Function abc::scaling::rank

pub fn rank() -> Box<ScalingFunction>

Chooses solutions according to their rank.

Rather than use the fitness directly, this formula ranks the N solutions 1 to N, in ascending order of fitness, then chooses in proportion to the value of the rank. So, the solution with rank 6 will be chosen twice as often as the solution with rank 3, and three times as often as the solution with rank 2.

scaled_i = rank_i