From a1dba40f806cac48601847c6f794b6311c25ab46 Mon Sep 17 00:00:00 2001
From: Him188 <Him188@mamoe.net>
Date: Wed, 25 Aug 2021 13:51:58 +0800
Subject: [PATCH] Add util `CollectionDiff`

---
 .../src/commonMain/kotlin/CollectionDiff.kt   | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 mirai-core-utils/src/commonMain/kotlin/CollectionDiff.kt

diff --git a/mirai-core-utils/src/commonMain/kotlin/CollectionDiff.kt b/mirai-core-utils/src/commonMain/kotlin/CollectionDiff.kt
new file mode 100644
index 000000000..2a7e1149f
--- /dev/null
+++ b/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) }
+    }
+}
\ No newline at end of file