The History of Rune Circuit: NetWalk, Net and the Pipes Puzzle

First, what it isn’t: Pipe Mania, 1989

In June 1989 a British studio, The Assembly Line, released a game known as Pipe Mania for the Amiga, Atari ST and MS-DOS. Lucasfilm Games distributed it in North America under the name Pipe Dream. It is the game most people think of when they see pipes on a grid, so it is the usual first guess at where this puzzle came from.

It is a different kind of game. In Pipe Mania the board starts empty. Pieces of pipe arrive from a dispenser in random order, and you place them against the clock to build a pipeline long enough before the liquid reaches the end. You are racing, and you choose where each piece goes.

In the rotation puzzle nothing is placed and nothing is timed by the rules. Every tile is already on the board and stays in its square. All you can do is turn it. The two share a look and very little else, and nothing in the record ties one to the other.

NetWalk and the Moscow puzzle makers

The origin usually given for the rotation puzzle is NetWalk, a Windows game from Gamos, a Moscow company founded in 1991 and known for its logic games. In NetWalk the tiles are pieces of network cable. You rotate them until every computer on the grid is connected to a central server.

The dates are less firm than the story. Game databases list NetWalk as a 1996 release. The long-running archive Home of the Underdogs files it under 2000 and mentions a handheld version. One database entry describes NetWalk as a variation of an earlier Gamos game called Vetka with a different theme. We found no first-hand account from Gamos itself, so treat “about 1996” as the best available estimate and not a settled fact.

What the descriptions agree on is the design. NetWalk had several skill levels on larger and larger grids, it scored you on how few moves you needed, and its hardest level let connections wrap from one side of the board to the other. All of those ideas turn up again in later versions.

KPlumber: the same idea with water

A second branch of the family used plumbing. KPlumber was a game for Linux in which each tile holds pipes running from its center to some of its sides, and one click rotates a tile by a quarter turn. Water leaks from any open pipe end, and the goal is a board with no leaks. A Windows game based on it, Linkz, is dated 2000, so KPlumber was in circulation by then.

KPlumber’s goal is looser than the one you play here. It asks only that every pipe end meets another. It does not ask for a single network, so a solved board can be several separate closed systems. Rune Circuit, like NetWalk, adds the rule that everything must join up, which is what makes an isolated group a mistake.

FreeNet in the browser

The puzzle reached a wider audience as a browser game. Pavils Jurjans published a Flash version called FreeNet on his personal site, jurjans.lv. The Internet Archive’s index shows a saved copy of that page from December 2002, so it was online by then. We could not open the page itself while researching this chapter, and we have no firm date for when it first appeared.

FreeNet matters because of who played it. It is the version that Simon Tatham names as the inspiration for his own, and through his collection the puzzle spread to nearly every kind of computer and phone. Academic papers on the puzzle still list FreeNet among its names.

Simon Tatham’s Net, April 2004

In April 2004 the British programmer Simon Tatham began a set of small puzzle games that would run the same way on Windows and Unix. He has written that he wanted little games you could play for two or three minutes during a break. The first puzzle in it was Net. His source history records the framework and Net arriving together, with a note the next day that Net was playable. The manual credits FreeNet and adds that other versions go by the name NetWalk.

Tatham’s collection grew to dozens of puzzles, and its permissive license let others port it widely. His Net fixed several features that players now expect. A middle click marks a tile as a locked tile so you can’t turn it by accident. Tiles joined to the center light up. Options add wrapping edges and barriers, which are fixed walls between two squares that act as hints.

In May 2005 he added a guarantee that each puzzle has a unique solution, switched on by default. His generator also never makes a tile with four arms, because such a tile looks the same whichever way it faces and would give itself away.

KNetWalk, browser versions and the name Pipes

Free software carried the puzzle further. KNetWalk, part of the KDE desktop’s games, credits Andi Peredri as its original author in 2004 and Thomas Nagy for the KDE port. It keeps the NetWalk story of terminals and a server, and its handbook calls it “a game for system administrators”. The handbook also promises one correct solution per game and lets connections wrap on the hardest level.

Ben Lynn wrote another open-source NetWalk and later rewrote it to run in web browsers. Versions like these kept the cables and computers. Many web versions and a 2022 Leiden University thesis call the same puzzle Pipes, and at least one puzzle site offers it in sizes from 4×4 to 25×25 with a wrapping option. The names differ and the rules barely do: rotate every tile, join everything, leave no loop.

The mathematics: trees and hard problems

A finished board has a shape mathematicians know well. Treat each tile as a point and each connection as a line. A network that reaches every tile and contains no loop is a spanning tree of the grid, which is also the shape of a maze. That is why programs often build these puzzles with maze-making methods and then scramble the tiles. A 5×5 grid has 557,568,000 different spanning trees.

How hard the puzzle is in general has been studied twice. In 2004 Daniel Král and four colleagues published It is tough to be a plumber in the journal Theoretical Computer Science. They showed that deciding whether a KPlumber board can be solved is NP-complete for most mixes of tile types, and found some restricted cases that can be solved quickly.

For Net itself, with its single network and no loops, Marzio De Biasi posted a proof of NP-completeness in December 2012. It is a self-published note that he describes as an amateur proof, though later academic work cites it. Results like these are about boards of unlimited size. They say no fast general method is known, and they say nothing against a 5×5 board, which always yields to the reasoning in the solving tips.

How LarkSpark plays it

Rune Circuit is LarkSpark’s own telling of the puzzle. The cables become carved stones, the server becomes a star, and the board is always 5×5, with the star in the top-left corner. Each day’s board is built from a seed, so everyone gets the same 25 runes. There is no wrapping and there are no barriers: the edge rule holds on all four sides.

Each puzzle starts life as a finished circuit with no loops, which is then scrambled. Any arrangement that connects all 25 runes to the star with no loose end counts as solved. Because the stones come from a loop-free circuit, their arms add up to exactly enough for 24 connections, and that has a neat consequence. If you ever close a loop, some other stone must be cut off.

Play is timed and hints are counted in your daily record. The full rules are on the rules page, and the words used here are collected in the glossary.

The history of Rune Circuit