I got really into esoteric languages at some point, which culminated
in me writing the Sieve of Erathostenes in Brainf*ck,
a programming language with only 8 commands.
Run the code here
or view on GitHub.

The GitHub README says “I wrote this in the summer of 2014 when I was bored on holiday. The whole thing was written from my phone.”
Weird flex, teenage me.

The program prints primes up to N, which it reads from input (terminated by 0).
Memory cells are divided into groups of five, each group represents one number.
Each cell of the group has a certain function:

The number itself

Number of divisors of this number minus 2. This value is 0 for primes and more for composite numbers

Whether the number is in range from 1 to N, 1 if yes, 0 if no

A robotic stringed instrument: one motor tunes the string, and another one
plucks it. Fretless, since the pitch is controlled by tuning. Controllable via MID...