Representing negative numbers in Ruby

This repo accompanies the article “Representations count”.

It contains a challenge: how would you add support for negative numbers on a (hypothetical) computer which only has natural numbers built in?

In practice, that means implementing the constructor, factory methods and instance methods of the SignedNumber class. That class includes a refinement which discourages you from accidentally using negative numbers.

Getting started

1. git clone https://github.com/tomstuart/negative-numbers.git
2. cd negative-numbers
3. bundle install
4. bundle exec rspec
5. Edit lib/signed_number.rb to make the next failing test pass
6. If more failing tests remain, go to 4
7. Celebrate! 🎉

Two solutions

One way to solve this challenge is to represent a signed number as the combination of a “sign” (positive or negative) and a “size” (absolute magnitude). See the sign-and-size branch for a worked solution.

An alternative is to represent a signed number as the difference between two natural numbers. See the left-and-right branch for a worked solution.

The first representation is easier to understand, but the second is arguably easier to implement correctly.