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diff --git a/ext/js/general/cache-map.js b/ext/js/general/cache-map.js
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+/*
+ * Copyright (C) 2020-2021 Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Class which caches a map of values, keeping the most recently accessed values.
+ */
+class CacheMap {
+ /**
+ * Creates a new CacheMap.
+ * @param maxSize The maximum number of entries able to be stored in the cache.
+ */
+ constructor(maxSize) {
+ if (!(
+ typeof maxSize === 'number' &&
+ Number.isFinite(maxSize) &&
+ maxSize >= 0 &&
+ Math.floor(maxSize) === maxSize
+ )) {
+ throw new Error('Invalid maxCount');
+ }
+
+ this._maxSize = maxSize;
+ this._map = new Map();
+ this._listFirst = this._createNode(null, null);
+ this._listLast = this._createNode(null, null);
+ this._resetEndNodes();
+ }
+
+ /**
+ * Returns the number of items in the cache.
+ */
+ get size() {
+ return this._map.size;
+ }
+
+ /**
+ * Returns the maximum number of items that can be added to the cache.
+ */
+ get maxSize() {
+ return this._maxSize;
+ }
+
+ /**
+ * Returns whether or not an element exists at the given key.
+ * @param key The key of the element.
+ * @returns `true` if an element with the specified key exists, `false` otherwise.
+ */
+ has(key) {
+ return this._map.has(key);
+ }
+
+ /**
+ * Gets an element at the given key, if it exists. Otherwise, returns undefined.
+ * @param key The key of the element.
+ * @returns The existing value at the key, if any; `undefined` otherwise.
+ */
+ get(key) {
+ const node = this._map.get(key);
+ if (typeof node === 'undefined') { return void 0; }
+ this._updateRecency(node);
+ return node.value;
+ }
+
+ /**
+ * Sets a value at a given key.
+ * @param key The key of the element.
+ * @param value The value to store in the cache.
+ */
+ set(key, value) {
+ let node = this._map.get(key);
+ if (typeof node !== 'undefined') {
+ this._updateRecency(node);
+ node.value = value;
+ } else {
+ if (this._maxSize <= 0) { return; }
+
+ node = this._createNode(key, value);
+ this._addNode(node, this._listFirst);
+ this._map.set(key, node);
+
+ // Remove
+ for (let removeCount = this._map.size - this._maxSize; removeCount > 0; --removeCount) {
+ node = this._listLast.previous;
+ this._removeNode(node);
+ this._map.delete(node.key);
+ }
+ }
+ }
+
+ /**
+ * Clears the cache.
+ */
+ clear() {
+ this._map.clear();
+ this._resetEndNodes();
+ }
+
+ // Private
+
+ _updateRecency(node) {
+ this._removeNode(node);
+ this._addNode(node, this._listFirst);
+ }
+
+ _createNode(key, value) {
+ return {key, value, previous: null, next: null};
+ }
+
+ _addNode(node, previous) {
+ const next = previous.next;
+ node.next = next;
+ node.previous = previous;
+ previous.next = node;
+ next.previous = node;
+ }
+
+ _removeNode(node) {
+ node.next.previous = node.previous;
+ node.previous.next = node.next;
+ }
+
+ _resetEndNodes() {
+ this._listFirst.next = this._listLast;
+ this._listLast.previous = this._listFirst;
+ }
+}