I disagree with you here. TDD does require evolutionary path for the entire system, but the minimum unit is a feature that is expected to be fully specified and implemented to pass the first test. You cannot evolve a data structure or an algorithm with TDD, because TDD by original definition allows only refactoring, not re-engineering (I.e. if your feature is addition, writing „return 4“ to pass test2plus2 isn’t meaningful TDD). So in your case properly applied TDD would require fully implemented structure or at least an atomic part of it within the known design to pass the first test (e.g. testing Feistel round function in an encryption algorithm is ok, but you will know the design of the entire algorithm from the start).