PlankwayHashiwokakero: a short history of the bridges puzzle
Plankway is a new game, but the puzzle underneath it has been around for decades. It is known as Hashiwokakero, Hashi for short, or simply Bridges.
From a Japanese puzzle magazine
Hashiwokakero was published by Nikoli, the Japanese puzzle company that also gave Sudoku its name. Nikoli built its magazine, Puzzle Communication Nikoli, around puzzles sent in by readers, and the bridges puzzle appeared there around 1990. The name means roughly "build bridges!".
Like many Nikoli puzzles, it has a short list of rules, no maths beyond counting, and a strong tradition: a good puzzle has exactly one solution and can be solved by reasoning alone. Hand-made puzzles were valued for the "aha" moments their designers built in.
Around the world
The puzzle spread through puzzle books and newspapers under different names: Bridges or Hashi in English, and similar translations in other languages. Today it is a staple of logic puzzle collections next to Sudoku, Nonograms and Slitherlink.
It has also drawn interest from computer scientists. Deciding whether an arbitrary bridges puzzle has a solution is known to be NP-complete, which roughly means there is no known shortcut that works for every possible puzzle. For the size of puzzles people solve, careful reasoning is more than enough; it is also what makes generating good puzzles an interesting problem.
What Plankway adds
- Reefs: rocks that block bridges, so islands in line do not always see each other. Read more in reefs.
- One shared puzzle a day, with a difficulty that rises from Monday to Sunday, and an endless mode with levels that get harder as you go.
- Puzzles generated and checked by a computer, with the same promise as the hand-made classics: one solution, no guessing. How that works is explained in how Plankway puzzles are made.
The rules of the classic remain: numbers, straight bridges, at most two, no crossings, one network. If you have solved Hashi before, you already know how to play Plankway; the reefs will just make you look twice.