euler/ruby/euler051.rb

require_relative 'euler'

def bitmasks(length)
  (1..2**length-1).map { |n| n.to_s(2) }.map { |s| s.rjust(4, '0')}.map { |s| s.split('').map(&:to_i)}
end

def prime_digit_replacement(number, mask)
  digits = number.to_digit_list
  related_primes = []
  ('0'..'9').each do |j|
    copied_digit_list = Array.new(digits)
    mask.each_with_index do |bit, index|
      copied_digit_list[index] = j if bit == 1
    end
    new_num = Euler.from_digit_list(copied_digit_list)
    related_primes.push(new_num) if new_num.prime? and !related_primes.include?(new_num) and new_num.to_digit_list.count == digits.count
  end
  related_primes
end

def solution(family_count)
  result = false
  Prime.take_while do |p|
    digits = p.to_digit_list
    bitmasks(digits.count).each do |mask|
      related_primes = prime_digit_replacement(p, mask)
      if related_primes.count == family_count and related_primes.include?(p)
        result = p
        break
      end
    end
    result == false
  end
  result
end