It’s my own puzzle. Oops.
The stub generator is slightly wrong. It currently is:
read N:int loop N read guess:int bulls:int cows:int
It really should be:
read N:int loop N read guess:word(4) bulls:int cows:int write answer
The [[guess]] variable should be a word, not an int. Some of the guesses in the tests start with leading zeros, which are significant and should not be dropped. The way it is now, it is more challenging to solve than intended.
Sorry for the screw-up.