Refactor: terrain improvement replacement logic (#844)

Minor readability improvement

Author: esbudylin

C7/Game.cs

@@ -809,11 +809,11 @@ private void ProcessAction(string currentAction) {
 			|| terraform == null || !CurrentlySelectedUnit.canPerformTerraformAction(terraform))
 			return;
 
-		TerrainImprovement currentImprovement = CurrentlySelectedUnit.location.overlays.ImprovementAtLayer(terraform);
-		if (currentImprovement != null && terraform.Improvement.upgradesFrom != currentImprovement) {
+		TerrainImprovement replacementTarget = CurrentlySelectedUnit.location.overlays.GetReplacementTarget(terraform);
+		if (replacementTarget != null) {
 			popupOverlay.ShowPopup(
 				new ConfirmationPopup(
-					$"A previous terrain enhancement ({currentImprovement.key.Capitalize()}) will be replaced \nby this operation. Do you wish to continue?",
+					$"A previous terrain enhancement ({replacementTarget.key.Capitalize()}) will be replaced \nby this operation. Do you wish to continue?",
 					"Continue.",
 					"Cancel action.",
 					() => {

C7Engine/C7GameData/Tile.cs

@@ -772,8 +772,19 @@ public TerrainImprovement ImprovementAtLayer(TerrainImprovement.Layer layer) {
 			return ti;
 		}
 
-		public TerrainImprovement ImprovementAtLayer(Terraform terraform) {
-			return terraform.Improvement == null ? null : ImprovementAtLayer(terraform.Improvement.layer);
+		// Returns an existing improvement that would be replaced by the given terraform.
+		// Returns null if there is no such improvement,
+		// or the new improvement upgrades from the existing one (upgrades don't count as replacements)
+		public TerrainImprovement GetReplacementTarget(Terraform terraform) {
+			var newImp = terraform.Improvement;
+			if (newImp == null)
+				return null;
+
+			var current = ImprovementAtLayer(newImp.layer);
+			if (current == null)
+				return null;
+
+			return newImp.upgradesFrom != current ? current : null;
 		}
 
 		public bool HasImprovement(TerrainImprovement improvement) {