implement tidal locking of moons

src/cmd.rs

@@ -91,6 +91,7 @@ struct ParserState {
     is_moon: bool,
     is_planet: bool,
     is_point_of_interest: bool,
+    is_tidally_locked: bool,
     orbit_distance: Option<f64>,
     orbit_object_id: Option<String>,
     orbit_phase: Option<f64>,
@@ -149,6 +150,7 @@ impl Default for ParserState {
             is_moon: false,
             is_planet: false,
             is_point_of_interest: false,
+            is_tidally_locked: false,
             orbit_distance: None,
             orbit_object_id: None,
             orbit_phase: None,
@@ -456,6 +458,9 @@ pub fn load_defs(mut ew_spawn: EventWriter<SpawnEvent>) {
             ["ring", "yes"] => {
                 state.has_ring = true;
             }
+            ["tidally", "locked"] => {
+                state.is_tidally_locked = true;
+            }
             ["pointofinterest", "yes"] => {
                 state.is_point_of_interest = true;
             }
@@ -848,6 +853,17 @@ fn spawn_entities(
                 None
             };
             absolute_pos += nature::pos_offset_for_orbiting_body(r, mass, state.orbit_phase);
+
+            if state.is_tidally_locked {
+                if let Some(mass) = mass {
+                    rotation =
+                        Quat::from_rotation_y(nature::rotation_for_orbiting_body(r, mass) as f32);
+                } else {
+                    error!("Attempted to use 'tidally locked' without specifying mass via 'orbitaround'");
+                }
+            }
+        } else if state.is_tidally_locked {
+            error!("Attempted to use 'tidally locked' without specifying orbital distance via 'orbitaround'");
         }
         let scale = Vec3::splat(
             if state.is_sun {

src/data/defs.txt

@@ -127,10 +127,11 @@ actor 0 0 0
             relativeto jupiter
             id thebe
             orbitaround jupiter 221900e3
+            tidally locked
             scale 50e3
             moon yes
             collider handcrafted
-            angularmomentum 0 0.0 0
+            angularmomentum 0 0 0
             actor 0 0 0 orbitring
                 relativeto jupiter
                 scale 221900e3

src/nature.rs

@@ -230,3 +230,18 @@ pub fn pos_offset_for_orbiting_body(
     }
     return phase_dist_to_coords(-phase_radians, r);
 }
+
+/// Assumes the "front" (as seen in blender) is pointing at the orbited mass
+pub fn rotation_for_orbiting_body(
+    orbit_distance: f64,
+    mass: f64,
+) -> f64 {
+    if let Ok(epoch) = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
+        let orbital_period = nature::simple_orbital_period(mass, orbit_distance);
+        let now = epoch.as_secs_f64() + ORBIT_TIME_OFFSET;
+        PI * 2.0 * (now % orbital_period) / orbital_period - PI * 0.5
+    } else {
+        eprintln!("WARNING: Can't determine current time in calculate_position_offset_for_orbiting_body().");
+        0.0
+    }
+}