Match loop on the client side: orders go to the server, a confirmed turn steps the world five times, and a checksum of the shared state catches worlds that diverge

src/game.rs

@@ -594,6 +594,8 @@ pub struct Game {
     #[serde(skip)]
     step_acc: f32, // real time not yet paid for with a step
     #[serde(skip)]
+    pub netplay: Option<crate::netplay::Netplay>, // a match: turns come in, the world steps per turn
+    #[serde(skip)]
     pub hints: bool, // show hint messages (setting)
     #[serde(skip, default = "hint_times")]
     hint_t: [f32; 32], // last time each hint slot was shown
@@ -774,6 +776,7 @@ impl Game {
             cmd_seq: 0,
             sim_steps: 0,
             step_acc: 0.0,
+            netplay: None,
             hint_t: hint_times(),
             reserve: 0.0,
             incoming: Vec::new(),
@@ -1176,13 +1179,19 @@ impl Game {
     // ---------------------------------------------------------------- update
 
     pub fn update(&mut self, dt: f32) {
-        self.step_acc = (self.step_acc + dt).min(STEP * MAX_STEPS_PER_FRAME as f32);
         let mut steps = 0usize;
-        while self.step_acc >= STEP {
-            self.step_acc -= STEP;
-            steps += 1;
-        }
-        let cmds = std::mem::take(&mut self.pending);
+        let cmds = if self.netplay.is_some() {
+            // a match runs turn by turn (see netplay.rs): without the next turn the world stands still,
+            // and local orders are not applied here - they travel to the server first
+            Vec::new()
+        } else {
+            self.step_acc = (self.step_acc + dt).min(STEP * MAX_STEPS_PER_FRAME as f32);
+            while self.step_acc >= STEP {
+                self.step_acc -= STEP;
+                steps += 1;
+            }
+            std::mem::take(&mut self.pending)
+        };
         self.apply_turn(&cmds, steps);
         // what only the camera cares about runs in real time: it is not part of the shared state
         self.update_particles(dt);

src/main.rs

@@ -13,6 +13,7 @@ mod gfx;
 mod i18n;
 mod map;
 mod menu;
+mod netplay;
 mod personality;
 mod platform;
 mod orders;

src/netplay.rs

@@ -0,0 +1,277 @@
+// Multiplayer, client side of deterministic lockstep: the local orders go to the server, the server
+// confirms a turn with the orders of all masters, and the world advances one turn at a time (five
+// steps of 20 ms). A client that receives nothing stands still. Every client computes the same
+// world from the same turns, so comparing a checksum of that world shows a divergence (desync).
+#![allow(dead_code)] // the server relay and the Multiplayer menu (steps C and D) call all of this
+
+use crate::commands::Command;
+use crate::game::Game;
+
+// one server turn of 100 ms: five fixed simulation steps
+pub const STEPS_PER_TURN: usize = 5;
+
+#[derive(Clone, Debug, Default)]
+pub struct Netplay {
+    pub turn: u32, // turns applied so far
+    pub desync: bool, // a peer reported a different world
+}
+
+const FNV_OFFSET: u64 = 0xcbf2_9ce4_8422_2325;
+const FNV_PRIME: u64 = 0x100_0000_01b3;
+
+fn mix(h: &mut u64, v: u64) {
+    *h ^= v;
+    *h = h.wrapping_mul(FNV_PRIME);
+}
+
+fn mixf(h: &mut u64, v: f32) {
+    mix(h, v.to_bits() as u64);
+}
+
+fn mixp(h: &mut u64, p: Option<(i32, i32)>) {
+    match p {
+        Some((x, y)) => {
+            mix(h, x as u32 as u64);
+            mix(h, y as u32 as u64);
+        }
+        None => mix(h, u64::MAX),
+    }
+}
+
+// a value whose type is not a plain number (Job carries which tile it works on)
+fn mixdbg(h: &mut u64, v: impl std::fmt::Debug) {
+    for b in format!("{v:?}").bytes() {
+        mix(h, b as u64);
+    }
+}
+
+impl Game {
+    // a match with masters on `sides` (of 0, 2, 3, 4), `local` is the master who plays at this computer
+    pub fn start_match(&mut self, sides: &[u8], local: u8) -> bool {
+        if !self.start_masters(sides, local) {
+            return false;
+        }
+        self.netplay = Some(Netplay::default());
+        true
+    }
+
+    pub fn in_match(&self) -> bool {
+        self.netplay.is_some()
+    }
+
+    // what the local master ordered since the last turn: these go to the server
+    pub fn take_orders(&mut self) -> Vec<Command> {
+        std::mem::take(&mut self.pending)
+    }
+
+    // the server confirmed a turn: run the orders of all masters, then the fixed steps of the turn.
+    // A client that is paused or finished leaves the turn out (a match pauses while somebody is away).
+    pub fn receive_turn(&mut self, cmds: &[Command]) -> u32 {
+        if self.netplay.is_none() || self.state != 0 || self.paused {
+            return 0;
+        }
+        self.apply_turn(cmds, STEPS_PER_TURN);
+        match self.netplay.as_mut() {
+            Some(np) => {
+                np.turn += 1;
+                np.turn
+            }
+            None => 0,
+        }
+    }
+
+    // a peer's checksum of turn `turn`: when we are standing at that turn as well, the numbers have
+    // to match. A mismatch is reported once and makes the match unusable (the message says it).
+    pub fn check_peer(&mut self, turn: u32, sum: u64) -> bool {
+        let same = match &self.netplay {
+            Some(np) => np.turn != turn || sum == self.checksum(),
+            None => true,
+        };
+        if same {
+            return true;
+        }
+        if let Some(np) = self.netplay.as_mut() {
+            np.desync = true;
+        }
+        self.msg("net.desync", vec![]);
+        self.sfx_ui(crate::game::Sfx::Error);
+        false
+    }
+
+    // the part of the world every client of a match has to agree on: the map, the creatures, the
+    // economy of every master, the wave and the random state. Fog, particles and the camera are
+    // local and stay out of it.
+    pub fn checksum(&self) -> u64 {
+        let mut h = FNV_OFFSET;
+        mix(&mut h, self.sim_steps);
+        mix(&mut h, self.state as u64);
+        mix(&mut h, self.next_id as u64);
+        mix(&mut h, self.rng.0 as u64);
+        mix(&mut h, self.wave as u64);
+        mixf(&mut h, self.time);
+        mixf(&mut h, self.gate_hp);
+        mixf(&mut h, self.last_wave_value);
+        mix(&mut h, self.towers.len() as u64);
+        mix(&mut h, self.traps.len() as u64);
+        mix(&mut h, self.rollers.len() as u64);
+        mix(&mut h, self.graves.len() as u64);
+        for t in self.map.t.iter() {
+            mix(&mut h, t.kind as u8 as u64);
+            mix(&mut h, t.owner as u8 as u64);
+            mix(&mut h, t.marked as u64);
+            mix(&mut h, t.reinforced as u64);
+            mix(&mut h, t.res as u64);
+            mix(&mut h, t.var as u64);
+            mixf(&mut h, t.hp);
+            mix(&mut h, t.gold as u32 as u64);
+            mix(&mut h, t.reserved as u64);
+            mix(&mut h, t.bed as u64);
+            mixf(&mut h, t.food);
+        }
+        for e in self.ents.iter() {
+            mix(&mut h, e.id as u64);
+            mix(&mut h, e.kind as u8 as u64);
+            mix(&mut h, e.side as u64);
+            mix(&mut h, e.hero as u64);
+            mix(&mut h, e.alive as u64);
+            mix(&mut h, e.held as u64);
+            mix(&mut h, e.level as u64);
+            mixf(&mut h, e.x);
+            mixf(&mut h, e.y);
+            mixf(&mut h, e.hp);
+            mixf(&mut h, e.max_hp);
+            mixf(&mut h, e.xp);
+            mixdbg(&mut h, &e.job);
+            mixf(&mut h, e.timer);
+            mixf(&mut h, e.cd);
+            mixf(&mut h, e.think);
+            mixf(&mut h, e.repath);
+            mix(&mut h, e.carry as u32 as u64);
+            mix(&mut h, e.carry_res as u64);
+            mixf(&mut h, e.hunger);
+            mixf(&mut h, e.energy);
+            mixf(&mut h, e.happy);
+            mixp(&mut h, e.bed.map(|b| (b as i32, 0)));
+            mix(&mut h, e.moving as u64);
+            mix(&mut h, e.working as u64);
+            mixf(&mut h, e.slow);
+            mix(&mut h, e.captive as u64);
+            mix(&mut h, e.carrier as u64);
+            mixf(&mut h, e.torture_t);
+            mixf(&mut h, e.skill_cd);
+            mixp(&mut h, e.waypoint);
+            mixf(&mut h, e.boost);
+            mixf(&mut h, e.stun);
+            mixf(&mut h, e.haste);
+            mixf(&mut h, e.shield);
+            mixf(&mut h, e.invis);
+            mixf(&mut h, e.plague);
+            mixf(&mut h, e.ab_cd);
+            mixf(&mut h, e.mood_t);
+            mix(&mut h, e.guard as u64);
+            mix(&mut h, e.path.len() as u64);
+        }
+        let mut sums: Vec<(u8, f32, f32, f32, f32, usize, usize, f32, usize, (i32, i32))> = Vec::new();
+        sums.push((self.me_side, self.gold, self.mana, self.core_hp, self.payday, self.hand.len(), self.counts.iter().sum(), self.reserve, self.springs, self.map.core));
+        for m in self.masters.iter() {
+            sums.push((m.side, m.gold, m.mana, m.core_hp, m.payday, m.hand.len(), m.counts.iter().sum(), m.reserve, m.springs, m.core));
+        }
+        sums.sort_by(|a, b| a.0.cmp(&b.0));
+        for (side, gold, mana, core_hp, payday, hand, count, reserve, springs, core) in sums.iter() {
+            mix(&mut h, *side as u64);
+            mixf(&mut h, *gold);
+            mixf(&mut h, *mana);
+            mixf(&mut h, *core_hp);
+            mixf(&mut h, *payday);
+            mix(&mut h, *hand as u64);
+            mix(&mut h, *count as u64);
+            mixf(&mut h, *reserve);
+            mix(&mut h, *springs as u64);
+            mixp(&mut h, Some(*core));
+        }
+        h
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::commands::{Act, Rect};
+    use crate::map::{Kind, Owner};
+
+    fn client(seed: u32, local: u8) -> Game {
+        let mut g = Game::new(seed, 96, 128);
+        g.init_fog();
+        assert!(g.start_match(&[0, 2, 3], local), "a match of three masters");
+        g
+    }
+
+    // the top-left corner of the core of `owner`
+    fn core_of(g: &Game, owner: Owner) -> (i32, i32) {
+        for y in 0..128i32 {
+            for x in 0..96i32 {
+                let t = &g.map.t[(y as usize) * 96 + x as usize];
+                if t.kind == Kind::Core && t.owner == owner {
+                    return (x, y);
+                }
+            }
+        }
+        panic!("no core of {owner:?}");
+    }
+
+    #[test]
+    fn two_clients_of_one_match_see_the_same_world() {
+        let (mut a, mut b) = (client(21, 0), client(21, 2));
+        assert_eq!(a.checksum(), b.checksum(), "the same start");
+        let (red, green) = (core_of(&a, Owner::Player), core_of(&a, Owner::Rival1));
+        for t in 0..40u32 {
+            let (d, o) = (t as i32 * 2, t as i32 * 2);
+            let turn = vec![
+                Command { side: 0, seq: t * 2, act: Act::Dig(Rect::new(red.0 + 7 + o, red.1 - 4, red.0 + 8 + o, red.1 - 3)) },
+                Command { side: 2, seq: t * 2 + 1, act: Act::Dig(Rect::new(green.0 + 4, green.1 + 5 + d, green.0 + 5, green.1 + 6 + d)) },
+            ];
+            a.receive_turn(&turn);
+            b.receive_turn(&turn);
+        }
+        assert_eq!(a.sim_steps, b.sim_steps);
+        assert_eq!(a.sim_steps, 40 * STEPS_PER_TURN as u64, "five steps in every turn");
+        assert_eq!(a.gold, b.gold);
+        assert_eq!(a.checksum(), b.checksum(), "the worlds stayed identical");
+        // real time alone never moves a match forward
+        a.update(1.0);
+        b.update(1.0);
+        assert_eq!(a.sim_steps, 40 * STEPS_PER_TURN as u64);
+        assert_eq!(a.checksum(), b.checksum());
+    }
+
+    #[test]
+    fn a_client_without_the_next_turn_stands_still_and_its_orders_wait() {
+        let mut g = client(22, 0);
+        let (cx, cy) = core_of(&g, Owner::Player);
+        g.queue(Act::Dig(Rect::new(cx + 7, cy, cx + 8, cy + 1)));
+        for _ in 0..10 {
+            g.update(0.2);
+        }
+        assert_eq!(g.sim_steps, 0, "no turn, no step");
+        assert_eq!(g.netplay.as_ref().unwrap().turn, 0);
+        let orders = g.take_orders();
+        assert_eq!(orders.len(), 1, "the order waits for the server");
+        assert!(g.pending.is_empty());
+        g.receive_turn(&orders);
+        assert_eq!(g.sim_steps, STEPS_PER_TURN as u64);
+        assert_eq!(g.netplay.as_ref().unwrap().turn, 1);
+    }
+
+    #[test]
+    fn a_world_that_develops_differently_is_caught_by_the_checksum() {
+        let (mut a, mut b) = (client(23, 0), client(23, 0));
+        let (cx, cy) = core_of(&a, Owner::Player);
+        let turn = vec![Command { side: 0, seq: 0, act: Act::Dig(Rect::new(cx + 7, cy, cx + 8, cy + 1)) }];
+        a.receive_turn(&turn);
+        b.receive_turn(&[]); // b lost that turn
+        assert_ne!(a.checksum(), b.checksum());
+        assert!(!b.check_peer(a.netplay.as_ref().unwrap().turn, a.checksum()), "b notices the difference");
+        assert!(b.netplay.as_ref().unwrap().desync, "and it is recorded");
+        assert!(b.msgs.iter().any(|m| m.key == "net.desync"));
+    }
+}