NumberLetterCounts Kata (Project Euler)

README.txt

If the numbers 1 to 5 are written out in words: one, two, three, four, five
then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.

If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?


NOTE:
Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of "and" when writing out numbers is in compliance with British usage.

numberlettercounts.rb

class NumberLetterCounts
  def self.calc(first, last)
    count = 0

    (first..last).each do |number|
      str_number = number_to_string(number)
      count += str_number.length
    end

    count
  end

  def self.number_to_string(number)
    NumberString.new(number).to_s
  end

  class NumberString
    NUMBER_TO_STRING = {
      1 => "one",
      2 => "two",
      3 => "three",
      4 => "four",
      5 => "five",
      6 => "six",
      7 => "seven",
      8 => "eight",
      9 => "nine",
      10 => "ten",
      11 => "eleven",
      12 => "twelve",
      13 => "thirteen",
      14 => "fourteen",
      15 => "fifteen",
      16 => "sixteen",
      17 => "seventeen",
      18 => "eighteen",
      19 => "nineteen",
      20 => "twenty",
      30 => "thirty",
      40 => "forty",
      50 => "fifty",
      60 => "sixty",
      70 => "seventy",
      80 => "eighty",
      90 => "ninety",
    }

    attr_reader :int, :parts

    def initialize(int)
      @int = int
      @parts = int.to_s.split('').reverse.map(&:to_i)
    end

    def thousands
      "#{NUMBER_TO_STRING[parts[3]]} thousand" if parts.length > 3
    end

    def hundreds
      "#{NUMBER_TO_STRING[parts[2]]} hundred" if (parts.length > 2 && parts[2] != 0)
    end

    def tens
      tens = parts[1] * 10
      units = parts[0]

      if ((tens + units) > 10 && (tens + units) < 20)
        return NUMBER_TO_STRING[tens + units] if NUMBER_TO_STRING.has_key?(tens + units)
      end
      NUMBER_TO_STRING[tens] if parts.length > 1
    end

    def units
      tens = parts[1] * 10
      units = parts[0]

      NUMBER_TO_STRING[parts[0]] unless ((tens + units) > 10 && (tens + units) < 20)
    end

    def to_s
      return NUMBER_TO_STRING[int] if NUMBER_TO_STRING.has_key?(int)
      "#{thousands} #{hundreds} #{tens} #{units}".strip.gsub(/\s+/, " ")
    end
  end
end

numberlettercounts_spec.rb

require 'rspec'
require 'numberlettercounts'

describe NumberLetterCounts do
  describe ".calc" do
    it 'should return 19 for 1 - 5' do
      described_class.calc(1, 5).should eq 19
    end

    it 'should return 74 for 1 - 15' do
      described_class.calc(1, 15).should eq 74
    end

    it 'should return 21098 for 1 - 1000' do
      described_class.calc(1, 1000).should eq 21098
    end
  end

  describe ".number_to_string" do
    it 'should return "twenty three" for 23' do
      described_class.number_to_string(23).should eq "twenty three"
    end

    it 'should return "seventy five" for 75' do
      described_class.number_to_string(75).should eq "seventy five"
    end

    it 'should return "one hundred fifty five" for 155' do
      described_class.number_to_string(155).should eq "one hundred fifty five"
    end

    it 'should return "one thousand one hundred fifty five" for 1155' do
      described_class.number_to_string(1155).should eq "one thousand one hundred fifty five"
    end
  end
end