Robotik language - entry for
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This repository contains the code for a compiler of Robotik language, one of the entries for PLT Games Contest

Language Description

The language is inspired by the game Ricochet Robots.

Indeed, the active units in the execution of a program in Robotik are some robots. A program consists of a description of the initial placement of the robots followed by a list of directives. Each directive is an order given to a robot to move in one direction and write a value at the stopping point.

Each robot has a modulus. It cannot pass through cells containing another robots or values which are exact multiples of this modulus. The following image is an example (for a robot named R1 with modulus 3 ordered to move to the right; assume a closed world).


If one robot has no stopper on the direction it is ordered to move, then the robot keeps his position but writes the value nonetheless.

There is one exception to the above rule. Robots with modulus equal to 0 cannot be stopped by cells with values. They are special robots and have two interesting properties. They always push the robot they collide with one cell forward, if this is possible. After this, the next directive to execute is one of the previous directives given to the pushed robot. More exactly, if the directive given to the 0-modulus robot has value v then the next directive to execute is the v-th latest directive for the pushed robot, if any. In case there are no directives for the pushed robot then execution continues normally. However, if there are directives for it but not enough, execution continues from the first directive of said robot. See the following image for an illustration (assume ... mean more directives and the arrows on the right show the jumping process).

The above exception was introduced to enrich the language with loops, thus making it Turing complete (taking together all other features)

The world is an infinite 2D lattice of integer values. Thus, the entire program is made from integer values only. The position of these integers and their relationships are the ingredients which make Robotik an interesting language.

Not all sequences of numbers are valid programs. Some values could be reduced to others using modular arithmetic. But others cannot. There are a few restrictions:

  1. The number of robots must be a positive value R.
  2. Each modulus should be a nonnegative value.
  3. There should be at least 3*R + 2 values in the program. If a directive is incomplete it will be padded with 0 values. However, there is no program with no directives or with an incomplete robot specification.

Nevertheless, even with these restrictions the language accepts almost all streams of numbers. If we restrict our input to non-negative numbers and to streams starting with low values then the ratio between valid inputs and possible inputs approaches 1. This can be useful for chaining multiple programs together, for considering the output of one program as being another program or for stenographic purposes. It is easy to embed a program inside an image and carry it through unsuspecting eyes. Even a little encryption can help a long way here :)

The output of such a program is the entire state of the lattice.

Language Implementation

Because we cannot work with infinite lattices the display is limited to the minimal axis-aligned rectangle enclosing all robots. This is the only modification done between the theoretical language illustrated above and the practical implementation given by this repository.

The compiler builds a board from the incoming numbers and moves robots on it according to the directives. Because only the final board is able to produce output endless running programs will loop forever without producing anything usable.

Busy Beaver example

In the srcs directory, there are two sources for the busy-beaver type of program. In order to implement looping we have to use 0-modulus robots an the reason for starting with this section is to give examples of their role.

First, we have srcs/busyBeaver which is a program which will print 4 values of 1 before stopping. R1 will constantly push R2 to the left while R2 will always write a 1 in the current cell.

Secondly, there is srcs/endlessOnes which will try to create the state where all values on Ox axis are 1. However, since that state is unreachable, the program will loop forever.

Hello World examples

The classical program which prints "Hello World" is illustrated by 4 examples. Of course, there could be more. Each of the 4 examples demonstrates one of the features of the Robotik language.

To start with, we have srcs/hello1. The initial situation can be illustrated by the following picture.

The only moving robot is R2. It sweeps the board between R1 and R0 writing a value at each stop, according to the ASCII value of the letter which needs to be written there. Because it has modulus 1 it will stop before each previously written value, thus reducing the width of the sweep. The following image is an illustration of this program in action.

Turning our attention to srcs/hello2 we see that it has the same length and overall structure as srcs/hello1 with two differences. The numbers are no longer on a single line but you already know that this is no significance to the language. What matters is that the numbers in the directives part are increasing. The program does the same thing as srcs/hello1 because each number is translated in the appropriate range during the building stage of the compiler using modular arithmetic.

The robots in srcs/hello3 are still printing "Hello World" but using a different strategy: the robot R2 first tries to move on the Oy axis. Since there is no blocking point on that axis the robot keeps its position and writes its value at that point. The R2 moves to R1 and backwards. Since its modulus is 1 it will stop at the next position.

Lastly, srcs/hello4 uses 4 robots but only R0 and R1 do actual work. As an exercise, try to deduce the translation from the code in srcs/hello4 to the desired message.

All of the example have used two robots which are only sitting idle. Their purpose is to extend the range of the output values such that the entire message is printed. Also, they have a role in limiting the initial walk of the active robots.

As a more interesting exercise, try to rewrite the last example using 0-modulus robots and looping. It is going to be a fascinating experience.

Turing completeness

The theoretical language can be easily translated into a classical Turing machine. The implementation has the same propriety but this is not as easily seen.

Anyway, proofs of the above claims are left as an exercise to the reader. As a hint, first you need to reduce this problem to a 2D Turing machine with multiple heads and proceed from there.


In order to exhibit randomness, the only valid solution is to place two or more robots at the same location. The builder of the internal structure used by the compiler will take care to randomly move robots around until no overlaps are found.

Of course, this random moving will create program species which will exhibit divergent behaviours. But this is intended :)

However, there are some programs which will give the same output, even if using randomness. See srcs/random example.

Open questions

  1. What is the minimum description of a program which outputs "Hello World"?
  2. What is the minimum description of a busy-beaver program?
  3. What is the minimum description of a program exhibiting the greatest variety due to randomness?
  4. Can someone implement Game of Life in Robotik?