add equality map size

src/equality-map.ts

@@ -2,6 +2,7 @@ import { Equater, MaybeAsyncEquater } from "./types.js";
 import { asAsyncGenerator } from "./utils.js";
 
 export interface EqualityMap<K, V> extends Iterable<[K, V]> {
+  readonly size: number;
   get(key: K): V | undefined;
   set(key: K, value: V): V | undefined;
   contains(key: K): boolean;
@@ -12,6 +13,10 @@ export interface EqualityMap<K, V> extends Iterable<[K, V]> {
 class NativeEqualityMap<K, V> implements EqualityMap<K, V> {
   readonly #map = new Map<K, V>();
 
+  get size() {
+    return this.#map.size;
+  }
+
   get(key: K) {
     return this.#map.get(key);
   }
@@ -49,6 +54,10 @@ class CustomEqualityMap<K, V> implements EqualityMap<K, V> {
     this.#keyComparer = keyComparer;
   }
 
+  get size() {
+    return this.#list.length;
+  }
+
   get(key: K) {
     for (const entry of this.#list) {
       if (this.#keyComparer(key, entry[0])) {