Program for solving and generating correct numberlink puzzles
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Numberlink is a small, but very fast, program for solving puzzles of Numberlink/Arukone/Nanbarinku/FlowFree. Instances of size 40x40 are casually solved, and the run time is often linear for sparse enough puzzles.

The Numberlink puzzle involves finding links to connect numbers or letters in a grid. See for a detailed description.

Running it

You can download the source code of Numberlink at

Numberlink is written in the Go Programming Language and is compiled using $ go install numberlink. This won't install anything on your system. For more information on compiling, see the INSTALL file.

When you have created the binary, you can run $ bin/numberlink [options]. Numberlink will then read puzzles from standard input in the following format:

5 4

The first line consists of the width and height of the puzzle. The following lines contains the puzzle where . represents an empty square and letters or digits are the sources that must be connected.

Numberlink then prints the solved puzzle to standard output, either in the format below, or as specified by command line flags:

5 4

If the puzzle wasn't solvable, IMPOSSIBLE is be printed.

To learn about the available command line flags, see $ bin/numberlink --help.

What Numberlink is not

You can't use numberlink for checking if a puzzle is unique. Indeed numberlink will assume the puzzle has just one solution and only it such that the solution uses 100% of the paper and no link touches itself. Hence some non unique puzzles will be solved, while others will be IMPOSSIBLE.

If you want to find the number of solution to a general numberlink puzzle, I suggest using this solver by ~imos:

How it works

Numberlink solves puzzles using a heavily pruned backtracking search. In particular the following pruning heuristics are used:

  • Partial links
  • A dual representation based on link corners
  • Optimistic validation

There are multiple ways to do backtracking on numberlink puzzles. The most obvious is to start at a source, choose a link to its other end and recurse. Alternatively one can start at all sources at the same time, or you can ignore the sources and systematically fill out the squares on the paper in some order.

Numberlink uses the later approach: It fills out the paper starting in the upper left corner and continuing along the SW-diagonals. For a 4x4 paper the order in which squares are visited is (in base 16):


Backtracking in this systematic give us a lot of advantages compared to starting at the sources:

  • We never get unconnected squares. We simply always connect a square as we go over it.
  • We never block a source from its other end. To see this notice that blocking a source requires us to have passed it. In passing it we must have connected it to a partial link. The other end of this partial link can not be connected to any other sources or to the side, so it must be in the 'active diagonal'.
  • We always know exactly what squares around us have already been connected. That's the one above us and the one to the left. The directions we need to care about is down and right.

The challenge with this approach is that we need to manage 'partial links' that aren't yet connected to anything. We don't want to accidentally connect a link to itself, or to connect the ends to different labeled sources. This problem can be solved efficiently by the disjoint-set data structure, but it is simpler for us to just keep an array such that if pos is a 'link head' then end[pos] is the current position of the other end. Initially end[pos]=pos and if pos has degree two, end[pos]=-1. (Actually the last part is unnecessary due to the systematic order of connection). This array is easily updated when two links are merged by no more than two array assignments.

The corner dual heuristic is the most important part of what makes Numberlink fast. It relies on the observation that if a square is filled out with a ┐ (a south turn of a link, we'll call it a SW 'corner') the lower left square will either have to be a source or to be a SW corner as well. Anything else will force a self-touching link.

Taking the inductive closure of the above observation, we see that all corners, must be found in 'spikes' rooted at the sources. Indeed a source can't even have such a spike in two opposite directions, as it would create a link surrounding the source. All in all we conclude that any solution to a numberlink puzzle can be represented uniquely as a set of signed integer pairs, one pair for each source, describing the length of its two spikes.

We don't directly use the above representation however, as it doesn't seem to suggest an easy way to backtrack. Instead we backtrack on the partial link representation, but make sure that no connections are made, which would create an illegal situation in the dual representation. It is worth noticing that the dual representation means especially very sparse puzzles can be efficiently solved.

The corner heuristic also protects us from a lot of illegal states in the primary representation, for example self touching links are very rarely explored. It isn't however totally safe to rely on, as this example shows:

4 4

Numberlinks approach to this kind of situation is to assume that they won't happen very often, and hence we don't need to check for them during solving. This makes searching more efficient and only once the entire paper is filled out do we check if we have done something illegal.

It's still unexplored however exactly how much extra pruning we could make, if we detected self-touch early on. Another kind of self touch we might be able to prune early is this one:


The last question one may ask is 'why search diagonally?' Instead one could have walked row by row, or with an expanding boundary like a bfs search. While the later approach may allow us to fill out some obvious squares higher up in the tree, it doesn't give us much predictability in the structure of the filled out squares, something that simplifies the search code greatly. Filling by rows is very similar to diagonals, but with diagonals the tree is often twice as high.


Numberlink was written by Thomas Dybdahl Ahle for a competition at Oxford University arranged by Michael Spivey ( The description of the competition is available at


Numberlink is released under the GPL3.

Read LICENSE for more details.