Roman Numerals

main.rb

#!/usr/bin/env ruby

########## GRAB AND VALIDATE INPUT ##########
num_str = STDIN.gets.chomp
if not (1..4999).include? num_str.to_i
  warn "number must be between 1 and 4999 (inclusive)"
  exit 1
end

########## PAIR UP DIGITS WITH NUMERALS ##########
# digits from the input, as numbers, lowest value first
digits = num_str.each_char.map(&:to_i).reverse
# each digit can be resolved independantly, given the ones, fives, and tens
# numerals for that digit's place
numerals = [
  ['I', 'V', 'X'],
  ['X', 'L', 'C'],
  ['C', 'D', 'M'],
  ['M',  '',  '']
]

########## MAIN ALGORITHM ##########
# pair each digit with the numerals need to resolve it
result = digits.zip(numerals).reduce('') do |result, (digit, (one, five, ten))|
  # 0..4 is nothing, 4..8 is the fives numeral, 9 is the tens numeral
  bases = ['', five, ten]
  base = bases[(digit+1) / 5]
  # avoid adding nil to strings
  # pre and post will both be blank if mod_five is 0
  pre = post = ''
  # we use `digit % 5` in a few places, so let's calculate it once
  mod_five = digit % 5
  if mod_five == 4
    # prepend a ones numeral for 4 and 9
    pre = one
  elsif mod_five > 0 or one == 'M'
    # this will match 1..3, or anything if we're on the thousands place
    # we need this special case to get 'MMMM'
    post = one * mod_five
  end
  # prepend to the result since this loop starts with the lowest value first
  pre + base + post + result
end

########## PRINT THE RESULT ##########
puts result