Implement throttle floor option

Source/Engines/ThrottleFloorHandler.cs

@@ -0,0 +1,107 @@
+using UnityEngine;
+
+namespace RealFuels
+{
+    /// <summary>
+    /// Keeps the throttle-down key from taking the throttle all the way to zero while an engine is
+    /// burning. ModuleEnginesRF drops out of its ignited state as soon as the main throttle hits
+    /// exactly zero, so with limited ignitions the slightest overshoot while feeling for minimum
+    /// throttle costs an ignition. While the key is held the throttle bottoms out at
+    /// <see cref="ThrottleFloor"/> instead; the throttle cutoff key still cuts to zero.
+    /// Opt-in through <see cref="RFGameParameters.throttleDownKeepsEnginesLit"/>.
+    /// </summary>
+    [KSPAddon(KSPAddon.Startup.Flight, false)]
+    public class ThrottleFloorHandler : MonoBehaviour
+    {
+        /// <summary>Lowest throttle the throttle-down key is allowed to reach: 0.1%.</summary>
+        public const float ThrottleFloor = 0.001f;
+
+        private Vessel _hookedVessel;
+        private float _lastThrottle;
+        private bool _holdingFloor;
+        private bool _floorEnabled;
+
+        private void Start()
+        {
+            GameEvents.OnGameSettingsApplied.Add(LoadSettings);
+            LoadSettings();
+        }
+
+        private void Update()
+        {
+            // Polled rather than driven off onVesselChange: the active vessel may not be set yet
+            // when a flight scene addon starts up, and this also picks up docking and undocking.
+            if (_hookedVessel != FlightGlobals.ActiveVessel)
+                HookVessel(FlightGlobals.ActiveVessel);
+        }
+
+        private void OnDestroy()
+        {
+            GameEvents.OnGameSettingsApplied.Remove(LoadSettings);
+            HookVessel(null);
+        }
+
+        private void LoadSettings()
+        {
+            _floorEnabled = HighLogic.CurrentGame?.Parameters.CustomParams<RFGameParameters>()?.throttleDownKeepsEnginesLit ?? false;
+        }
+
+        private void HookVessel(Vessel v)
+        {
+            if (_hookedVessel != null)
+                _hookedVessel.OnPreAutopilotUpdate -= ApplyThrottleFloor;
+
+            _hookedVessel = v;
+            _holdingFloor = false;
+
+            if (_hookedVessel != null)
+            {
+                // Runs from FlightInputHandler.FixedUpdate() right after the keyboard input has been
+                // pushed into ctrlState, and before any autopilot gets to touch it.
+                _hookedVessel.OnPreAutopilotUpdate += ApplyThrottleFloor;
+                _lastThrottle = _hookedVessel.ctrlState.mainThrottle;
+            }
+        }
+
+        private void ApplyThrottleFloor(FlightCtrlState st)
+        {
+            if (!GameSettings.THROTTLE_DOWN.GetKey() || GameSettings.THROTTLE_CUTOFF.GetKey())
+            {
+                _holdingFloor = false;
+            }
+            // Requiring the previous throttle to be above the floor means a throttle that is already
+            // at zero stays there, and that a cutoff from any other source is not undone.
+            else if (_floorEnabled && st.mainThrottle < ThrottleFloor && _lastThrottle >= ThrottleFloor)
+            {
+                // The engines can't shut themselves off while we hold the floor, so the vessel only
+                // needs to be scanned on the frame the key first bottoms the throttle out.
+                if (!_holdingFloor)
+                    _holdingFloor = AnyEngineBurning(_hookedVessel);
+
+                if (_holdingFloor)
+                {
+                    st.mainThrottle = ThrottleFloor;
+                    // FlightInputHandler keeps the master copy and re-derives the throttle from it
+                    // every physics frame, so it has to be corrected too or the floor lasts one frame.
+                    FlightInputHandler.state.mainThrottle = ThrottleFloor;
+                }
+            }
+
+            _lastThrottle = st.mainThrottle;
+        }
+
+        private static bool AnyEngineBurning(Vessel v)
+        {
+            for (int i = v.parts.Count - 1; i >= 0; --i)
+            {
+                PartModuleList modules = v.parts[i].Modules;
+                for (int j = modules.Count - 1; j >= 0; --j)
+                {
+                    if (modules[j] is ModuleEngines engine && engine.EngineIgnited && !engine.flameout && engine.currentThrottle > 0f)
+                        return true;
+                }
+            }
+            return false;
+        }
+    }
+}

Source/RFGameParameters.cs

@@ -6,7 +6,7 @@ namespace RealFuels
     /// </summary>
     public class RFGameParameters : GameParameters.CustomParameterNode
     {
-        public override string Title => "Engine Config GUI";
+        public override string Title => "General";
         public override GameParameters.GameMode GameMode => GameParameters.GameMode.ANY;
         public override string Section => "RealFuels";
         public override string DisplaySection => "RealFuels";
@@ -16,5 +16,9 @@ public class RFGameParameters : GameParameters.CustomParameterNode
         [GameParameters.CustomParameterUI("Auto-open Engine Config window",
             toolTip = "Automatically open the engine config selection window when right-clicking an engine part.")]
         public bool autoOpenEngineGUI = true;
+
+        [GameParameters.CustomParameterUI("Throttle-down key keeps engines lit",
+            toolTip = "Holding the throttle-down key (Left Ctrl by default) stops at 0.1% throttle instead of zero while an engine is burning, so throttling down to minimum will not shut the engine off. Use the throttle cutoff key (X by default) to shut engines down.")]
+        public bool throttleDownKeepsEnginesLit = false;
     }
 }

Source/RealFuels.csproj

@@ -105,6 +105,7 @@
     <Compile Include="Engines\RFSettings.cs" />
     <Compile Include="Engines\ModuleEnginesRF.cs" />
     <Compile Include="Engines\SolverRF.cs" />
+    <Compile Include="Engines\ThrottleFloorHandler.cs" />
     <Compile Include="EntryCosts\TLUpgrade.cs" />
     <Compile Include="EntryCosts\Initializer.cs" />
     <Compile Include="EntryCosts\UpgradeManager.cs" />