MastodonWriting a brainf**k interpreter with networking functionality - Stefano Vazzoler

Writing a brainf**k interpreter with networking functionality

I wrote Toy Language, a bf interpreter written in Go with TCP networking functionality.

The language

I wanted to write a simple interpreter for a while so recently I've taken a look at writing one for an esoteric programming language called Brainfuck.

To give the reader an idea of how this language looks like, let's take a look at an example from Wikipedia:

Hello World

The language operates with a byte array and a pointer to a value in such array, all operations revolve around manipulating the pointer and the array value at the pointer head. < and > allow us to add or subtract 1 from the pointer, + and - allow adding or subtracting 1 from the byte at the data pointer, . and , allow us to print the value at the data pointer to StdOut or write from StdIn, and finally [ and ] are equivalent to a while (*ptr) {} in C.

Writing a interpreter

To speed up my interpreter, I first wrote a parser that goes over an input file and removes all invalid characters, so we can ignore them while iterating over the code.

Now, with some cleaned up code, I iterate over the instructions and execute them one by one.

I used a series of if statements instead of a switch/case to decide what instructions to execute since the former is 3x faster in go 1.19 and, at least in my opinion, is still quite readable.

Extending the language

The problem

Adding a TCP networking extension to the language was a bit of a design challenge. I wanted commands not to feel too foreign to the original language while avoiding a custom messaging protocol on top of TCP.

At first I thought of grouping consecutive selectors - for example, to send a 4 byte packet you write ^^^^ but this felt a bit of a cheat where the first few ^ do a different task than the last ^ and doesn't easily allow variable sizes. Similar challenges came to selecting the connection target and more.


To make this work well, I had to compromise.

First, the networking extension needs to keep an internal state that the user code can interact with.

Second, I decided on a pre-set listening address and a pre-set connection target address.

Then, I allow the user to set (@) a port value 42000 plus the value at the data pointer, so a single byte can represent a range of ports - and similarly, the timeout is set (*) to 0.1 seconds times the value at the data pointer.

Finally, the user can read a byte from the connection with ? or queue a byte write with ^ or flush the write queue with ;.

The connection is setup in server mode (listener) when using ? and in client mode (connect to) when using ;, but once a connection is setup any subsequent ? and ; will interact with it.

Essentially this design allows our brainfuck code to operate like a feature complete TCP client/server while offering a coding experience which still breaks your brain.

Sample code

Sample code with comments is available in the repo, but two quick examples for networking are:

Netcat commandBF + net extension
nc -k -l 42002tl:net -*+++@[>?.<]
nc 42001tl:net -*++@[>,^;<]


Try it out for yourself - what are you going to build? a chat app? a simple http server? surprise me!