Implement collision detection (#8)

* Implement collision detection

* Minor cleanups.

* Add laser/player collisions

* Add laser collision with bullets and fix fire button repeating

* Add basic shield collisions

* Refactor collision and debug

* Simplify collision and debug by not tracking laser indices

- We don't care about which laser collided, because they get destroyed immediately.

* Don't track laser indicies against bullets either

* DRY and docs

* Adjust the fleet bounding box as invaders are shot

examples/invaders/main.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::time::Instant;
 
 use pixels::{Error, Pixels, SurfaceTexture};
@@ -9,6 +10,12 @@ fn main() -> Result<(), Error> {
     env_logger::init();
     let event_loop = EventLoop::new();
 
+    // Enable debug mode with `DEBUG=true` environment variable
+    let debug = env::var("DEBUG")
+        .unwrap_or_else(|_| "false".to_string())
+        .parse()
+        .unwrap_or(false);
+
     let (window, surface, width, height) = {
         let scale = 3.0;
         let width = SCREEN_WIDTH as f64 * scale;
@@ -27,9 +34,13 @@ fn main() -> Result<(), Error> {
 
     let surface_texture = SurfaceTexture::new(width, height, &surface);
     let mut fb = Pixels::new(224, 256, surface_texture)?;
-    let mut invaders = World::new();
+    let mut invaders = World::new(debug);
     let mut last = Instant::now();
+
     let mut controls = Controls::default();
+    let mut last_state = false;
+    let mut button_state = false;
+    let mut rising_edge = false;
 
     event_loop.run(move |event, _, control_flow| match event {
         event::Event::WindowEvent { event, .. } => match event {
@@ -57,7 +68,7 @@ fn main() -> Result<(), Error> {
             } => match virtual_code {
                 event::VirtualKeyCode::Left => controls.direction = Direction::Left,
                 event::VirtualKeyCode::Right => controls.direction = Direction::Right,
-                event::VirtualKeyCode::Space => controls.fire = true,
+                event::VirtualKeyCode::Space => button_state = true,
                 _ => (),
             },
 
@@ -72,7 +83,7 @@ fn main() -> Result<(), Error> {
             } => match virtual_code {
                 event::VirtualKeyCode::Left => controls.direction = Direction::Still,
                 event::VirtualKeyCode::Right => controls.direction = Direction::Still,
-                event::VirtualKeyCode::Space => controls.fire = false,
+                event::VirtualKeyCode::Space => button_state = false,
                 _ => (),
             },
 
@@ -87,6 +98,13 @@ fn main() -> Result<(), Error> {
             let dt = now.duration_since(last);
             last = now;
 
+            // Compute rising edge based on current and last button states
+            rising_edge = button_state && !last_state;
+            last_state = button_state;
+
+            // Fire button only uses rising edge
+            controls.fire = rising_edge;
+
             // Update the game logic and request redraw
             invaders.update(&dt, &controls);
             window.request_redraw();

simple-invaders/src/collision.rs

@@ -0,0 +1,190 @@
+//! Collision detection primitives.
+
+use crate::geo::{Point, Rect};
+use crate::{Bullet, Invaders, Laser, Player, Shield, COLS, GRID, ROWS};
+use std::collections::HashSet;
+
+/// Store information about collisions (for debug mode).
+#[derive(Debug, Default)]
+pub(crate) struct Collision {
+    pub(crate) bullet_details: HashSet<BulletDetail>,
+    pub(crate) laser_details: HashSet<LaserDetail>,
+}
+
+/// Information regarding collisions between bullets and invaders, lasers, or shields.
+#[derive(Debug, Eq, Hash, PartialEq)]
+pub(crate) enum BulletDetail {
+    /// A grid position (col, row) for an invader.
+    Invader(usize, usize),
+    /// A shield index.
+    Shield(usize),
+    /// Collided with a laser.
+    Laser,
+}
+
+/// Information regarding collisions between lasers and shields or the player.
+#[derive(Debug, Eq, Hash, PartialEq)]
+pub(crate) enum LaserDetail {
+    /// A shield index.
+    Shield(usize),
+    /// Collided with the player.
+    Player,
+}
+
+impl Collision {
+    /// Clear the collision details.
+    pub(crate) fn clear(&mut self) {
+        self.bullet_details.clear();
+        self.laser_details.clear();
+    }
+
+    /// Handle collisions between bullets and invaders.
+    pub(crate) fn bullet_to_invader(
+        &mut self,
+        bullet: &mut Option<Bullet>,
+        invaders: &mut Invaders,
+    ) -> bool {
+        // Broad phase collision detection
+        let (top, right, bottom, left) = invaders.get_bounds();
+        let invaders_rect = Rect::new(&Point::new(left, top), &Point::new(right, bottom));
+        let bullet_rect = {
+            let bullet = bullet.as_ref().unwrap();
+            Rect::from_drawable(&bullet.pos, &bullet.sprite)
+        };
+        if bullet_rect.intersects(&invaders_rect) {
+            // Narrow phase collision detection
+            let corners = [
+                (bullet_rect.p1.x, bullet_rect.p1.y),
+                (bullet_rect.p1.x, bullet_rect.p2.y),
+                (bullet_rect.p2.x, bullet_rect.p1.y),
+                (bullet_rect.p2.x, bullet_rect.p2.y),
+            ];
+
+            for (x, y) in corners.iter() {
+                let col = (x - left) / GRID.x + invaders.bounds.left_col;
+                let row = (y - top) / GRID.y + invaders.bounds.top_row;
+
+                if col < COLS && row < ROWS && invaders.grid[row][col].is_some() {
+                    let detail = BulletDetail::Invader(col, row);
+                    self.bullet_details.insert(detail);
+                }
+            }
+
+            // If any collision candidate is a hit, kill the bullet and invader
+            for detail in self.bullet_details.iter() {
+                if let BulletDetail::Invader(x, y) = *detail {
+                    let invader = invaders.grid[y][x].as_ref().unwrap();
+                    let invader_rect = Rect::from_drawable(&invader.pos, &invader.sprite);
+                    if bullet_rect.intersects(&invader_rect) {
+                        // TODO: Explosion! Score!
+                        invaders.grid[y][x] = None;
+
+                        // Destroy bullet
+                        *bullet = None;
+
+                        return true;
+                    }
+                }
+            }
+        }
+
+        false
+    }
+
+    /// Handle collisions between bullets and shields.
+    pub(crate) fn bullet_to_shield(&mut self, bullet: &mut Option<Bullet>, shields: &mut [Shield]) {
+        if bullet.is_some() {
+            let shield_rects = create_shield_rects(shields);
+            let bullet_rect = {
+                let bullet = bullet.as_ref().unwrap();
+                Rect::from_drawable(&bullet.pos, &bullet.sprite)
+            };
+
+            for (i, shield_rect) in shield_rects.iter().enumerate() {
+                // broad phase collision detection
+                if bullet_rect.intersects(&shield_rect) {
+                    // TODO: Narrow phase (per-pixel) collision detection
+                    // TODO: Break shield
+
+                    // TODO: Explosion!
+                    let detail = BulletDetail::Shield(i);
+                    self.bullet_details.insert(detail);
+
+                    // Destroy bullet
+                    *bullet = None;
+                }
+            }
+        }
+    }
+
+    /// Handle collisions between lasers and the player.
+    pub(crate) fn laser_to_player(&mut self, laser: &Laser, player: &Player) -> bool {
+        let laser_rect = Rect::from_drawable(&laser.pos, &laser.sprite);
+        let player_rect = Rect::from_drawable(&player.pos, &player.sprite);
+        if laser_rect.intersects(&player_rect) {
+            self.laser_details.insert(LaserDetail::Player);
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Handle collisions between lasers and bullets.
+    pub(crate) fn laser_to_bullet(&mut self, laser: &Laser, bullet: &mut Option<Bullet>) -> bool {
+        let mut destroy = false;
+        if bullet.is_some() {
+            let laser_rect = Rect::from_drawable(&laser.pos, &laser.sprite);
+
+            if let Some(bullet) = &bullet {
+                let bullet_rect = Rect::from_drawable(&bullet.pos, &bullet.sprite);
+                if bullet_rect.intersects(&laser_rect) {
+                    // TODO: Explosion!
+                    let detail = BulletDetail::Laser;
+                    self.bullet_details.insert(detail);
+
+                    // Destroy laser and bullet
+                    destroy = true;
+                }
+            }
+
+            if destroy {
+                *bullet = None;
+            }
+        }
+
+        destroy
+    }
+
+    /// Handle collisions between lasers and shields.
+    pub(crate) fn laser_to_shield(&mut self, laser: &Laser, shields: &mut [Shield]) -> bool {
+        let laser_rect = Rect::from_drawable(&laser.pos, &laser.sprite);
+        let shield_rects = create_shield_rects(shields);
+        let mut destroy = false;
+
+        for (i, shield_rect) in shield_rects.iter().enumerate() {
+            // broad phase collision detection
+            if laser_rect.intersects(&shield_rect) {
+                // TODO: Narrow phase (per-pixel) collision detection
+                // TODO: Break shield
+
+                // TODO: Explosion!
+                let detail = LaserDetail::Shield(i);
+                self.laser_details.insert(detail);
+
+                // Destroy laser
+                destroy = true;
+            }
+        }
+
+        destroy
+    }
+}
+
+fn create_shield_rects(shields: &[Shield]) -> [Rect; 4] {
+    [
+        Rect::from_drawable(&shields[0].pos, &shields[0].sprite),
+        Rect::from_drawable(&shields[1].pos, &shields[1].sprite),
+        Rect::from_drawable(&shields[2].pos, &shields[2].sprite),
+        Rect::from_drawable(&shields[3].pos, &shields[3].sprite),
+    ]
+}

simple-invaders/src/debug.rs

@@ -0,0 +1,106 @@
+use crate::collision::{BulletDetail, Collision, LaserDetail};
+use crate::geo::Point;
+use crate::sprites::{rect, Drawable};
+use crate::{Bullet, Invaders, Laser, Player, Shield, GRID};
+
+// Colors
+const RED: [u8; 4] = [255, 0, 0, 255];
+const GREEN: [u8; 4] = [0, 255, 0, 255];
+const BLUE: [u8; 4] = [0, 0, 255, 255];
+const YELLOW: [u8; 4] = [255, 255, 0, 255];
+
+/// Draw bounding boxes for the invader fleet and each invader.
+pub(crate) fn draw_invaders(screen: &mut [u8], invaders: &Invaders, collision: &Collision) {
+    // Draw invaders bounding box
+    {
+        let (top, right, bottom, left) = invaders.get_bounds();
+        let p1 = Point::new(left, top);
+        let p2 = Point::new(right, bottom);
+
+        rect(screen, &p1, &p2, BLUE);
+    }
+
+    // Draw bounding boxes for each invader
+    for (y, row) in invaders.grid.iter().enumerate() {
+        for (x, col) in row.iter().enumerate() {
+            let detail = BulletDetail::Invader(x, y);
+            if let Some(invader) = col {
+                let p1 = invader.pos;
+                let p2 = p1 + Point::new(invader.sprite.width(), invader.sprite.height());
+
+                // Select color based on proximity to bullet
+                let color = if collision.bullet_details.contains(&detail) {
+                    YELLOW
+                } else {
+                    GREEN
+                };
+
+                rect(screen, &p1, &p2, color);
+            } else if collision.bullet_details.contains(&detail) {
+                let x = x - invaders.bounds.left_col;
+                let y = y - invaders.bounds.top_row;
+                let p1 = invaders.bounds.pos + Point::new(x, y) * GRID;
+                let p2 = p1 + GRID;
+
+                rect(screen, &p1, &p2, RED);
+            }
+        }
+    }
+}
+
+/// Draw bounding box for bullet.
+pub(crate) fn draw_bullet(screen: &mut [u8], bullet: Option<&Bullet>) {
+    if let Some(bullet) = bullet {
+        let p1 = bullet.pos;
+        let p2 = p1 + Point::new(bullet.sprite.width(), bullet.sprite.height());
+
+        rect(screen, &p1, &p2, GREEN);
+    }
+}
+
+/// Draw bounding box for lasers.
+pub(crate) fn draw_lasers(screen: &mut [u8], lasers: &[Laser]) {
+    for laser in lasers {
+        let p1 = laser.pos;
+        let p2 = p1 + Point::new(laser.sprite.width(), laser.sprite.height());
+
+        rect(screen, &p1, &p2, GREEN);
+    }
+}
+
+/// Draw bounding box for player.
+pub(crate) fn draw_player(screen: &mut [u8], player: &Player, collision: &Collision) {
+    let p1 = player.pos;
+    let p2 = p1 + Point::new(player.sprite.width(), player.sprite.height());
+
+    // Select color based on collisions
+    let detail = LaserDetail::Player;
+    let color = if collision.laser_details.contains(&detail) {
+        RED
+    } else {
+        GREEN
+    };
+
+    rect(screen, &p1, &p2, color);
+}
+
+/// Draw bounding boxes for shields.
+pub(crate) fn draw_shields(screen: &mut [u8], shields: &[Shield], collision: &Collision) {
+    for (i, shield) in shields.iter().enumerate() {
+        let p1 = shield.pos;
+        let p2 = p1 + Point::new(shield.sprite.width(), shield.sprite.height());
+
+        // Select color based on collisions
+        let laser_detail = LaserDetail::Shield(i);
+        let bullet_detail = BulletDetail::Shield(i);
+        let color = if collision.laser_details.contains(&laser_detail)
+            || collision.bullet_details.contains(&bullet_detail)
+        {
+            RED
+        } else {
+            GREEN
+        };
+
+        rect(screen, &p1, &p2, color);
+    }
+}

simple-invaders/src/geo.rs

@@ -0,0 +1,127 @@
+//! Simple geometry primitives.
+
+use crate::sprites::Drawable;
+
+/// A tiny position vector.
+#[derive(Copy, Clone, Debug, Default)]
+pub(crate) struct Point {
+    pub(crate) x: usize,
+    pub(crate) y: usize,
+}
+
+/// A tiny rectangle based on two absolute `Point`s.
+#[derive(Copy, Clone, Debug, Default)]
+pub(crate) struct Rect {
+    pub(crate) p1: Point,
+    pub(crate) p2: Point,
+}
+
+impl Point {
+    /// Create a new point.
+    pub(crate) const fn new(x: usize, y: usize) -> Point {
+        Point { x, y }
+    }
+}
+
+impl std::ops::Add for Point {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self {
+        Self::new(self.x + other.x, self.y + other.y)
+    }
+}
+
+impl std::ops::Mul for Point {
+    type Output = Self;
+
+    fn mul(self, other: Self) -> Self {
+        Self::new(self.x * other.x, self.y * other.y)
+    }
+}
+
+impl Rect {
+    /// Create a rectangle from two `Point`s.
+    pub(crate) fn new(p1: &Point, p2: &Point) -> Rect {
+        let p1 = *p1;
+        let p2 = *p2;
+
+        Rect { p1, p2 }
+    }
+
+    /// Create a rectangle from a `Point` and a `Drawable`.
+    pub(crate) fn from_drawable<D>(pos: &Point, drawable: &D) -> Rect
+    where
+        D: Drawable,
+    {
+        let p1 = *pos;
+        let p2 = p1 + Point::new(drawable.width(), drawable.height());
+
+        Rect { p1, p2 }
+    }
+
+    /// Test for intersections between two rectangles.
+    ///
+    /// Rectangles intersect when the geometry of either overlaps.
+    pub(crate) fn intersects(&self, other: &Rect) -> bool {
+        let (top1, right1, bottom1, left1) = self.get_bounds();
+        let (top2, right2, bottom2, left2) = other.get_bounds();
+
+        bottom1 > top2 && bottom2 > top1 && right1 > left2 && right2 > left1
+    }
+
+    /// Compute the bounding box for this rectangle.
+    ///
+    /// # Returns
+    ///
+    /// Tuple of `(top, right, bottom, left)`, e.g. in CSS clockwise order.
+    fn get_bounds(&self) -> (usize, usize, usize, usize) {
+        (self.p1.y, self.p2.x, self.p2.y, self.p1.x)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_rect_intersect() {
+        let rect_size = Point::new(10, 10);
+        let r1 = Rect::new(&rect_size, &(rect_size + rect_size));
+
+        // Test intersection between equal-sized rectangles
+        for y in 0..3 {
+            for x in 0..3 {
+                let x = x * 5 + 5;
+                let y = y * 5 + 5;
+
+                let r2 = Rect::new(&Point::new(x, y), &(Point::new(x, y) + rect_size));
+
+                assert!(r1.intersects(&r2), "Should intersect");
+                assert!(r2.intersects(&r1), "Should intersect");
+            }
+        }
+
+        // Test non-intersections
+        for y in 0..3 {
+            for x in 0..3 {
+                if x == 1 && y == 1 {
+                    continue;
+                }
+
+                let x = x * 10;
+                let y = y * 10;
+
+                let r2 = Rect::new(&Point::new(x, y), &(Point::new(x, y) + rect_size));
+
+                assert!(!r1.intersects(&r2), "Should not intersect");
+                assert!(!r2.intersects(&r1), "Should not intersect");
+            }
+        }
+
+        // Test intersection between different-sized rectangles
+        let r2 = Rect::new(&Point::new(0, 0), &Point::new(30, 30));
+
+        assert!(r1.intersects(&r2), "Should intersect");
+        assert!(r2.intersects(&r1), "Should intersect");
+    }
+}

simple-invaders/src/lib.rs

@@ -5,14 +5,18 @@
 //! to understand the code.
 
 use rand_core::{OsRng, RngCore};
-use std::env;
 use std::time::Duration;
 
-pub use controls::{Controls, Direction};
+pub use crate::controls::{Controls, Direction};
-use loader::{load_assets, Assets};
+use crate::geo::Point;
-use sprites::{blit, line, Animation, Frame, Sprite, SpriteRef};
+use crate::loader::{load_assets, Assets};
+use crate::sprites::{blit, Animation, Drawable, Frame, Sprite, SpriteRef};
+use collision::Collision;
 
+mod collision;
 mod controls;
+mod debug;
+mod geo;
 mod loader;
 mod sprites;
 
@@ -27,6 +31,13 @@ const GRID: Point = Point::new(16, 16);
 const ROWS: usize = 5;
 const COLS: usize = 11;
 
+// Player positioning
+const PLAYER_START: Point = Point::new(80, 216);
+
+// Projectile positioning
+const LASER_OFFSET: Point = Point::new(4, 10);
+const BULLET_OFFSET: Point = Point::new(7, 0);
+
 #[derive(Debug)]
 pub struct World {
     invaders: Invaders,
@@ -34,6 +45,7 @@ pub struct World {
     shields: Vec<Shield>,
     player: Player,
     bullet: Option<Bullet>,
+    collision: Collision,
     score: u32,
     assets: Assets,
     screen: Vec<u8>,
@@ -43,13 +55,6 @@ pub struct World {
     debug: bool,
 }
 
-/// A tiny position vector.
-#[derive(Debug, Default, Eq, PartialEq)]
-struct Point {
-    x: usize,
-    y: usize,
-}
-
 /// A fleet of invaders.
 #[derive(Debug)]
 struct Invaders {
@@ -73,9 +78,11 @@ struct Invader {
 /// Used for collision detection and minor optimizations.
 #[derive(Debug)]
 struct Bounds {
+    pos: Point,
     left_col: usize,
     right_col: usize,
-    px: usize,
+    top_row: usize,
+    bottom_row: usize,
 }
 
 /// The player entity.
@@ -112,7 +119,7 @@ struct Bullet {
 
 impl World {
     /// Create a new simple-invaders `World`.
-    pub fn new() -> World {
+    pub fn new(debug: bool) -> World {
         use Frame::*;
 
         // Load assets first
@@ -126,40 +133,43 @@ impl World {
             descend: false,
             bounds: Bounds::default(),
         };
-        let player = Player {
+        let lasers = Vec::new();
-            sprite: SpriteRef::new(&assets, Player1, Duration::from_millis(100)),
-            pos: Point::new(80, 216),
-            dt: 0,
-        };
         let shields = (0..4)
             .map(|i| Shield {
                 sprite: Sprite::new(&assets, Shield1),
                 pos: Point::new(i * 45 + 32, 192),
             })
             .collect();
+        let player = Player {
+            sprite: SpriteRef::new(&assets, Player1, Duration::from_millis(100)),
+            pos: PLAYER_START,
+            dt: 0,
+        };
+        let bullet = None;
+        let collision = Collision::default();
+        let score = 0;
 
         // Create a screen with the correct size
         let mut screen = Vec::new();
         screen.resize_with(SCREEN_WIDTH * SCREEN_HEIGHT * 4, Default::default);
 
-        // Enable debug mode with `DEBUG=true` environment variable
+        let dt = Duration::default();
-        let debug = env::var("DEBUG")
+        let gameover = false;
-            .unwrap_or_else(|_| "false".to_string())
+        let random = OsRng;
-            .parse()
-            .unwrap_or(false);
 
         World {
             invaders,
-            lasers: Vec::new(),
+            lasers,
             shields,
             player,
-            bullet: None,
+            bullet,
-            score: 0,
+            collision,
+            score,
             assets,
             screen,
-            dt: Duration::default(),
+            dt,
-            gameover: false,
+            gameover,
-            random: OsRng,
+            random,
             debug,
         }
     }
@@ -181,6 +191,9 @@ impl World {
         // Advance the timer by the delta time
         self.dt += *dt;
 
+        // Clear the collision details
+        self.collision.clear();
+
         // Step the invaders one by one
         while self.dt >= one_frame {
             self.dt -= one_frame;
@@ -190,13 +203,25 @@ impl World {
         // Handle player movement and animation
         self.step_player(controls, dt);
 
-        // Handle bullet movement
         if let Some(bullet) = &mut self.bullet {
+            // Handle bullet movement
             let velocity = update_dt(&mut bullet.dt, dt) * 4;
 
             if bullet.pos.y > velocity {
                 bullet.pos.y -= velocity;
                 bullet.sprite.animate(&self.assets, dt);
+
+                // Handle collisions
+                if self
+                    .collision
+                    .bullet_to_invader(&mut self.bullet, &mut self.invaders)
+                {
+                    // One of the end scenarios
+                    self.gameover = self.invaders.shrink_bounds();
+                } else {
+                    self.collision
+                        .bullet_to_shield(&mut self.bullet, &mut self.shields);
+                }
             } else {
                 self.bullet = None;
             }
@@ -210,18 +235,32 @@ impl World {
             if laser.pos.y < self.player.pos.y {
                 laser.pos.y += velocity;
                 laser.sprite.animate(&self.assets, dt);
+
+                // Handle collisions
+                if self.collision.laser_to_player(laser, &self.player) {
+                    // One of the end scenarios
+                    self.gameover = true;
+
+                    destroy.push(i);
+                } else if self.collision.laser_to_bullet(laser, &mut self.bullet)
+                    || self.collision.laser_to_shield(laser, &mut self.shields)
+                {
+                    destroy.push(i);
+                }
             } else {
                 destroy.push(i);
             }
         }
 
         // Destroy dead lasers
-        for i in destroy.iter().rev() {
+        for &i in destroy.iter().rev() {
-            self.lasers.remove(*i);
+            self.lasers.remove(i);
         }
     }
 
     /// Draw the internal state to the screen.
+    ///
+    /// Calling this method more than once without an `update` call between is a no-op.
     pub fn draw(&mut self) -> &[u8] {
         // Clear the screen
         self.clear();
@@ -253,25 +292,20 @@ impl World {
             blit(&mut self.screen, &laser.pos, &laser.sprite);
         }
 
+        // Draw debug information
         if self.debug {
-            // Draw invaders bounding box
+            debug::draw_invaders(&mut self.screen, &self.invaders, &self.collision);
-            let (left, right) = self.invaders.get_bounds();
+            debug::draw_bullet(&mut self.screen, self.bullet.as_ref());
-            let red = [255, 0, 0, 255];
+            debug::draw_lasers(&mut self.screen, &self.lasers);
-
+            debug::draw_player(&mut self.screen, &self.player, &self.collision);
-            let p1 = Point::new(left, START.y);
+            debug::draw_shields(&mut self.screen, &self.shields, &self.collision);
-            let p2 = Point::new(left, self.player.pos.y);
-            line(&mut self.screen, &p1, &p2, red);
-
-            let p1 = Point::new(right, START.y);
-            let p2 = Point::new(right, self.player.pos.y);
-            line(&mut self.screen, &p1, &p2, red);
         }
 
         &self.screen
     }
 
     fn step_invaders(&mut self) {
-        let (left, right) = self.invaders.get_bounds();
+        let (_, right, _, left) = self.invaders.get_bounds();
         let (invader, is_leader) =
             next_invader(&mut self.invaders.grid, &mut self.invaders.stepper);
 
@@ -284,20 +318,22 @@ impl World {
             match self.invaders.direction {
                 Direction::Left => {
                     if left < 2 {
-                        self.invaders.bounds.px += 2;
+                        self.invaders.bounds.pos.x += 2;
+                        self.invaders.bounds.pos.y += 8;
                         self.invaders.descend = true;
                         self.invaders.direction = Direction::Right;
                     } else {
-                        self.invaders.bounds.px -= 2;
+                        self.invaders.bounds.pos.x -= 2;
                     }
                 }
                 Direction::Right => {
                     if right > SCREEN_WIDTH - 2 {
-                        self.invaders.bounds.px -= 2;
+                        self.invaders.bounds.pos.x -= 2;
+                        self.invaders.bounds.pos.y += 8;
                         self.invaders.descend = true;
                         self.invaders.direction = Direction::Left;
                     } else {
-                        self.invaders.bounds.px += 2;
+                        self.invaders.bounds.pos.x += 2;
                     }
                 }
                 _ => unreachable!(),
@@ -334,7 +370,7 @@ impl World {
 
             let laser = Laser {
                 sprite: SpriteRef::new(&self.assets, Frame::Laser1, Duration::from_millis(16)),
-                pos: Point::new(invader.pos.x + 4, invader.pos.y + 10),
+                pos: invader.pos + LASER_OFFSET,
                 dt: 0,
             };
             self.lasers.push(laser);
@@ -343,17 +379,18 @@ impl World {
 
     fn step_player(&mut self, controls: &Controls, dt: &Duration) {
         let frames = update_dt(&mut self.player.dt, dt);
+        let width = self.player.sprite.width();
 
         match controls.direction {
             Direction::Left => {
-                if self.player.pos.x >= frames {
+                if self.player.pos.x > width {
                     self.player.pos.x -= frames;
                     self.player.sprite.animate(&self.assets, dt);
                 }
             }
 
             Direction::Right => {
-                if self.player.pos.x < SCREEN_WIDTH - 15 - frames {
+                if self.player.pos.x < SCREEN_WIDTH - width * 2 {
                     self.player.pos.x += frames;
                     self.player.sprite.animate(&self.assets, dt);
                 }
@@ -364,7 +401,7 @@ impl World {
         if controls.fire && self.bullet.is_none() {
             self.bullet = Some(Bullet {
                 sprite: SpriteRef::new(&self.assets, Frame::Bullet1, Duration::from_millis(32)),
-                pos: Point::new(self.player.pos.x + 7, self.player.pos.y),
+                pos: self.player.pos + BULLET_OFFSET,
                 dt: 0,
             });
         }
@@ -380,40 +417,77 @@ impl World {
 
 impl Default for World {
     fn default() -> Self {
-        World::new()
+        World::new(false)
-    }
-}
-
-impl Point {
-    const fn new(x: usize, y: usize) -> Point {
-        Point { x, y }
-    }
-}
-
-impl std::ops::Add for Point {
-    type Output = Self;
-
-    fn add(self, other: Self) -> Self {
-        Self::new(self.x + other.x, self.y + other.y)
-    }
-}
-
-impl std::ops::Mul for Point {
-    type Output = Self;
-
-    fn mul(self, other: Self) -> Self {
-        Self::new(self.x * other.x, self.y * other.y)
     }
 }
 
 impl Invaders {
-    fn get_bounds(&self) -> (usize, usize) {
+    /// Compute the bounding box for the Invader fleet.
+    ///
+    /// # Returns
+    ///
+    /// Tuple of `(top, right, bottom, left)`, e.g. in CSS clockwise order.
+    fn get_bounds(&self) -> (usize, usize, usize, usize) {
         let width = (self.bounds.right_col - self.bounds.left_col + 1) * GRID.x;
+        let height = (self.bounds.bottom_row - self.bounds.top_row + 1) * GRID.y;
 
-        let left = self.bounds.px;
+        let top = self.bounds.pos.y;
+        let bottom = top + height;
+        let left = self.bounds.pos.x;
         let right = left + width;
 
-        (left, right)
+        (top, right, bottom, left)
+    }
+
+    /// Resize the bounds to fit the live invaders.
+    ///
+    /// # Returns
+    ///
+    /// `true` when all invaders have been destroyed.
+    fn shrink_bounds(&mut self) -> bool {
+        let mut top = ROWS;
+        let mut right = 0;
+        let mut bottom = 0;
+        let mut left = COLS;
+
+        // Scan through the entire grid
+        for (y, row) in self.grid.iter().enumerate() {
+            for (x, col) in row.iter().enumerate() {
+                if col.is_some() {
+                    // Build a boundary box of invaders in the grid
+                    if top > y {
+                        top = y;
+                    }
+                    if bottom < y {
+                        bottom = y;
+                    }
+                    if left > x {
+                        left = x;
+                    }
+                    if right < x {
+                        right = x;
+                    }
+                }
+            }
+        }
+
+        if top > bottom || left > right {
+            // No more invaders left alive
+            return true;
+        }
+
+        // Adjust the bounding box position
+        self.bounds.pos.x += (left - self.bounds.left_col) * GRID.x;
+        self.bounds.pos.y += (top - self.bounds.top_row) * GRID.y;
+
+        // Adjust the bounding box columns and rows
+        self.bounds.left_col = left;
+        self.bounds.right_col = right;
+        self.bounds.top_row = top;
+        self.bounds.bottom_row = bottom;
+
+        // No more changes
+        false
     }
 
     fn get_closest_invader(&self, mut col: usize) -> &Invader {
@@ -439,9 +513,11 @@ impl Invaders {
 impl Default for Bounds {
     fn default() -> Self {
         Self {
+            pos: START,
             left_col: 0,
             right_col: COLS - 1,
-            px: START.x,
+            top_row: 0,
+            bottom_row: ROWS - 1,
         }
     }
 }
@@ -451,7 +527,7 @@ fn make_invader_grid(assets: &Assets) -> Vec<Vec<Option<Invader>>> {
     use Frame::*;
 
     const BLIPJOY_OFFSET: Point = Point::new(3, 4);
-    const FERRIS_OFFSET: Point = Point::new(3, 5);
+    const FERRIS_OFFSET: Point = Point::new(2, 5);
     const CTHULHU_OFFSET: Point = Point::new(1, 3);
 
     (0..1)

simple-invaders/src/sprites.rs

@@ -203,9 +203,9 @@ where
 
         // Merge pixels from sprite into screen
         let zipped = screen[i..i + width].iter_mut().zip(&pixels[s..s + width]);
-        for (left, right) in zipped {
+        for (left, &right) in zipped {
-            if *right > 0 {
+            if right > 0 {
-                *left = *right;
+                *left = right;
             }
         }
 
@@ -228,12 +228,13 @@ pub(crate) fn line(screen: &mut [u8], p1: &Point, p2: &Point, color: [u8; 4]) {
 }
 
 /// Draw a rectangle to the pixel buffer using two points in opposite corners.
-pub(crate) fn _rect(screen: &mut [u8], p1: &Point, p2: &Point, color: [u8; 4]) {
+pub(crate) fn rect(screen: &mut [u8], p1: &Point, p2: &Point, color: [u8; 4]) {
+    let p2 = Point::new(p2.x - 1, p2.y - 1);
     let p3 = Point::new(p1.x, p2.y);
     let p4 = Point::new(p2.x, p1.y);
 
     line(screen, p1, &p3, color);
-    line(screen, &p3, p2, color);
+    line(screen, &p3, &p2, color);
-    line(screen, p2, &p4, color);
+    line(screen, &p2, &p4, color);
     line(screen, &p4, p1, color);
 }