1 files changed, 140 insertions, 0 deletions
diff --git a/ext/js/general/cache-map.js b/ext/js/general/cache-map.js
new file mode 100644
index 00000000..c7d72e6b
--- /dev/null
+++ b/ext/js/general/cache-map.js
@@ -0,0 +1,140 @@
+/*
+ * 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;
+    }
+}