ruby's string class extension conting the length of multi-byte string as 2

string.rb

class String
  # number of characters where the multi-byte string is counted by 2
  def mbsize
    chars.sum{|str| str.bytesize == 1 ? 1 : 2 }
  end

  def mbljust(num, char=" ")
    return self if mbsize > num

    self + char * (num - mbsize)
  end

  def mbrjust(num, char = " ")
    return self if mbsize > num

    char * (num - mbsize) + self
  end

  def mbcenter(num, char = " ")
    return self if mbsize > num

    padsize = num - mbsize
    half = char * (padsize / 2)
    padded = half + self + half

    padsize.even? ? padded : padded + char
  end
end

p "北海道".mbljust(11)
p "会津若松市".mbljust(11)
p '🐧🐰'.mbljust(11)
p '🐧🐰'.mbrjust(11)
p '🐧1🐰'.mbcenter(11)
p '1'.mbcenter(11)

# =>
# "北海道     "
# "会津若松市 "
# "🐧🐰       "
# "       🐧🐰"
# "   🐧1🐰   "
# "     1     "