add util.ts (Immutable, DeepPath, DeepIndex)

src/util.ts

@@ -0,0 +1,35 @@
+// https://stackoverflow.com/a/58993872/4941530
+// eslint-disable-next-line @typescript-eslint/ban-types
+type ImmutablePrimitive = undefined | null | boolean | string | number | Function;
+
+export type Immutable<T> =
+    T extends ImmutablePrimitive ? T :
+    T extends Array<infer U> ? ImmutableArray<U> :
+    T extends Map<infer K, infer V> ? ImmutableMap<K, V> :
+    T extends Set<infer M> ? ImmutableSet<M> : ImmutableObject<T>;
+
+type ImmutableArray<T> = ReadonlyArray<Immutable<T>>;
+type ImmutableMap<K, V> = ReadonlyMap<Immutable<K>, Immutable<V>>;
+type ImmutableSet<T> = ReadonlySet<Immutable<T>>;
+type ImmutableObject<T> = {readonly [K in keyof T]: Immutable<T[K]>};
+
+
+// https://stackoverflow.com/a/65963590
+type PathTree<T> = {
+    [P in keyof T]-?: T[P] extends object
+    ? [P] | [P, ...DeepPath<T[P]>]
+    : [P];
+};
+
+export type DeepPath<T> = PathTree<T>[keyof PathTree<T>];
+
+// https://stackoverflow.com/a/61648690
+type Keys = readonly PropertyKey[];
+export type DeepIndex<T, KS extends Keys, Fail = undefined> =
+    KS extends [infer F, ...infer R] ? F extends keyof T ? R extends Keys ?
+    DeepIndex<T[F], R, Fail> : Fail : Fail : T;
+
+
+export type Im<T> = Immutable<T>;
+export type DP<T> = DeepPath<T>;
+export type DI<T, KS extends Keys, Fail = undefined> = DeepIndex<T, KS, Fail>;