Add `reduce` iterables utility

ts/ConversationController.ts

@@ -1,7 +1,7 @@
 // Copyright 2020-2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
-import { debounce, reduce, uniq, without } from 'lodash';
+import { debounce, uniq, without } from 'lodash';
 import PQueue from 'p-queue';
 
 import dataInterface from './sql/Client';
@@ -13,6 +13,7 @@ import { SendOptionsType, CallbackResultType } from './textsecure/SendMessage';
 import { ConversationModel } from './models/conversations';
 import { maybeDeriveGroupV2Id } from './groups';
 import { assert } from './util/assert';
+import { map, reduce } from './util/iterables';
 import { isGroupV1, isGroupV2 } from './util/whatTypeOfConversation';
 import { deprecated } from './util/deprecated';
 import { getSendOptions } from './util/getSendOptions';
@@ -103,7 +104,7 @@ export function start(): void {
       };
 
       const newUnreadCount = reduce(
-        this.map((m: ConversationModel) =>
+        map(this, (m: ConversationModel) =>
           canCount(m) ? getUnreadCount(m) : 0
         ),
         (item: number, memo: number) => (item || 0) + memo,

ts/test-both/util/iterables_test.ts

@@ -11,6 +11,7 @@ import {
   groupBy,
   isIterable,
   map,
+  reduce,
   size,
   take,
 } from '../../util/iterables';
@@ -311,6 +312,23 @@ describe('iterable utilities', () => {
     });
   });
 
+  describe('reduce', () => {
+    it('returns the accumulator when passed an empty iterable', () => {
+      const fn = sinon.fake();
+
+      assert.strictEqual(reduce([], fn, 123), 123);
+
+      sinon.assert.notCalled(fn);
+    });
+
+    it('iterates over the iterable, ultimately returning a result', () => {
+      assert.strictEqual(
+        reduce(new Set([1, 2, 3, 4]), (result, n) => result + n, 89),
+        99
+      );
+    });
+  });
+
   describe('take', () => {
     it('returns the first n elements from an iterable', () => {
       const everyNumber = {

ts/util/iterables.ts

@@ -155,6 +155,18 @@ class MapIterator<T, ResultT> implements Iterator<ResultT> {
   }
 }
 
+export function reduce<T, TResult>(
+  iterable: Iterable<T>,
+  fn: (result: TResult, value: T) => TResult,
+  accumulator: TResult
+): TResult {
+  let result = accumulator;
+  for (const value of iterable) {
+    result = fn(result, value);
+  }
+  return result;
+}
+
 export function take<T>(iterable: Iterable<T>, amount: number): Iterable<T> {
   return new TakeIterable(iterable, amount);
 }