I have 3 different understandings of the target value:
1. Whole number of bits in all bytes to be changed to make bit substrings equal (any bit(s) of each byte can be changed);
2. The same as p. 1, but only last consecutive bits of byte (suffix) can be changed;
3. The number from the 0..8 range which is the minimum number of bits that can be changed in all bytes.
Tests do not clarify this. Which one is right?

It's allowed to change only last bit in every byte.

OK)), thanks!

Use the FFTW.