Lets start with that old promotion rant I use all the time.

You should have a look at puzzl, if you haven’t yet.

What this is all about?

This post is about development of a sliding-puzzle game I made recently, named puzzl. I refer it as an intelligent version of the classical sliding puzzle game, and I’ll explain the reason behind this. Stay awake.

Puzzl in action

For What Joy?

Yes, that’s the best question. Why did I do that?

There is a force behind everything we do, so the same goes for any new software product we see in the jungle or a new medicine or whatever.

They all try to solve some problems. A new version of a software has some bugs fixed from the older one and so on.

But hey! you can develop a game without such force anytime.

Specially, when you are supposed to study for your exams. Tested, rule of thumb.

Exam-time bamboozlement

The session when your mind’s stream is *grep*ing too many *event*s.

You feel like, put this exam thing in left and consider this problem-blah, because the world needs you. Right now.

It’s gone after a nice sleep, like hangover. But it may attack you again on the next exam day, depending on your performance in last exam.

You think about too many things, however some are out of your domain though.

For example, A bicycle for old aged people. I found it an actual problem by the way, as I often see them having trouble riding the native big bicycles.

Then you think that it should run on its own, so you draw a sketch and invest your time in adding solar panels at proper orientation, so that charging is not something to worry about, no matter where the sun is.

Then you suddenly realize that you’re about to transform into Tony Stark. You’re working on a problem which comes under Mechanical Engineering. And there is no chance you can complete this without either having a degree in it or a girlfriend from that stream. By the way, both the solutions are difficult.

A similar bamboozlement was the reason behind the game puzzl.

I was setting up my camp for the mid-term exam of Artificial Intelligence course. And then I stumbled upon this sliding-puzzle game thing in book.

Then I thought, what about a sliding-puzzle game for terminals?

That’s it. I was done with further preparation. I even spent the entire exam time divising the scoring mechanism and logo for the game. The last page of the solution paper and my low score is the proof.

The problem statement

Sliding-puzzle games have many variants. I decided to move ahead with the one having a 3x3 board, the 8-puzzle. 8 out of 9 tiles have some numbers on them, one tile is blank.

All you have to do is, arrange all tiles in a particular order. Lets say, in increasing order with the blank tile being the last one.

8 puzzle game states

This sounds a little easy, taking user inputs, moving the tiles accordingly and checking that whether we are done with the game or not. So, I decided to enhance both the development and playing processes.

To solve the game in background and track user moves accordingly or give them hints. Poor humans.

The solution

The solution is pretty simple actually. There is a blank tile in the game board and you can reach to a different configuration by moving that blank tile in atmost 4 directions, according to its current position.

Now, how do you make a computer solve this, efficiently?

This can be reduced to a problem where any game board configuration is a node and you can reach to a node from its parent node by applying a successor function, which is the movement of the blank tile in this case.

So, this is a graph search problem now. You have a start state and you have to reach at a goal state in minimum steps.

In puzzl, the algoirthm which solves this graph search problem is A* algorithm. As of now, it uses the easiest Heuristic Function, Misplaced Tiles Count.

The auto solving part sounds pretty easy, right? Like a walk while eating a cake.

But there were some other interesting challenges.

Primary Language

I opted for Golang as the primary language of puzzl, just because I’m loving it and I wanted it to takeoff comparatively faster than other solutions, let’s say Python.

After some work, I started getting ideas of other features and I was happy with my language choice, as Go is the best solution I can think of for them. I’ll talk about those features in a while.

Solvability of the problem

This is one of those interesting parts I faced while making it.

Not all 8- puzzle game configurations are solvable.

Yes, for a 3x3 game board, there are a total of 362880 configurations possible(9!), but only half of them are actually solvable.

Initially it took me a while to get it that why only some of them are solvable? But after going through “Notes on the 15 puzzle” paper by Wm. Woolsey Johnson and William E. Story it was clear to me.

Using the Mathematical Induction on what it was there in the paper, I have a sample game for you. You can spend rest of your life solving this, if you want to.

Unsolvable sliding puzzle

Then I added a package named scanner in the game, with the help of “Analysis of the Sixteen Puzzle”. It makes sure that any game configuration we are using, is really solvable. It uses another for the game, if one is not actually solvable.

Notification System

And this was the most interesting part. Developing a Real time Notification System according to the game state.

When I was done with writing the auto-solving background process for the game, I noticed that sometimes it takes a while to solve from the initial game configuration.

For example, the hardest game configuration is solvable in 31 moves. So, when the game is solving it, it doesn’t properly respond to user inputs.

It won’t respond in this way because the control flow architecture is Synchronous now. It means that firstoff the game will solve itself in background and then stars accepting user inputs.

And this goes worse when the game is solving itself on each move because user input was wrong. A serious problem it was.

Here comes the concurrency primitives of Golang.

Concurrent programming is the beauty in Go. The language comes with built-in weapons, goroutines and channels.

The solution works like this, we have to change the control flow architecture into Asynchronous mode. Where the game would be notifying us that OK, now I’m done with solving and you can give any input, and avoiding all user inputs gracefully till then without a crash.

And this can be used in many other aspects, notifying on a particular internal event. Like the game is complete, you just did a right/wrong move, there are no hints left etc.

There are two goroutines, one for handling the notification events and another for solving the game in background, both are non-blocking in nature to other routines like user inputs, game display etc. A channel named Tunnel was the messenger for these goroutines.

Asynchronous Gouroutines Architecture

This is like feeding each other. Best friends forever.

Game Interface

Thanks to Box-drawing characters, the best old way to develop text based user interfaces. And thanks to termbox-go, an awesome library for such interface development in Golang.

arXiv submission

I loved working on it so much that I even framed its development in shape of a research paper, sort of. It’s available on the arXiv digital library, here arxiv-1503.08345.

Implementing an intelligent version of the classical sliding-puzzle game for unix terminals using Golang’s concurrency primitives.

It’s not in LaTeX, so the arXiv stamp is absent in PDF format there. They replied that they don’t have any technique yet to stamp other formats. I’ll probably port it in LaTeX one day.


  1. Notes on the 15 puzzle
  2. Analysis of the Sixteen Puzzle
  3. ai-php.com for the 8-puzzle game states image
  4. Abhay Rana for teaching me what For What Joy is. #FWJ?

Behind the scene

I framed all this story for you, the truth is something else.

Guess what? It’s my exam-time again that made me write this.

The source code for puzzl is on GitHub. Do whatever you want, just respect licensing and other respectful things.