feat: Add CachedResourceSet

Uses a WeakMap on the identifier to cache resource existence.

src/index.ts

@@ -349,6 +349,7 @@ export * from './storage/validators/QuotaValidator';
 export * from './storage/AtomicResourceStore';
 export * from './storage/BaseResourceStore';
 export * from './storage/BasicConditions';
+export * from './storage/CachedResourceSet';
 export * from './storage/Conditions';
 export * from './storage/DataAccessorBasedStore';
 export * from './storage/IndexRepresentationStore';

src/storage/CachedResourceSet.ts

@@ -0,0 +1,24 @@
+import type { ResourceIdentifier } from '../http/representation/ResourceIdentifier';
+import type { ResourceSet } from './ResourceSet';
+
+/**
+ * Caches resource existence in a `WeakMap` tied to the `ResourceIdentifier` object.
+ */
+export class CachedResourceSet implements ResourceSet {
+  private readonly source: ResourceSet;
+  private readonly cache: WeakMap<ResourceIdentifier, boolean>;
+
+  public constructor(source: ResourceSet) {
+    this.source = source;
+    this.cache = new WeakMap();
+  }
+
+  public async hasResource(identifier: ResourceIdentifier): Promise<boolean> {
+    if (this.cache.has(identifier)) {
+      return this.cache.get(identifier)!;
+    }
+    const result = await this.source.hasResource(identifier);
+    this.cache.set(identifier, result);
+    return result;
+  }
+}