feat: Add CachedResourceSet

Uses a WeakMap on the identifier to cache resource existence.

test/unit/storage/CachedResourceSet.test.ts

@@ -0,0 +1,39 @@
+import type { ResourceIdentifier } from '../../../src/http/representation/ResourceIdentifier';
+import { CachedResourceSet } from '../../../src/storage/CachedResourceSet';
+import type { ResourceSet } from '../../../src/storage/ResourceSet';
+
+describe('A CachedResourceSet', (): void => {
+  const identifier: ResourceIdentifier = { path: 'http://example.com/foo' };
+  let source: jest.Mocked<ResourceSet>;
+  let set: CachedResourceSet;
+
+  beforeEach(async(): Promise<void> => {
+    source = {
+      hasResource: jest.fn().mockResolvedValue(true),
+    };
+
+    set = new CachedResourceSet(source);
+  });
+
+  it('calls the source.', async(): Promise<void> => {
+    await expect(set.hasResource(identifier)).resolves.toBe(true);
+    expect(source.hasResource).toHaveBeenCalledTimes(1);
+    expect(source.hasResource).toHaveBeenLastCalledWith(identifier);
+  });
+
+  it('caches the result.', async(): Promise<void> => {
+    await expect(set.hasResource(identifier)).resolves.toBe(true);
+    await expect(set.hasResource(identifier)).resolves.toBe(true);
+    expect(source.hasResource).toHaveBeenCalledTimes(1);
+    expect(source.hasResource).toHaveBeenLastCalledWith(identifier);
+  });
+
+  it('caches on the identifier object itself.', async(): Promise<void> => {
+    const copy = { ...identifier };
+    await expect(set.hasResource(identifier)).resolves.toBe(true);
+    await expect(set.hasResource(copy)).resolves.toBe(true);
+    expect(source.hasResource).toHaveBeenCalledTimes(2);
+    expect(source.hasResource).toHaveBeenNthCalledWith(1, identifier);
+    expect(source.hasResource).toHaveBeenNthCalledWith(2, copy);
+  });
+});