Prime Number Test in the Piet language
The effect of the program is to take in a number, and output either “PRIME” or “NOT PRIME” depending on which it is. The first part finds the whole number lower than the input’s square root (a surprising amount of work in Piet) to save on runtime. Then, the program goes input-modulus-(2, 3, etc) to see if it has any factors. If it does, it goes through that big light magenta square to write out NOT, then into the dark cyan square. If none of the numbers below the square root are factors, it enters the dark cyan square. From there the program writes PRIME.
via DM’s Esoteric Programming Languages