1 # election library -- a ruby library for elections
2 # copyright © 2005 MIT Media Lab and Benjamin Mako Hill
4 # This program is free software; you can redistribute it and/or modify
5 # it under the terms of the GNU General Public License as published by
6 # the Free Software Foundation; either version 2 of the License, or
7 # (at your option) any later version.
9 # This program is distributed in the hope that it will be useful, but
10 # WITHOUT ANY WARRANTY; without even the implied warranty of
11 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 # General Public License for more details.
14 # You should have received a copy of the GNU General Public License
15 # along with this program; if not, write to the Free Software
16 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
19 #################################################################
20 ## ==== election.rb ====
22 ## This file contains the core ElectionVote and ElectionResults
23 ## classes and the most common and simple election methods including
24 ## plurality and approval voting.
25 #################################################################
27 ##################################################################
28 ## ElectionVote Classes and SubClasses
30 ## There classes are used to store, verify, and "tally" (i.e. count
31 ## votes for the standard Election superclass and for the most common
32 ## types of elections.
36 attr_reader :candidates
38 def initialize(votes=nil)
39 @votes = Hash.new unless defined?(@votes)
40 @candidates = Array.new unless defined?(@candidates)
43 if votes.instance_of?( Array )
45 self.tally_vote(vote) if self.verify_vote(vote)
48 raise ElectionError, "Votes must be in the form of an array.", caller
54 # by default, this is set to look if the vote is defined. it should
55 # be overridden in each class
56 def verify_vote(vote=nil)
60 # by default, this does nothing. it must be redefined in any subclass
62 self.verify_vote(vote)
65 def filter_out(winner)
66 if winner.winners[0].class == Array
67 to_filter = winner.winners[0]
69 to_filter = [winner.winners[0]]
71 @candidates.delete_if {|x| to_filter.include?(x)}
76 class PluralityVote < ElectionVote
78 PluralityResult.new(self)
82 def verify_vote(vote=nil)
83 vote.instance_of?( String )
86 def tally_vote(candidate)
87 if @votes.has_key?(candidate)
88 @votes[candidate] += 1
91 @candidates << candidate
96 class ApprovalVote < PluralityVote
98 ApprovalResult.new(self)
102 def verify_vote(vote=nil)
103 vote.instance_of?( Array ) and vote.length >= 1
106 def tally_vote(approvals)
107 approvals.each {|candidate| super(candidate)}
112 ##################################################################
113 ## Election Result Classes
116 ## There classes are used to compute and report the results of an
117 ## election. In almost all cases, these will be returned by the
118 ## #results method of a corresponding ElectionVote subclass.
122 attr_accessor :full_results
124 def initialize(voteobj=nil)
125 unless voteobj and voteobj.kind_of?( ElectionVote )
126 raise ArgumentError, "You must pass a ElectionVote array.", caller
131 @full_results = Array.new
135 @winners[0] if @winners.length > 0
143 @full_results.collect {|x| x.winners}
148 class PluralityResult < ElectionResult
149 attr_reader :ranked_candidates
151 def initialize(voteobj=nil)
154 votes = @election.votes
155 candidates = @election.candidates
157 @ranked_candidates = votes.sort do |a, b|
159 end.collect {|a| a[0]}
161 # winners are anyone who has the same number of votes as the
163 @winners = @ranked_candidates.find_all do |i|
164 votes[i] == votes[@ranked_candidates[0]]
169 # this class is complete because results for approval are computed
170 # identically to results from plurality
171 class ApprovalResult < PluralityResult
174 class ElectionError < ArgumentError