msglist diffing tests: Fix a flake with stream-subscribed status changing

src/__tests__/lib/exampleData.js

@@ -80,16 +80,16 @@ const makeTime: () => number = (() => {
  * The factory will throw if it ever runs out of integers in its set. Both
  * initialization and use should be O(1).
  */
-const makeUniqueRandInt = (itemsType: string, end: number): (() => number) => {
+const makeUniqueRandInt = (itemsType: string, end: number, start: number = 0): (() => number) => {
   // Sparse array. Pretends to be initialized to iota.
   const deck = new Array(end);
 
   return () => {
-    if (deck.length === 0) {
+    if (deck.length === start) {
       throw new Error(`ran out of ${itemsType}`);
     }
     // Perform a single step of the Fisher-Yates shuffle...
-    const leftIndex = randInt(deck.length);
+    const leftIndex = randInt(deck.length, start);
     const rightIndex = deck.length - 1;
     const leftValue = deck[leftIndex] ?? leftIndex;
     const rightValue = deck[rightIndex] ?? rightIndex;
@@ -142,10 +142,12 @@ const makeAvatarUrl = (tag: string) =>
   // here with a full-blown URL object in the first place to prevent that.
   new UploadedAvatarURL(new URL(`https://zulip.example.org/yo/avatar-${tag}.png`));
 
+// Why start at 11? To reserve IDs 1 through 10 for use as constants in
+// snapshot tests.
 const randUserId: () => UserId = (
   mk => () =>
     makeUserId(mk())
-)(makeUniqueRandInt('user IDs', 10000));
+)(makeUniqueRandInt('user IDs', 10000, 11));
 const userOrBotProperties = (args: UserOrBotPropertiesArgs) => {
   const user_id = args.user_id != null ? makeUserId(args.user_id) : randUserId();
   const randName = randString();
@@ -288,7 +290,9 @@ export const crossRealmBot: CrossRealmBot = makeCrossRealmBot({
  * Streams and subscriptions
  */
 
-const randStreamId: () => number = makeUniqueRandInt('stream IDs', 1000);
+// Why start at 11? To reserve IDs 1 through 10 for use as constants in
+// snapshot tests.
+const randStreamId: () => number = makeUniqueRandInt('stream IDs', 1000, 11);
 export const makeStream = (
   args: {|
     stream_id?: number,
@@ -418,7 +422,9 @@ const messagePropertiesFromSender = (user: User) => {
   });
 };
 
-const randMessageId: () => number = makeUniqueRandInt('message ID', 10000000);
+// Why start at 11? To reserve IDs 1 through 10 for use as constants in
+// snapshot tests.
+const randMessageId: () => number = makeUniqueRandInt('message ID', 10000000, 11);
 
 /**
  * A PM, by default a 1:1 from eg.otherUser to eg.selfUser.

src/utils/misc.js

@@ -29,8 +29,9 @@ export function deeperMerge<K, V>(
 
 export const isValidEmailFormat = (email: string = ''): boolean => /\S+@\S+\.\S+/.test(email);
 
-/** Return an integer 0 <= N < end, roughly uniformly at random. */
-export const randInt = (end: number): number => Math.floor(Math.random() * end);
+/** Return an integer start <= N < end, roughly uniformly at random. */
+export const randInt = (end: number, start: number = 0): number =>
+  Math.floor(Math.random() * (end - start) + start);
 
 /** Return a string that's almost surely different every time. */
 export const randString = (): string => randInt(2 ** 54).toString(36);