//! Provides [`force_order`], a tool for fixing the order of nodes in a Hugr.
use std::{cmp::Reverse, collections::BinaryHeap, iter};

use hugr_core::{
    HugrView as _, Node,
    hugr::{HugrError, hugrmut::HugrMut},
    ops::{OpTag, OpTrait},
    types::EdgeKind,
};
use itertools::Itertools as _;
use petgraph::{
    Direction::Incoming,
    visit::{
        GraphBase, GraphRef, IntoNeighbors as _, IntoNeighborsDirected, IntoNodeIdentifiers,
        NodeFiltered, VisitMap, Visitable, Walker,
    },
};

/// Insert order edges into a Hugr according to a rank function.
///
/// All dataflow parents which are transitive children of `root`, including
/// `root` itself, will have their dataflow regions ordered.
///
/// Dataflow regions are ordered by inserting order edges between their
/// immediate children. A dataflow parent with `C` children will have at most
/// `C-1` edges added. Any node that can be ordered will be.
///
/// Nodes are ordered according to the `rank` function but respecting the order
/// required for their dependencies (edges). The algorithm will put nodes of
/// lower rank earlier in their parent whenever (transitive) dependencies allow.
/// If `rank(n1) < rank(n2)` then `n1` will be ordered before `n2` so long as
/// there is no path from `n2` to `n1` (otherwise this would invalidate `hugr`).
/// Nodes of equal rank will be ordered arbitrarily, although that arbitrary
/// order is deterministic.
pub fn force_order<H: HugrMut<Node = Node>>(
    hugr: &mut H,
    root: Node,
    rank: impl Fn(&H, Node) -> i64,
) -> Result<(), HugrError> {
    force_order_by_key(hugr, root, rank)
}

/// As [`force_order`], but allows a generic [Ord] choice for the result of the
/// `rank` function.
pub fn force_order_by_key<H: HugrMut<Node = Node>, K: Ord>(
    hugr: &mut H,
    root: Node,
    rank: impl Fn(&H, Node) -> K,
) -> Result<(), HugrError> {
    let dataflow_parents = hugr
        .descendants(root)
        .filter(|n| hugr.get_optype(*n).tag() <= OpTag::DataflowParent)
        .collect_vec();
    for dp in dataflow_parents {
        // we filter out the input and output nodes from the topological sort
        let [i, o] = hugr.get_io(dp).unwrap();
        let ordered_nodes = {
            let sg = hugr.scheduling_graph(dp);
            let rank = |n| rank(hugr, sg.pg_to_node(n));
            let i_pg = sg.node_to_pg(i);
            let o_pg = sg.node_to_pg(o);
            let petgraph = NodeFiltered::from_fn(sg.petgraph(), |x| x != i_pg && x != o_pg);
            ForceOrder::<_, portgraph::NodeIndex, _, _>::new(&petgraph, &rank)
                .iter(&petgraph)
                .filter_map(|x| {
                    let x = sg.pg_to_node(x);
                    let expected_edge = Some(EdgeKind::StateOrder);
                    let optype = hugr.get_optype(x);
                    if optype.other_input() == expected_edge
                        || optype.other_output() == expected_edge
                    {
                        assert_eq!(
                            optype.other_input(),
                            optype.other_output(),
                            "Optype does not have both input and output order edge: {optype}"
                        );
                        Some(x)
                    } else {
                        None
                    }
                })
                .collect_vec()
        };

        // we iterate over the topologically sorted nodes, prepending the input
        // node and suffixing the output node.
        for (&n1, &n2) in iter::once(&i)
            .chain(ordered_nodes.iter())
            .chain(iter::once(&o))
            .tuple_windows()
        {
            let (n1_ot, n2_ot) = (hugr.get_optype(n1), hugr.get_optype(n2));
            if !hugr.output_neighbours(n1).contains(&n2) {
                hugr.connect(
                    n1,
                    n1_ot.other_output_port().unwrap(),
                    n2,
                    n2_ot.other_input_port().unwrap(),
                );
            }
        }
    }

    Ok(())
}

/// An adaption of [`petgraph::visit::Topo`]. We differ only in that we sort nodes
/// by the rank function before adding them to the internal work stack. This
/// ensures we visit lower ranked nodes before higher ranked nodes whenever the
/// topology of the graph allows.
#[derive(Clone)]
struct ForceOrder<K, N, VM, F> {
    tovisit: BinaryHeap<(Reverse<K>, N)>,
    ordered: VM,
    rank: F,
}

impl<K: Ord, N: Ord, VM, F: Fn(N) -> K> ForceOrder<K, N, VM, F>
where
    N: Copy + PartialEq,
    VM: VisitMap<N>,
{
    pub fn new<G>(graph: G, rank: F) -> Self
    where
        G: IntoNodeIdentifiers + IntoNeighborsDirected + Visitable<NodeId = N, Map = VM>,
    {
        let mut topo = Self::empty(graph, rank);
        topo.extend_with_initials(graph);
        topo
    }

    fn empty<G>(graph: G, rank: F) -> Self
    where
        G: GraphRef + Visitable<NodeId = N, Map = VM>,
    {
        Self {
            ordered: graph.visit_map(),
            tovisit: Default::default(),
            rank,
        }
    }

    fn extend_with_initials<G>(&mut self, g: G)
    where
        G: IntoNodeIdentifiers + IntoNeighborsDirected<NodeId = N>,
    {
        // find all initial nodes (nodes without incoming edges)
        self.extend(
            g.node_identifiers()
                .filter(move |&a| g.neighbors_directed(a, Incoming).next().is_none()),
        );
    }

    fn extend(&mut self, new_nodes: impl IntoIterator<Item = N>) {
        self.tovisit
            .extend(new_nodes.into_iter().map(|x| (Reverse((self.rank)(x)), x)));
    }

    /// Return the next node in the current topological order traversal, or
    /// `None` if the traversal is at the end.
    ///
    /// *Note:* The graph may not have a complete topological order, and the only
    /// way to know is to run the whole traversal and make sure it visits every node.
    pub fn next<G>(&mut self, g: G) -> Option<N>
    where
        G: IntoNeighborsDirected + Visitable<NodeId = N, Map = VM>,
    {
        // Take an unvisited element and find which of its neighbors are next
        while let Some((_, nix)) = self.tovisit.pop() {
            if self.ordered.is_visited(&nix) {
                continue;
            }
            self.ordered.visit(nix);
            // Look at each neighbor, and those that only have incoming edges
            // from the already ordered list, they are the next to visit.
            let new_nodes = g
                .neighbors(nix)
                .filter(|&n| {
                    petgraph::visit::Reversed(g)
                        .neighbors(n)
                        .all(|b| self.ordered.is_visited(&b))
                })
                .collect_vec();

            self.extend(new_nodes);
            return Some(nix);
        }
        None
    }
}

impl<K: Ord, G: Visitable + IntoNeighborsDirected, F: Fn(G::NodeId) -> K> Walker<G>
    for ForceOrder<K, G::NodeId, G::Map, F>
where
    G::NodeId: Ord,
{
    type Item = <G as GraphBase>::NodeId;

    fn walk_next(&mut self, g: G) -> Option<Self::Item> {
        self.next(g)
    }
}

#[cfg(test)]
mod test {
    use std::collections::HashMap;

    use super::*;
    use hugr_core::builder::{BuildHandle, Dataflow, DataflowHugr, endo_sig};
    use hugr_core::ops::handle::{DataflowOpID, NodeHandle};

    use hugr_core::ops::{self, Value};
    use hugr_core::std_extensions::arithmetic::int_ops::IntOpDef;
    use hugr_core::std_extensions::arithmetic::int_types::INT_TYPES;
    use hugr_core::types::{Signature, Type};
    use hugr_core::{HugrView, Wire};
    use hugr_core::{builder::DFGBuilder, hugr::Hugr};

    use petgraph::visit::Topo;

    const I: u8 = 3;

    fn build_neg(builder: &mut impl Dataflow, wire: Wire) -> BuildHandle<DataflowOpID> {
        builder
            .add_dataflow_op(IntOpDef::ineg.with_log_width(I), [wire])
            .unwrap()
    }

    fn build_add(builder: &mut impl Dataflow, w1: Wire, w2: Wire) -> BuildHandle<DataflowOpID> {
        builder
            .add_dataflow_op(IntOpDef::iadd.with_log_width(I), [w1, w2])
            .unwrap()
    }

    /// Our tests use the following hugr:
    ///
    /// a DFG with sig: [i8,i8] -> [i8,i8]
    ///
    ///         Input
    ///    |        |
    ///    | iw1    | iw2
    ///   v0(neg)  v1(neg)
    ///    |   \    |
    ///    |    \   |
    ///    |     \  |
    ///   v2(neg)  v3(add)
    ///    |        |
    ///      Output
    fn test_hugr() -> (Hugr, [Node; 4]) {
        let t = INT_TYPES[I as usize].clone();
        let mut builder = DFGBuilder::new(endo_sig(vec![t.clone(), t.clone()])).unwrap();
        let [iw1, iw2] = builder.input_wires_arr();
        let v0 = build_neg(&mut builder, iw1);
        let v1 = build_neg(&mut builder, iw2);
        let v2 = build_neg(&mut builder, v0.out_wire(0));
        let v3 = build_add(&mut builder, v0.out_wire(0), v1.out_wire(0));
        let nodes = [v0, v1, v2, v3]
            .into_iter()
            .map(|x| x.handle().node())
            .collect_vec()
            .try_into()
            .unwrap();
        (
            builder
                .finish_hugr_with_outputs([v2.out_wire(0), v3.out_wire(0)])
                .unwrap(),
            nodes,
        )
    }

    type RankMap = HashMap<Node, i64>;

    fn force_order_test_impl(hugr: &mut Hugr, rank_map: RankMap) -> Vec<Node> {
        force_order(hugr, hugr.entrypoint(), |_, n| {
            *rank_map.get(&n).unwrap_or(&0)
        })
        .unwrap();

        let sg = hugr.scheduling_graph(hugr.entrypoint());

        let topo_sorted = Topo::new(sg.petgraph())
            .iter(sg.petgraph())
            .map(|n| sg.pg_to_node(n))
            .filter(|n| rank_map.contains_key(n))
            .collect_vec();
        hugr.validate().unwrap();

        topo_sorted
    }

    #[test]
    fn test_force_order_1() {
        let (mut hugr, [v0, v1, v2, v3]) = test_hugr();

        // v0 has a higher rank than v2, but v0 dominates v2, so cannot be
        // ordered before it.
        //
        // v1 and v3 are pushed to the bottom of the graph with high weights.
        let rank_map = [(v0, 2), (v2, 1), (v1, 10), (v3, 9)].into_iter().collect();

        let topo_sort = force_order_test_impl(&mut hugr, rank_map);
        assert_eq!(vec![v0, v2, v1, v3], topo_sort);
    }

    #[test]
    fn test_force_order_2() {
        let (mut hugr, [v0, v1, v2, v3]) = test_hugr();

        // v1 and v3 are pulled to the top of the graph with low weights.
        // v3 cannot ascend past v0 because it is dominated by v0
        let rank_map = [(v0, 2), (v2, 1), (v1, -10), (v3, -9)]
            .into_iter()
            .collect();
        let topo_sort = force_order_test_impl(&mut hugr, rank_map);
        assert_eq!(vec![v1, v0, v3, v2], topo_sort);
    }

    #[test]
    fn test_force_order_3() {
        let (mut hugr, [v0, v1, v2, v3]) = test_hugr();
        let rank_map = [(v0, 0), (v1, 1), (v2, 2), (v3, 3)].into_iter().collect();
        let topo_sort = force_order_test_impl(&mut hugr, rank_map);
        assert_eq!(vec![v0, v1, v2, v3], topo_sort);
    }

    #[test]
    fn test_force_order_const() {
        let mut hugr = {
            let mut builder =
                DFGBuilder::new(Signature::new(Type::EMPTY_TYPEROW, [Type::UNIT])).unwrap();
            let unit = builder.add_load_value(Value::unary_unit_sum());
            builder.finish_hugr_with_outputs([unit]).unwrap()
        };
        let root = hugr.entrypoint();
        force_order(&mut hugr, root, |_, _| 0).unwrap();
    }

    #[test]
    /// test for <https://github.com/CQCL/hugr/issues/2005>
    fn call_indirect_bug() {
        let fn_type = Signature::new_endo([Type::UNIT]);
        let mut hugr = {
            let mut builder = DFGBuilder::new(Signature::new(
                vec![Type::new_function(fn_type.clone()), Type::UNIT],
                vec![Type::UNIT, Type::UNIT],
            ))
            .unwrap();
            let out = builder
                .add_dataflow_op(
                    ops::CallIndirect { signature: fn_type },
                    builder.input_wires(),
                )
                .unwrap()
                .out_wire(0);
            // requires another op to induce an order edge
            let other_unit = builder.add_load_value(Value::unary_unit_sum());
            builder.finish_hugr_with_outputs([out, other_unit]).unwrap()
        };
        let root = hugr.entrypoint();

        force_order(&mut hugr, root, |_, _| 0).unwrap();
    }
}
