//! Semantic regions and connectivity (spec §4.5).
//!
//! Where [`crate::grid::Grid`] answers *what tile is at a cell*, this module
//! answers *what areas exist and how they connect*. A [`Region`] is a named
//! semantic area (a room, a corridor) with a bounding box and the exact set of
//! cells it occupies; a [`ConnGraph`] records which regions are linked and,
//! once doors are placed, where the link physically sits.
//!
//! The shared vocabulary here is deliberately map-type agnostic: a dungeon is
//! rooms joined by corridor edges, while a future town would be districts and
//! buildings joined by road edges — same types, different passes producing
//! them.
//!
//! All types derive `serde::{Serialize, Deserialize}` so they can travel inside
//! a [`crate::map::Map`] through JSON (spec §3).

use serde::{Deserialize, Serialize};

use crate::geometry::{Point, Rect};

/// A handle to a [`Region`], equal to its index in the region list.
///
/// **Invariant:** `RegionId(n)` is always the index `n` of that region in
/// `ctx.regions` / [`crate::map::Map::regions`]. Regions are append-only — never
/// removed or reordered — so this id-as-index relationship holds for the life of
/// the map. Edges in the [`ConnGraph`] and any lookup that turns an id back into
/// a region rely on it.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
pub struct RegionId(pub usize);

/// What kind of semantic area a [`Region`] represents.
///
/// Starts minimal; new variants are additive (later: `District`, `Plaza`,
/// `Building`). Passes match only on the kinds they care about.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub enum RegionKind {
    /// A room: a carved, typically rectangular open area.
    Room,
    /// A corridor: a thin passage carved to connect rooms.
    Corridor,
}

/// A semantic area of the map: an id, a kind, a bounding box, and its cells.
///
/// `bounds` is the axis-aligned bounding box; `cells` is the exact set of
/// occupied cells, which lets non-rectangular regions (e.g. cave blobs) be
/// represented faithfully rather than only as rectangles. Fields are `pub`
/// because passes and visualization read them directly across modules.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Region {
    /// This region's stable id, equal to its index in the region list.
    pub id: RegionId,
    /// The semantic category of this region.
    pub kind: RegionKind,
    /// The axis-aligned bounding box enclosing all of `cells`.
    pub bounds: Rect,
    /// The exact cells this region occupies, in the order they were added.
    pub cells: Vec<Point>,
}

/// A connection between two regions in the [`ConnGraph`].
///
/// `a` and `b` are the linked regions. `at` is the physical location of the
/// connection (the door cell): it is [`None`] when the edge is first created by
/// `MstConnect`, and is filled in later by `DoorPlacer`. Refines spec §4.5,
/// where `at` was sketched as a bare `Point` — the door location simply is not
/// known until placement runs. Fields are `pub` because `DoorPlacer` writes `at`
/// and visualization reads all three.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Edge {
    /// One endpoint region.
    pub a: RegionId,
    /// The other endpoint region.
    pub b: RegionId,
    /// Where the connection physically sits, once known (e.g. the door cell).
    pub at: Option<Point>,
}

/// The region connectivity graph: nodes are [`RegionId`]s, edges are [`Edge`]s.
///
/// Built incrementally — `MstConnect` adds the edges that should exist (with no
/// location yet), then `DoorPlacer` populates each edge's `at`. The graph stores
/// edges in insertion order, which is deterministic and the order
/// [`ConnGraph::edges`] yields them in.
#[derive(Clone, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct ConnGraph {
    edges: Vec<Edge>,
}

impl ConnGraph {
    /// Creates an empty connectivity graph.
    pub fn new() -> Self {
        ConnGraph::default()
    }

    /// Records that regions `a` and `b` are connected.
    ///
    /// The new edge's location is left as `at: None`; `DoorPlacer` fills it in
    /// later. Edges are appended, so the order they were added is preserved.
    pub fn add_edge(&mut self, a: RegionId, b: RegionId) {
        self.edges.push(Edge { a, b, at: None });
    }

    /// Returns the edges in insertion order.
    pub fn edges(&self) -> &[Edge] {
        &self.edges
    }

    /// Returns the edges mutably, so `DoorPlacer` can populate each `at`.
    pub fn edges_mut(&mut self) -> &mut [Edge] {
        &mut self.edges
    }

    /// Iterates the regions directly connected to `id`.
    ///
    /// For every edge touching `id`, yields the *other* endpoint. An undirected
    /// link `{a, b}` therefore appears from both sides: `neighbors(a)` yields `b`
    /// and `neighbors(b)` yields `a`. Self-loops (`a == b == id`) yield `id`. The
    /// iteration order follows the edges' insertion order.
    pub fn neighbors(&self, id: RegionId) -> impl Iterator<Item = RegionId> + '_ {
        self.edges.iter().filter_map(move |e| {
            if e.a == id {
                Some(e.b)
            } else if e.b == id {
                Some(e.a)
            } else {
                None
            }
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn add_edge_stores_none_location() {
        let mut g = ConnGraph::new();
        g.add_edge(RegionId(0), RegionId(1));

        assert_eq!(g.edges().len(), 1);
        let e = g.edges()[0];
        assert_eq!(e.a, RegionId(0));
        assert_eq!(e.b, RegionId(1));
        assert_eq!(e.at, None, "MstConnect leaves the door location unknown");
    }

    #[test]
    fn neighbors_returns_both_endpoints_counterparts() {
        let mut g = ConnGraph::new();
        g.add_edge(RegionId(0), RegionId(1));
        g.add_edge(RegionId(1), RegionId(2));

        // From region 1 we can reach both of its neighbors.
        let n1: Vec<RegionId> = g.neighbors(RegionId(1)).collect();
        assert_eq!(n1, vec![RegionId(0), RegionId(2)]);

        // The link is undirected: each endpoint sees the other.
        let n0: Vec<RegionId> = g.neighbors(RegionId(0)).collect();
        assert_eq!(n0, vec![RegionId(1)]);
        let n2: Vec<RegionId> = g.neighbors(RegionId(2)).collect();
        assert_eq!(n2, vec![RegionId(1)]);
    }

    #[test]
    fn edges_mut_lets_door_placer_populate_location() {
        let mut g = ConnGraph::new();
        g.add_edge(RegionId(0), RegionId(1));

        // Simulate DoorPlacer writing the door cell into the edge.
        g.edges_mut()[0].at = Some(Point::new(4, 7));
        assert_eq!(g.edges()[0].at, Some(Point::new(4, 7)));
    }
}