How it solves
Each cell starts as a candidate set {1–9}. Every wave the automaton applies three constraint rules across all 27 units (rows, columns, boxes):
Solve effort (how long it lasts)
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Last wave
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Rules applied, in order:
1. Eliminate — a placed digit clears itself from every peer's candidates.
2. Naked single — a cell with one candidate locks in.
3. Hidden single — a digit with one home in a unit locks in.
If logic stalls, the solver guesses the cell with the fewest candidates and backtracks if wrong — shown with a ⚡ tint.