add default maps

src/lib.ts

@@ -26,3 +26,4 @@ export { Range, range } from "./lib/Range.ts";
 export { Event } from "./lib/Event.ts";
 export { AsyncPool } from "./lib/AsyncPool.ts";
 export { ClassMap } from "./lib/ClassMap.ts";
+export { DefaultMap } from "./lib/DefaultMap.ts";

src/lib/DefaultMap.ts

@@ -0,0 +1,18 @@
+export class DefaultMap<K = string, V = string> {
+	inner = new Map<K, V>();
+
+	constructor(
+		private def: () => V,
+	) {}
+
+	get(key: K) {
+		if (!this.inner.has(key)) this.inner.set(key, this.def());
+		return this.inner.get(key)!;
+	}
+
+	set(key: K, value: V) {
+		this.inner.set(key, value);
+	}
+
+	[Symbol.iterator] = () => this.inner[Symbol.iterator]();
+}