//! The rules of flight around one place: steering, thrust and drag, the
//! mining laser, breaking rocks, and scooping up the ore they drop. Nothing
//! here draws or reads keys; the scene turns keys into `Controls` and draws
//! the result.
//!
//! The feel is arcade rather than physics: you turn on the spot, thrust
//! pushes along your heading, a little drag stops you drifting forever, and
//! speed is capped. To stop, turn to face backwards (`Controls::retro` does
//! that for you) and thrust. Nothing pulls on you; planets and stations are
//! places to land or dock, not wells to fall into.

use std::f64::consts::TAU;

use rand::RngExt;

use super::locale::{DOCK_RADIUS, Locale, Ore, Rock, Size};
use super::{Vec2, turn_between};
use crate::seed::{self, Rng};

/// Units per second per second at full thrust.
pub const THRUST: f64 = 26.0;
pub const MAX_SPEED: f64 = 32.0;
/// The fraction of your speed lost each second.
const DRAG: f64 = 0.12;
/// Radians per second.
pub const TURN_RATE: f64 = 3.6;
pub const LASER_RANGE: f64 = 24.0;
/// Seconds before uncollected ore drifts off for good.
pub const FLOTSAM_LIFE: f64 = 45.0;
const PICKUP_RANGE: f64 = 2.5;
/// Land, dock or board only when slower than this.
pub const LANDING_SPEED: f64 = 8.0;
/// Board a wreck only when this close.
pub const BOARDING_RANGE: f64 = 4.0;
pub const SPARK_LIFE: f64 = 0.45;
/// How hard the edge of the map pushes you back, per unit you're past it.
const EDGE_PUSH: f64 = 3.0;

#[derive(Clone, Copy, Debug, Default)]
pub struct Controls {
    /// 0 (engines off) to 1 (full).
    pub thrust: f64,
    /// -1 (anticlockwise) to 1 (clockwise), as a fraction of `TURN_RATE`.
    pub turn: f64,
    /// Turn to face against your direction of travel, ready to brake.
    pub retro: bool,
    pub firing: bool,
}

pub struct Ship {
    pub pos: Vec2,
    pub vel: Vec2,
    /// 0 is up the screen; see `Vec2::from_heading`.
    pub heading: f64,
    pub thrusting: bool,
}

/// Ore floating free after a rock breaks, waiting to be scooped up.
pub struct Flotsam {
    pub pos: Vec2,
    pub vel: Vec2,
    pub ore: &'static Ore,
    pub age: f64,
}

/// A spark of engine exhaust.
pub struct Spark {
    pub pos: Vec2,
    pub vel: Vec2,
    pub age: f64,
}

/// The mining laser, as fired this step.
pub struct Beam {
    pub from: Vec2,
    pub to: Vec2,
    /// Whether it ended on something rather than at full range.
    pub hit: bool,
}

#[derive(Debug, PartialEq)]
pub enum Event {
    Shattered {
        size: Size,
        ore: Option<&'static str>,
    },
    Collected(&'static Ore),
    /// Flew over ore with no room for it.
    HoldFull,
}

pub struct Flight {
    pub locale: Locale,
    /// Seconds since you arrived.
    pub time: f64,
    pub ship: Ship,
    pub flotsam: Vec<Flotsam>,
    pub sparks: Vec<Spark>,
    pub beam: Option<Beam>,
    /// How fast the laser cuts: 1 as fitted, more with better lenses.
    pub laser_power: f64,
    rng: Rng,
}

impl Flight {
    /// Arrives at `locale`, facing whatever's in the middle of it.
    pub fn new(locale: Locale, seed: u64) -> Flight {
        let pos = locale.arrival;
        let heading = if pos.length() > 0.0 { (-pos).heading() } else { 0.0 };
        Flight {
            locale,
            time: 0.0,
            ship: Ship { pos, vel: Vec2::ZERO, heading, thrusting: false },
            flotsam: Vec::new(),
            sparks: Vec::new(),
            beam: None,
            laser_power: 1.0,
            rng: seed::rng(seed::derive(seed, "flight", 0)),
        }
    }

    pub fn speed(&self) -> f64 {
        self.ship.vel.length()
    }

    /// Whether you're over the planet or moon in the middle of the map.
    pub fn over_body(&self) -> bool {
        self.locale.body.as_ref().is_some_and(|b| self.ship.pos.length() <= b.radius)
    }

    /// Whether you're close enough to the station to dock.
    pub fn at_dock(&self) -> bool {
        self.locale.dock.is_some() && self.ship.pos.length() <= DOCK_RADIUS
    }

    /// The wreck close enough to board, if any, as an index into
    /// `locale.wrecks`.
    pub fn wreck_alongside(&self) -> Option<usize> {
        self.locale.wrecks.iter().position(|w| w.pos.distance(self.ship.pos) <= BOARDING_RANGE)
    }

    /// Whether you're beyond the edge of the map, being pushed back.
    pub fn past_edge(&self) -> bool {
        let e = self.locale.extent;
        self.ship.pos.x.abs() > e || self.ship.pos.y.abs() > e
    }

    /// Whether you're pointing (near enough) against your direction of travel.
    pub fn facing_retro(&self) -> bool {
        let backwards = (-self.ship.vel).heading();
        self.speed() < 0.5 || turn_between(self.ship.heading, backwards).abs() < 0.05
    }

    /// Advances the world by `dt` seconds. `room` is how many more tons of
    /// ore the hold can take.
    pub fn step(&mut self, dt: f64, controls: Controls, room: u32) -> Vec<Event> {
        let mut events = Vec::new();
        self.time += dt;
        let extent = self.locale.extent;
        for rock in &mut self.locale.rocks {
            rock.step(dt, extent);
        }
        self.steer(dt, controls);
        self.fire(dt, controls.firing, &mut events);

        for spark in &mut self.sparks {
            spark.pos += spark.vel * dt;
            spark.age += dt;
        }
        self.sparks.retain(|s| s.age < SPARK_LIFE);

        let mut collected = 0;
        for f in &mut self.flotsam {
            f.pos += f.vel * dt;
            f.vel = f.vel * (1.0 - 0.3 * dt);
            f.age += dt;
            if f.pos.distance(self.ship.pos) <= PICKUP_RANGE {
                if collected < room {
                    collected += 1;
                    f.age = f64::INFINITY;
                    events.push(Event::Collected(f.ore));
                } else if !events.contains(&Event::HoldFull) {
                    events.push(Event::HoldFull);
                }
            }
        }
        self.flotsam.retain(|f| f.age < FLOTSAM_LIFE);
        events
    }

    fn steer(&mut self, dt: f64, controls: Controls) {
        let ship = &mut self.ship;
        ship.heading += controls.turn.clamp(-1.0, 1.0) * TURN_RATE * dt;
        if controls.retro && ship.vel.length() > 0.5 {
            let off = turn_between(ship.heading, (-ship.vel).heading());
            let most = TURN_RATE * dt;
            ship.heading += off.clamp(-most, most);
        }
        ship.heading = ship.heading.rem_euclid(TAU);

        let ahead = Vec2::from_heading(ship.heading);
        let thrust = controls.thrust.clamp(0.0, 1.0);
        ship.thrusting = thrust > 0.0;
        if ship.thrusting {
            ship.vel += ahead * (THRUST * thrust * dt);
            // Exhaust: a spark out the back, thrown off a little to one side.
            let side = Vec2::new(-ahead.y, ahead.x) * self.rng.random_range(-2.5..2.5);
            self.sparks.push(Spark {
                pos: ship.pos - ahead,
                vel: ship.vel - ahead * self.rng.random_range(10.0..16.0) + side,
                age: 0.0,
            });
        }
        // The edge of the map: a spring that pushes you back in.
        let e = self.locale.extent;
        let over = |v: f64| (v.abs() - e).max(0.0) * v.signum();
        ship.vel -= Vec2::new(over(ship.pos.x), over(ship.pos.y)) * (EDGE_PUSH * dt);

        ship.vel = ship.vel * (1.0 - DRAG * dt);
        if ship.vel.length() > MAX_SPEED {
            ship.vel = ship.vel.normalised() * MAX_SPEED;
        }
        ship.pos += ship.vel * dt;
    }

    /// Casts the laser along your heading, wearing down the first rock it
    /// meets. The planet or station in the middle stops it too, unharmed.
    fn fire(&mut self, dt: f64, firing: bool, events: &mut Vec<Event>) {
        self.beam = None;
        if !firing {
            return;
        }
        let ahead = Vec2::from_heading(self.ship.heading);
        let from = self.ship.pos + ahead;
        let mut nearest: Option<(f64, Option<usize>)> = None;
        let mut consider = |centre: Vec2, r: f64, what: Option<usize>| {
            if let Some(d) = ray_hits_circle(from, ahead, centre, r)
                && d <= LASER_RANGE
                && nearest.is_none_or(|(n, _)| d < n)
            {
                nearest = Some((d, what));
            }
        };
        for (i, rock) in self.locale.rocks.iter().enumerate() {
            consider(rock.pos, rock.size.radius(), Some(i));
        }
        if let Some(r) = self.locale.solid_radius() {
            consider(Vec2::ZERO, r, None);
        }
        let reach = nearest.map_or(LASER_RANGE, |(d, _)| d);
        self.beam = Some(Beam { from, to: from + ahead * reach, hit: nearest.is_some() });
        if let Some((_, Some(i))) = nearest {
            self.locale.rocks[i].integrity -= dt * self.laser_power;
            if self.locale.rocks[i].integrity <= 0.0 {
                self.shatter(i, events);
            }
        }
    }

    /// Breaks a rock into two smaller ones drifting apart, and spills its
    /// ore, if it had any.
    fn shatter(&mut self, i: usize, events: &mut Vec<Event>) {
        let rock = self.locale.rocks.swap_remove(i);
        let rng = &mut self.rng;
        let vel = rock.velocity();
        if let Some(size) = rock.size.smaller() {
            let apart = Vec2::from_heading(rng.random_range(0.0..TAU));
            for side in [-1.0, 1.0] {
                let pos = rock.pos + apart * (side * rock.size.radius() * 0.6);
                let child = if rock.circling == 0.0 {
                    let kick = apart * (side * rng.random_range(0.5..1.5));
                    Rock::new(pos, rock.vel + kick, 0.0, size, rock.ore)
                } else {
                    // Keep circling at about the parent's distance.
                    let circling = rock.circling * rng.random_range(0.9..1.1);
                    Rock::new(pos, Vec2::ZERO, circling, size, rock.ore)
                };
                self.locale.rocks.push(child);
            }
        }
        if let Some(ore) = rock.ore {
            let chunks = match rock.size {
                Size::Small => 1,
                Size::Medium => 2,
                Size::Large => 3,
            };
            for _ in 0..chunks {
                let drift =
                    Vec2::from_heading(rng.random_range(0.0..TAU)) * rng.random_range(1.0..4.0);
                self.flotsam.push(Flotsam { pos: rock.pos, vel: vel + drift, ore, age: 0.0 });
            }
        }
        events.push(Event::Shattered { size: rock.size, ore: rock.ore.map(|o| o.name) });
    }
}

/// How far along a ray (from `from`, along unit vector `dir`) it first
/// touches a circle, if it does. Zero if it starts inside.
fn ray_hits_circle(from: Vec2, dir: Vec2, centre: Vec2, r: f64) -> Option<f64> {
    let to_centre = centre - from;
    let along = to_centre.dot(dir);
    let miss_sq = to_centre.dot(to_centre) - along * along;
    if along < -r || miss_sq > r * r {
        return None;
    }
    Some((along - (r * r - miss_sq).sqrt()).max(0.0))
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::space::locale::{self, ORES};
    use crate::space::system::{self, PlaceKind};

    const DT: f64 = 1.0 / 30.0;

    /// Flying in the belt of system 1, with the rocks cleared away.
    fn flight() -> Flight {
        let s = system::generate(1);
        let belt = s.places.iter().position(|p| p.kind == PlaceKind::Belt).unwrap();
        let mut locale = locale::generate(&s, belt, 0.0, None, &Default::default());
        locale.rocks.clear();
        Flight::new(locale, 1)
    }

    fn run(f: &mut Flight, seconds: f64, controls: Controls) -> Vec<Event> {
        let mut events = Vec::new();
        for _ in 0..(seconds / DT).round() as usize {
            events.extend(f.step(DT, controls, 99));
        }
        events
    }

    /// A rock that sits still at `pos`.
    fn rock_at(pos: Vec2, size: Size) -> Rock {
        Rock::new(pos, Vec2::ZERO, 0.0, size, Some(&ORES[0]))
    }

    #[test]
    fn thrust_pushes_along_the_heading_up_to_a_limit() {
        let mut f = flight();
        f.ship.heading = TAU / 4.0;
        run(&mut f, 1.0, Controls { thrust: 1.0, ..Controls::default() });
        assert!(f.ship.vel.x > 15.0 && f.ship.vel.y.abs() < 1e-6, "heading right goes right");
        run(&mut f, 3.0, Controls { thrust: 1.0, ..Controls::default() });
        assert!((f.speed() - MAX_SPEED).abs() < 1e-6, "speed is capped");
    }

    #[test]
    fn drag_slows_you_when_coasting() {
        let mut f = flight();
        f.ship.vel = Vec2::new(20.0, 0.0);
        run(&mut f, 3.0, Controls::default());
        assert!(f.speed() < 15.0 && f.speed() > 5.0, "coasting decays, slowly: {}", f.speed());
    }

    #[test]
    fn retro_turns_you_to_face_backwards() {
        let mut f = flight();
        f.ship.vel = Vec2::new(0.0, -10.0);
        f.ship.heading = 0.0;
        assert!(!f.facing_retro());
        run(&mut f, 1.5, Controls { retro: true, ..Controls::default() });
        assert!(f.facing_retro(), "heading {} after retro", f.ship.heading);
    }

    #[test]
    fn the_edge_pushes_you_back() {
        let mut f = flight();
        f.ship.pos = Vec2::new(f.locale.extent - 5.0, 0.0);
        f.ship.vel = Vec2::new(MAX_SPEED, 0.0);
        run(&mut f, 5.0, Controls::default());
        assert!(f.ship.vel.x < 0.0, "turned round");
        assert!(f.ship.pos.x < f.locale.extent + 30.0);
    }

    #[test]
    fn mining_breaks_rocks_and_ore_can_be_scooped_up() {
        let mut f = flight();
        f.ship.pos = Vec2::new(50.0, 0.0);
        f.ship.heading = 0.0;
        f.locale.rocks = vec![rock_at(Vec2::new(50.0, -10.0), Size::Medium)];
        // Long enough to break the rock, not long enough to break its pieces.
        let events = run(&mut f, 1.1, Controls { firing: true, ..Controls::default() });
        assert!(events.contains(&Event::Shattered { size: Size::Medium, ore: Some("iron ore") }));
        assert_eq!(f.locale.rocks.len(), 2, "a medium rock breaks in two");
        assert!(f.locale.rocks.iter().all(|r| r.size == Size::Small));
        assert_eq!(f.flotsam.len(), 2, "a medium rock spills two chunks");

        f.ship.pos = f.flotsam[0].pos;
        let events = f.step(DT, Controls::default(), 1);
        assert!(events.iter().any(|e| matches!(e, Event::Collected(_))));
    }

    #[test]
    fn a_full_hold_leaves_ore_floating() {
        let mut f = flight();
        f.flotsam = vec![Flotsam { pos: f.ship.pos, vel: Vec2::ZERO, ore: &ORES[0], age: 0.0 }];
        let events = f.step(DT, Controls::default(), 0);
        assert_eq!(events, vec![Event::HoldFull]);
        assert_eq!(f.flotsam.len(), 1);
    }

    #[test]
    fn the_laser_stops_at_the_first_rock() {
        let mut f = flight();
        f.ship.pos = Vec2::new(50.0, 0.0);
        f.ship.heading = 0.0;
        f.locale.rocks = vec![
            rock_at(Vec2::new(50.0, -15.0), Size::Small),
            rock_at(Vec2::new(50.0, -6.0), Size::Small),
        ];
        f.step(DT, Controls { firing: true, ..Controls::default() }, 0);
        let beam = f.beam.as_ref().unwrap();
        assert!(beam.hit && beam.to.y > -6.0 && beam.to.y < -5.0, "beam ends at {:?}", beam.to);
        assert!(f.locale.rocks[1].integrity < f.locale.rocks[0].integrity);
    }

    #[test]
    fn planets_stop_the_laser_and_can_be_landed_on() {
        let s = system::generate(1);
        let mut f =
            Flight::new(locale::generate(&s, s.home_place(), 0.0, None, &Default::default()), 1);
        f.locale.rocks.clear();
        let radius = f.locale.body.as_ref().unwrap().radius;
        assert!(!f.over_body());
        f.step(DT, Controls { firing: true, ..Controls::default() }, 0);
        let beam = f.beam.as_ref().unwrap();
        assert!(beam.hit, "you arrive facing the planet, in range");
        assert!((beam.to.length() - radius).abs() < 1e-6);
        f.ship.pos = Vec2::new(0.0, radius - 1.0);
        assert!(f.over_body());
    }
}