voting_systems

Ruby gem implementing various voting systems.

Each voting system (e.g. borda) takes a set of preferences as input. Each preference is a ranking of alternatives. The output is the winning alternative(s).

Ties and omissions are allowed in the preferences. Omitted alternatives are considered least preferred.

All of the systems are deterministic. There is no random tie-breaking. If the result is a tie, all of the tied alternatives will be returned.

Systems

method name	description
baldwin	Baldwin method
borda	Borda count
bucklin	Bucklin voting
coombs	Coombs' method
copeland	Copeland's method
instant_runoff	Instant-runoff voting
ranked_pairs	Ranked pairs

Installation

gem install voting_systems

Usage

require 'voting_systems'

There are two input formats. The first looks like:

votes = '4:A>B>C
         5:B>C>A
         2:C=A>B'

This means 4 voters have the preference A>B>C, 5 voters have B>C>A, and 2 voters have C=A>B (meaning that C and A are equally preferred, and both are preferred to B).

The second looks like:

votes = [
  [4, ['A','B','C']],
  [5, ['B','C','A']],
  [2, [['C','A'],'B']],
]

Here, the alternatives need not be strings, but can be any object.

To find the winner(s) for a given system (e.g. borda):

winners = borda votes
=> ["B"]

Author

Tim Smith (timlabs.org)