add LRU Cache utility

we'll use it to cache members later on

src/utils/LRUCache.js

@@ -0,0 +1,146 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * Very simple least-recently-used cache implementation
+ * that should be fast enough for very small cache sizes
+ */
+export class BaseLRUCache {
+    constructor(limit) {
+        this._limit = limit;
+        this._entries = [];
+    }
+
+    _get(findEntryFn) {
+        const idx = this._entries.findIndex(findEntryFn);
+        if (idx !== -1) {
+            const entry = this._entries[idx];
+            // move to top
+            if (idx > 0) {
+                this._entries.splice(idx, 1);
+                this._entries.unshift(entry);
+            }
+            return entry;
+        }
+    }
+
+    _set(value, findEntryFn) {
+        let indexToRemove = this._entries.findIndex(findEntryFn);
+        this._entries.unshift(value);
+        if (indexToRemove === -1) {
+            if (this._entries.length > this._limit) {
+                indexToRemove = this._entries.length - 1;
+            }
+        } else {
+            // we added the entry at the start since we looked for the index
+            indexToRemove += 1;
+        }
+        if (indexToRemove !== -1) {
+            this._onEvictEntry(this._entries[indexToRemove]);
+            this._entries.splice(indexToRemove, 1);
+        }
+    }
+
+    _onEvictEntry() {}
+}
+
+export class LRUCache extends BaseLRUCache {
+    constructor(limit, keyFn) {
+        super(limit);
+        this._keyFn = keyFn;
+    }
+
+    get(key) {
+        return this._get(e => this._keyFn(e) === key);
+    }
+
+    set(value) {
+        const key = this._keyFn(value);
+        this._set(value, e => this._keyFn(e) === key);
+    }
+}
+
+export function tests() {
+    return {
+        "can retrieve added entries": assert => {
+            const cache = new LRUCache(2, e => e.id);
+            cache.set({id: 1, name: "Alice"});
+            cache.set({id: 2, name: "Bob"});
+            assert.equal(cache.get(1).name, "Alice");
+            assert.equal(cache.get(2).name, "Bob");
+        },
+        "first entry is evicted first": assert => {
+            const cache = new LRUCache(2, e => e.id);
+            cache.set({id: 1, name: "Alice"});
+            cache.set({id: 2, name: "Bob"});
+            cache.set({id: 3, name: "Charly"});
+            assert.equal(cache.get(1), undefined);
+            assert.equal(cache.get(2).name, "Bob");
+            assert.equal(cache.get(3).name, "Charly");
+            assert.equal(cache._entries.length, 2);
+        },
+        "second entry is evicted if first is requested": assert => {
+            const cache = new LRUCache(2, e => e.id);
+            cache.set({id: 1, name: "Alice"});
+            cache.set({id: 2, name: "Bob"});
+            cache.get(1);
+            cache.set({id: 3, name: "Charly"});
+            assert.equal(cache.get(1).name, "Alice");
+            assert.equal(cache.get(2), undefined);
+            assert.equal(cache.get(3).name, "Charly");
+            assert.equal(cache._entries.length, 2);
+        },
+        "setting an entry twice removes the first": assert => {
+            const cache = new LRUCache(2, e => e.id);
+            cache.set({id: 1, name: "Alice"});
+            cache.set({id: 2, name: "Bob"});
+            cache.set({id: 1, name: "Al Ice"});
+            cache.set({id: 3, name: "Charly"});
+            assert.equal(cache.get(1).name, "Al Ice");
+            assert.equal(cache.get(2), undefined);
+            assert.equal(cache.get(3).name, "Charly");
+            assert.equal(cache._entries.length, 2);
+        },
+        "evict callback is called": assert => {
+            let evictions = 0;
+            class CustomCache extends LRUCache {
+                _onEvictEntry(entry) {
+                    assert.equal(entry.name, "Alice");
+                    evictions += 1;
+                }
+            }
+            const cache = new CustomCache(2, e => e.id);
+            cache.set({id: 1, name: "Alice"});
+            cache.set({id: 2, name: "Bob"});
+            cache.set({id: 3, name: "Charly"});
+            assert.equal(evictions, 1);
+        },
+        "evict callback is called when replacing entry with same identity": assert => {
+            let evictions = 0;
+            class CustomCache extends LRUCache {
+                _onEvictEntry(entry) {
+                    assert.equal(entry.name, "Alice");
+                    evictions += 1;
+                }
+            }
+            const cache = new CustomCache(2, e => e.id);
+            cache.set({id: 1, name: "Alice"});
+            cache.set({id: 1, name: "Bob"});
+            assert.equal(evictions, 1);
+        },
+        
+    };
+}