Advent of Code, day 11

day11.rb

#!/usr/bin/env ruby

# http://adventofcode.com/day/11

INPUT = "hepxcrrq"

THREE_LETTER_RUN = Regexp.union(("a".."z").each_cons(3).map(&:join))
ILLEGAL_LETTER = /[iol]/

def unique_pairs(s)
  s.scan(/(.)\1/).uniq
end

def good_password?(password)
  unique_pairs(password).count > 1 &&
    password =~ THREE_LETTER_RUN &&
    password !~ ILLEGAL_LETTER
end

def next_good_password(password)
  loop do
    password = password.succ
    return password if good_password?(password)
  end
end

password = INPUT
2.times do
  password = next_good_password(password)
  puts password
end