GA hello world (ruby) -- http://www.generation5.org/content/2003/gahelloworld.asp

ga-hello-world.rb

#!/usr/bin/env ruby

POPSIZE = 2048
MAXITER = 16384
ELITRATE = 0.10
MUTATIONRATE = 0.25
TARGET = "Hello world!"

class Ga
  attr_accessor :str, :fitness
end

def init_pop
  pop = Array.new
  POPSIZE.times do |i|
    citizen = Ga.new
    citizen.fitness = 0
    citizen.str = ""
    TARGET.length.times do |j|
      citizen.str += (32 + Random.rand(90)).chr
    end
    pop.push citizen
  end
  return pop
end

def calc_fitness(pop)
  pop.each do |citizen|
    citizen.fitness = 0
    TARGET.length.times do |i|
      citizen.fitness += (TARGET[i].ord - citizen.str[i].ord).abs
    end
  end
end

def sort_by_fitness(pop)
  pop.sort { |a,b| a.fitness <=> b.fitness }
end

def elitism(pop)
  return pop[0, (ELITRATE*POPSIZE).to_i]
end

def mutate(citizen)
  ipos = Random.rand(TARGET.length)
  delta = Random.rand(90) + 32
  citizen.str[ipos] = ((citizen.str[ipos].ord + delta) % 122).chr
end

def mate(pop)
  buf = elitism(pop)
  (pop.length - buf.length).times do |i|
    one = Random.rand(POPSIZE / 2)
    two = Random.rand(POPSIZE / 2)
    spos = Random.rand(TARGET.length)
    citizen = Ga.new
    citizen.fitness = 0
    citizen.str = pop[one].str[0, spos] + pop[two].str[spos, TARGET.length - spos]

    mutate(citizen) if rand() < MUTATIONRATE
    buf.push citizen
  end
  buf
end

# main

pop = init_pop
MAXITER.times do |i|
  calc_fitness pop
  pop = sort_by_fitness(pop)
  puts "Best: #{pop[0].str} (#{pop[0].fitness})"
  break if pop[0].fitness == 0
  pop = mate(pop)
end