From d2e6bfac2bef502e434c4a6c1b92c2ade1f42f6e Mon Sep 17 00:00:00 2001
From: copygirl <copygirl@mcft.net>
Date: Sun, 2 Nov 2025 14:42:42 +0100
Subject: [PATCH] Predict player entity, unify movement system

---
 client/src/main.rs             |  6 ++---
 common/src/network/protocol.rs | 17 ++++++++++++-
 common/src/network/server.rs   | 10 ++++----
 common/src/player.rs           | 44 ++++++++++++++++++----------------
 4 files changed, 47 insertions(+), 30 deletions(-)

diff --git a/client/src/main.rs b/client/src/main.rs
index 4a498ae..baac5c9 100644
--- a/client/src/main.rs
+++ b/client/src/main.rs
@@ -90,7 +90,7 @@ fn main() -> Result {
     );
 
     app.add_observer(spawn_initial_blocks);
-    app.add_observer(setup_player);
+    app.add_observer(handle_predicted_spawn);
 
     app.run();
     Ok(())
@@ -135,8 +135,8 @@ fn spawn_initial_blocks(_event: On<Add, Started>, mut blocks: Blocks) {
     }
 }
 
-/// When the player we control is being spawned, insert necessary components.
-fn setup_player(event: On<Add, Controlled>, mut commands: Commands) {
+/// When the `Predicted` player we control is being spawned, insert necessary components.
+fn handle_predicted_spawn(event: On<Add, Predicted>, mut commands: Commands) {
     let player = event.entity;
     commands
         .entity(player)
diff --git a/common/src/network/protocol.rs b/common/src/network/protocol.rs
index 287f5ea..4c26c96 100644
--- a/common/src/network/protocol.rs
+++ b/common/src/network/protocol.rs
@@ -25,8 +25,23 @@ impl Plugin for ProtocolPlugin {
     fn build(&self, app: &mut App) {
         app.add_plugins(InputPlugin::<Inputs>::default());
 
-        app.register_component::<Transform>();
+        // marker components
         app.register_component::<Player>();
         app.register_component::<Block>();
+
+        app.register_component::<Transform>()
+            .add_prediction()
+            .add_interpolation_with(interpolate_transform);
+
+        // unified update systems
+        app.add_systems(FixedUpdate, crate::player::movement);
+    }
+}
+
+fn interpolate_transform(start: Transform, other: Transform, t: f32) -> Transform {
+    Transform {
+        translation: start.translation.lerp(other.translation, t),
+        rotation: start.rotation.slerp(other.rotation, t),
+        scale: start.scale.lerp(other.scale, t),
     }
 }
diff --git a/common/src/network/server.rs b/common/src/network/server.rs
index e093222..58b26b5 100644
--- a/common/src/network/server.rs
+++ b/common/src/network/server.rs
@@ -26,8 +26,6 @@ impl Plugin for ServerPlugin {
 
         #[cfg(not(target_family = "wasm"))]
         app.add_observer(super::server_webtransport::print_certificate_digest);
-
-        app.add_systems(FixedUpdate, crate::player::server_movement);
     }
 }
 
@@ -57,11 +55,11 @@ fn on_server_stopped(event: On<Add, Stopped>) {
 
 fn handle_client_connected(
     event: On<Add, Connected>,
-    clients: Query<&LinkOf>,
+    clients: Query<(&LinkOf, &RemoteId)>,
     mut commands: Commands,
 ) {
     let client = event.entity;
-    let Ok(LinkOf { server }) = clients.get(client) else {
+    let Ok((LinkOf { server }, RemoteId(peer_id))) = clients.get(client) else {
         return; // Not a client of the server. (client-side?)
     };
     info!("Client '{client}' connected to server '{server}'");
@@ -74,11 +72,13 @@ fn handle_client_connected(
     commands.spawn((
         Player,
         Name::from("Player"),
-        Replicate::to_clients(NetworkTarget::All),
         ControlledBy {
             owner: client,
             lifetime: Lifetime::SessionBased,
         },
+        Replicate::to_clients(NetworkTarget::All),
+        PredictionTarget::to_clients(NetworkTarget::Single(*peer_id)),
+        InterpolationTarget::to_clients(NetworkTarget::AllExceptSingle(*peer_id)),
     ));
 }
 
diff --git a/common/src/player.rs b/common/src/player.rs
index 52f7f16..4e24d58 100644
--- a/common/src/player.rs
+++ b/common/src/player.rs
@@ -1,4 +1,5 @@
 use bevy::prelude::*;
+use lightyear::prelude::*;
 
 use lightyear::prelude::input::native::ActionState;
 use serde::{Deserialize, Serialize};
@@ -11,28 +12,29 @@ const MOVEMENT_SPEED: f32 = 5.0;
 #[require(Transform)]
 pub struct Player;
 
-pub fn server_movement(
+pub fn movement(
     time: Res<Time<Fixed>>,
-    mut players: Query<(&mut Transform, &ActionState<Inputs>)>,
+    mut players: Query<
+        (&mut Transform, &ActionState<Inputs>),
+        // Must be a `Player` which is either be `ControlledBy` a remote
+        // client (server-side) or its movement `Predicted` on the client.
+        (With<Player>, Or<(With<ControlledBy>, With<Predicted>)>),
+    >,
 ) {
-    for (mut transform, inputs) in players.iter_mut() {
-        shared_movement(&mut transform, *time, inputs);
+    for (mut transform, input) in players.iter_mut() {
+        let dt = time.delta_secs();
+        let mut translation = Vec3::ZERO;
+        if input.movement != Vec2::ZERO {
+            let movement = input.movement.clamp_length_max(1.0);
+            translation.x += movement.x;
+            translation.z += movement.y;
+        }
+        if input.up {
+            translation.y += 1.0;
+        }
+        if input.down {
+            translation.y -= 1.0;
+        }
+        transform.translation += translation * MOVEMENT_SPEED * dt;
     }
 }
-
-pub fn shared_movement(transform: &mut Transform, time: Time<Fixed>, input: &Inputs) {
-    let dt = time.delta_secs();
-    let mut translation = Vec3::ZERO;
-    if input.movement != Vec2::ZERO {
-        let movement = input.movement.clamp_length_max(1.0);
-        translation.x += movement.x;
-        translation.z += movement.y;
-    }
-    if input.up {
-        translation.y += 1.0;
-    }
-    if input.down {
-        translation.y -= 1.0;
-    }
-    transform.translation += translation * MOVEMENT_SPEED * dt;
-}