Floating bodies: FloatingBodyId, KinematicBody.Step, Zone.NextBodyId and the three drift settings

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/FloatingBodies.cs

@@ -0,0 +1,42 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+using System;
+using CultMath;
+using static CultMath.math;
+using float2 = CultMath.float2;
+
+// A free-floating body in a zone: a floating item or a mine. Zone-scoped and never reused within a zone;
+// runtime state only, never saved.
+public readonly struct FloatingBodyId : IEquatable<FloatingBodyId>
+{
+    public readonly uint Value;
+    public FloatingBodyId(uint value) => Value = value;
+    public bool Equals(FloatingBodyId other) => Value == other.Value;
+    public override bool Equals(object obj) => obj is FloatingBodyId other && Equals(other);
+    public override int GetHashCode() => (int) Value;
+    public override string ToString() => Value.ToString();
+}
+
+// The one drift integrator of every free body in a zone. A launch velocity decays on its own drag; the zone's
+// force accumulates into a separate drift velocity that has its own drag, so a body settles to
+// force * gravity / drag. Engine-free: the caller supplies dt and the zone force at the body's position.
+public struct KinematicBody
+{
+    public float2 Position;
+    // The launch velocity, decaying.
+    public float2 Velocity;
+    public float2 Drift;
+
+    // What a presenter draws ahead with.
+    public float2 TotalVelocity => Velocity + Drift;
+
+    public void Step(float dt, float2 force, GameplaySettings s)
+    {
+        Velocity *= exp(-s.FloatingBodyLaunchDrag * dt);
+        Drift += force * s.FloatingBodyGravity * dt;
+        Drift *= exp(-s.FloatingBodyDrag * dt);
+        Position += (Velocity + Drift) * dt;
+    }
+}

Assets/Scripts/ServerShared/FloatingBodies.cs.meta

@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 274c5573e2634fb58c52dbbf0406ce79
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData:
+  assetBundleName:
+  assetBundleVariant:

Assets/Scripts/ServerShared/Settings.cs

@@ -173,6 +173,11 @@ public class GameplaySettings
     public float ThermalQualityMin = 2;
     public float ThermalQualityMax = .25f;
     public float DefaultShutdownPerformance = .25f;
+    // Free-floating bodies (floating items, mines): drag on a launch velocity, drag on the force-driven drift, and
+    // how strongly the zone force accelerates it.
+    public float FloatingBodyDrag = .05f;
+    public float FloatingBodyLaunchDrag = .2f;
+    public float FloatingBodyGravity = 1f;
     public float SevereHeatstrokeRiskThreshold = .25f;
     public float WormholeDepth = 1000;
     public float WormholeExitVelocity = 20;

Assets/Scripts/ServerShared/Zone.cs

@@ -81,6 +81,10 @@ public class Zone
     // of touching a shared stream.
     public uint CombatSeed { get; }
 
+    // The one source of floating item and mine ids: zone-scoped, from 1, never reused. Runtime state, not saved.
+    private uint _lastBodyId;
+    public FloatingBodyId NextBodyId() => new FloatingBodyId(++_lastBodyId);
+
     public Zone(ItemManager itemManager, PlanetSettings settings, ZonePack pack, GalaxyZone galaxyZone, Galaxy galaxy)
     {
         _time = pack.Time;