Swapotron

Swapotron is a gem for reallocating stuff based on preferences.

It uses the Top Trading Cycle algorithm to generate a stable allocation of resources, which means that:

• there are no further swaps that would make both sides happier
• nothing can be gained by misrepresenting your preferences

Installation

NOTE: this is not published to RubyGems yet.

Add this line to your application's Gemfile:

gem 'swapotron'

And then execute:

$ bundle

Or install it yourself as:

$ gem install swapotron

Usage

Initialise the swapper with a hash:

# Alice owns item 1, Bob owns item 2, Carol owns item 3
swapper = Swapotron::Swapper.new(alice: 1, bob: 2, carol: 3)

If a person isn't happy with the thing they have, set their preferred items:

swapper.set_preferences(:alice, [2, 3])
swapper.set_preferences(:bob, [1, 3])
swapper.set_preferences(:carol, [2, 1])

Call #proposed_swaps at any time to get possible swaps that are in line with the preferences.

swapper.proposed_swaps
=> [[:bob, :alice]]

Call #swap with an array of people to swap their items:

# Alice and Bob's preferences will be cleared.
swapper.swap([:alice, :bob])
=> {:alice=>2, :bob=>1, :carol=>3}

# There are no more proposed swaps because nobody wants to swap with Carol
swapper.proposed_swaps
=> []

Swapping items clears all preferences related to the swap, so even if the swap was not a recommended one, Swapotron will not suggest further swaps that undo it.

If the array is more than two items, the items get rotated between people:

swapper.swap([:alice, :bob, :carol])
=> {:alice=>1, :bob=>3, :carol=>2}

Calling #swap_all will automatically swap items until no more swaps are proposed.

swapper = Swapotron::Swapper.new(alice: 1, bob: 2, carol: 3)
swapper.set_preferences(:alice, [2, 3])
swapper.set_preferences(:bob, [1, 3])
swapper.set_preferences(:carol, [2, 1])
swapper.swap_all
=> {:alice=>2, :bob=>1, :carol=>3}

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome.

License

The gem is available as open source under the terms of the MIT License.