diff --git a/package.json b/package.json
index 60c6a0978..811a60814 100644
--- a/package.json
+++ b/package.json
@@ -55,7 +55,7 @@
 		"@types/koa-send": "4.1.1",
 		"@types/koa-views": "2.0.3",
 		"@types/koa__cors": "2.2.3",
-		"@types/minio": "6.0.2",
+		"@types/minio": "7.0.0",
 		"@types/mkdirp": "0.5.2",
 		"@types/mocha": "5.2.3",
 		"@types/mongodb": "3.1.4",
@@ -80,7 +80,7 @@
 		"@types/webpack": "4.4.11",
 		"@types/webpack-stream": "3.2.10",
 		"@types/websocket": "0.0.40",
-		"@types/ws": "6.0.0",
+		"@types/ws": "6.0.1",
 		"animejs": "2.2.0",
 		"autosize": "4.0.2",
 		"autwh": "0.1.0",
diff --git a/src/client/app/boot.js b/src/client/app/boot.js
index dd2cf93a8..f14cebe7d 100644
--- a/src/client/app/boot.js
+++ b/src/client/app/boot.js
@@ -140,7 +140,7 @@
 		// Random
 		localStorage.setItem('salt', Math.random().toString());
 
-		// Clear cache (serive worker)
+		// Clear cache (service worker)
 		try {
 			navigator.serviceWorker.controller.postMessage('clear');
 
diff --git a/src/client/app/common/scripts/check-for-update.ts b/src/client/app/common/scripts/check-for-update.ts
index 4445eefc3..91b165b45 100644
--- a/src/client/app/common/scripts/check-for-update.ts
+++ b/src/client/app/common/scripts/check-for-update.ts
@@ -9,7 +9,7 @@ export default async function(mios: MiOS, force = false, silent = false) {
 		localStorage.setItem('should-refresh', 'true');
 		localStorage.setItem('v', newer);
 
-		// Clear cache (serive worker)
+		// Clear cache (service worker)
 		try {
 			if (navigator.serviceWorker.controller) {
 				navigator.serviceWorker.controller.postMessage('clear');
diff --git a/src/client/app/common/scripts/streaming/stream-manager.ts b/src/client/app/common/scripts/streaming/stream-manager.ts
index 568b8b037..8dd06f67d 100644
--- a/src/client/app/common/scripts/streaming/stream-manager.ts
+++ b/src/client/app/common/scripts/streaming/stream-manager.ts
@@ -1,6 +1,7 @@
 import { EventEmitter } from 'eventemitter3';
 import * as uuid from 'uuid';
 import Connection from './stream';
+import { erase } from '../../../../../prelude/array';
 
 /**
  * ストリーム接続を管理するクラス
@@ -89,7 +90,7 @@ export default abstract class StreamManager<T extends Connection> extends EventE
 	 * @param userId use で発行したユーザーID
 	 */
 	public dispose(userId) {
-		this.users = this.users.filter(id => id != userId);
+		this.users = erase(userId, this.users);
 
 		this._connection.user = `Managed (${ this.users.length })`;
 
diff --git a/src/client/app/common/views/components/poll-editor.vue b/src/client/app/common/views/components/poll-editor.vue
index 115c934c8..30d9799fe 100644
--- a/src/client/app/common/views/components/poll-editor.vue
+++ b/src/client/app/common/views/components/poll-editor.vue
@@ -20,6 +20,7 @@
 
 <script lang="ts">
 import Vue from 'vue';
+import { erase } from '../../../../../prelude/array';
 export default Vue.extend({
 	data() {
 		return {
@@ -53,7 +54,7 @@ export default Vue.extend({
 
 		get() {
 			return {
-				choices: this.choices.filter(choice => choice != '')
+				choices: erase('', this.choices)
 			}
 		},
 
diff --git a/src/client/app/desktop/views/components/post-form.vue b/src/client/app/desktop/views/components/post-form.vue
index f6f52c8f1..65dc9eb9c 100644
--- a/src/client/app/desktop/views/components/post-form.vue
+++ b/src/client/app/desktop/views/components/post-form.vue
@@ -62,6 +62,7 @@ import getFace from '../../../common/scripts/get-face';
 import MkVisibilityChooser from '../../../common/views/components/visibility-chooser.vue';
 import parse from '../../../../../mfm/parse';
 import { host } from '../../../config';
+import { erase } from '../../../../../prelude/array';
 
 export default Vue.extend({
 	components: {
@@ -346,7 +347,7 @@ export default Vue.extend({
 		},
 
 		removeVisibleUser(user) {
-			this.visibleUsers = this.visibleUsers.filter(u => u != user);
+			this.visibleUsers = erase(user, this.visibleUsers);
 		},
 
 		post() {
diff --git a/src/client/app/desktop/views/pages/welcome.vue b/src/client/app/desktop/views/pages/welcome.vue
index d55029fb5..1bcb0ecb2 100644
--- a/src/client/app/desktop/views/pages/welcome.vue
+++ b/src/client/app/desktop/views/pages/welcome.vue
@@ -85,6 +85,7 @@
 <script lang="ts">
 import Vue from 'vue';
 import { host, copyright } from '../../../config';
+import { concat } from '../../../../../prelude/array';
 
 export default Vue.extend({
 	data() {
@@ -119,8 +120,8 @@ export default Vue.extend({
 		(this as any).api('notes/local-timeline', {
 			fileType: image,
 			limit: 6
-		}).then(notes => {
-			const files = [].concat(...notes.map(n => n.files));
+		}).then((notes: any[]) => {
+			const files = concat(notes.map((n: any): any[] => n.files));
 			this.photos = files.filter(f => image.includes(f.type)).slice(0, 6);
 		});
 	},
diff --git a/src/client/app/mios.ts b/src/client/app/mios.ts
index c2ec7f175..0f72cd2f3 100644
--- a/src/client/app/mios.ts
+++ b/src/client/app/mios.ts
@@ -17,6 +17,7 @@ import Err from './common/views/components/connect-failed.vue';
 import { LocalTimelineStreamManager } from './common/scripts/streaming/local-timeline';
 import { HybridTimelineStreamManager } from './common/scripts/streaming/hybrid-timeline';
 import { GlobalTimelineStreamManager } from './common/scripts/streaming/global-timeline';
+import { erase } from '../../prelude/array';
 
 //#region api requests
 let spinner = null;
@@ -537,7 +538,7 @@ export default class MiOS extends EventEmitter {
 	}
 
 	public unregisterStreamConnection(connection: Connection) {
-		this.connections = this.connections.filter(c => c != connection);
+		this.connections = erase(connection, this.connections);
 	}
 }
 
diff --git a/src/client/app/mobile/views/components/post-form.vue b/src/client/app/mobile/views/components/post-form.vue
index 644e27cce..8107c1f3a 100644
--- a/src/client/app/mobile/views/components/post-form.vue
+++ b/src/client/app/mobile/views/components/post-form.vue
@@ -59,6 +59,7 @@ import MkVisibilityChooser from '../../../common/views/components/visibility-cho
 import getFace from '../../../common/scripts/get-face';
 import parse from '../../../../../mfm/parse';
 import { host } from '../../../config';
+import { erase } from '../../../../../prelude/array';
 
 export default Vue.extend({
 	components: {
@@ -262,7 +263,7 @@ export default Vue.extend({
 		},
 
 		removeVisibleUser(user) {
-			this.visibleUsers = this.visibleUsers.filter(u => u != user);
+			this.visibleUsers = erase(user, this.visibleUsers);
 		},
 
 		clear() {
diff --git a/src/client/app/mobile/views/pages/welcome.vue b/src/client/app/mobile/views/pages/welcome.vue
index 1856731d8..7446cc700 100644
--- a/src/client/app/mobile/views/pages/welcome.vue
+++ b/src/client/app/mobile/views/pages/welcome.vue
@@ -40,6 +40,7 @@
 <script lang="ts">
 import Vue from 'vue';
 import { apiUrl, copyright, host } from '../../../config';
+import { concat } from '../../../../../prelude/array';
 
 export default Vue.extend({
 	data() {
@@ -79,8 +80,8 @@ export default Vue.extend({
 		(this as any).api('notes/local-timeline', {
 			fileType: image,
 			limit: 6
-		}).then(notes => {
-			const files = [].concat(...notes.map(n => n.files));
+		}).then((notes: any[]) => {
+			const files = concat(notes.map((n: any): any[] => n.files));
 			this.photos = files.filter(f => image.includes(f.type)).slice(0, 6);
 		});
 	}
diff --git a/src/client/app/store.ts b/src/client/app/store.ts
index 53f3eefc0..08dd9f992 100644
--- a/src/client/app/store.ts
+++ b/src/client/app/store.ts
@@ -4,6 +4,7 @@ import * as nestedProperty from 'nested-property';
 
 import MiOS from './mios';
 import { hostname } from './config';
+import { erase } from '../../prelude/array';
 
 const defaultSettings = {
 	home: null,
@@ -195,7 +196,7 @@ export default (os: MiOS) => new Vuex.Store({
 
 				removeDeckColumn(state, id) {
 					state.deck.columns = state.deck.columns.filter(c => c.id != id);
-					state.deck.layout = state.deck.layout.map(ids => ids.filter(x => x != id));
+					state.deck.layout = state.deck.layout.map(ids => erase(id, ids));
 					state.deck.layout = state.deck.layout.filter(ids => ids.length > 0);
 				},
 
@@ -266,7 +267,7 @@ export default (os: MiOS) => new Vuex.Store({
 
 				stackLeftDeckColumn(state, id) {
 					const i = state.deck.layout.findIndex(ids => ids.indexOf(id) != -1);
-					state.deck.layout = state.deck.layout.map(ids => ids.filter(x => x != id));
+					state.deck.layout = state.deck.layout.map(ids => erase(id, ids));
 					const left = state.deck.layout[i - 1];
 					if (left) state.deck.layout[i - 1].push(id);
 					state.deck.layout = state.deck.layout.filter(ids => ids.length > 0);
@@ -274,7 +275,7 @@ export default (os: MiOS) => new Vuex.Store({
 
 				popRightDeckColumn(state, id) {
 					const i = state.deck.layout.findIndex(ids => ids.indexOf(id) != -1);
-					state.deck.layout = state.deck.layout.map(ids => ids.filter(x => x != id));
+					state.deck.layout = state.deck.layout.map(ids => erase(id, ids));
 					state.deck.layout.splice(i + 1, 0, [id]);
 					state.deck.layout = state.deck.layout.filter(ids => ids.length > 0);
 				},
diff --git a/src/client/app/sw.js b/src/client/app/sw.js
index ac7ea20ac..d381bfb7a 100644
--- a/src/client/app/sw.js
+++ b/src/client/app/sw.js
@@ -3,6 +3,7 @@
  */
 
 import composeNotification from './common/scripts/compose-notification';
+import { erase } from '../../prelude/array';
 
 // キャッシュするリソース
 const cachee = [
@@ -24,8 +25,7 @@ self.addEventListener('activate', ev => {
 	// Clean up old caches
 	ev.waitUntil(
 		caches.keys().then(keys => Promise.all(
-			keys
-				.filter(key => key != _VERSION_)
+			erase(_VERSION_, keys)
 				.map(key => caches.delete(key))
 		))
 	);
diff --git a/src/games/reversi/core.ts b/src/games/reversi/core.ts
index 9199efa09..e724917fb 100644
--- a/src/games/reversi/core.ts
+++ b/src/games/reversi/core.ts
@@ -1,4 +1,4 @@
-import { count, countIf } from "../../prelude/array";
+import { count, concat } from "../../prelude/array";
 
 // MISSKEY REVERSI ENGINE
 
@@ -110,7 +110,7 @@ export default class Reversi {
 	 * 白石の数
 	 */
 	public get whiteCount() {
-		return count(BLACK, this.board);
+		return count(WHITE, this.board);
 	}
 
 	/**
@@ -238,87 +238,55 @@ export default class Reversi {
 	/**
 	 * 指定のマスに石を置いた時の、反転させられる石を取得します
 	 * @param color 自分の色
-	 * @param pos 位置
+	 * @param initPos 位置
 	 */
-	public effects(color: Color, pos: number): number[] {
+	public effects(color: Color, initPos: number): number[] {
 		const enemyColor = !color;
 
-		// ひっくり返せる石(の位置)リスト
-		let stones: number[] = [];
+		const diffVectors: [number, number][] = [
+			[  0,  -1], // 上
+			[ +1,  -1], // 右上
+			[ +1,   0], // 右
+			[ +1,  +1], // 右下
+			[  0,  +1], // 下
+			[ -1,  +1], // 左下
+			[ -1,   0], // 左
+			[ -1,  -1]  // 左上
+		];
 
-		const initPos = pos;
-
-		// 走査
-		const iterate = (fn: (i: number) => number[]) => {
-			let i = 1;
-			const found = [];
+		const effectsInLine = ([dx, dy]: [number, number]): number[] => {
+			const nextPos = (x: number, y: number): [number, number] => [x + dx, y + dy];
 
+			const found: number[] = []; // 挟めるかもしれない相手の石を入れておく配列
+			let [x, y] = this.transformPosToXy(initPos);
 			while (true) {
-				let [x, y] = fn(i);
+				[x, y] = nextPos(x, y);
 
 				// 座標が指し示す位置がボード外に出たとき
 				if (this.opts.loopedBoard) {
-					if (x <  0             ) x = this.mapWidth  - ((-x) % this.mapWidth);
-					if (y <  0             ) y = this.mapHeight - ((-y) % this.mapHeight);
-					if (x >= this.mapWidth ) x = x % this.mapWidth;
-					if (y >= this.mapHeight) y = y % this.mapHeight;
+					x = ((x % this.mapWidth) + this.mapWidth) % this.mapWidth;
+					y = ((y % this.mapHeight) + this.mapHeight) % this.mapHeight;
 
-					// for debug
-					//if (x < 0 || y < 0 || x >= this.mapWidth || y >= this.mapHeight) {
-					//	console.log(x, y);
-					//}
-
-					// 一周して自分に帰ってきたら
 					if (this.transformXyToPos(x, y) == initPos) {
-						// ↓のコメントアウトを外すと、「現時点で自分の石が隣接していないが、
-						// そこに置いたとするとループして最終的に挟んだことになる」というケースを有効化します。(Test4のマップで違いが分かります)
-						// このケースを有効にした方が良いのか無効にした方が良いのか判断がつかなかったためとりあえず無効としておきます
-						// (あと無効な方がゲームとしておもしろそうだった)
-						stones = stones.concat(found);
-						break;
+						// 盤面の境界でループし、自分が石を置く位置に戻ってきたとき、挟めるようにしている (ref: Test4のマップ)
+						return found;
 					}
 				} else {
-					if (x == -1 || y == -1 || x == this.mapWidth || y == this.mapHeight) break;
+					if (x == -1 || y == -1 || x == this.mapWidth || y == this.mapHeight) {
+						return []; // 挟めないことが確定 (盤面外に到達)
+					}
 				}
 
 				const pos = this.transformXyToPos(x, y);
-
-				//#region 「配置不能」マスに当たった場合走査終了
-				const pixel = this.mapDataGet(pos);
-				if (pixel == 'null') break;
-				//#endregion
-
-				// 石取得
+				if (this.mapDataGet(pos) === 'null') return []; // 挟めないことが確定 (配置不可能なマスに到達)
 				const stone = this.board[pos];
-
-				// 石が置かれていないマスなら走査終了
-				if (stone === null) break;
-
-				// 相手の石なら「ひっくり返せるかもリスト」に入れておく
-				if (stone === enemyColor) found.push(pos);
-
-				// 自分の石なら「ひっくり返せるかもリスト」を「ひっくり返せるリスト」に入れ、走査終了
-				if (stone === color) {
-					stones = stones.concat(found);
-					break;
-				}
-
-				i++;
+				if (stone === null) return []; // 挟めないことが確定 (石が置かれていないマスに到達)
+				if (stone === enemyColor) found.push(pos); // 挟めるかもしれない (相手の石を発見)
+				if (stone === color) return found; // 挟めることが確定 (対となる自分の石を発見)
 			}
 		};
 
-		const [x, y] = this.transformPosToXy(pos);
-
-		iterate(i => [x    , y - i]); // 上
-		iterate(i => [x + i, y - i]); // 右上
-		iterate(i => [x + i, y    ]); // 右
-		iterate(i => [x + i, y + i]); // 右下
-		iterate(i => [x    , y + i]); // 下
-		iterate(i => [x - i, y + i]); // 左下
-		iterate(i => [x - i, y    ]); // 左
-		iterate(i => [x - i, y - i]); // 左上
-
-		return stones;
+		return concat(diffVectors.map(effectsInLine));
 	}
 
 	/**
diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index aee17640e..abef6ca03 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -6,6 +6,18 @@ export function count<T>(x: T, xs: T[]): number {
 	return countIf(y => x === y, xs);
 }
 
-export function intersperse<T>(sep: T, xs: T[]): T[] {
-	return [].concat(...xs.map(x => [sep, x])).slice(1);
+export function concat<T>(xss: T[][]): T[] {
+	return ([] as T[]).concat(...xss);
+}
+
+export function intersperse<T>(sep: T, xs: T[]): T[] {
+	return concat(xs.map(x => [sep, x])).slice(1);
+}
+
+export function erase<T>(x: T, xs: T[]): T[] {
+	return xs.filter(y => x !== y);
+}
+
+export function unique<T>(xs: T[]): T[] {
+	return [...new Set(xs)];
 }
diff --git a/src/server/api/endpoints/hashtags/trend.ts b/src/server/api/endpoints/hashtags/trend.ts
index 01dfccc71..e7c08ca9f 100644
--- a/src/server/api/endpoints/hashtags/trend.ts
+++ b/src/server/api/endpoints/hashtags/trend.ts
@@ -1,4 +1,5 @@
 import Note from '../../../../models/note';
+import { erase } from '../../../../prelude/array';
 
 /*
 トレンドに載るためには「『直近a分間のユニーク投稿数が今からa分前～今からb分前の間のユニーク投稿数のn倍以上』のハッシュタグの上位5位以内に入る」ことが必要
@@ -85,8 +86,7 @@ export default () => new Promise(async (res, rej) => {
 	//#endregion
 
 	// タグを人気順に並べ替え
-	let hots = (await Promise.all(hotsPromises))
-		.filter(x => x != null)
+	let hots = erase(null, await Promise.all(hotsPromises))
 		.sort((a, b) => b.count - a.count)
 		.map(tag => tag.name)
 		.slice(0, max);
diff --git a/src/server/api/endpoints/notes/search_by_tag.ts b/src/server/api/endpoints/notes/search_by_tag.ts
index 82f11a977..77082c260 100644
--- a/src/server/api/endpoints/notes/search_by_tag.ts
+++ b/src/server/api/endpoints/notes/search_by_tag.ts
@@ -5,6 +5,7 @@ import Mute from '../../../../models/mute';
 import { getFriendIds } from '../../common/get-friends';
 import { pack } from '../../../../models/note';
 import getParams from '../../get-params';
+import { erase } from '../../../../prelude/array';
 
 export const meta = {
 	desc: {
@@ -103,23 +104,23 @@ export default (params: any, me: ILocalUser) => new Promise(async (res, rej) =>
 	if (psErr) throw psErr;
 
 	if (ps.includeUserUsernames != null) {
-		const ids = (await Promise.all(ps.includeUserUsernames.map(async (username) => {
+		const ids = erase(null, await Promise.all(ps.includeUserUsernames.map(async (username) => {
 			const _user = await User.findOne({
 				usernameLower: username.toLowerCase()
 			});
 			return _user ? _user._id : null;
-		}))).filter(id => id != null);
+		})));
 
 		ids.forEach(id => ps.includeUserIds.push(id));
 	}
 
 	if (ps.excludeUserUsernames != null) {
-		const ids = (await Promise.all(ps.excludeUserUsernames.map(async (username) => {
+		const ids = erase(null, await Promise.all(ps.excludeUserUsernames.map(async (username) => {
 			const _user = await User.findOne({
 				usernameLower: username.toLowerCase()
 			});
 			return _user ? _user._id : null;
-		}))).filter(id => id != null);
+		})));
 
 		ids.forEach(id => ps.excludeUserIds.push(id));
 	}
diff --git a/src/services/note/create.ts b/src/services/note/create.ts
index 11e375586..c08836c94 100644
--- a/src/services/note/create.ts
+++ b/src/services/note/create.ts
@@ -24,6 +24,7 @@ import isQuote from '../../misc/is-quote';
 import { TextElementMention } from '../../mfm/parse/elements/mention';
 import { TextElementHashtag } from '../../mfm/parse/elements/hashtag';
 import { updateNoteStats } from '../update-chart';
+import { erase, unique } from '../../prelude/array';
 
 type NotificationType = 'reply' | 'renote' | 'quote' | 'mention';
 
@@ -103,7 +104,7 @@ export default async (user: IUser, data: Option, silent = false) => new Promise<
 	if (data.viaMobile == null) data.viaMobile = false;
 
 	if (data.visibleUsers) {
-		data.visibleUsers = data.visibleUsers.filter(x => x != null);
+		data.visibleUsers = erase(null, data.visibleUsers);
 	}
 
 	if (data.reply && data.reply.deletedAt != null) {
@@ -384,7 +385,7 @@ function extractHashtags(tokens: ReturnType<typeof parse>): string[] {
 		.map(t => (t as TextElementHashtag).hashtag)
 		.filter(tag => tag.length <= 100);
 
-	return [...new Set(hashtags)];
+	return unique(hashtags);
 }
 
 function index(note: INote) {
@@ -541,20 +542,20 @@ function incNotesCount(user: IUser) {
 async function extractMentionedUsers(tokens: ReturnType<typeof parse>): Promise<IUser[]> {
 	if (tokens == null) return [];
 
-	const mentionTokens = [...new Set(
+	const mentionTokens = unique(
 		tokens
 			.filter(t => t.type == 'mention') as TextElementMention[]
-	)];
+	);
 
-	const mentionedUsers = [...new Set(
-		(await Promise.all(mentionTokens.map(async m => {
+	const mentionedUsers = unique(
+		erase(null, await Promise.all(mentionTokens.map(async m => {
 			try {
 				return await resolveUser(m.username, m.host);
 			} catch (e) {
 				return null;
 			}
-		}))).filter(x => x != null)
-	)];
+		})))
+	);
 
 	return mentionedUsers;
 }
diff --git a/tslint.json b/tslint.json
index ae0df46b9..1adc0a2ae 100644
--- a/tslint.json
+++ b/tslint.json
@@ -17,6 +17,7 @@
 		"no-empty":false,
 		"ordered-imports": [false],
 		"arrow-parens": false,
+		"array-type": false,
 		"object-literal-shorthand": false,
 		"object-literal-key-quotes": false,
 		"triple-equals": [false],