Iterables: add and use `filter`

ts/models/conversations.ts

@@ -50,6 +50,7 @@ import { handleMessageSend } from '../util/handleMessageSend';
 import { getConversationMembers } from '../util/getConversationMembers';
 import { sendReadReceiptsFor } from '../util/sendReadReceiptsFor';
 import { updateConversationsWithUuidLookup } from '../updateConversationsWithUuidLookup';
+import { filter, map, take } from '../util/iterables';
 
 /* eslint-disable more/no-then */
 window.Whisper = window.Whisper || {};
@@ -2855,53 +2856,54 @@ export class ConversationModel extends window.Backbone
     sticker: WhatIsThis
   ): Promise<WhatIsThis> {
     if (attachments && attachments.length) {
-      return Promise.all(
-        attachments
-          .filter(
-            attachment => attachment && !attachment.pending && !attachment.error
-          )
-          .slice(0, 1)
-          .map(async attachment => {
-            const { fileName, thumbnail, contentType } = attachment;
+      const validAttachments = filter(
+        attachments,
+        attachment => attachment && !attachment.pending && !attachment.error
+      );
+      const attachmentsToUse = take(validAttachments, 1);
 
-            return {
-              contentType,
-              // Our protos library complains about this field being undefined, so we
-              //   force it to null
-              fileName: fileName || null,
-              thumbnail: thumbnail
-                ? {
-                    ...(await loadAttachmentData(thumbnail)),
-                    objectUrl: getAbsoluteAttachmentPath(thumbnail.path),
-                  }
-                : null,
-            };
-          })
+      return Promise.all(
+        map(attachmentsToUse, async attachment => {
+          const { fileName, thumbnail, contentType } = attachment;
+
+          return {
+            contentType,
+            // Our protos library complains about this field being undefined, so we force
+            //   it to null
+            fileName: fileName || null,
+            thumbnail: thumbnail
+              ? {
+                  ...(await loadAttachmentData(thumbnail)),
+                  objectUrl: getAbsoluteAttachmentPath(thumbnail.path),
+                }
+              : null,
+          };
+        })
       );
     }
 
     if (preview && preview.length) {
-      return Promise.all(
-        preview
-          .filter(item => item && item.image)
-          .slice(0, 1)
-          .map(async attachment => {
-            const { image } = attachment;
-            const { contentType } = image;
+      const validPreviews = filter(preview, item => item && item.image);
+      const previewsToUse = take(validPreviews, 1);
 
-            return {
-              contentType,
-              // Our protos library complains about this field being undefined, so we
-              //   force it to null
-              fileName: null,
-              thumbnail: image
-                ? {
-                    ...(await loadAttachmentData(image)),
-                    objectUrl: getAbsoluteAttachmentPath(image.path),
-                  }
-                : null,
-            };
-          })
+      return Promise.all(
+        map(previewsToUse, async attachment => {
+          const { image } = attachment;
+          const { contentType } = image;
+
+          return {
+            contentType,
+            // Our protos library complains about this field being undefined, so we
+            //   force it to null
+            fileName: null,
+            thumbnail: image
+              ? {
+                  ...(await loadAttachmentData(image)),
+                  objectUrl: getAbsoluteAttachmentPath(image.path),
+                }
+              : null,
+          };
+        })
       );
     }
 

ts/test-both/util/iterables_test.ts

@@ -4,7 +4,7 @@
 import { assert } from 'chai';
 import * as sinon from 'sinon';
 
-import { isIterable, size, map, take } from '../../util/iterables';
+import { isIterable, size, filter, map, take } from '../../util/iterables';
 
 describe('iterable utilities', () => {
   describe('isIterable', () => {
@@ -82,6 +82,61 @@ describe('iterable utilities', () => {
     });
   });
 
+  describe('filter', () => {
+    it('returns an empty iterable when passed an empty iterable', () => {
+      const fn = sinon.fake();
+
+      assert.deepEqual([...filter([], fn)], []);
+      assert.deepEqual([...filter(new Set(), fn)], []);
+      assert.deepEqual([...filter(new Map(), fn)], []);
+
+      sinon.assert.notCalled(fn);
+    });
+
+    it('returns a new iterator with some values removed', () => {
+      const isOdd = sinon.fake((n: number) => Boolean(n % 2));
+      const result = filter([1, 2, 3, 4], isOdd);
+
+      sinon.assert.notCalled(isOdd);
+
+      assert.deepEqual([...result], [1, 3]);
+      assert.notInstanceOf(result, Array);
+
+      sinon.assert.callCount(isOdd, 4);
+    });
+
+    it('can filter an infinite iterable', () => {
+      const everyNumber = {
+        *[Symbol.iterator]() {
+          for (let i = 0; true; i += 1) {
+            yield i;
+          }
+        },
+      };
+
+      const isOdd = (n: number) => Boolean(n % 2);
+      const result = filter(everyNumber, isOdd);
+      const iterator = result[Symbol.iterator]();
+
+      assert.deepEqual(iterator.next(), { value: 1, done: false });
+      assert.deepEqual(iterator.next(), { value: 3, done: false });
+      assert.deepEqual(iterator.next(), { value: 5, done: false });
+      assert.deepEqual(iterator.next(), { value: 7, done: false });
+    });
+
+    it('respects TypeScript type assertion signatures', () => {
+      // This tests TypeScript, not the actual runtime behavior.
+      function isString(value: unknown): value is string {
+        return typeof value === 'string';
+      }
+
+      const input: Array<unknown> = [1, 'two', 3, 'four'];
+      const result: Iterable<string> = filter(input, isString);
+
+      assert.deepEqual([...result], ['two', 'four']);
+    });
+  });
+
   describe('map', () => {
     it('returns an empty iterable when passed an empty iterable', () => {
       const fn = sinon.fake();

ts/util/iterables.ts

@@ -28,6 +28,49 @@ export function size(iterable: Iterable<unknown>): number {
   return result;
 }
 
+export function filter<T, S extends T>(
+  iterable: Iterable<T>,
+  predicate: (value: T) => value is S
+): Iterable<S>;
+export function filter<T>(
+  iterable: Iterable<T>,
+  predicate: (value: T) => unknown
+): Iterable<T>;
+export function filter<T>(
+  iterable: Iterable<T>,
+  predicate: (value: T) => unknown
+): Iterable<T> {
+  return new FilterIterable(iterable, predicate);
+}
+
+class FilterIterable<T> implements Iterable<T> {
+  constructor(
+    private readonly iterable: Iterable<T>,
+    private readonly predicate: (value: T) => unknown
+  ) {}
+
+  [Symbol.iterator](): Iterator<T> {
+    return new FilterIterator(this.iterable[Symbol.iterator](), this.predicate);
+  }
+}
+
+class FilterIterator<T> implements Iterator<T> {
+  constructor(
+    private readonly iterator: Iterator<T>,
+    private readonly predicate: (value: T) => unknown
+  ) {}
+
+  next(): IteratorResult<T> {
+    // eslint-disable-next-line no-constant-condition
+    while (true) {
+      const nextIteration = this.iterator.next();
+      if (nextIteration.done || this.predicate(nextIteration.value)) {
+        return nextIteration;
+      }
+    }
+  }
+}
+
 export function map<T, ResultT>(
   iterable: Iterable<T>,
   fn: (value: T) => ResultT