1 files changed, 193 insertions, 0 deletions
diff --git a/planetwars-rules/src/rules.rs b/planetwars-rules/src/rules.rs
new file mode 100644
index 0000000..7c13e58
--- /dev/null
+++ b/planetwars-rules/src/rules.rs
@@ -0,0 +1,193 @@
+/// The planet wars game rules.
+#[derive(Debug)]
+pub struct PwState {
+    pub players: Vec<Player>,
+    pub planets: Vec<Planet>,
+    pub expeditions: Vec<Expedition>,
+    // How many expeditions were already dispatched.
+    // This is needed for assigning expedition identifiers.
+    pub expedition_num: u64,
+    pub turn_num: u64,
+    pub max_turns: u64,
+}
+
+#[derive(Debug)]
+pub struct Player {
+    pub id: usize,
+    pub alive: bool,
+}
+
+#[derive(Debug)]
+pub struct Fleet {
+    pub owner: Option<usize>,
+    pub ship_count: u64,
+}
+
+#[derive(Debug)]
+pub struct Planet {
+    pub id: usize,
+    pub name: String,
+    pub fleets: Vec<Fleet>,
+    pub x: f64,
+    pub y: f64,
+}
+
+#[derive(Debug)]
+pub struct Expedition {
+    pub id: u64,
+    pub origin: usize,
+    pub target: usize,
+    pub fleet: Fleet,
+    pub turns_remaining: u64,
+}
+
+#[derive(Debug)]
+pub struct Dispatch {
+    pub origin: usize,
+    pub target: usize,
+    pub ship_count: u64,
+}
+
+impl PwState {
+    pub fn dispatch(&mut self, dispatch: &Dispatch) {
+        let distance = self.planets[dispatch.origin]
+            .distance(&self.planets[dispatch.target]);
+
+        let origin = &mut self.planets[dispatch.origin];
+        origin.fleets[0].ship_count -= dispatch.ship_count;
+
+        let expedition = Expedition {
+            id: self.expedition_num,
+            origin: dispatch.origin,
+            target: dispatch.target,
+            turns_remaining: distance,
+            fleet: Fleet {
+                owner: origin.owner(),
+                ship_count: dispatch.ship_count,
+            },
+        };
+
+        // increment counter
+        self.expedition_num += 1;
+        self.expeditions.push(expedition);
+    }
+
+    // Play one step of the game
+    pub fn step(&mut self) {
+        self.turn_num += 1;
+
+        // Initially mark all players dead, re-marking them as alive once we
+        // encounter a sign of life.
+        for player in self.players.iter_mut() {
+            player.alive = false;
+        }
+
+        self.step_expeditions();
+        self.resolve_combat();
+    }
+
+    pub fn repopulate(&mut self) {
+        for planet in self.planets.iter_mut() {
+            if planet.owner().is_some() {
+                planet.fleets[0].ship_count += 1;
+            }
+        }
+    }
+
+    fn step_expeditions(&mut self) {
+        let mut i = 0;
+        let exps = &mut self.expeditions;
+        while i < exps.len() {
+            // compare with 1 to avoid issues with planet distance 0
+            if exps[i].turns_remaining <= 1 {
+                // remove expedition from expeditions, and add to fleet
+                let exp = exps.swap_remove(i);
+                let planet = &mut self.planets[exp.target];
+                planet.orbit(exp.fleet);
+            } else {
+                exps[i].turns_remaining -= 1;
+                if let Some(owner_num) = exps[i].fleet.owner {
+                    // owner has an expedition in progress; this is a sign of life.
+                    self.players[owner_num].alive = true;
+                }
+
+                // proceed to next expedition
+                i += 1;
+            }
+        }
+    }
+
+    fn resolve_combat(&mut self) {
+        for planet in self.planets.iter_mut() {
+            planet.resolve_combat();
+            if let Some(owner_num) = planet.owner() {
+                // owner owns a planet; this is a sign of life.
+                self.players[owner_num].alive = true;
+            }
+        }
+    }
+
+    pub fn is_finished(&self) -> bool {
+        let remaining = self.players.iter().filter(|p| p.alive).count();
+        return remaining < 2 || self.turn_num >= self.max_turns;
+    }
+
+    pub fn living_players(&self) -> Vec<usize> {
+        self.players
+            .iter()
+            .filter_map(|p| if p.alive { Some(p.id) } else { None })
+            .collect()
+    }
+}
+
+impl Planet {
+    pub fn owner(&self) -> Option<usize> {
+        self.fleets.first().and_then(|f| f.owner)
+    }
+
+    pub fn ship_count(&self) -> u64 {
+        self.fleets.first().map_or(0, |f| f.ship_count)
+    }
+
+    /// Make a fleet orbit this planet.
+    fn orbit(&mut self, fleet: Fleet) {
+        // If owner already has a fleet present, merge
+        for other in self.fleets.iter_mut() {
+            if other.owner == fleet.owner {
+                other.ship_count += fleet.ship_count;
+                return;
+            }
+        }
+        // else, add fleet to fleets list
+        self.fleets.push(fleet);
+    }
+
+    fn resolve_combat(&mut self) {
+        // The player owning the largest fleet present will win the combat.
+        // Here, we resolve how many ships he will have left.
+        // note: in the current implementation, we could resolve by doing
+        // winner.ship_count -= second_largest.ship_count, but this does not
+        // allow for simple customizations (such as changing combat balance).
+
+        self.fleets
+            .sort_by(|a, b| a.ship_count.cmp(&b.ship_count).reverse());
+        while self.fleets.len() > 1 {
+            let fleet = self.fleets.pop().unwrap();
+            // destroy some ships
+            for other in self.fleets.iter_mut() {
+                other.ship_count -= fleet.ship_count;
+            }
+
+            // remove dead fleets
+            while self.fleets.last().map(|f| f.ship_count) == Some(0) {
+                self.fleets.pop();
+            }
+        }
+    }
+
+    fn distance(&self, other: &Planet) -> u64 {
+        let dx = self.x - other.x;
+        let dy = self.y - other.y;
+        return (dx.powi(2) + dy.powi(2)).sqrt().ceil() as u64;
+    }
+}