Backfill the grid as people leave by moving tiles along paths

src/useReactiveState.ts

@@ -0,0 +1,46 @@
+import {
+  DependencyList,
+  Dispatch,
+  SetStateAction,
+  useCallback,
+  useRef,
+  useState,
+} from "react";
+
+export const useReactiveState = <T>(
+  updateFn: (prevState?: T) => T,
+  deps: DependencyList
+): [T, Dispatch<SetStateAction<T>>] => {
+  const state = useRef<T>();
+  if (state.current === undefined) state.current = updateFn();
+  const prevDeps = useRef<DependencyList>();
+
+  // Since we store the state in a ref, we use this counter to force an update
+  // when someone calls setState
+  const [, setNumUpdates] = useState(0);
+
+  // If this is the first render or the deps have changed, recalculate the state
+  if (
+    prevDeps.current === undefined ||
+    deps.length !== prevDeps.current.length ||
+    deps.some((d, i) => d !== prevDeps.current![i])
+  ) {
+    state.current = updateFn(state.current);
+  }
+  prevDeps.current = deps;
+
+  return [
+    state.current,
+    useCallback(
+      (action) => {
+        if (typeof action === "function") {
+          state.current = (action as (prevValue: T) => T)(state.current!);
+        } else {
+          state.current = action;
+        }
+        setNumUpdates((n) => n + 1); // Force an update
+      },
+      [setNumUpdates]
+    ),
+  ];
+};