require 'digest/md5'
def crypt_private(password, setting)
  output = '*0'
  itoa64 = './0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
  output = '*1' if setting[0..1] == output
  return output if setting[0..2] != '$P$'
  salt = setting[4..11]
  return output if salt.length != 8

  count_log2 = itoa64.index(setting[3])
  return output if count_log2 < 7 || count_log2 > 30
  count = 1 << count_log2
  hash = Digest::MD5.digest("#{salt}#{password}");
  count.times do |i|
    hash = Digest::MD5.digest("#{hash}#{password}");
  end
  output = setting[0..11]
  output += b64encode(hash, 16, itoa64)
  return output == setting
end

def b64encode(input, count, itoa64)
  output = '';
  i = 0;
  ary = itoa64.split(//)
  while (i < count)
    value   = input[i];
    i += 1
    output += ary[value & 0x3f];
    value |= input[i] << 8 if i < count
    output += ary[(value >> 6) & 0x3f]
    i += 1
    return output if i >= count
    value |= input[i] << 16 if i < count
    output += ary[(value >> 12) & 0x3f]
    i += 1
    return output if i >= count
    output += ary[(value >> 18) & 0x3f]
  end
  return output
end

puts crypt_private("test-password", "$P$Bd68Jgt9eSsoeqqq9uOSviaJxBUBy./")