diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..25f9131
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,5 @@
+[workspace]
+
+members = [
+    "planetwars-rules",
+]
\ No newline at end of file
diff --git a/planetwars-rules/.gitignore b/planetwars-rules/.gitignore
new file mode 100644
index 0000000..869df07
--- /dev/null
+++ b/planetwars-rules/.gitignore
@@ -0,0 +1,2 @@
+/target
+Cargo.lock
\ No newline at end of file
diff --git a/planetwars-rules/Cargo.toml b/planetwars-rules/Cargo.toml
new file mode 100644
index 0000000..d42da14
--- /dev/null
+++ b/planetwars-rules/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "planetwars-rules"
+version = "0.1.0"
+authors = ["Ilion Beyst <ilion.beyst@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+serde_json = "1.0"
+serde = { version = "1.0", features = ["derive"] }
diff --git a/planetwars-rules/src/config.rs b/planetwars-rules/src/config.rs
new file mode 100644
index 0000000..32c23f5
--- /dev/null
+++ b/planetwars-rules/src/config.rs
@@ -0,0 +1,84 @@
+use std::fs::File;
+use std::io;
+use std::io::Read;
+
+use serde_json;
+
+use super::protocol as proto;
+use super::rules::*;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Config {
+    pub map_file: String,
+    pub max_turns: u64,
+}
+
+impl Config {
+    pub fn create_state(&self, num_players: usize) -> PwState {
+        let planets = self.load_map(num_players);
+        let players = (0..num_players)
+            .map(|player_num| Player {
+                id: player_num + 1,
+                alive: true,
+            })
+            .collect();
+
+        PwState {
+            players: players,
+            planets: planets,
+            expeditions: Vec::new(),
+            expedition_num: 0,
+            turn_num: 0,
+            max_turns: self.max_turns,
+        }
+    }
+
+    fn load_map(&self, num_players: usize) -> Vec<Planet> {
+        let map = self.read_map().expect("[PLANET_WARS] reading map failed");
+
+        return map
+            .planets
+            .into_iter()
+            .enumerate()
+            .map(|(num, planet)| {
+                let mut fleets = Vec::new();
+                let owner = planet.owner.and_then(|owner_num| {
+                    // in the current map format, player numbers start at 1.
+                    // TODO: we might want to change this.
+                    // ignore players that are not in the game
+                    if owner_num > 0 && owner_num <= num_players {
+                        Some(owner_num - 1)
+                    } else {
+                        None
+                    }
+                });
+                if planet.ship_count > 0 {
+                    fleets.push(Fleet {
+                        owner: owner,
+                        ship_count: planet.ship_count,
+                    });
+                }
+                return Planet {
+                    id: num,
+                    name: planet.name,
+                    x: planet.x,
+                    y: planet.y,
+                    fleets: fleets,
+                };
+            })
+            .collect();
+    }
+
+    fn read_map(&self) -> io::Result<Map> {
+        let mut file = File::open(&self.map_file)?;
+        let mut buf = String::new();
+        file.read_to_string(&mut buf)?;
+        let map = serde_json::from_str(&buf)?;
+        return Ok(map);
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Map {
+    pub planets: Vec<proto::Planet>,
+}
diff --git a/planetwars-rules/src/lib.rs b/planetwars-rules/src/lib.rs
new file mode 100644
index 0000000..29c7b28
--- /dev/null
+++ b/planetwars-rules/src/lib.rs
@@ -0,0 +1,111 @@
+#[macro_use]
+extern crate serde;
+extern crate serde_json;
+
+pub mod config;
+pub mod protocol;
+pub mod rules;
+pub mod serializer;
+
+use std::collections::HashMap;
+pub use rules::{PwState, Dispatch};
+pub use protocol::CommandError;
+pub use config::Config as PwConfig;
+
+pub struct PlanetWars {
+    /// Game state
+    state: rules::PwState,
+    /// Map planet names to their ids
+    planet_map: HashMap<String, usize>
+}
+
+impl PlanetWars {
+    pub fn create(config: PwConfig, num_players: usize) -> Self {
+        let state = config.create_state(num_players);
+
+        let planet_map = state
+            .planets
+            .iter()
+            .map(|p| (p.name.clone(), p.id))
+            .collect();
+        
+        PlanetWars { state, planet_map }
+    }
+
+    /// Proceed to next turn
+    pub fn step(&mut self) {
+        self.state.repopulate();
+        self.state.step();
+    }
+
+    pub fn is_finished(&self) -> bool {
+        self.state.is_finished()
+    }
+
+    pub fn serialize_state(&self) -> protocol::State {
+        serializer::serialize(&self.state)
+    }
+
+    pub fn serialize_player_state(&self, player_id: usize) -> protocol::State {
+        serializer::serialize_rotated(&self.state, player_id - 1)
+    }
+
+    pub fn state<'a>(&'a self) -> &'a PwState {
+        &self.state
+    }
+
+    /// Execute a command
+    pub fn execute_command(
+        &mut self,
+        player_num: usize,
+        cmd: &protocol::Command
+    ) -> Result<(), CommandError>
+    {
+        let dispatch = self.parse_command(player_num, cmd)?;
+        self.state.dispatch(&dispatch);
+        return Ok(());
+    }
+
+    /// Check the given command for validity.
+    /// If it is valid, return an internal representation of the dispatch
+    /// described by the command.
+    pub fn parse_command(&self, player_id: usize, cmd: &protocol::Command)
+        -> Result<Dispatch, CommandError>
+    {
+        let origin_id = *self
+        .planet_map
+        .get(&cmd.origin)
+        .ok_or(CommandError::OriginDoesNotExist)?;
+
+        let target_id = *self
+            .planet_map
+            .get(&cmd.destination)
+            .ok_or(CommandError::DestinationDoesNotExist)?;
+
+        if self.state.planets[origin_id].owner() != Some(player_id - 1) {
+            println!("owner was {:?}", self.state.planets[origin_id].owner());
+            return Err(CommandError::OriginNotOwned);
+        }
+
+        if self.state.planets[origin_id].ship_count() < cmd.ship_count {
+            return Err(CommandError::NotEnoughShips);
+        }
+
+        if cmd.ship_count == 0 {
+            return Err(CommandError::ZeroShipMove);
+        }
+
+        Ok(Dispatch {
+            origin: origin_id,
+            target: target_id,
+            ship_count: cmd.ship_count,
+        })
+    }
+
+    /// Execute a dispatch.
+    /// This assumes the dispatch is valid. You should check this yourself
+    /// or use `parse_command` to obtain a valid dispatch.
+    pub fn execute_dispatch(&mut self, dispatch: &Dispatch) {
+        self.state.dispatch(dispatch);
+    }
+}
\ No newline at end of file
diff --git a/planetwars-rules/src/protocol.rs b/planetwars-rules/src/protocol.rs
new file mode 100644
index 0000000..23612d0
--- /dev/null
+++ b/planetwars-rules/src/protocol.rs
@@ -0,0 +1,79 @@
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Expedition {
+    pub id: u64,
+    pub ship_count: u64,
+    pub origin: String,
+    pub destination: String,
+    pub owner: usize,
+    pub turns_remaining: u64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Planet {
+    pub ship_count: u64,
+    pub x: f64,
+    pub y: f64,
+    pub owner: Option<usize>,
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Action {
+    #[serde(rename = "moves")]
+    pub commands: Vec<Command>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Command {
+    pub origin: String,
+    pub destination: String,
+    pub ship_count: u64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct State {
+    pub planets: Vec<Planet>,
+    pub expeditions: Vec<Expedition>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct GameInfo {
+    pub players: Vec<String>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum CommandError {
+    NotEnoughShips,
+    OriginNotOwned,
+    ZeroShipMove,
+    OriginDoesNotExist,
+    DestinationDoesNotExist,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PlayerCommand {
+    pub command: Command,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub error: Option<CommandError>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+#[serde(tag = "type", content = "value")]
+pub enum PlayerAction {
+    Timeout,
+    ParseError(String),
+    Commands(Vec<PlayerCommand>),
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+#[serde(tag = "type", content = "content")]
+pub enum ServerMessage {
+    /// Game state in current turn
+    GameState(State),
+    /// The action that was performed
+    PlayerAction(PlayerAction),
+    /// The game is over, and this is the concluding state.
+    FinalState(State),
+}
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;
+    }
+}
diff --git a/planetwars-rules/src/serializer.rs b/planetwars-rules/src/serializer.rs
new file mode 100644
index 0000000..380433e
--- /dev/null
+++ b/planetwars-rules/src/serializer.rs
@@ -0,0 +1,76 @@
+use super::protocol as proto;
+use super::rules::{Expedition, Planet, PwState};
+
+/// Serialize given gamestate
+pub fn serialize(state: &PwState) -> proto::State {
+    serialize_rotated(state, 0)
+}
+
+/// Serialize given gamestate with player numbers rotated by given offset.
+pub fn serialize_rotated(state: &PwState, offset: usize) -> proto::State {
+    let serializer = Serializer::new(state, offset);
+    serializer.serialize_state()
+}
+
+struct Serializer<'a> {
+    state: &'a PwState,
+    player_num_offset: usize,
+}
+
+impl<'a> Serializer<'a> {
+    fn new(state: &'a PwState, offset: usize) -> Self {
+        Serializer {
+            state: state,
+            player_num_offset: offset,
+        }
+    }
+
+    fn serialize_state(&self) -> proto::State {
+        proto::State {
+            planets: self
+                .state
+                .planets
+                .iter()
+                .map(|planet| self.serialize_planet(planet))
+                .collect(),
+            expeditions: self
+                .state
+                .expeditions
+                .iter()
+                .map(|exp| self.serialize_expedition(exp))
+                .collect(),
+        }
+    }
+
+    /// Gets the player number for given player id.
+    /// Player numbers are 1-based (as opposed to player ids), They will also be
+    /// rotated based on the number offset for this serializer.
+    fn player_num(&self, player_id: usize) -> usize {
+        let num_players = self.state.players.len();
+        let rotated_id =
+            (player_id + num_players - self.player_num_offset) % num_players;
+        // protocol player ids start at 1
+        return rotated_id + 1;
+    }
+
+    fn serialize_planet(&self, planet: &Planet) -> proto::Planet {
+        proto::Planet {
+            name: planet.name.clone(),
+            x: planet.x,
+            y: planet.y,
+            owner: planet.owner().map(|id| self.player_num(id)),
+            ship_count: planet.ship_count(),
+        }
+    }
+
+    fn serialize_expedition(&self, exp: &Expedition) -> proto::Expedition {
+        proto::Expedition {
+            id: exp.id,
+            owner: self.player_num(exp.fleet.owner.unwrap()),
+            ship_count: exp.fleet.ship_count,
+            origin: self.state.planets[exp.origin as usize].name.clone(),
+            destination: self.state.planets[exp.target as usize].name.clone(),
+            turns_remaining: exp.turns_remaining,
+        }
+    }
+}