ported from perforce

RobotAndDonkey.Game/Modifiers/RainModifier.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using RobotAndDonkey.Game.Board;
+using RobotAndDonkey.Game.Execution;
+using RobotAndDonkey.Game.Execution.Results;
+using RobotAndDonkey.Game.GameState;
+using RobotAndDonkey.Game.Intents;
+
+namespace RobotAndDonkey.Game.Modifiers;
+
+public record RainModifier(EModifierDuration Duration) : Modifier(EModifierKind.Cell, EModifierId.Rain, Duration)
+{
+    protected override RainModifier CreateInstance() => new(Duration);
+
+    public override void OnAdded(Entity owner, List<Intent> newIntents)
+    {
+        if (owner is not Cell cell)
+            throw new NotSupportedException("Rain is only allowed in cells.");
+
+        cell.Type = cell.Type switch
+        {
+            ECellType.Grass   => ECellType.Mud,
+            ECellType.Rocky   => ECellType.Dry,
+            _                 => throw new NotSupportedException("Rain only works on Rocky and Grass")
+        };
+    }
+
+    public override void OnRemoved(Entity owner, List<Intent> newIntents)
+    {
+        if (owner is not Cell cell)
+            throw new NotSupportedException("Rain is only allowed in cells.");
+
+        cell.Type = cell.Type switch
+        {
+            ECellType.Mud => ECellType.Grass,
+            ECellType.Dry => ECellType.Rocky,
+            _             => throw new NotSupportedException("Rain can only be removed from Dry and Mud")
+        };
+    }
+
+    public override string ToolTip => "Humidity was reduced from this land, reducing fertility and maybe drying out.";
+}