[Auth] Break down event ID digits to a max Math.random() digit count

Author: sam-gc

packages-exp/auth-exp/src/core/util/event_id.test.ts

@@ -0,0 +1,16 @@
+import { expect } from 'chai';
+import { _generateEventId } from './event_id';
+
+describe('core/util/event_id', () => {
+  it('sub-15 digit id', () => {
+    expect(_generateEventId('', 10)).to.have.length(10);
+  });
+
+  it('15 digit id', () => {
+    expect(_generateEventId('', 15)).to.have.length(15);
+  });
+
+  it('above-15 digit id', () => {
+    expect(_generateEventId('', 20)).to.have.length(20);
+  });
+});

packages-exp/auth-exp/src/core/util/event_id.ts

@@ -15,6 +15,16 @@
  * limitations under the License.
  */
 
-export function _generateEventId(prefix?: string): string {
-  return `${prefix ? prefix : ''}${Math.floor(Math.random() * 1000000000)}`;
+const MAX_RANDOM_DIGITS = 15;
+
+export function _generateEventId(prefix = '', digits = 10): string {
+  // This array breaks down digits into a list of numbers, maxed at 15, that
+  // sums to digits. For example, 10 becomes [10] and 16 becomes [15, 1].
+  const digitBreakdown = Array(Math.floor(digits / MAX_RANDOM_DIGITS)).fill(MAX_RANDOM_DIGITS);
+  if (digits % MAX_RANDOM_DIGITS) {
+    digitBreakdown.push(digits % MAX_RANDOM_DIGITS);
+  }
+
+  const random = digitBreakdown.map(digits => Math.floor(Math.random() * Math.pow(10, digits)).toString().padStart(digits, '0')).join('');
+  return prefix + random;
 }

packages-exp/auth-exp/src/platform_browser/messagechannel/sender.ts

@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+import { _generateEventId } from '../../core/util/event_id';
 import {
   _SenderRequest,
   _EventType,
@@ -32,10 +33,6 @@ interface MessageHandler {
   onMessage: EventListenerOrEventListenerObject;
 }
 
-function generateEventId(prefix = '', digits = 20): string {
-  return `${prefix}${Math.floor(Math.random() * Math.pow(10, digits))}`;
-}
-
 /**
  * Interface for sending messages and waiting for a completion response.
  *
@@ -91,7 +88,7 @@ export class Sender {
     let completionTimer: any;
     let handler: MessageHandler;
     return new Promise<_ReceiverMessageResponse<T>>((resolve, reject) => {
-      const eventId = generateEventId();
+      const eventId = _generateEventId('', 20);
       messageChannel.port1.start();
       const ackTimer = setTimeout(() => {
         reject(new Error(_MessageError.UNSUPPORTED_EVENT));