But C++11 solution is not functional. It uses state variables: look at the while loop, it updates variable. Look at the next_forests.reserve, std::back_inserter. These are not clear functional constructs. Of course it is possible to model memory state with the monads :) but... The C++ code is quite different from other codes. So i was surprised by Java which is only 3 times slower than C++ while running a much less effective code
Glad someone pointed that out. It is cheating. It is doing a lot of state transformations like in-place sorting / filtering. I'd like to see some real C++ functional code here, using immutable data structures, not an imperative program using lambdas. I guess it would be both much harder to write (C++ is not really a functional language) and much slower, because dynamic memory allocation in C++ is more costly than in Java.
Isn't this just evidence of how broad of a definition "functional" programming is? It is the scottsman of programming debates. (Though, I suspect any "paradigm" example will fall into this trap.)
