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 ## ==== condorcet.rb ====
22 ## This file contains Condorcet election methods. Currently this
23 ## includes a pure condorcet and a CloneproofSSD implementation
24 ## modeled after the Python-based Debian project election code and
25 ## that gives the same results in several tested corner cases.
26 #################################################################
28 ##################################################################
29 ## CondorcetVote Classes and SubClasses
31 ## The CondorcetVote class is subclassed by the PureCondorcetVote and
32 ## the CloneproofSSDVote classes but should not be used directly.
34 class CondorcetVote < ElectionVote
36 def initialize(votes=nil)
37 unless defined?(@candidates)
38 @candidates = Array.new
39 votes.each do |vote_row|
40 vote_row = vote_row.flatten if vote_row.class == Array
41 vote_row.each do |vote|
42 @candidates << vote unless @candidates.include?(vote)
49 def tally_vote(vote=nil)
51 vote.each_with_index do |winner, index|
52 if vote.flatten.length < @candidates.length
53 implied_losers = @candidates.select { |c| not vote.include?(c) }
54 vote.push(implied_losers)
56 if vote.length - 1 == index
59 losers = vote.flatten.last( vote.flatten.length - index - 1)
62 losers.each do |place|
63 place = [place] unless place.class == Array
67 @votes[winner] = Hash.new unless @votes.has_key?(winner)
68 @votes[loser] = Hash.new unless @votes.has_key?(loser)
70 if @votes[winner].has_key?(loser)
71 @votes[winner][loser] += 1
73 @votes[winner][loser] = 1
76 # make sure we have a comparable object
77 @votes[loser][winner] = 0 unless @votes[loser].has_key?( winner )
88 def verify_vote(vote=nil)
89 vote.instance_of?( Array ) and
95 class PureCondorcetVote < CondorcetVote
96 def resultFactory(init)
97 PureCondorcetResult.new(init)
101 class CloneproofSSDVote < CondorcetVote
102 def resultFactory(init)
103 CloneproofSSDResult.new(init)
107 top_result = resultFactory( self )
108 until @candidates.empty?
109 aResult = resultFactory( self )
110 top_result.full_results << aResult
119 ##################################################################
120 ## CondorcetResult Classes and SubClasses
122 ## The CondorcetResult class is subclassed by the PureCondorcetResult
123 ## and the CloneproofSSDResult classes but should not be used
126 class CondorcetResult < ElectionResult
127 def initialize(voteobj=nil)
128 unless voteobj and voteobj.kind_of?( CondorcetVote )
129 raise ArgumentError, "You must pass a CondorcetVote array.", caller
135 def defeats(candidates=nil, votes=nil)
136 candidates = @election.candidates unless candidates
137 votes = @election.votes unless votes
140 candidates = [candidates] unless candidates.class == Array
141 candidates.each do |candidate|
142 candidates.each do |challenger|
143 next if candidate == challenger
144 if votes[candidate][challenger] > votes[challenger][candidate]
145 defeats << [ candidate, challenger ]
155 class PureCondorcetResult < CondorcetResult
156 def initialize(voteobj=nil)
163 votes = @election.votes
164 candidates = @election.candidates
167 candidates.each do |candidate|
168 victors[candidate] = Array.new
171 self.defeats.each do |pair|
172 winner, loser = *pair
173 victors[winner] << loser
176 winners = @election.candidates.find_all do |candidate|
177 victors[candidate].length == @election.candidates.length - 1
180 @winners << winners if winners.length == 1
184 class CloneproofSSDResult < CondorcetResult
185 def initialize(voteobj=nil)
187 @winners = self.cpssd()
192 votes = @election.votes
193 candidates = *@election.candidates
195 def in_schwartz_set?(candidate, candidates, transitive_defeats)
196 candidates.each do |challenger|
197 next if candidate == challenger
199 if transitive_defeats.include?( [ challenger, candidate ] ) and
200 not transitive_defeats.include?( [ candidate, challenger ] )
209 # see the array with the standard defeats
210 transitive_defeats = self.defeats(candidates, votes)
212 candidates = [candidates] unless candidates.class == Array
213 candidates.each do |cand1|
214 candidates.each do |cand2|
215 candidates.each do |cand3|
216 if transitive_defeats.include?( [ cand2, cand1 ] ) and
217 transitive_defeats.include?( [ cand1, cand3 ] ) and
218 not transitive_defeats.include?( [ cand2, cand3 ] ) and
220 transitive_defeats << [ cand2, cand3 ]
226 schwartz_set = Array.new
227 candidates.each do |candidate|
228 if in_schwartz_set?(candidate, candidates, transitive_defeats)
229 schwartz_set << candidate
233 candidates = schwartz_set
235 # look through the schwartz set now for defeats
236 defeats = self.defeats(candidates, votes)
238 # it's a tie or there's only one option
239 break if defeats.length == 0
241 def is_weaker_defeat?( pair1, pair2 )
242 votes = @election.votes
243 if votes[pair1[0]][pair1[1]] > votes[pair2[0]][pair2[1]]
245 elsif votes[pair1[0]][pair1[1]] == votes[pair2[0]][pair2[1]] and
246 votes[pair1[1]][pair1[0]] > votes[pair2[1]][pair2[0]]
253 defeats.sort! do |pair1, pair2|
254 if is_weaker_defeat?( pair1, pair2 )
256 elsif is_weaker_defeat?( pair2, pair1 )
263 votes[defeats[0][0]][defeats[0][1]] = 0
264 votes[defeats[0][1]][defeats[0][0]] = 0