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Normally "Good luck!" = "Удачи!"
It is shortened from "Я желаю тебе удачи!" = "I wish you good luck!"
Very much like in English.

about a person - везучий, удачливый
about an inanimate object which brought luck once = (lucky day, lucky deal, lucky strike) - удачный
about an inanimate object which is expected (superstitiously)...

I can't think about a word in Russian which is a full equivalent of what you describe as snap. Maybe if you will provide more exact context (several sentences) someone can give you ways to translate...