Convert arabic to roman numerals

roman_numerals.rb

def possible_reps(input)
  lambda {|k,_| k <= input}
end

def build_roman(input)
  lambda do |result, (arabic, roman)|
    if (input >= arabic)
      quotient, input = input.divmod(arabic)
      result << roman*quotient
    end
    result
  end
end

def romanize(input)
  arabic_roman = {
    1000 => 'M',
    900  => 'CM',
    500  => 'D',
    400  => 'CD',
    100  => 'C',
    90   => 'XC',
    50   => 'L',
    40   => 'XL',
    10   => 'X',
    9    => 'IX',
    5    => 'V',
    4    => 'IV',
    1    => 'I'
  }.freeze

  arabic_roman.select( &possible_reps(input) ).reduce( [], &build_roman(input) ).join
end