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 |winners, index|
52 if vote.flatten.length < @candidates.length
53 implied_losers = @candidates.select { |c| not vote.flatten.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
66 winners = [winners] unless winners.class == Array
67 next if winners.include?(loser)
68 winners.each do |winner|
69 @votes[winner] = Hash.new unless @votes.has_key?(winner)
70 @votes[loser] = Hash.new unless @votes.has_key?(loser)
72 if @votes[winner].has_key?(loser)
73 @votes[winner][loser] += 1
75 @votes[winner][loser] = 1
78 # make sure we have a comparable object
79 @votes[loser][winner] = 0 unless @votes[loser].has_key?( winner )
88 def verify_vote(vote=nil)
89 vote.instance_of?( Array ) and
94 class PureCondorcetVote < CondorcetVote
96 PureCondorcetResult.new(self)
100 class CloneproofSSDVote < CondorcetVote
102 CloneproofSSDResult.new(self)
107 ##################################################################
108 ## CondorcetResult Classes and SubClasses
110 ## The CondorcetResult class is subclassed by the PureCondorcetResult
111 ## and the CloneproofSSDResult classes but should not be used
114 class CondorcetResult < ElectionResult
117 def initialize(voteobj=nil)
118 unless voteobj and voteobj.kind_of?( CondorcetVote )
119 raise ArgumentError, "You must pass a CondorcetVote array.", caller
122 @matrix = voteobj.votes
125 def victories_and_ties
127 candidates = @matrix.keys.sort
129 candidates.each do |candidate|
130 candidates.each do |challenger|
131 next if candidate == challenger
132 diff = @matrix[candidate][challenger] - @matrix[challenger][candidate]
133 victories_ties[candidate] = {} unless victories_ties.key?(candidate)
135 victories_ties[candidate][challenger] = diff
140 return victories_ties
143 def ranked_candidates
144 if not defined?(@ranked_candidates)
145 @ranked_candidates = build_ranked_candidates()
152 def defeats(candidates=nil, votes=nil)
153 candidates ||= @election.candidates || []
154 # we're assumign that if there are candidates, there must be at
155 # least one vote for them
156 votes ||= @election.votes
159 candidates.each do |candidate|
160 candidates.each do |challenger|
161 next if candidate == challenger
162 if votes[candidate][challenger] > votes[challenger][candidate]
163 defeats << [ candidate, challenger ]
171 def build_ranked_candidates
172 # build a lis of ranked candidates by dropping the winner and
175 ranked_candidates = []
178 candidates = self.election.candidates
180 until candidates.empty?
181 ranked_candidates << resultobj.winners
183 new_voteobj = resultobj.election.dup
184 candidates = new_voteobj.candidates
185 new_voteobj.candidates.delete_if {|x| resultobj.winners.include?(x)}
186 resultobj = new_voteobj.result
194 class PureCondorcetResult < CondorcetResult
195 def initialize(voteobj=nil)
203 votes = @election.votes
204 candidates = @election.candidates
206 unless votes.length > 0 and candidates.length > 0
211 candidates.each do |candidate|
212 victors[candidate] = Array.new
215 self.defeats.each do |pair|
216 winner, loser = *pair
217 victors[winner] << loser
222 winners = @election.candidates.find_all do |candidate|
223 victors[candidate].length == @election.candidates.length - victory_margin
225 if winners.length > 0
235 class CloneproofSSDResult < CondorcetResult
236 def initialize(voteobj=nil)
238 @winners = self.cpssd()
244 votes = @election.votes
245 candidates = @election.candidates.dup
247 def in_schwartz_set?(candidate, candidates, transitive_defeats)
248 candidates.each do |challenger|
249 next if candidate == challenger
251 if transitive_defeats.include?( [ challenger, candidate ] ) and
252 not transitive_defeats.include?( [ candidate, challenger ] )
261 # see the array with the standard defeats
262 transitive_defeats = self.defeats(candidates, votes)
263 defeats_hash = Hash.new
264 transitive_defeats.each { |td| defeats_hash[td] = 1 }
266 candidates = [candidates] unless candidates.class == Array
267 candidates.each do |cand1|
268 candidates.each do |cand2|
269 unless cand1 == cand2
270 candidates.each do |cand3|
271 if not cand2 == cand3 and
272 not cand1 == cand3 and
273 defeats_hash[[cand2, cand1]] and
274 defeats_hash[[cand1, cand3]] and
275 not defeats_hash[[cand2, cand3]]
276 transitive_defeats << [cand2, cand3]
277 defeats_hash[[cand2, cand3]] = 1
284 schwartz_set = Array.new
285 candidates.each do |candidate|
286 if in_schwartz_set?(candidate, candidates, transitive_defeats)
287 schwartz_set << candidate
291 candidates = schwartz_set
293 # look through the schwartz set now for defeats
294 defeats = self.defeats(candidates, votes)
296 # it's a tie or there's only one option
297 break if defeats.length == 0
299 def is_weaker_defeat?( pair1, pair2 )
300 votes = @election.votes
301 if votes[pair1[0]][pair1[1]] > votes[pair2[0]][pair2[1]]
303 elsif votes[pair1[0]][pair1[1]] == votes[pair2[0]][pair2[1]] and
304 votes[pair1[1]][pair1[0]] > votes[pair2[1]][pair2[0]]
311 defeats.sort! do |pair1, pair2|
312 if is_weaker_defeat?( pair1, pair2 )
314 elsif is_weaker_defeat?( pair2, pair1 )
321 votes[defeats[0][0]][defeats[0][1]] = 0
322 votes[defeats[0][1]][defeats[0][0]] = 0