How the Randomness Works

Where the randomness comes from

ChalkSpin uses crypto.getRandomValues, part of the Web Crypto standard that every modern browser supports. It's a cryptographically secure random number generator, seeded by your device's operating system from unpredictable physical events. Browsers use the same source to create security keys, so it's built to be impossible to guess.

Many simple spinners use Math.random() instead. That's fine for games, but it isn't designed to be unpredictable: its sequence can be worked out from a handful of earlier results. ChalkSpin never uses it to decide anything.

Strictly, both are "pseudo-random": computed rather than measured from nature. For a cryptographic generator the difference can't be detected, which is why banks and browsers trust it, and why it's more than fair enough for choosing who reads next.

No modulo bias: rejection sampling

The generator hands out random 32-bit numbers: one of 4,294,967,296 equally likely values. To turn that into a pick from 30 names, the obvious move is to take the remainder after dividing by 30. But 4,294,967,296 isn't a multiple of 30. It leaves 16 values over, so 16 of the names would get one extra chance in 4.3 billion. Tiny, but not equal.

ChalkSpin removes that bias completely with rejection sampling: if the number lands in the leftover range at the very top, it's thrown away and a new one is drawn. For 30 names that happens about once in 268 million draws, and every name ends up with exactly the same chance. This is the code every ChalkSpin tool uses:

/** Fair integer in [0, n): rejection sampling, so there is no modulo bias. */
export function randInt(n) {
  const limit = Math.floor(4294967296 / n) * n;
  const u = new Uint32Array(1);
  do crypto.getRandomValues(u);
  while (u[0] >= limit);
  return u[0] % n;
}

Nothing hidden: why the wheel can't be rigged

• The winner is chosen before the wheel moves. Spin length, the number of turns, the theme, sounds and confetti only change how the result looks.
• There are no hidden weights. A weight shows as a wider slice and a ×2 badge in the list. There's no secret "next winner" or "always skip" setting anywhere in ChalkSpin.
• The draw happens in your browser. No request goes to a server to pick a winner, so nobody, including us, can steer it from outside.
• Everyone sees the same wheel. A share link carries the entries and their weights, so anyone who opens it sees the same odds you do.
• Every pick is on the record. The spin history lists each winner with the time, and can be copied as proof for a giveaway.

The only ways to change the odds are ones the room can see: giving an entry a weight, adding the same name twice (ChalkSpin flags duplicates), or removing a name, for example a student who's absent today.

Why the same name can come up twice

Each spin is independent: the wheel doesn't remember who won last time, the same way a coin doesn't. So repeats are normal, and they happen more often than most people expect.

• With 25 names, the next spin has a 1 in 25 (4%) chance of picking the name that just won.
• Over 5 spins, there's about a 35% chance that some name comes up twice.
• Over 10 spins, it's about 88%.

If you want everyone to have a turn before anyone goes again, switch on Remove winner: each winner leaves the wheel after their spin (and Undo brings them back). The random name picker is built around the same idea: no repeats until everyone has had a turn.

Instant result and reduced motion

Instant result skips the animation and shows the winner straight away. It uses exactly the same draw, so the odds don't change. If your device is set to reduce motion, ChalkSpin shows results instantly on its own and turns off the celebrations.

The other tools use the same fair draw

Every ChalkSpin tool builds on the same three functions: a fair whole number, a fair weighted pick, and a fair shuffle.

• Random name picker: draws from the names still waiting, so nobody repeats until everyone has been picked.
• Group generator: shuffles the class with a Fisher–Yates shuffle, where every possible order is equally likely, then deals people into groups.
• Random number generator: every number in your range has the same chance; with no repeats, each number comes up once.
• Yes or no wheel: two equal slices, a true 50/50 (or three equal ways with Maybe).
• Bingo cards: each card's squares and the calling order are shuffled the same fair way.