February 2, 2026
Ho-ho-hold my spellcheck
Solvingn the Santa Claus concurrency puzzle with a model checker
Santa’s puzzle cracked, but readers fixate on a typo
TLDR: A blogger used a model checker to prove Santa only wakes for the right-sized groups and always prioritizes reindeer. The comments? Half roast the headline typo, half applaud the “fail first” teaching style—highlighting how thorough checks catch bugs that normal tests miss.
Waqas Younas dives into the classic Santa Claus concurrency puzzle—Santa only wakes for nine reindeer or three elves, with reindeer taking priority—and uses a model checker to catch sneaky timing bugs. He explores three failure cases where Santa does the impossible: delivering toys with too few reindeer, consulting elves and delivering at the same time, or ignoring ready reindeer. It’s nerdy, but explained in simple terms: think of a model checker like a robot that tries every possible order of actions to find hidden mistakes. The tech crowd nodded along, but the comments turned spicy fast. One user instantly zeroed in on the headline typo—“Solvingn”—and the thread basically became a spellcheck showdown. Meanwhile, others praised the “fail first, then fix” approach as a smart way to learn. A few explained that SPIN and its language Promela are tools that check all the tricky timing combos your regular tests miss. So yes, the post is about taming chaos with math and patience—but the community couldn’t resist the real chaos: a typo overshadowing Santa’s sleigh of logic. Classic internet energy: brilliant content, zero mercy for spelling.
Key Points
- •The article models the Santa Claus concurrency puzzle and validates solutions using the SPIN model checker with Promela.
- •It outlines core constraints: Santa cannot marshal groups, reindeer have priority, and exact group sizes (nine reindeer or three elves) are required.
- •A failure-first approach models incorrect designs to reveal unsafe assumptions, using correctness properties to catch bugs.
- •Three failure scenarios are demonstrated: insufficient reindeer, simultaneous delivery and consultation, and violating reindeer-first priority.
- •Model checking offers coverage across interleavings that typical Python or Go implementations may miss, enabling proofs or counterexamples.