Add a LRUMap test, fix a bug in LRUMap

Bug: When appending a node, we weren't setting the tail's `next` value
correctly.

src/renderer/atlas/LRUMap.test.ts

@@ -0,0 +1,65 @@
+/**
+ * Copyright (c) 2017 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { assert } from 'chai';
+import LRUMap from './LRUMap';
+
+describe('LRUMap', () => {
+  it('can be used to store and retrieve values', () => {
+    const map = new LRUMap(10);
+    map.set('keya', 'valuea');
+    map.set('keyb', 'valueb');
+    map.set('keyc', 'valuec');
+    assert.strictEqual(map.get('keya'), 'valuea');
+    assert.strictEqual(map.get('keyb'), 'valueb');
+    assert.strictEqual(map.get('keyc'), 'valuec');
+  });
+
+  it('maintains a size from insertions', () => {
+    const map = new LRUMap(10);
+    assert.strictEqual(map.size, 0);
+    map.set('a', 'value');
+    assert.strictEqual(map.size, 1);
+    map.set('b', 'value');
+    assert.strictEqual(map.size, 2);
+  });
+
+  it('deletes the oldest entry when the capacity is exceeded', () => {
+    const map = new LRUMap(4);
+    map.set('a', 'value');
+    map.set('b', 'value');
+    map.set('c', 'value');
+    map.set('d', 'value');
+    map.set('e', 'value');
+    assert.isNull(map.get('a'));
+    assert.isNotNull(map.get('b'));
+    assert.isNotNull(map.get('c'));
+    assert.isNotNull(map.get('d'));
+    assert.isNotNull(map.get('e'));
+    assert.strictEqual(map.size, 4);
+  });
+
+  it('prevents a recently accessed entry from getting deleted', () => {
+    const map = new LRUMap(2);
+    map.set('a', 'value');
+    map.set('b', 'value');
+    map.get('a');
+    // a would normally get deleted here, except that we called get()
+    map.set('c', 'value');
+    assert.isNotNull(map.get('a'));
+    // b got deleted instead of a
+    assert.isNull(map.get('b'));
+    assert.isNotNull(map.get('c'));
+  });
+
+  it('supports mutation', () => {
+    const map = new LRUMap(10);
+    map.set('keya', 'oldvalue');
+    map.set('keya', 'newvalue');
+    // mutation doesn't change the size
+    assert.strictEqual(map.size, 1);
+    assert.strictEqual(map.get('keya'), 'newvalue');
+  });
+});

src/renderer/atlas/LRUMap.ts

@@ -37,7 +37,11 @@ export default class LRUMap<T> {
   }
 
   private _appendNode(node: ILinkedListNode<T>): void {
-    node.prev = this._tail;
+    const tail = this._tail;
+    if (tail !== null) {
+      tail.next = node;
+    }
+    node.prev = tail;
     node.next = null;
     this._tail = node;
     if (this._head === null) {