ages.rb

require 'active_support'
require 'active_model'
require 'pry'
require 'awesome_print'

AwesomePrint.pry!

class Ages
  extend ActiveModel::Naming
  include ActiveModel::Validations

  attr_reader :m1, :m2, :m3
  attr_reader :f1, :f2, :f3

  PAIRS = {
    m1: :f3,
    m2: :f1,
    m3: :f2
  }

  validate :ages_constraints

  def initialize
    @m1, @m2, @m3 = [5, 5, 5]
    @f1, @f2, @f3 = [0, 0, 0]
  end

  def set hash
    changes = []

    hash.each do |key, value|
      next unless /[mf][1-3]/ === key.to_s
      instance_variable_set "@#{key}", value.to_i

      if /m/ === key.to_s
        opposite_key = PAIRS[key]
        instance_variable_set "@#{opposite_key}", send(key) - 5
        changes << key.to_sym
      else
        opposite_key = PAIRS.invert[key]
        instance_variable_set "@#{opposite_key}", send(key) + 5
        changes << opposite_key
      end
    end

    unchanged = PAIRS.keys - changes.uniq

    if unchanged.size == 1
      sum = changes.uniq.map(&method(:send)).reduce &:+
      set unchanged[0] => (83 - sum)
    end

    self
  end

  def ages_constraints
    errors.add :base, 'male_should_be_older_5'    unless male_older_5?
    errors.add :base, 'm1 == f2 + 1'              unless constraint_1?
    errors.add :base, 'm1 + f1 == 48'             unless constraint_2?
    errors.add :base, 'm3 + f1 == 52'             unless constraint_3?
    errors.add :base, 'Males ages should be 83'   unless males_equal?
    errors.add :base, 'Females ages should be 68' unless females_equal?
  end

  def male_older_5?
    PAIRS.all? do |male, female|
      send(male) == send(female) + 5
    end
  end

  def constraint_1?
    m1 == f2 + 1
  end

  def constraint_2?
    m1 + f1 == 48
  end

  def constraint_3?
    m3 + f1 == 52
  end

  def males_equal?
    PAIRS.keys.map(&method(:send)).reduce(&:+) == 83
  end

  def females_equal?
    PAIRS.values.map(&method(:send)).reduce(&:+) == 68
  end
end

q = Ages.new

50.times do |m1|
  50.times do |m2|
    print '.'
    q.set m1: m1, m2: m2
    break if q.valid?
  end
  break if q.valid?
end

puts
puts q.valid?
puts q.errors.full_messages
puts q.inspect

__END__

OUTPUT:
................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
true
#<Ages:0x007f9cca1dabf0 @m3=30, @m2=27, @m1=26, @f1=22, @f2=25, @f3=21, @validation_context=nil, @errors=#<ActiveModel::Errors:0x007f9cca2f9130 @base=#<Ages:0x007f9cca1dabf0 ...>, @messages={}>>
[Finished in 0.3s]