http://en.wikipedia.org/wiki/Alphametics

alphametics.rb

require 'set'

DIGITS = ('0'..'9').to_a

def solve alphametics
  firsts, rests = Set.new, Set.new

  alphametics.scan(/\w+/) do |w|
    firsts << w[0]
    rests += w[1..-1].chars
  end

  chars = firsts + rests
  fail "chars.size > 10" if chars.size > 10
  size = firsts.size

  DIGITS.permutation(chars.size).each do |p|
    begin
      am = alphametics.dup
      chars.zip(p).each {|s, d| am.tr!(s, d)}
      return am if eval(am)
    end unless p[0..size].include?('0')
  end
  fail "unsolvable: #{alphametics}"
end

puts solve("mad * man == asylum")