+# election library -- a ruby library for elections
+# copyright © 2005 MIT Media Lab and Benjamin Mako Hill
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+# 02110-1301, USA.
+
+#################################################################
+## ==== condorcet.rb ====
+##
+## This file contains Condorcet election methods. Currently this
+## includes a pure condorcet and a CloneproofSSD implementation
+## modeled after the Python-based Debian project election code and
+## that gives the same results in several tested corner cases.
+#################################################################
+
+##################################################################
+## CondorcetVote Classes and SubClasses
+##
+## The CondorcetVote class is subclassed by the PureCondorcetVote and
+## the CloneproofSSDVote classes but should not be used directly.
+
+class CondorcetVote < ElectionVote
+
+ def tally_vote(vote=nil)
+
+ vote.each_with_index do |winner, index|
+ # only run through this if this *is* preferred to something
+ break if vote.length - 1 == index
+ losers = vote.last( vote.length - index )
+
+ losers.each do |loser|
+ next if winner == loser
+
+ @votes[winner] = Hash.new unless @votes.has_key?(winner)
+ @votes[loser] = Hash.new unless @votes.has_key?(loser)
+
+ if @votes[winner].has_key?(loser)
+ @votes[winner][loser] += 1
+ else
+ @votes[winner][loser] = 1
+ end
+
+ # make sure we have a comparable object
+ @votes[loser][winner] = 0 unless @votes[loser].has_key?( winner )
+
+ @candidates << loser unless @candidates.include?( loser )
+ end
+
+ @candidates << winner unless @candidates.include?( winner )
+ end
+ end
+
+ protected
+ def verify_vote(vote=nil)
+ vote.instance_of?( Array ) and
+ vote == vote.uniq
+ end
+
+end
+
+class PureCondorcetVote < CondorcetVote
+ def result
+ PureCondorcetResult.new( self )
+ end
+end
+
+class CloneproofSSDVote < CondorcetVote
+ def result
+ CloneproofSSDResult.new( self )
+ end
+end
+
+
+##################################################################
+## CondorcetResult Classes and SubClasses
+##
+## The CondorcetResult class is subclassed by the PureCondorcetResult
+## and the CloneproofSSDResult classes but should not be used
+## directly.
+
+class CondorcetResult < ElectionResult
+ def initialize(voteobj=nil)
+ unless voteobj and voteobj.kind_of?( CondorcetVote )
+ raise ArgumentError, "You must pass a CondorcetVote array.", caller
+ end
+ super(voteobj)
+ end
+
+ def defeats(candidates=nil, votes=nil)
+ candidates = @election.candidates unless candidates
+ votes = @election.votes unless votes
+
+ defeats = Array.new
+ candidates.each do |candidate|
+ candidates.each do |challenger|
+ next if candidate == challenger
+ if votes[candidate][challenger] > votes[challenger][candidate]
+ defeats << [ candidate, challenger ]
+ end
+ end
+ end
+
+ defeats
+ end
+
+end
+
+class PureCondorcetResult < CondorcetResult
+ def initialize(voteobj=nil)
+ super(voteobj)
+ self.condorcet()
+ end
+
+ protected
+ def condorcet
+ votes = @election.votes
+ candidates = @election.candidates
+
+ victors = Hash.new
+ candidates.each do |candidate|
+ victors[candidate] = Array.new
+ end
+
+ self.defeats.each do |pair|
+ winner, loser = *pair
+ victors[winner] << loser
+ end
+
+ winners = @election.candidates.find_all do |candidate|
+ victors[candidate].length == @election.candidates.length - 1
+ end
+
+ @winners << winners if winners.length == 1
+ end
+end
+
+class CloneproofSSDResult < CondorcetResult
+ def initialize(voteobj=nil)
+ super(voteobj)
+ @winners = self.cpssd()
+ end
+
+ protected
+ def cpssd
+ votes = @election.votes
+ candidates = *@election.candidates
+
+ def in_schwartz_set?(candidate, candidates, transitive_defeats)
+ candidates.each do |challenger|
+ next if candidate == challenger
+
+ if transitive_defeats.include?( [ challenger, candidate ] ) and
+ not transitive_defeats.include?( [ candidate, challenger ] )
+ return false
+ end
+ end
+ return true
+ end
+
+ loop do
+
+ # see the array with the standard defeats
+ transitive_defeats = self.defeats(candidates, votes)
+
+ candidates.each do |cand1|
+ candidates.each do |cand2|
+ candidates.each do |cand3|
+ if transitive_defeats.include?( [ cand2, cand1 ] ) and
+ transitive_defeats.include?( [ cand1, cand3 ] ) and
+ not transitive_defeats.include?( [ cand2, cand3 ] ) and
+ not cand2 == cand3
+ transitive_defeats << [ cand2, cand3 ]
+ end
+ end
+ end
+ end
+
+ schwartz_set = Array.new
+ candidates.each do |candidate|
+ if in_schwartz_set?(candidate, candidates, transitive_defeats)
+ schwartz_set << candidate
+ end
+ end
+
+ candidates = schwartz_set
+
+ # look through the schwartz set now for defeats
+ defeats = self.defeats(candidates, votes)
+
+ # it's a tie or there's only one option
+ break if defeats.length == 0
+
+ def is_weaker_defeat?( pair1, pair2 )
+ votes = @election.votes
+ if votes[pair1[0]][pair1[1]] > votes[pair2[0]][pair2[1]]
+ return true
+ elsif votes[pair1[0]][pair1[1]] == votes[pair2[0]][pair2[1]] and
+ votes[pair1[1]][pair1[0]] > votes[pair2[1]][pair2[0]]
+ return true
+ else
+ false
+ end
+ end
+
+ defeats.sort! do |pair1, pair2|
+ if is_weaker_defeat?( pair1, pair2 )
+ +1
+ elsif is_weaker_defeat?( pair2, pair1 )
+ -1
+ else
+ 0
+ end
+ end
+
+ votes[defeats[0][0]][defeats[0][1]] = 0
+ votes[defeats[0][1]][defeats[0][0]] = 0
+
+ end
+
+ return candidates
+ end
+
+end
projects / selectricity / commitdiff