2026-09-15 · history
Hitori (1990) — Why a Puzzle Too Hard to Catch On for 8 Years Is Still an AI Test Case in 2025
Hitori — first published as a reader submission in Puzzle Communication Nikoli issue #29 in March 1990 — is a logic puzzle built from just three rules: no duplicate numbers, no adjacent black cells, and no disconnection. It was too hard to catch on at first, taking eight years to find its footing. Yet it was ported to the Nintendo DS in 2007, proven NP-complete by Robert Hearn and Erik Demaine in 2009, and in 2025 became the subject of an AI research paper on explaining puzzle solutions. This traces how far three reader-submitted rules have carried across 35 years.
