Selection in a moth population

A wood has dark moths and pale moths. Birds eat more of one kind than the other. You will write the three steps that follow the moths from generation to generation.

The idea. Each kind keeps only the survivors, and the population is then scaled back to its old size. The kind that survives better becomes more common.

Your tasks.

1. Finish survivors. It gives how many moths of one kind survive.
2. Finish next_generation. It gives the new numbers of dark and pale moths, with the total kept the same.
3. Finish generations_to_half. It counts the generations until the dark moths outnumber the pale moths.