hexadecimal

hexadecimal.rb

def hexadecimal(hex_number)
  hex = 0
  letters = { "a" => 10, "b" => 11, "c" => 12, "d" => 13, "e" => 14, "f" => 15}
  
  hex_number = hex_number.split("").map do |digit|
    if letters.keys.include?(digit)
      letters[digit] 
    else
      digit
    end
  end

  hex_number.reverse.each_with_index do |digit, index|
    hex += digit.to_i * 16 ** index
  end
  hex
end  

puts hexadecimal("-1b3a")

hexadecimal_spec.rb

require 'rspec'
require 'hexadecimal'

describe 'hexadecimal' do
  it 'translates digits' do
    hexadecimal('0').should eq 0
    hexadecimal('2').should eq 2
  end

  it 'translates a-f' do
    hexadecimal('a').should eq 10
  end

  it 'raises an exception for negative values' do
    expect {hexadecimal('-1b3a')}.to raise_exception
  end
end