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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.
Sudoku (1979) — How a Nameless Invention Conquered the World's Morning Papers
In 1979, a puzzle called 'Number Place' appeared, uncredited, in an American magazine. This piece traces the 25 years it took to become Sudoku — refined by Japan's Nikoli, carried by a retired Hong Kong judge's homemade program, and finally landing in a London newspaper in 2004 — and the 'logic only, no guessing' design principle that still lives on in the 2024 Steam puzzle collection Islands of Insight.
Hashiwokakero (1990) — Thirty-Six Years of Logic Grown from Numbers Joined by Lines
In 1990, Hashiwokakero debuted in Puzzle Communication Nikoli #31. Its rule — connect numbered islands with bridges until everything forms one network — grew out of an earlier prototype, "Ketsugoshu," that the same contributor "renin" had published in issue #28 the year before. In 2009 it was proven NP-complete; in 2004 it reached Simon Tatham's Portable Puzzle Collection ahead of any Steam release; and in 2023 Nikoli itself ported it to Switch, PC, and Xbox. This article verifies a 36-year lineage.
Nurikabe (1991) — A Ban on Two-by-Two, Borrowed from a Yōkai's Name
In 1991, Nurikabe debuted under the pen name "renin" in the reader-development column of Puzzle Communication Nikoli #33. Its four rules — size islands by number, connect the rest into a single black sea, and never allow a two-by-two black block — weren't even expected to catch on by the magazine's own editors. In 2004 it was proven NP-complete; its "no two-by-two" rule was inherited by LITS; and in 2025, Steam's Nurikabe World brought the puzzle back under its own name. This article verifies a 34-year lineage.
Masyu (2000) — The Loop of Black and White Pearls Born from a Misreading
In April 1999, Puzzle Communication Nikoli issue #84 published a puzzle format called "Pearl Necklace," using nothing but white circles. In 2000, issue #90 added black circles and renamed it "White Pearl, Black Pearl," completing the logical skeleton Masyu still uses today. The official name "Masyu" wasn't settled until 2003 — the result of Nikoli's president misreading a kanji, an accident that simply stuck. With sources, we trace how a grid using no numbers or letters survived 26 years across an NP-completeness proof, official console ports, and an open-source implementation.
Numberlink (1897) — From a Newspaper Puzzle Column to the Age of Flow Free and Zip
In 1897, puzzle-maker Sam Loyd printed 'The Puzzled Neighbors' in the Brooklyn Daily Eagle. In 1917, Henry Ernest Dudeney refined it into a motorist's puzzle in his own book; in the 1980s, Nikoli gave it a home in Japan as Arukone and Numberlink. In 2012, Big Duck Games' Flow Free turned it into colored dots and pipes and passed 100 million downloads; by 2025, the lineage reaches LinkedIn's daily puzzle Zip. This essay traces how a 120-plus-year-old rule — draw a single line without crossing — has survived.
Yajilin (1999) — A Board Where Two Rule-Systems Were Grafted Together
In July 1999, Puzzle Communication Nikoli issue #86 introduced Yajilin, a hybrid pencil puzzle that grafted the "draw exactly one loop" grammar established by 1989's Slitherlink onto a black-cell-counting rule. It reused the arrow-and-number cell from a puzzle published just before it, Yajisan-Kazusan, and by 2005 had already been adopted, as "Arrow Ring," into the US qualifying round of the World Puzzle Championship. From official Xbox and PS Vita ports to a dedicated 2025 mobile app called Loopr, we trace, with sources, 26 years of this grid's survival.
Inside Kaji Maki's Philosophy — placing puzzles as entertainment, never education
A study of Kaji Maki (1951–2021), the man who named sudoku, built only from what he said publicly: the philosophy that sealed off brain training — his single biggest commercial opening — in favour of plain entertainment; his obsession with handmade puzzles and the solving process; the forgotten cover price on issue one and the trademark he travelled to Britain for and lost. It covers the dilemmas of reach versus revenue and a small company facing global demand, and closes with Kizuki's reading.
Kakuro (1966) — From Cross Sums to Addition Cross, Sixty Years of an Unmoved Grid
Kakuro, known in English as Cross Sums, began in 1966 as a Dell Magazines feature. Nikoli brought it to Japan in the 1980s and renamed it 'kasan kurosu' (addition cross); in 2005 it was re-exported to the world via The Guardian. This essay traces why this grid of arithmetic — neither crossword nor Sudoku, but a hybrid of both — kept its rule unchanged across sixty years of shifting media, from paper magazines to a Nintendo DS cartridge to today's Steam store pages.
Slitherlink (1989) — Loop-Closing Logic Born From a Two-Player Paper Game
In June 1989, barely half a year into the Heisei era, Puzzle Communication Nikoli #26 introduced a new puzzle format. Adapted from a two-player paper game called Slither into a single-player pencil puzzle, Slitherlink offers a paradoxical solving pleasure — proving where lines cannot go rather than where they can — and carries a nearly 38-year lineage running through NP-completeness proofs to Steam storefronts in the 2020s. We trace, with sources, the loop-logic Nikoli founder Maki Kaji once called "the son of Sudoku."
“Writing the rules of a puzzle as mathematics”: an attempt to systematize pencil-puzzle rules
One piece today. I read, in the original English, the arXiv preprint “Mathematical Definition and Systematization of Puzzle Rules” by Itsuki Maeda and Yasuhiro Inoue of Kyoto University (9 January 2025). Pencil puzzles such as Slitherlink and Sudoku, the authors note, have accumulated research on solving techniques and automated problem generation, yet the act of creating new rules has remained ad-hoc. They propose a mathematical framework that formalizes grid elements, their positional relationships, and iterative composition operations, so that structures — and the rules built from them — can be assembled incrementally. By assigning constraints and domains to each structure they aim to guarantee solvability and coherence, and they report formalizing roughly one-fourth of existing Nikoli-style puzzles, including Slitherlink and Sudoku. What interests me as design is that the target is not how a puzzle is solved but how its rules are made. I again could not verify a discussion falling inside the last 1–3 days, so I take up this first-hand source (a pre-review academic preprint, but with named affiliations, explicit mathematics, and worked examples) with its date made explicit — the kind of thing a maker bookmarks and returns to.
Nikoli (1980) — The Reader-Grown Puzzle Magazine, and the Road Sudoku Took Around the World
Puzzle Communication Nikoli, launched in 1980, was Japan's first magazine devoted to puzzles. Its name came from a racehorse; the prototype of Sudoku came from America. This essay traces forty years in which Nikoli's mode of production—reader submissions and handmade problems—gave birth to Slitherlink (1989) and Nurikabe (1991), and sent Sudoku around the world via Hong Kong judge Wayne Gould's generator program, rereading the source of a question at the heart of modern puzzle design: who should make a good problem?
