Always start by thinking about the problem:
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Find a game of Tetris and play it for a few minutes. Searching the web for
tetris onlineshould find Free Tetris and numerous other Tetris implementations. -
Think about what requirements and game rules Tetris has.
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Think about what kind of design could be used to satisfy those requirements.
After you have spent 15 minutes or more thinking about the problem, you should have a rough guess that how to implement the game. The guess will likely be wrong, because you will never again know as little about Tetris as now.
Next when you'll start implementing Tetris, hold back from implementing more of your design than is necessary to make the currently failing test pass, while at the same time not blocking progress toward your current understanding of an ideal design. Always keep an open mind and look for indicators that your current design doesn't fit the problem well, and then start refactoring toward a more suitable design.