triangle(self.s_mut(), t, face);
        self.legalize_vertex(new_handle);
        new_handle
    }

    fn insert_on_edge(
        &mut self,
        edge: FixedDirectedEdgeHandle,
        new_vertex: Self::Vertex,
    ) -> (FixedVertexHandle, [FixedDirectedEdgeHandle; 2]) {
        let edge_handle = self.directed_edge(edge);
        if edge_handle.is_outer_edge() {
            let (new_vertex, [e0, e1]) =
                dcel_operations::split_half_edge(self.s_mut(), edge.rev(), new_vertex);
            (new_vertex, [e1.rev(), e0.rev()])
        } else if edge_handle.rev().is_outer_edge() {
            dcel_operations::split_half_edge(self.s_mut(), edge, new_vertex)
        } else {
            dcel_operations::split_edge(self.s_mut(), edge, new_vertex)
        }
    }

    fn legalize_vertex(&mut self, new_handle: FixedVertexHandle) {
        let edges: SmallVec<[_; 4]> = self
            .vertex(new_handle)
            .out_edges()
            .filter(|e| !e.is_outer_edge())
            .map(|edge| edge.next().fix())
            .collect();

        for edge_to_legalize in edges {
            self.legalize_edge(edge_to_legalize);
        }
    }

    /// The Delaunay property refers to the property that no point lies inside
    /// the circumcircle of any of the triangulation's triangles. Adding a
    /// new point into the triangulations may violate this property, this method
    /// "repairs" it by strategically flipping edges until the property
    /// holds again. Every flip produces more "illegal" edges that may have to
    /// be flipped. However, since any edge is flipped at most once, this
    /// algorithm is known to terminate.
    ///
    /// The given position is the position of a new point may have
    /// invalidated the Delaunay property, the point must be on the left side
    /// of the given edge.
    ///
    /// "Flipping an edge" refers to switching to the other diagonal in a
    /// four sided polygon.
    ///
    /// Returns `true` if at least one edge was flipped. This will always include the initial edge.
    fn legalize_edge(&mut self, edge: FixedDirectedEdgeHandle) -> bool {
        let mut edges: SmallVec<[FixedDirectedEdgeHandle; 8]> = Default::default();
        edges.push(edge);

        let mut result = false;

        while let Some(e) = edges.pop() {
            if !self.is_defined_legal(e.as_undirected()) {
                let edge = self.directed_edge(e);

                //         v2------ v0
                //          |     / |
                //          |    /  |
                //          |   /<-edge that might be flipped ("edge")
                //          |  /    |
                //          | V     |
                //         v1-------v3
                //
                //
                // If the edge is flipped, the new quad will look like this:
                //
                //      New illegal edge
                //              |
                //              V
                //         v2-------v0
                //          | ^     |
                // New      |  \    |
                // illegal->|   \   |
                // edge     |    \  |
                //          |     \ |
                //         v1-------v3
                let v2 = edge.rev().opposite_position();
                let v3 = edge.opposite_position();

                if let (Some(v2), Some(v3)) = (v2, v3) {
                    let v0 = edge.from().position();
                    let v1 = edge.to().position();
                    debug_assert!(math::is_ordered_ccw(v2, v1, v0));
                    let should_flip = math::contained_in_circumference(v2, v1, v0, v3);
                    result |= should_flip;

                    if should_flip {
                        let e1 = edge.rev().next().fix();
                        let e2 = edge.rev().prev().fix();

                        dcel_operations::flip_cw(self.s_mut(), e.as_undirected());
                        edges.push(e1);
                        edges.push(e2);
                    }
                }
            }
        }
        result
    }

    fn validate_vertex_handle(&self, handle: FixedVertexHandle) -> FixedVertexHandle {
        if handle.index() < self.num_vertices() {
            handle
        } else {
            FixedVertexHandle::new(0)
        }
    }

    fn walk_to_nearest_neighbor(
        &self,
        start: FixedVertexHandle,
        position: Point2<<Self::Vertex as HasPosition>::Scalar>,
    ) -> VertexHandle<Self::Vertex, Self::DirectedEdge, Self::UndirectedEdge, Self::Face> {
        let start_position = self.vertex(start).position();

        if start_position == position {
            return self.vertex(start);
        }

        let mut current_minimal_distance = position.distance_2(start_position);
        let mut current_minimum_vertex = self.vertex(start);

        while let Some((next_minimum_index, next_minimal_distance)) = current_minimum_vertex
            .out_edges()
            .filter_map(|out_edge| {
                let next_candidate = out_edge.to();
                let new_distance = next_candidate.position().distance_2(position);
                if new_distance < current_minimal_distance {
                    Some((next_candidate, new_distance))
                } else {
                    None
                }
            })
            .next()
        {
            current_minimal_distance = next_minimal_distance;
            current_minimum_vertex = next_minimum_index;
        }

        current_minimum_vertex
    }

    /// "Walks" through the triangulation until it finds the target point.
    fn locate_with_hint_fixed_core(
        &self,
        target_position: Point2<<Self::Vertex as HasPosition>::Scalar>,
        start: FixedVertexHandle,
    ) -> PositionInTriangulation {
        if self.num_vertices() < 2 {
            return match self.vertices().next() {
                Some(single_vertex) if single_vertex.position() == target_position => {
                    PositionInTriangulation::OnVertex(single_vertex.fix())
                }
                _ => PositionInTriangulation::NoTriangulation,
            };
        }

        let start = self.validate_vertex_handle(start);

        let closest_vertex = self.walk_to_nearest_neighbor(start, target_position);

        let out_edge = closest_vertex
            .out_edge()
            .expect("No out edge found. This is a bug.");

        let mut query = out_edge.side_query(target_position);
        let mut edge = if query.is_on_right_side() {
            out_edge.rev()
        } else {
            out_edge
        };

        let mut loop_counter = self.s().num_directed_edges();

        // Invariant: position is on the left side or on the line of edge.
        loop {
            // Prevent accidental infinite loops (easier to debug). Should never happen
            if loop_counter == 0 {
                panic!("Failed to locate position. This is a bug in spade.")
            }
            loop_counter -= 1;

            let prev = edge.prev();
            if !edge.is_outer_edge() {
                let next = edge.next();
                let next_query = next.side_query(target_position);

                if next_query.is_on_right_side() {
                    edge = next.rev();
                    query = edge.side_query(target_position);
                    continue;
                }

                let prev_query = prev.side_query(target_position);
                if prev_query.is_on_right_side() {
                    edge = prev.rev();
                    query = edge.side_query(target_position);
                    continue;
                }

                self.hint_generator().notify_vertex_lookup(edge.to().fix());

                // Point must be in triangle or on its lines
                return match (
                    query.is_on_line(),
                    next_query.is_on_line(),
                    prev_query.is_on_line(),
                ) {
                    // Point lies on no line and must be inside the face
                    (false, false, false) => {
                        PositionInTriangulation::OnFace(next.face().fix().adjust_inner_outer())
                    }
                    // Point lies on exactly one line
                    (false, false, true) => PositionInTriangulation::OnEdge(prev.fix()),
                    (false, true, false) => PositionInTriangulation::OnEdge(next.fix()),
                    (true, false, false) => PositionInTriangulation::OnEdge(edge.fix()),
                    // Point lies on exactly two lines. Since the edges cannot be collinear,
                    // the point lies on the intersection
                    (false, true, true) => PositionInTriangulation::OnVertex(prev.from().fix()),
                    (true, false, true) => PositionInTriangulation::OnVertex(edge.from().fix()),
                    (true, true, false) => PositionInTriangulation::OnVertex(next.from().fix()),
                    (true, true, true) => panic!("Invalid triangle. This is a bug"),
                };
            } else {
                // Edge is part of the convex hull
                if query.is_on_line() {
                    let rev = edge.rev();
                    if rev.is_outer_edge() {
                        // Both edge and its rev are part of the convex hull,
                        // hence all points must lie on a line
                        return self.locate_if_all_points_on_line(rev, target_position);
                    }
                    // Triangulation is not degenerated. Just continue with the
                    // inner triangle
                    edge = rev;
                    continue;
                } else {
                    self.hint_generator()
                        .notify_vertex_lookup(edge.from().fix());
                    return PositionInTriangulation::OutsideOfConvexHull(edge.fix());
                }
            }
        }
    }

    fn locate_if_all_points_on_line(
        &self,
        start_edge: DirectedEdgeHandle<
            Self::Vertex,
            Self::DirectedEdge,
            Self::UndirectedEdge,
            Self::Face,
        >,
        position: Point2<<Self::Vertex as HasPosition>::Scalar>,
    ) -> PositionInTriangulation {
        let mut current_edge = start_edge;

        let current_projection = start_edge.project_point(position);

        if current_projection.is_before_edge() {
            current_edge = current_edge.rev();
        }

        loop {
            if current_edge.from().position() == position {
                return PositionInTriangulation::OnVertex(current_edge.from().fix());
            }
            if current_edge.to().position() == position {
                return PositionInTriangulation::OnVertex(current_edge.to().fix());
            }

            let current_projection = current_edge.project_point(position);

            if current_projection.is_behind_edge() {
                if current_edge.next() == current_edge.rev() {
                    return PositionInTriangulation::OutsideOfConvexHull(current_edge.fix());
                }
                current_edge = current_edge.next();
            } else {
                // 0.0 <= current_projection <= 1.0
                return PositionInTriangulation::OnEdge(current_edge.fix());
            }
        }
    }

    fn remove_and_notify(&mut self, vertex_to_remove: FixedVertexHandle) -> Self::Vertex {
        let position = self.vertex(vertex_to_remove).position();
        let removal_result = self.remove_core(vertex_to_remove);

        let swapped_in_point = removal_result
            .swapped_in_vertex
            .map(|_| self.vertex(vertex_to_remove).position());

        self.hint_generator_mut().notify_vertex_removed(
            swapped_in_point,
            vertex_to_remove,
            position,
        );

        removal_result.removed_vertex
    }

    fn remove_core(&mut self, vertex_to_remove: FixedVertexHandle) -> RemovalResult<Self::Vertex> {
        if self.num_all_faces() <= 1 {
            return dcel_operations::remove_when_degenerate(self.s_mut(), vertex_to_remove);
        }
        let vertex = self.vertex(vertex_to_remove);

        let mut border_loop = Vec::with_capacity(10);
        let mut convex_hull_edge = None;
        for edge in vertex.out_edges().rev() {
            if edge.is_outer_edge() {
                convex_hull_edge = Some(edge.fix());
                break;
            }
            let border_edge = edge.next();
            border_loop.push(border_edge.fix());
        }

        if let Some(convex_hull_edge) = convex_hull_edge {
            let mut isolation_result = self.isolate_convex_hull_vertex(convex_hull_edge);

            dcel_operations::cleanup_isolated_vertex(self.s_mut(), &mut isolation_result);
            dcel_operations::swap_remove_vertex(self.s_mut(), vertex_to_remove)
        } else {
            let mut isolation_result = dcel_operations::isolate_vertex_and_fill_hole(
                self.s_mut(),
                border_loop,
                vertex_to_remove,
            );
            // Not exactly elegant. IsolateVertexResult should maybe be split into two parts
            let mut new_edges = core::mem::take(&mut isolation_result.new_edges);
            self.legalize_edges_after_removal(&mut new_edges, |edge| {
                !isolation_result.is_new_edge(edge)
            });
            dcel_operations::cleanup_isolated_vertex(self.s_mut(), &mut isolation_result);
            dcel_operations::swap_remove_vertex(self.s_mut(), vertex_to_remove)
        }
    }

    fn isolate_convex_hull_vertex(
        &mut self,
        convex_hull_out_edge: FixedDirectedEdgeHandle,
    ) -> IsolateVertexResult {
        let mut edges_to_validate = Vec::with_capacity(10);
        let mut convex_edges: Vec<FixedDirectedEdgeHandle> = Vec::with_capacity(10);

        let loop_end = self.directed_edge(convex_hull_out_edge);
        let loop_start = loop_end.ccw().fix();
        let mut current = loop_start;
        let loop_end_next = loop_end.next().fix();
        let loop_end = loop_end.fix();

        loop {
            let current_handle = self.directed_edge(current);
            current = current_handle.ccw().fix();
            let edge = current_handle.next().fix();
            convex_edges.push(edge);

            while let &[.., edge1, edge2] = &*convex_edges {
                let edge1 = self.directed_edge(edge1);
                let edge2 = self.directed_edge(edge2);

                let target_position = edge2.to().position();
                // Check if the new edge would violate the convex hull property by turning left
                // The convex hull must only contain curves turning right
                if edge1.side_query(target_position).is_on_left_side() {
                    // Violation detected. It is resolved by flipping the edge that connects
                    // the most recently added edge (edge2) with the point that was removed
                    let edge_to_flip = edge2.prev().fix().rev();
                    dcel_operations::flip_cw(self.s_mut(), edge_to_flip.as_undirected());
                    convex_edges.pop();
                    convex_edges.pop();
                    convex_edges.push(edge_to_flip);
                    edges_to_validate.push(edge_to_flip.as_undirected());
                } else {
                    break;
                }
            }

            if current == loop_end {
                break;
            }
        }

        convex_edges.push(loop_end_next);
        let result = dcel_operations::disconnect_edge_strip(self.s_mut(), convex_edges);
        self.legalize_edges_after_removal(&mut edges_to_validate, |_| false);
        result
    }

    /// After a vertex removal, the created hole is stitched together by connecting
    /// all vertices to a single vertex at the border (triangle fan).
    /// Since these new edges can violate the Delaunay property, it must be restored.
    ///
    /// The algorithm works like this:
    ///  - Add all new edges to an "invalid" list
    ///  - While the invalid list is not empty: Determine if flipping the top edge is
    ///    required to restore the Delaunay property locally.
    ///  - If the edge was flipped: Determine the flip polygon. A flip refers
    ///    to switching the diagonal in a four sided polygon which defines the
    ///    flip polygon.
    ///  - Add all edges of the flip polygon to the invalid list if they were
    ///    newly created. Otherwise, the edge is part of the border loop surrounding
    ///    the hole created after the vertex removal. These are known to be valid and
    ///    need not to be checked
    ///
    /// For more details, refer to
    /// Olivier Devillers. Vertex Removal in Two Dimensional Delaunay Triangulation:
    /// Speed-up by Low Degrees Optimization.
    /// <https://doi.org/10.1016/j.comgeo.2010.10.001>
    ///
    /// Note that the described low degrees optimization is not yet part of this library.
    fn legalize_edges_after_removal<F>(
        &mut self,
        edges_to_validate: &mut Vec<FixedUndirectedEdgeHandle>,
        edge_must_not_be_flipped_predicate: F,
    ) where
        F: Fn(FixedUndirectedEdgeHandle) -> bool,
    {
        while let Some(next_edge) = edges_to_validate.pop() {
            // left----to
            //  |     ^ |
            //  |    /  |
            //  |   /<-edge that might be flipped ("next_edge")
            //  |  /    |
            //  | /     |
            // from----right
            //
            // left, from, right and to define the flip polygon.
            if self.is_defined_legal(next_edge) || edge_must_not_be_flipped_predicate(next_edge) {
                continue;
            }

            let edge = self.directed_edge(next_edge.as_directed());
            let e2 = edge.prev();
            let e4 = edge.rev().prev();

            let from = edge.from().position();
            let to = edge.to().position();
            let left = edge.opposite_position();
            let right = edge.rev().opposite_position();

            let should_flip = match (left, right) {
                (Some(left), Some(right)) => {
                    math::contained_in_circumference(from, to, left, right)
                }
                // Handle special cases when evaluating edges next to the convex hull
                (None, Some(right)) => math::is_ordered_ccw(right, from, to),
                (Some(left), None) => math::is_ordered_ccw(left, to, from),
                (None, None) => {
                    panic!("Unexpected geometry. This is a bug in spade.")
                }
            };

            if should_flip {
                let e1 = edge.next();
                let e3 = edge.rev().next();

                let mut push_if_not_contained = |handle| {
                    if !edges_to_validate.contains(&handle) {
                        edges_to_validate.push(handle);
                    }
                };

                push_if_not_contained(e1.fix().as_undirected());
                push_if_not_contained(e2.fix().as_undirected());
                push_if_not_contained(e3.fix().as_undirected());
                push_if_not_contained(e4.fix().as_undirected());

                let fixed = edge.fix();
                dcel_operations::flip_cw(self.s_mut(), fixed.as_undirected());
            }
        }
    }

    #[cfg(any(test, fuzzing))]
    fn basic_sanity_check(&self) {
        self.s().sanity_check();
        let all_vertices_on_line = self.s().num_faces() <= 1;

        for face in self.s().inner_faces() {
            let triangle = face.vertices();
            // Check that all vertices are stored in ccw orientation
            assert!(math::side_query(
                triangle[0].position(),
                triangle[1].position(),
                triangle[2].position()
            )
            .is_on_left_side(),);
        }

        for edge in self.s().directed_edges() {
            if all_vertices_on_line {
                assert_eq!(edge.face().fix(), dcel_operations::OUTER_FACE_HANDLE)
            } else {
                assert_ne!(edge.face(), edge.rev().face());
            }
            assert_ne!(edge.from(), edge.to());
        }

        if all_vertices_on_line {
            if self.s().num_vertices() > 1 {
                assert_eq!(self.s().num_undirected_edges(), self.s().num_vertices() - 1);
            } else {
                assert_eq!(self.s().num_undirected_edges(), 0);
            }
            assert_eq!(self.s().num_faces(), 1);
        } else {
            let num_inner_edges = self
                .s()
                .directed_edges()
                .filter(|e| !e.face().is_outer())
                .count();

            let num_inner_faces = self.s().num_faces() - 1;
            assert_eq!(num_inner_faces * 3, num_inner_edges);
        }
    }

    #[cfg(any(test, fuzzing))]
    fn sanity_check(&self) {
        self.basic_sanity_check();

        for edge in self.undirected_edges() {
            let edge = edge.as_directed();
            let rev = edge.rev();

            if let (Some(edge_opposite), Some(rev_opposite)) =
                (edge.opposite_position(), rev.opposite_position())
            {
                let from = edge.from().position();
                let to = edge.to().position();
                assert!(!math::contained_in_circumference(
                    from,
                    to,
                    edge_opposite,
                    rev_opposite
                ))
            }
        }
    }
}

#[cfg(test)]
mod test {
    use crate::test_utilities::SEED;
    use crate::test_utilities::*;
    use crate::PositionInTriangulation;
    use crate::TriangulationExt;
    use crate::{
        handles::FixedVertexHandle, DelaunayTriangulation, InsertionError, Point2, Triangulation,
    };
    use rand::distributions::{Distribution, Uniform};
    use rand::{seq::SliceRandom, Rng, SeedableRng};

    use alloc::{vec, vec::Vec};

    #[test]
    fn test_empty() {
        let d = DelaunayTriangulation::<Point2<f32>>::default();
        assert_eq!(d.num_vertices(), 0);
        assert_eq!(d.num_all_faces(), 1);
        assert_eq!(d.num_undirected_edges(), 0);
    }

    #[test]
    fn test_insert_first() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<Point2<f32>>::default();
        d.insert(Point2::default())?;
        assert_eq!(d.num_vertices(), 1);
        assert_eq!(d.num_all_faces(), 1);
        assert_eq!(d.num_undirected_edges(), 0);
        Ok(())
    }

    #[test]
    fn test_insert_second() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::default())?;
        d.insert(Point2::new(0.123, 1.234))?;
        assert_eq!(d.num_vertices(), 2);
        assert_eq!(d.num_all_faces(), 1);
        assert_eq!(d.num_undirected_edges(), 1);
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_third_point() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(1f64, 0f64))?;
        d.insert(Point2::new(0f64, 1f64))?;
        d.insert(Point2::new(1f64, 1f64))?;

        assert_eq!(d.num_vertices(), 3);
        assert_eq!(d.num_all_faces(), 2);
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_five_points() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(1f64, 0f64))?;
        d.insert(Point2::new(0f64, 1f64))?;

        let v3 = Point2::new(0.433_833_144_214_401f64, 0.900_993_231_373_602_9f64);
        let v4 = Point2::new(2.0, 2.0);
        let v5 = Point2::new(0.5, 0.25);
        d.insert(v3)?;
        d.sanity_check();
        d.insert(v4)?;
        d.s().sanity_check();
        d.insert(v5)?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_small_triangulation_iterators() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        assert_eq!(d.all_faces().count(), 1);
        assert_eq!(d.inner_faces().count(), 0);

        d.insert(Point2::new(1f64, 1f64))?;
        assert_eq!(d.all_faces().count(), 1);
        assert_eq!(d.inner_faces().count(), 0);

        d.insert(Point2::new(-1f64, 1f64))?;
        assert_eq!(d.all_faces().count(), 1);
        assert_eq!(d.inner_faces().count(), 0);
        Ok(())
    }

    #[test]
    fn test_iterate_faces() -> Result<(), InsertionError> {
        const SIZE: usize = 1000;
        let points = random_points_with_seed(SIZE, SEED);
        let mut d = DelaunayTriangulation::<Point2<f64>>::bulk_load(points)?;
        d.sanity_check();

        assert_eq!(d.all_faces().count(), d.num_all_faces());
        assert_eq!(d.inner_faces().count(), d.num_inner_faces());

        for _ in 0..SIZE / 2 {
            d.remove(FixedVertexHandle::new(5));
        }

        assert_eq!(d.all_faces().count(), d.num_all_faces());
        assert_eq!(d.inner_faces().count(), d.num_inner_faces());

        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_many_points() -> Result<(), InsertionError> {
        const SIZE: usize = 10000;
        let points = random_points_with_seed(SIZE, SEED);

        let mut d = DelaunayTriangulation::<_>::new();
        for point in points {
            d.insert(point)?;
        }

        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_outside_convex_hull() -> Result<(), InsertionError> {
        const NUM: usize = 100;
        let mut rng = rand::rngs::StdRng::from_seed(*SEED);
        let range = Uniform::new(0., 2.0 * ::core::f64::consts::PI);

        let mut d = DelaunayTriangulation::<_>::default();

        // Insert points on a circle. Every new point lies outside the convex hull.
        for _ in 0..NUM {
            let ang = range.sample(&mut rng);
            let vec = Point2::new(ang.sin(), ang.cos());
            d.insert(vec)?;
        }
        assert_eq!(d.num_vertices(), NUM);
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_same_point_small() -> Result<(), InsertionError> {
        let points = vec![
            Point2::new(0.2, 0.1),
            Point2::new(1.3, 2.2),
            Point2::new(0.0, 0.0),
        ];
        let mut d = DelaunayTriangulation::<_>::bulk_load(points.clone())?;

        for p in &points {
            d.insert(*p)?;
            d.sanity_check();
        }
        assert_eq!(d.num_vertices(), points.len());
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_same_point() -> Result<(), InsertionError> {
        const SIZE: usize = 300;
        let points = random_points_with_seed(SIZE, SEED);
        let mut d = DelaunayTriangulation::<_>::bulk_load(points.clone())?;
        for p in points {
            d.insert(p)?;
        }
        assert_eq!(d.num_vertices(), SIZE);
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_point_on_ch_edge() -> Result<(), InsertionError> {
        let points = vec![
            Point2::new(0., 0f64),
            Point2::new(1., 0.),
            Point2::new(0., 1.),
            Point2::new(0., 0.4),
        ];
        let d = DelaunayTriangulation::<_>::bulk_load(points)?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_on_edges() -> Result<(), InsertionError> {
        let points = vec![Point2::new(0., 0f64), Point2::new(1., 0.)];
        let mut d = DelaunayTriangulation::<_>::bulk_load(points)?;

        d.insert(Point2::new(1., 1.))?;
        d.sanity_check();
        d.insert(Point2::new(0.5, 0.5))?;
        d.sanity_check();
        d.insert(Point2::new(0., 0.4))?;
        d.sanity_check();
        d.insert(Point2::new(1., 0.5))?;
        d.sanity_check();
        d.insert(Point2::new(0.5, 1.))?;
        d.sanity_check();
        d.insert(Point2::new(0.7, 0.))?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_degenerate_triangulation() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        for i in -50..50 {
            d.insert(Point2::new(f64::from(i), 0.))?;
        }

        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_points_on_line() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(0.0, 1.0))?;
        for i in -50..50 {
            d.insert(Point2::new(f64::from(i), 0.))?;
        }
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_points_on_line_2() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(0.0, 1.0))?;

        for i in -50..50 {
            d.insert(Point2::new(f64::from(i), 0.))?;
            d.sanity_check();
        }

        for i in -10..10 {
            d.insert(Point2::new(f64::from(i), 0.5 * f64::from(i)))?;
            d.sanity_check();
        }
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_points_on_grid2() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();

        for y in 0..20 {
            for x in 0..7 {
                d.insert(Point2::new(f64::from(x), f64::from(y)))?;
                d.sanity_check();
            }
        }
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_points_with_increasing_distance() -> Result<(), InsertionError> {
        let mut points = random_points_with_seed(1000, SEED);
        points.sort_by(|p1, p2| p1.length2().partial_cmp(&p2.length2()).unwrap());
        let mut d = DelaunayTriangulation::<_>::new();
        for point in points {
            d.insert(point)?;
        }
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_insert_points_on_grid_with_increasing_distance() -> Result<(), InsertionError> {
        // This test inserts points on a grid with increasing distance from (0., 0.)
        let mut points = Vec::new();
        const SIZE: i64 = 7;
        for x in -SIZE..SIZE {
            for y in -SIZE..SIZE {
                let point = Point2::new(x as f64, y as f64);
                points.push(point);
            }
        }
        points.sort_by(|p1, p2| p1.length2().partial_cmp(&p2.length2()).unwrap());
        let d = DelaunayTriangulation::<_>::bulk_load(points)?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_in_triangle() -> Result<(), InsertionError> {
        let points = vec![
            Point2::new(-1.0, 0.0f64),
            Point2::new(1.0, 0.0f64),
            Point2::new(0.0, 1.0f64),
        ];
        let mut d = DelaunayTriangulation::<_>::bulk_load(points)?;
        let to_remove = d.insert(Point2::new(0.0, 0.5))?;
        d.remove(to_remove);
        assert_eq!(d.num_vertices(), 3);
        // Reinsert the last point, just to see if a crash occurs
        d.insert(Point2::new(0.0, 0.5))?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_complex_single_outer_vertex() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(0.0, 0.0))?;
        d.insert(Point2::new(0.0, 1.0))?;
        d.insert(Point2::new(4.0, 0.0))?;
        d.insert(Point2::new(4.0, 1.0))?;
        d.insert(Point2::new(2.0, 0.5))?;
        d.insert(Point2::new(1.0, 0.5))?;
        d.insert(Point2::new(3.0, 0.5))?;

        let v4_position = Point2::new(2.5, 2.0);
        let v4 = d.insert(v4_position)?;

        let removed = d.remove(v4);
        d.sanity_check();
        assert_eq!(removed, v4_position);
        assert_eq!(d.num_vertices(), 7);
        Ok(())
    }

    #[test]
    fn test_remove_single_outer() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(0.0, 0.0))?;
        d.insert(Point2::new(0.0, 1.0))?;
        d.insert(Point2::new(1.0, 0.0))?;
        let v4_position = Point2::new(1.5, 1.5);
        let v4 = d.insert(v4_position)?;

        let removed = d.remove(v4);
        d.sanity_check();
        assert_eq!(removed, v4_position);
        assert_eq!(d.num_vertices(), 3);
        Ok(())
    }

    #[test]
    fn test_remove_in_quad() -> Result<(), InsertionError> {
        let points = vec![
            Point2::new(0.0, 0.0f64),
            Point2::new(1.0, 0.0f64),
            Point2::new(0.0, 1.0f64),
            Point2::new(1.0, 1.0f64),
        ];

        let mut d = DelaunayTriangulation::<_>::bulk_load(points)?;

        let to_remove = d.insert(Point2::new(0.5, 0.6))?;
        d.remove(to_remove);
        assert_eq!(d.num_vertices(), 4);
        let to_remove = d.insert(Point2::new(0.5, 0.6))?;
        d.remove(to_remove);
        assert_eq!(d.num_vertices(), 4);
        d.insert(Point2::new(0.5, 0.6))?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_star_shaped() -> Result<(), InsertionError> {
        let mut rng = rand::rngs::StdRng::from_seed(*SEED);
        let mut points = vec![
            Point2::new(0.0, 0.0),
            Point2::new(1.0, 1.0),
            Point2::new(1.0, -1.0),
            Point2::new(-1.0, 1.0),
            Point2::new(-1.0, -1.0),
            Point2::new(0.0, 3.0),
            Point2::new(0.0, -3.0),
            Point2::new(3.0, 0.0),
            Point2::new(-3.0, 0.0),
        ];

        points.shuffle(&mut rng);
        points.shuffle(&mut rng);
        points.shuffle(&mut rng);
        for _ in 0..20 {
            points.shuffle(&mut rng);
            let mut d = DelaunayTriangulation::<_>::bulk_load(points.clone())?;
            d.locate_and_remove(Point2::new(0.0, 0.0));
            d.sanity_check();
        }
        Ok(())
    }

    #[test]
    fn test_remove_inner() -> Result<(), InsertionError> {
        use ::rand::SeedableRng;
        use PositionInTriangulation::OnVertex;

        let mut points = random_points_with_seed(1000, SEED);
        let mut d = DelaunayTriangulation::<_>::bulk_load(points.clone())?;

        // Insert an outer quad since we don't want to remove vertices from
        // the convex hull.
        d.insert(Point2::new(-2.0, -2.0))?;
        d.insert(Point2::new(-2.0, 2.0))?;
        d.insert(Point2::new(2.0, -2.0))?;
        d.insert(Point2::new(2.0, 2.0))?;
        // Now remove all inner points
        let mut rng = rand::rngs::StdRng::from_seed(*SEED);
        points.shuffle(&mut rng);
        assert_eq!(d.num_vertices(), 1004);
        for point in points {
            match d.locate(point) {
                OnVertex(handle) => {
                    d.remove(handle);
                }
                _ => panic!("Point lookup failed: {:?}", point),
            }
        }
        assert_eq!(d.num_vertices(), 4);
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_outer() -> Result<(), InsertionError> {
        use PositionInTriangulation::OnVertex;

        let mut points = random_points_with_seed(1000, SEED);
        let mut d = DelaunayTriangulation::<_>::bulk_load(points.clone())?;

        points.sort_by(|p1, p2| p1.length2().partial_cmp(&p2.length2()).unwrap());
        for (index, point) in points[3..].iter().rev().enumerate() {
            match d.locate(*point) {
                OnVertex(handle) => {
                    d.remove(handle);
                    if index % 50 == 0 {
                        // Check only every 50 iterations to reduce test runtime
                        d.sanity_check();
                    }
                }
                _ => panic!("Point lookup failed: {:?}", point),
            }
        }
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_removal_and_insertion() -> Result<(), InsertionError> {
        let points = random_points_with_seed(1000, SEED);
        let mut d = DelaunayTriangulation::<_>::bulk_load(points)?;

        let mut rng = rand::rngs::StdRng::from_seed(*SEED);
        for _ in 0..1000 {
            if rng.gen() {
                // Insert new random point
                let x = rng.gen();
                let y = rng.gen();
                d.insert(Point2::new(x, y))?;
            } else {
                // Remove random point
                let range = Uniform::new(1, d.num_vertices());
                let handle = range.sample(&mut rng);
                d.remove(FixedVertexHandle::new(handle));
            }
        }
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_until_empty() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::bulk_load(vec![
            Point2::new(0.0, 0.0),
            Point2::new(0.0, 1.0),
            Point2::new(1.0, 2.0),
        ])?;

        while let Some(to_remove) = d.vertices().next() {
            let to_remove = to_remove.fix();
            d.remove(to_remove);
            d.sanity_check();
        }

        assert_eq!(d.num_vertices(), 0);

        d.insert(Point2::new(1.0, 0.0))?;
        d.insert(Point2::new(1.0, 1.0))?;
        d.insert(Point2::new(1.0, 2.0))?;
        d.insert(Point2::new(2.3, 1.4))?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_until_degenerate() -> Result<(), InsertionError> {
        let points = vec![
            Point2::new(0., 0f64),
            Point2::new(1., 0.),
            Point2::new(0., 1.),
            Point2::new(0., 0.5),
            Point2::new(0., 0.25),
            Point2::new(0., 0.75),
        ];
        let mut d = DelaunayTriangulation::<_>::bulk_load(points)?;

        assert_eq!(d.num_all_faces(), 5);
        d.locate_and_remove(Point2::new(1., 0.));
        d.sanity_check();
        assert!(d.all_vertices_on_line());

        while let Some(to_remove) = d.vertices().next() {
            let to_remove = to_remove.fix();
            d.remove(to_remove);
            d.sanity_check();
        }

        d.sanity_check();
        d.insert(Point2::new(0.5, 0.5))?;
        d.insert(Point2::new(0.2, 0.5))?;
        d.insert(Point2::new(1.5, 0.0))?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_few_points() -> Result<(), InsertionError> {
        let mut triangulation = DelaunayTriangulation::<_>::bulk_load(vec![
            Point2::new(0.0, 1.0),
            Point2::new(100.0, 1.0),
            Point2::new(0.0, 110.0),
            Point2::new(110.110, 110.0),
            Point2::new(50.0, 50.0),
            Point2::new(50.0, 80.0),
            Point2::new(75.0, 80.0),
        ])?;

        triangulation.remove(FixedVertexHandle::new(5));
        triangulation.sanity_check();
        triangulation.remove(FixedVertexHandle::new(5));
        triangulation.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_on_line_small() -> Result<(), InsertionError> {
        let mut triangulation = DelaunayTriangulation::<_>::bulk_load(vec![
            Point2::new(0.0, 0.0),
            Point2::new(1.0, 0.0), // This point will be removed
            Point2::new(2.0, 0.0),
        ])?;
        triangulation.remove(FixedVertexHandle::new(2));
        triangulation.sanity_check();
        Ok(())
    }

    #[test]
    fn test_remove_on_line_big() -> Result<(), InsertionError> {
        let mut triangulation = DelaunayTriangulation::<_>::default();
        for x in 2..100 {
            triangulation.insert(Point2::new(f64::from(x), 0.0))?;
        }
        let mut rng = rand::rngs::StdRng::from_seed(*SEED);
        while triangulation.num_vertices() > 3 {
            if rng.gen() {
                triangulation.remove(FixedVertexHandle::new(1));
            } else {
                triangulation.remove(FixedVertexHandle::new(2));
            }

            triangulation.sanity_check();
        }
        Ok(())
    }

    #[test]
    fn test_small_insert_on_line() -> Result<(), InsertionError> {
        let mut d = DelaunayTriangulation::<_>::default();
        d.insert(Point2::new(0.0, 0.0))?;
        d.insert(Point2::new(2.0, 0.0))?;
        d.insert(Point2::new(1.0, 0.0))?;
        d.sanity_check();
        Ok(())
    }

    #[test]
    fn test_locate_when_empty() {
        let triangulation = DelaunayTriangulation::<Point2<_>>::default();
        assert_eq!(
            triangulation.locate(Point2::new(0.0, 0.0)),
            PositionInTriangulation::NoTriangulation
        )
    }

    #[test]
    fn test_locate_with_single_vertex() -> Result<(), InsertionError> {
        let mut triangulation = DelaunayTriangulation::<_>::default();
        let point = Point2::new(0.0, 0.0);
        triangulation.insert(point)?;
        assert_eq!(
            triangulation.locate(point),
            PositionInTriangulation::OnVertex(FixedVertexHandle::new(0))
        );
        assert_eq!(
            triangulation.locate(Point2::new(1.0, 1.0)),
            PositionInTriangulation::NoTriangulation
        );
        Ok(())
    }

    #[test]
    fn test_remove_on_line_end() -> Result<(), InsertionError> {
        let mut triangulation = DelaunayTriangulation::<_>::bulk_load(vec![
            Point2::new(0.0, 0.0),
            Point2::new(1.0, 0.0),
            Point2::new(2.0, 0.0),
        ])?;
        triangulation.remove(FixedVertexHandle::new(2));
        triangulation.sanity_check();
        Ok(())
    }
}