feat: Implement basic general folder support (#2051)

- Related: #1748

src/cache/expiring-cache.ts

@@ -0,0 +1,70 @@
+import { CacheInterface } from './types';
+import { EqualityCache } from './equality-cache';
+
+interface ExpiringValue<Value> {
+  value: Value;
+  expires?: Date;
+}
+
+export class ExpiringEqualityCache<Key, Value> implements CacheInterface<Key, Value> {
+  protected _data: EqualityCache<Key, ExpiringValue<Value>> = new EqualityCache();
+
+  public get(key: Key): Value | null {
+    const value = this._data.get(key);
+    const now = new Date();
+    return value && (!value.expires || now <= value.expires) ? value.value : null;
+  }
+
+  public has(key: Key): boolean {
+    return !!this.get(key);
+  }
+
+  public set(key: Key, value: Value, expiry?: Date): void {
+    this._data.set(key, {
+      value: value,
+      expires: expiry,
+    });
+
+    // Clean up old requests on set.
+    this._expireOldValues();
+  }
+
+  public delete(key: Key): boolean {
+    return this._data.delete(key);
+  }
+
+  public clear(): void {
+    this._data.clear();
+  }
+
+  public *entries(): MapIterator<[Key, Value]> {
+    const now = new Date();
+    for (const [key, value] of this._data.entries()) {
+      if (!value.expires || now <= value.expires) {
+        yield [key, value.value];
+      }
+    }
+  }
+
+  public getMatches(predicate: (value: Value) => boolean): Value[] {
+    const out: Value[] = [];
+
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
+    for (const [_key, value] of this.entries()) {
+      if (predicate(value)) {
+        out.push(value);
+      }
+    }
+    return out;
+  }
+
+  protected _expireOldValues(): void {
+    const now = new Date();
+
+    for (const [key, value] of this._data.entries()) {
+      if (value.expires && now > value.expires) {
+        this._data.delete(key);
+      }
+    }
+  }
+}