24.rb

#!/usr/bin/env ruby

OPS = [:+, :-, :*, :/]

ALL_OPS = OPS.product(OPS).product(OPS).map do |n| n.flatten end
ALL_FOUR_CARDS = (0..9999).map do |n|
  a = n % 10 + 1
  b = n/10 % 10 + 1
  c = n/100 % 10 + 1
  d = n/1000 % 10 + 1
  [a,b,c,d].sort
end
UNIQUE_FOUR_CARDS = ALL_FOUR_CARDS.uniq

def findsol cards
  cards.permutation.each do |cards|
    a, b, c, d = *cards
    ALL_OPS.each do |o1, o2, o3|
      begin
        x = a.to_f.send(o1, b)
        y = c.to_f.send(o2, d)
        z = x.send(o3, y)
        return "(#{o3}(#{o1}#{a} #{b})(#{o2} #{c} #{d}))" if z == 24
      rescue ZeroDivisionError
      end
    end
  end
  nil
end

def repr card
  sprintf '%2d', card
end

ALL_FOUR_CARDS.each do |cards|
  sol = findsol cards
  r = cards.map { |c| repr c }.join ', '
  if sol
    puts "#{r}: #{sol}"
  else
    puts "#{r}: nothing"
  end
end