Add delete terrain command

src/commands/delete-terrain-command.ts

@@ -0,0 +1,73 @@
+import { createOpaqueId } from "../core/ids";
+import { cloneEditorSelection, type EditorSelection } from "../core/selection";
+import type { ToolMode } from "../core/tool-mode";
+import { cloneTerrain, type Terrain } from "../document/terrains";
+
+import type { EditorCommand } from "./command";
+
+export function createDeleteTerrainCommand(terrainId: string): EditorCommand {
+  let deletedTerrain: Terrain | null = null;
+  let previousSelection: EditorSelection | null = null;
+  let previousToolMode: ToolMode | null = null;
+
+  return {
+    id: createOpaqueId("command"),
+    label: "Delete terrain",
+    execute(context) {
+      const currentDocument = context.getDocument();
+      const terrain = currentDocument.terrains[terrainId];
+
+      if (terrain === undefined) {
+        throw new Error(`Terrain ${terrainId} does not exist.`);
+      }
+
+      if (deletedTerrain === null) {
+        deletedTerrain = cloneTerrain(terrain);
+      }
+
+      if (previousSelection === null) {
+        previousSelection = cloneEditorSelection(context.getSelection());
+      }
+
+      if (previousToolMode === null) {
+        previousToolMode = context.getToolMode();
+      }
+
+      const nextTerrains = {
+        ...currentDocument.terrains
+      };
+      delete nextTerrains[terrainId];
+
+      context.setDocument({
+        ...currentDocument,
+        terrains: nextTerrains
+      });
+      context.setSelection({
+        kind: "none"
+      });
+      context.setToolMode("select");
+    },
+    undo(context) {
+      if (deletedTerrain === null) {
+        return;
+      }
+
+      const currentDocument = context.getDocument();
+      context.setDocument({
+        ...currentDocument,
+        terrains: {
+          ...currentDocument.terrains,
+          [deletedTerrain.id]: cloneTerrain(deletedTerrain)
+        }
+      });
+
+      if (previousSelection !== null) {
+        context.setSelection(previousSelection);
+      }
+
+      if (previousToolMode !== null) {
+        context.setToolMode(previousToolMode);
+      }
+    }
+  };
+}