use bevy::prelude::*;

use crate::{
    building::{Blueprints, BuildKind},
    colonist::Colonist,
    sim::{SimSystems, SimTick, TICKS_PER_HOUR},
    structures::Structure,
    world::{Terrain, TileChanged, TilePos, WorldMap},
    zones::Zones,
};

pub struct PathsPlugin;

impl Plugin for PathsPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(
            FixedUpdate,
            (
                plan_paving.in_set(SimSystems::Decide),
                update_paths.in_set(SimSystems::Act),
            ),
        );
    }
}

pub const PATH_WEAR: f32 = 8.0;
pub const PAVE_WEAR: f32 = 20.0;
pub const BUSY_WEAR: f32 = PAVE_WEAR;

pub fn busy_path(map: &WorldMap, tile: IVec2) -> bool {
    map.get(tile) == Some(Terrain::Paved) || map.wear(tile) >= BUSY_WEAR
}
pub const REGROW_WEAR: f32 = 1.0;
pub const WEAR_KEPT_PER_HOUR: f32 = 0.98;
pub const PAVE_BATCH: usize = 8;
const UPDATE_INTERVAL_TICKS: u64 = TICKS_PER_HOUR;

pub fn wears_into_path(terrain: Terrain) -> bool {
    matches!(terrain, Terrain::Grass | Terrain::Soil | Terrain::Sand)
}

pub fn evolve(map: &mut WorldMap, zones: &Zones, mut on_change: impl FnMut(UVec2)) {
    for pos in map.worn_tiles() {
        let p = pos.as_ivec2();
        let wear = map.wear(p) * WEAR_KEPT_PER_HOUR;
        map.set_wear(pos, wear);
        let Some(terrain) = map.get(p) else {
            continue;
        };
        let becomes = if wears_into_path(terrain)
            && wear >= PATH_WEAR
            && map.plant(p).is_none()
            && !zones.is_farm(pos)
        {
            Terrain::Path
        } else if terrain == Terrain::Path && wear < REGROW_WEAR {
            Terrain::Grass
        } else {
            continue;
        };
        map.set(pos, becomes);
        on_change(pos);
    }
}

fn update_paths(
    tick: Res<SimTick>,
    zones: Res<Zones>,
    mut map: ResMut<WorldMap>,
    mut changed: MessageWriter<TileChanged>,
) {
    if !tick.0.is_multiple_of(UPDATE_INTERVAL_TICKS) {
        return;
    }
    evolve(&mut map, &zones, |pos| {
        changed.write(TileChanged(pos));
    });
}

pub fn paving_candidates(
    map: &WorldMap,
    blueprints: &Blueprints,
    occupied: &[UVec2],
) -> Vec<UVec2> {
    let pending = blueprints
        .0
        .values()
        .filter(|k| **k == BuildKind::Pave)
        .count();
    let mut busiest: Vec<UVec2> = map
        .worn_tiles()
        .into_iter()
        .filter(|&p| {
            map.get(p.as_ivec2()) == Some(Terrain::Path)
                && map.wear(p.as_ivec2()) >= PAVE_WEAR
                && blueprints.get(p).is_none()
                && !occupied.contains(&p)
        })
        .collect();
    busiest.sort_by(|a, b| {
        map.wear(b.as_ivec2())
            .total_cmp(&map.wear(a.as_ivec2()))
            .then((a.y, a.x).cmp(&(b.y, b.x)))
    });
    busiest.truncate(PAVE_BATCH.saturating_sub(pending));
    busiest
}

fn plan_paving(
    tick: Res<SimTick>,
    map: Res<WorldMap>,
    structures: Query<&TilePos, (With<Structure>, Without<Colonist>)>,
    mut blueprints: ResMut<Blueprints>,
) {
    if !tick.0.is_multiple_of(UPDATE_INTERVAL_TICKS) {
        return;
    }
    let occupied: Vec<UVec2> = structures.iter().map(|p| p.0).collect();
    for tile in paving_candidates(&map, &blueprints, &occupied) {
        blueprints.0.insert(tile, BuildKind::Pave);
    }
}

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

    fn worn(terrain: Terrain, wear: f32) -> WorldMap {
        let mut map = WorldMap::filled(UVec2::splat(3), terrain);
        map.set_wear(UVec2::ONE, wear);
        map
    }

    #[test]
    fn busy_ground_wears_into_a_path() {
        let mut map = worn(Terrain::Grass, PATH_WEAR * 2.0);
        let mut changes = 0;
        evolve(&mut map, &Zones::default(), |_| changes += 1);
        assert_eq!(map.get(IVec2::ONE), Some(Terrain::Path));
        assert_eq!(map.get(IVec2::ZERO), Some(Terrain::Grass));
        assert_eq!(changes, 1);
    }

    #[test]
    fn farms_plants_and_rock_floors_keep_their_ground() {
        let mut farm_map = worn(Terrain::Soil, PATH_WEAR * 2.0);
        let mut zones = Zones::default();
        zones.add_farm_tiles(&farm_map, [UVec2::ONE]);
        evolve(&mut farm_map, &zones, |_| {});
        assert_eq!(farm_map.get(IVec2::ONE), Some(Terrain::Soil));

        let mut bush_map = worn(Terrain::Grass, PATH_WEAR * 2.0);
        bush_map.set_plant(UVec2::ONE, Some(crate::world::Plant::BerryBush));
        evolve(&mut bush_map, &Zones::default(), |_| {});
        assert_eq!(bush_map.get(IVec2::ONE), Some(Terrain::Grass));

        let mut gravel = worn(Terrain::Gravel, PATH_WEAR * 2.0);
        evolve(&mut gravel, &Zones::default(), |_| {});
        assert_eq!(gravel.get(IVec2::ONE), Some(Terrain::Gravel));
    }

    #[test]
    fn unused_paths_grow_back() {
        let mut map = worn(Terrain::Path, 0.5);
        evolve(&mut map, &Zones::default(), |_| {});
        assert_eq!(map.get(IVec2::ONE), Some(Terrain::Grass));
    }

    // Only worn tiles are visited each hour (a full-map pass every game hour dominated frame time
    // at high speeds), so faded wear must drop out of the worn set and stray paths must still
    // regrow before that happens.
    #[test]
    fn faded_paths_regrow_and_leave_the_worn_set() {
        let mut map = worn(Terrain::Path, PATH_WEAR);
        for _ in 0..500 {
            evolve(&mut map, &Zones::default(), |_| {});
        }
        assert_eq!(map.get(IVec2::ONE), Some(Terrain::Grass));
        assert!(map.worn_tiles().is_empty());
    }

    #[test]
    fn wear_fades_over_time() {
        let mut map = worn(Terrain::Grass, 10.0);
        evolve(&mut map, &Zones::default(), |_| {});
        assert!(map.wear(IVec2::ONE) < 10.0);
    }

    #[test]
    fn the_busiest_paths_are_paved_first_in_batches() {
        let mut map = WorldMap::filled(UVec2::new(20, 1), Terrain::Path);
        for x in 0..20 {
            map.set_wear(UVec2::new(x, 0), PAVE_WEAR + x as f32);
        }
        let mut blueprints = Blueprints::default();
        blueprints.0.insert(UVec2::new(19, 0), BuildKind::Pave);
        let picked = paving_candidates(&map, &blueprints, &[UVec2::new(18, 0)]);
        assert_eq!(picked.len(), PAVE_BATCH - 1);
        assert_eq!(picked[0], UVec2::new(17, 0));
        assert!(!picked.contains(&UVec2::new(18, 0)));
    }
}