Add recently used section and scroll to category

Signed-off-by: Tulir Asokan <tulir@maunium.net>

src/components/views/emojipicker/recent.js

@@ -0,0 +1,35 @@
+/*
+Copyright 2019 Tulir Asokan
+
+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.
+*/
+
+const REACTION_COUNT = JSON.parse(window.localStorage.mx_reaction_count || '{}');
+let sorted = null;
+
+export function add(emoji) {
+    const [count] = REACTION_COUNT[emoji] || [0];
+    REACTION_COUNT[emoji] = [count + 1, Date.now()];
+    window.localStorage.mx_reaction_count = JSON.stringify(REACTION_COUNT);
+    sorted = null;
+}
+
+export function get(limit = 24) {
+    if (sorted === null) {
+        sorted = Object.entries(REACTION_COUNT)
+            .sort(([, [count1, date1]], [, [count2, date2]]) =>
+                count2 === count1 ? date2 - date1 : count2 - count1)
+            .map(([emoji, count]) => emoji);
+    }
+    return sorted.slice(0, limit);
+}