gistfile1.rb

POPULATION_SIZE = 50
NUM_BITS = 64
GENERATIONS = 1000
SAMPLE_SIZE = 100

CROSSOVER_PROBABILITY = 0.7
MUTATION_RATE = 0.001

class Chromosome
  
  attr_accessor :genes

  def initialize(genes = "")
    if genes == ""
      self.genes = (1..NUM_BITS).map{ rand(2) }.join
    else
      self.genes = genes
    end
  end
  
  def to_s
    genes.to_s
  end
  
  def count
    genes.count
  end
  
  def fitness
    genes.count("1")
  end
  
  def mutate!
    mutated = ""
    0.upto(genes.length - 1).each do |i|
      allele = genes[i, 1]
      if rand <= MUTATION_RATE
        mutated += (allele == "0") ? "1" : "0"
      else
        mutated += allele
      end
    end
  
    self.genes = mutated    
  end
  
  def crossover_with(other)
    locus = rand(genes.length) + 1

    child1 = genes[0, locus] + other.genes[locus, other.genes.length]
    child2 = other.genes[0, locus] + genes[locus, other.genes.length]

    return [
      Chromosome.new(child1),
      Chromosome.new(child2),
    ]
  end
end

class Population
  
  attr_accessor :chromosomes
  
  def initialize
    self.chromosomes = Array.new
  end
  
  def inspect
    chromosomes.join(" ")
  end
  
  def seed!
    chromosomes = Array.new
    1.upto(POPULATION_SIZE).each do
      chromosomes << Chromosome.new
    end

    self.chromosomes = chromosomes    
  end
  
  def count
    chromosomes.count
  end
  
  def fitness_values
    chromosomes.collect(&:fitness)
  end
  
  def total_fitness
    fitness_values.inject{|total, value| total + value }
  end
  
  def max_fitness
    fitness_values.max
  end
  
  def average_fitness
    total_fitness.to_f / chromosomes.length.to_f
  end
  
  def select
    rand_selection = rand(total_fitness)

    total = 0
    chromosomes.each_with_index do |chromosome, index|
      total += chromosome.fitness
      return chromosome if total > rand_selection || index == chromosomes.count - 1
    end
  end
end

population = Population.new
population.seed!

1.upto(GENERATIONS).each do |generation|

  offspring = Population.new
  while offspring.count < population.count
    parent1 = population.select
    parent2 = population.select

    if rand <= CROSSOVER_PROBABILITY
      child1, child2 = parent1.crossover_with(parent2)
    else
      child1 = parent1
      child2 = parent2
    end

    child1.mutate!
    child2.mutate!

    offspring.chromosomes << child1 << child2
  end

  puts "Generation: #{generation} - Average: #{population.average_fitness} - Max: #{population.max_fitness}"

  population = offspring
end

puts "Final population: " + population.inspect