I've always liked these code-breaking games. The concept behind them predates mastermind by many years. You can look it up in wikipedia under "bulls and cows". In some Spanish-speaking countries it is called "picas y fijas".
We are glad you liked it. It seems that this mastermind game have a lot of versions: some of them unaware of the father! :) Anyway we hope you liked also this version! :D
I just uploaded a new version and it should fix all upgrades issues. You will have to refresh the game unfortunately for the fix to take effect :( Sorry about that!
OH NO SPOILERS!! hahaha. I'm glad you liked it