password_generator.rb

# ・8-18文字であること
# ・大文字小文字アルファベットと数字が含まれていること
# ・.@_- (ドット、アットマーク、アンダースコア、ハイフン)のうち少なくとも1
# つ含まれていること

class PasswordGeneratorAttributeError < StandardError; end
class PasswordGenerator
  NUMBERS = [*0..9]
  UPPER_CHARS = [*'A'..'Z']
  LOWER_CHARS = [*'a'..'z']
  SYMBOLS = ['.', '@', '_', '-']

  MAX_LENGTH = 18
  MIN_LENGTH = 8

  attr_accessor :length, :max_syms, :max_nums

  def initialize(length = 8)
    @length = length

    if length < MIN_LENGTH || length > MAX_LENGTH
      raise PasswordGeneratorAttributeError,
            "Password length should be in between #{MIN_LENGTH} and #{MAX_LENGTH}."
    end

    @max_syms = (length * 0.25).to_i
    @max_nums = (length * 0.25).to_i
  end

  def self.new_password(length)
    new(length).generate
  end

  def generate
    (SYMBOLS.sample(sym_length) +
     NUMBERS.sample(num_length) +
     UPPER_CHARS.sample(upper_char_length) +
     LOWER_CHARS.sample(lower_char_length)
    ).shuffle.join
  end

  private

  def sym_length
    if length - 3 < max_syms
      raise PasswordGeneratorAttributeError, 'max_syms too long'
    end

    @sym_length ||= rand(max_syms - 1) + 1
  end

  def num_length
    if length - 3 < max_nums
      raise PasswordGeneratorAttributeError, 'max_nums too long'
    end

    @num_length ||= rand(max_nums - 1) + 1
  end

  def upper_char_length
    if length - 2 < max_syms + max_nums
      raise PasswordGeneratorAttributeError, 'max_syms + max_nums too long'
    end

    length_left = length - sym_length - num_length
    @upper_char_length ||= length_left - (rand(length_left - 2) + 1)
  end

  def lower_char_length
    @lower_char_length ||= length - sym_length - num_length - upper_char_length
  end
end

pass = PasswordGenerator.new_password(8)
p pass
# => "EbVG7.Nq"

# or

pwgen = PasswordGenerator.new
pwgen.length = 10
pwgen.max_syms = 4
pwgen.max_nums = 4
p pwgen.generate
#=> "D-TH4q@_Gf"