use ES6 Map if we can (#602)

src/Common/ObjectCache.ts

@@ -1,56 +1,27 @@
-import { HashMap } from "./HashMap";
-
-export class ObjectCache<T> extends HashMap<T> {
-  private keyQueue: string[]; // Last touched key FIFO to purge cache if too big.
-  private maxNbElements: number;
-
-  public constructor(maxNbElements: number) {
+export class ObjectCache<T> extends Map<string, T> {
+  constructor(private limit: number) {
     super();
-    this.keyQueue = [];
-    this.maxNbElements = maxNbElements;
-    this.clear();
   }
 
-  public clear(): void {
-    super.clear();
-    this.keyQueue = [];
+  public get(key: string): T | undefined {
+    return this.touch(key);
   }
 
-  public get(key: string): T {
-    this.markKeyAsTouched(key);
-    return super.get(key);
-  }
-
-  public set(key: string, value: T): void {
-    super.set(key, value);
-
-    this.markKeyAsTouched(key);
-
-    if (super.size() > this.maxNbElements && key !== this.keyQueue[0]) {
-      this.reduceCacheSize();
+  public set(key: string, value: T): this {
+    if (this.size === this.limit) {
+      this.delete(this.keys().next().value);
     }
+
+    return this.touch(key, value), this;
   }
 
-  /**
-   * Invalidate elements to keep the total number below the limit
-   */
-  private reduceCacheSize(): void {
-    // remove a key
-    const oldKey = this.keyQueue.shift();
-    if (oldKey) {
-      super.delete(oldKey);
+  private touch(key: string, value = super.get(key)) {
+    // Map keeps (re) insertion order according to ES6 spec
+    if (value) {
+      this.delete(key);
+      super.set(key, value);
     }
-  }
 
-  /**
-   * Bubble up this key as new.
-   * @param key
-   */
-  private markKeyAsTouched(key: string) {
-    const n = this.keyQueue.indexOf(key);
-    if (n > -1) {
-      this.keyQueue.splice(n, 1);
-    }
-    this.keyQueue.push(key);
+    return value;
   }
 }