number_to_words.rb

require 'rspec'

NUMBERS = {
  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'
}

TENS = {
  2 => 'twenty',
  3 => 'thirty',
  4 => 'fourty',
  5 => 'fifty',
  6 => 'sixty',
  7 => 'seventy',
  8 => 'eighty',
  9 => 'ninety'
}

def number_to_words(num)
  return nil if num == 0
  return NUMBERS[num] if (1..15).include?(num)
  return "#{NUMBERS[num % 10]}teen" if (16..19).include?(num)
  return [ TENS[num / 10], NUMBERS[num % 10] ].join if num < 100
  return [ "#{NUMBERS[num / 100]} hundred", number_to_words(num % 100) ].compact.join(' and ') if num < 1000
  [ "#{NUMBERS[num / 1000]} thousand", number_to_words(num % 1000) ].compact.join(' ')
end

RSpec.describe 'number to words' do
  it 'converts 0 to nil' do
    expect(number_to_words(0)).to eq(nil)
  end

  NUMBERS.each_pair do |num, word|
    it "converts #{num} to #{word}" do
      expect(number_to_words(num)).to eq(word)
    end
  end

  it 'converts 16 to sixteen' do
    expect(number_to_words(16)).to eq('sixteen')
  end

  it 'converts 17 to seventeen' do
    expect(number_to_words(17)).to eq('seventeen')
  end

  TENS.each_pair do |multiple, word|
    it "converts #{multiple * 10} to #{word}" do
      expect(number_to_words(multiple * 10)).to eq(word)
    end

    it "converts #{multiple * 10 + 5} to #{word}five" do
      expect(number_to_words(multiple * 10 + 5)).to eq(word + 'five')
    end
  end

  it 'converts 100 to one hundred' do
    expect(number_to_words(100)).to eq('one hundred')
  end

  it 'converts 120 to one hundred and twenty' do
    expect(number_to_words(120)).to eq('one hundred and twenty')
  end

  it 'converts 125 to one hundred and twentyfive' do
    expect(number_to_words(125)).to eq('one hundred and twentyfive')
  end

  it 'converts 200 to two hundred' do
    expect(number_to_words(200)).to eq('two hundred')
  end

  it 'converts 250 to two hundred and fifty' do
    expect(number_to_words(250)).to eq('two hundred and fifty')
  end
  
   it 'converts 351 to three hundred and fiftyone' do
    expect(number_to_words(351)).to eq('three hundred and fiftyone')
  end

  it 'converts 1000 to one thousand' do
    expect(number_to_words(1000)).to eq('one thousand')
  end

  it 'converts 1276 to one thousand two hundred and seventysix' do
    expect(number_to_words(1276)).to eq('one thousand two hundred and seventysix')
  end
end