Skyline puzzle

One of my colleagues brought the following puzzle to work:

Skyline puzzle

The puzzle is called Skyline and it’s a packing puzzle. The objective is to place the metal rod in one of the holes in the base and place the nine wooden pieces around it. It was designed by Jean Claude Constantin. When solved, the puzzle looks something like this:

Skyline solution

Sometimes with these kinds of puzzles it’s quicker to write a program that finds a solution than trying to solve it by hand. Check out this github repository for a Prolog program that finds solutions for a given rod location.

To use this program open the file in your favorite Prolog interpreter (e.g. SWI-Prolog) and execute the following:

You can press ; to find alternative solutions. The pos(X,Y) part refers to the location of the metal rod.

Statisticians are evil

I’ve made it my life’s goal to replace all statisticians with cute little robot bunnies. Watch the following video for a demo of my first prototype.

I developed a server in Prolog for the Nabaztag:tag bunny and hooked it up with a dialogue system I created during my masters. It uses an unofficial Google API for speech recognition and generation. It’s quite slow sometimes because of the poor Wi-Fi connection, the inefficient polling of the Nabaztag and the speech recognition. I have some ideas though for improving its speed. Read on for a transcript of the dialogue with comments.

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Bridge and torch puzzle

Four people need to cross a bridge at night which only supports two people at the same time. Person A needs 1 minute to cross the bridge, B needs 2 minutes, C needs 5 minutes and D needs 10 minutes. When two people cross the bridge they move at the slowest person’s pace.  They have a torch which has battery left for only 17 minutes. They can’t cross the bridge without light. How can they manage to cross the bridge?

One might guess that an obvious solution would be to let the fastest person (A) shuttle each other person over the bridge and return alone with the torch. This would give the following schedule:

A, B -> 2
A <- 1
A,C -> 5
A <- 1
A,D -> 10

The total duration of this schedule would be 19 minutes, so the torch would run out of battery while person A and D are still on the bridge.

The optimal solution consists of letting the two slowest people (C and D) cross the bridge together, giving the following schedule:

A, B -> 2
B <- 2
C,D -> 10
A <- 1
A,B -> 2

Which gives a total crossing time of exactly 17 minutes.

Writing a prolog program to solve this kind of river crossing problems is a walk in the park. Check it out if you want to know an alternative solution.

Prolog solution to Einstein’s riddle

The following puzzle is said to be invented by Einstein. Supposedly, he also claimed that only 2% of the world’s population would be smart enough to solve it.

There are 5 houses in 5 different colors in a row. In each house lives a person with a different nationality. These 5 owners drink a certain drink, smoke a certain brand of cigar, and keep a certain pet. No owners have the same pet, smoke the same brand of cigar or drink the same drink.

The question is: WHO OWNS THE FISH?


  • the Brit lives in the red house
  • the Swede keeps dogs as pets
  • the Dane drinks tea
  • the green house is on the immediate left of the white house
  • the green house owner drinks coffee
  • the person who smokes Pall Mall rears birds
  • the owner of the yellow house smokes Dunhill
  • the man living in the house right in the center drinks milk
  • the Norwegian lives in the first house
  • the man who smokes blends lives next to the one who keeps cats
  • the man who keeps horses lives next to the one who smokes Dunhill
  • the owner who smokes Bluemaster drinks beer
  • the German smokes prince
  • the Norwegian lives next to the blue house
  • the man who smokes blends has a neighbor who drinks water

Working out the solution with nothing more that a pen and some paper is certainly doable by, I suspect hope, a larger percentage of people than the 2 % mentioned above. But as an example of how to solve these kinds of logic puzzles using Prolog, I wrote this code.

Sudoku solver

It’s pretty straightforward to make a Sudoku solver in Prolog especially when applying CLP (Constraint Logic Programming).

Here is how to use my program:

Then you can enter the known numbers one by one.

When complete, the program determines and prints the solution.



By pressing ; over and over again, you could enumerate all 6,670,903,752,021,072,936,960 possible Sudoku solution grids, but this might take a while..

It shouldn’t be too hard to extend this program to actually create new puzzles. If anyone does, let me know.

The prolog environment I used here is SWI-Prolog.

A brainf*ck interpreter in Prolog

It’s pretty easy to define an interpreter in Prolog. How this can be done is shown in this paper.

The language brainfuck is extremely minimalistic, consists of only eight commands and is Turing complete. Writing an interpreter for it in Prolog is a breeze.

You can download my interpreter here. It takes a slightly different approach than described in the paper to allow for tail recursion. Without this optimization, running any brainfuck program more complex than ‘hello world’, would result in a Prolog ‘Out of Stack’ error. For some tips on writing efficient Prolog programs check out this document. The Prolog environment I used is SWI-Prolog.

I’ve included some brainfuck programs I found at ‘The brainfuck archive’.

To start a brainfuck program from Prolog do the following:

It’s not the fastest interpreter around, but it’s a nice example of how to build one in Prolog.