Add util CollectionDiff

mirai-core-utils/src/commonMain/kotlin/CollectionDiff.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2019-2021 Mamoe Technologies and contributors.
+ *
+ * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
+ * Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
+ *
+ * https://github.com/mamoe/mirai/blob/dev/LICENSE
+ */
+
+package net.mamoe.mirai.utils
+
+public class CollectionDiff<E> {
+    private var save: Collection<E> = listOf()
+
+    public fun save(collection: Collection<E>) {
+        save = collection.toList()
+    }
+
+    public fun subtract(collection: Collection<E>): Collection<E> = collection subtract save
+
+    public fun subtractAndSave(collection: Collection<E>): Collection<E> {
+        return subtract(collection).also { save(collection) }
+    }
+}