From: Benjamin Mako Hill Date: Fri, 27 Jan 2006 18:21:52 +0000 (+0000) Subject: Initial import into SVN of rubyvote library X-Git-Url: https://projects.mako.cc/source/rubyvote/commitdiff_plain/0ecf724c8d154069cb372e4ef87e0eae9fe872a0 Initial import into SVN of rubyvote library git-svn-id: svn://rubyforge.org/var/svn/rubyvote/trunk@1 1440c7f4-e209-0410-9a04-881b5eb134a8 --- 0ecf724c8d154069cb372e4ef87e0eae9fe872a0 diff --git a/COPYING b/COPYING new file mode 100644 index 0000000..b7b5f53 --- /dev/null +++ b/COPYING @@ -0,0 +1,340 @@ + GNU GENERAL PUBLIC LICENSE + Version 2, June 1991 + + Copyright (C) 1989, 1991 Free Software Foundation, Inc. + 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA + Everyone is permitted to copy and distribute verbatim copies + of this license document, but changing it is not allowed. + + Preamble + + The licenses for most software are designed to take away your +freedom to share and change it. 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If this is what you want to do, use the GNU Library General +Public License instead of this License. diff --git a/README b/README new file mode 100644 index 0000000..7029d84 --- /dev/null +++ b/README @@ -0,0 +1,196 @@ +RubyVote: Election Methods Library in Ruby +=========================================== + +:Author: Benjamin Mako Hill +:Copyright: MIT Media Lab and MIT +:License: GNU GPL + +Summary +-------- + +**Version:** 0.1 + +This software includes an set of classes to tally votes and compute +winners in elections or votes using a series of different +methods. Currrently these include: + +* Plurality or "winner-take-all" +* Approval +* Borda +* Simple Condorcet +* Condorcet with Cloneproof SSD + +More information can be found at the following pages: + +* http://en.wikipedia.org/wiki/Plurality_electoral_system +* http://en.wikipedia.org/wiki/Approval_voting +* http://en.wikipedia.org/wiki/Borda_count +* http://en.wikipedia.org/wiki/Condorcet_method +* http://en.wikipedia.org/wiki/Shulze_method + +Writing support for a new module (e.g., instant runnof voting) is a +fantastic way to to contribute to this module. + +This software is prerelease software. I am both a relatively new Ruby +progreammming and relatively new to election methods. These election +methods have bugs and may have bugs that skew results. If you are +understand Ruby and election methods, please do me a favor and audit +my code. + + + +How To Use This Library +------------------------- + +Using this library is relatively simple but will differ per election +methods. In each case, you will need to ``require`` the appropriate +file for the type of election you will be running and then create a +new vote object. You should then either pass an array of votes to the +object upon creation or pass votes in one at at a time. + + +.. Note:: *You* are responsible for ensuring that the votes are in + correct form before you hand them to this module. This will + not currently check for most types of invalid votes and does + not (currently) accept a list of candidates at creation from + which it checks all votes. As such, new candidates will be + created when seen. If you think this is a meaningful + addition to this library, please send a patch. Otherwise, + please check for the validity of votes BEFORE you pass them + to this election module. + +Examples of each type of election currently supported can be seen in +the test.rb file distributed in this archive. + +ElectionVote Objects +********************* + +Each ElectionVote object has the following exposed attributions: + +* ElectionVote#votes -- returns a list of votes that have been tallied +* ElectionVote#candidates -- returns a list of candidates + +Additionally, each subclass will create a #results method which will +return an ElectionResult subclass of the appropriate type. + +Currently, you use this module by creating any of the following types +of vote objects: + +Plurality +^^^^^^^^^^ + +This is the most simple "winner-take-all" system. The array passed to +the new vote object should be an array of strings. Each string is +counted as one vote for a candidate. + +Example:: + + require 'election' + vote_array = [ "A", "B", "B", "A" ] + resultobject = PluralityVote.new(vote_array).result + +Approval +^^^^^^^^^ + +Approval is similar to plurality voting except that users can vote for +more than one candidate at once naming all of the candidates that they +approve of. + +Example:: + + require 'election' + vote_array = [ ["A", "B"], ["B", "A"], ["B"] ] + resultobject = ApprovalVote.new(vote_array).result + +Borda +^^^^^^ + +Borda is a positional voting system and, as a result, takes a list of +ranked candidates and assigns points to each candidates based on their +order. In Borda, there are *n* candidate and the first candidates is +assigned *n* - 1 points and each subsequent candidate is assigned one +less point. The candidate is assigned no points. + +Currently, all candidates should be ranked in each ballot. + +Example:: + + require 'positional' + vote_array = [ ["A", "B"], ["B", "A"], ["B", "A"] ] + resultobject = BordaVote.new(vote_array).result + +Pure Condorcet +^^^^^^^^^^^^^^^^ + +Condorcet is a preferential system and, as such, each vote must list +of ranked preferences from most to least preferred. Currently, all +candidates must be listed. No ties are allowed on ballots with the +current implementation. + +Example:: + + require 'condorcet' + vote_array = [ ["A", "B"], ["B", "A"], ["B", "A"] ] + resultobject = PureCondorcetVote.new(vote_array).result + +Cloneproof Schwartz Sequential Dropping +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +`Cloneproof SSD` is a Condorcet variant with the ability to create +winners in circular defeats (e.g., A beats B, B beats C, C beats A) +where this is no clear winner in Condorcet. It is used identically to +Pure Condorcet. + +Example:: + + require 'condorcet' + vote_array = [ ["A", "B"], ["B", "A"], ["B", "A"] ] + resultobject = CloneproofSSDVote.new(vote_array).result + +ElectionResult Objects +*********************** + +Each election result object will have the following methods: + +* #winner? -- return boolean as to the winner or winners of an election +* #winners -- an array of winners of the election +* #ranked_candidates -- (where available) a list of ranked candidates + + +License +-------- + +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. + +Look in the COPYING file for the text of the GNU GPL. + +Authors +-------- + +Currently, the only contributor to this program is Benjamin Mako Hill +working at the MIT Media Lab. Please feel free to contribute to this +module and get your name added here. + +For more information about Mako and his programs, you can seee his +homepage here: + + http://mako.cc + +For more information about the MIT Media Lab, you can see its homepage +here: + + http://www.media.mit.edu + diff --git a/lib/condorcet.rb b/lib/condorcet.rb new file mode 100644 index 0000000..9733989 --- /dev/null +++ b/lib/condorcet.rb @@ -0,0 +1,238 @@ +# 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. + +require 'election' + +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 + + protected + 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 diff --git a/lib/election.rb b/lib/election.rb new file mode 100644 index 0000000..7fd8396 --- /dev/null +++ b/lib/election.rb @@ -0,0 +1,160 @@ +# 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. + +################################################################# +## ==== election.rb ==== +## +## This file contains the core ElectionVote and ElectionResults +## classes and the most common and simple election methods including +## plurality and approval voting. +################################################################# + +################################################################## +## ElectionVote Classes and SubClasses +## +## There classes are used to store, verify, and "tally" (i.e. count +## votes for the standard Election superclass and for the most common +## types of elections. + +class ElectionVote + attr_reader :votes + attr_reader :candidates + + def initialize(votes=nil) + @votes = Hash.new unless defined?(@votes) + @candidates = Array.new unless defined?(@candidates) + + if votes + if votes.instance_of?( Array ) + votes.each do |vote| + self.tally_vote(vote) if self.verify_vote(vote) + end + else + raise ElectionError, "Votes must be in the form of an array.", caller + end + end + end + + protected + # by default, this is set to look if the vote is defined. it should + # be overridden in each class + def verify_vote(vote=nil) + vote ? true : false + end + + # by default, this does nothing. it must be redefined in any subclass + def tally_vote + self.verify_vote(vote) + end +end + +class PluralityVote < ElectionVote + def result + PluralityResult.new(self) + end + + protected + def verify_vote(vote=nil) + vote.instance_of?( String ) + end + + def tally_vote(candidate) + if @votes.has_key?(candidate) + @votes[candidate] += 1 + else + @votes[candidate] = 1 + @candidates << candidate + end + end +end + +class ApprovalVote < PluralityVote + def result + ApprovalResult.new(self) + end + + protected + def verify_vote(vote=nil) + vote.instance_of?( Array ) and vote.length >= 1 + end + + def tally_vote(approvals) + approvals.each {|candidate| super(candidate)} + end +end + + +################################################################## +## Election Result Classes +## + +## There classes are used to compute and report the results of an +## election. In almost all cases, these will be returned by the +## #results method of a corresponding ElectionVote subclass. + +class ElectionResult + attr_reader :winners + + def initialize(voteobj=nil) + unless voteobj and voteobj.kind_of?( ElectionVote ) + raise ArgumentError, "You must pass a ElectionVote array.", caller + end + + @election = voteobj + @winners = Array.new + end + + def winner + @winners[0] if @winners.length > 0 + end + + def winner? + @winners.length > 0 + end + +end + +class PluralityResult < ElectionResult + attr_reader :ranked_candidates + + def initialize(voteobj=nil) + super(voteobj) + + votes = @election.votes + candidates = @election.candidates + + @ranked_candidates = votes.sort do |a, b| + b[1] <=> a[1] + end.collect {|a| a[0]} + + # winners are anyone who has the same number of votes as the + # first person + @winners = @ranked_candidates.find_all do |i| + votes[i] == votes[@ranked_candidates[0]] + end + end +end + +# this class is complete because results for approval are computed +# identically to results from plurality +class ApprovalResult < PluralityResult +end + +class ElectionError < ArgumentError +end + diff --git a/lib/positional.rb b/lib/positional.rb new file mode 100644 index 0000000..5136aec --- /dev/null +++ b/lib/positional.rb @@ -0,0 +1,76 @@ +# 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. + +################################################################# +## ==== positional.rb ==== +## +## This file contains positional election methods. Currently only +## includes an implementation of the Borda count election method. +################################################################# + +################################################################## +## BordaVote and BordaResult Classes +## +## These classes inherit from and/or are modeled after the classes in +## election.rb and condorcet.rb + +require 'election' + +class BordaVote < ElectionVote + + def initialize(votes=nil) + @candidates = Array.new + votes.each do |vote| + @candidates = vote.uniq if vote.uniq.length > candidates.length + end + super(votes) + end + + def tally_vote(vote) + points = candidates.length - 1 + vote.each do |candidate| + @votes[candidate] = points + points -= 1 + end + end + + def verify_vote(vote=nil) + vote.instance_of?( Array ) and + vote == vote.uniq + end + + def result + BordaResult.new(self) + end +end + +class BordaResult < ElectionResult + def initialize(voteobj=nil) + super(voteobj) + votes = @election.votes + + @ranked_candidates = votes.sort do |a, b| + b[1] <=> a[1] + end.collect {|i| i[0]} + + @winners = @ranked_candidates.find_all do |i| + votes[i] == votes[@ranked_candidates[0]] + end + end + +end diff --git a/test.rb b/test.rb new file mode 100755 index 0000000..3be4176 --- /dev/null +++ b/test.rb @@ -0,0 +1,138 @@ +#!/usr/bin/ruby -I./lib + +# 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. + +require 'election' +require 'condorcet' +require 'positional' + +def print_winner(result) + if not result.winner? + puts "There is no winner." + elsif result.winners.length == 1 + puts "The winner is %s" % result.winners[0] + else + puts "There is a tie between the following candidates: %s" % result.winners.join(", ") + end +end + +def condorcet_test1 + puts "USING CONDORCET..." + puts "The winner should be: B" + + vote_array = Array.new + 3.times {vote_array << "ABC".split("")} + 3.times {vote_array << "CBA".split("")} + 2.times {vote_array << "BAC".split("")} + + print_winner( PureCondorcetVote.new(vote_array).result ) +end + +def ssd_test1 + puts "USING CloneProofSSD..." + puts "The winner should be: E" + + vote_array = Array.new + 5.times {vote_array << "ACBED".split("")} + 5.times {vote_array << "ADECB".split("")} + 8.times {vote_array << "BEDAC".split("")} + 3.times {vote_array << "CABED".split("")} + 7.times {vote_array << "CAEBD".split("")} + 2.times {vote_array << "CBADE".split("")} + 7.times {vote_array << "DCEBA".split("")} + 8.times {vote_array << "EBADC".split("")} + + print_winner( CloneproofSSDVote.new(vote_array).result ) +end + +def ssd_test2 + puts "USING CloneProofSSD..." + puts "The winner should be: D" + + vote_array = Array.new + 5.times {vote_array << "ACBD".split("")} + 2.times {vote_array << "ACDB".split("")} + 3.times {vote_array << "ADCB".split("")} + 4.times {vote_array << "BACD".split("")} + 3.times {vote_array << "CBDA".split("")} + 3.times {vote_array << "CDBA".split("")} + 1.times {vote_array << "DACB".split("")} + 5.times {vote_array << "DBAC".split("")} + 4.times {vote_array << "DCBA".split("")} + + print_winner( CloneproofSSDVote.new(vote_array).result ) +end + +def ssd_test3 + puts "USING CloneProofSSD..." + puts "The winner should be: B(?)" + + vote_array = Array.new + 3.times {vote_array << "ABCD".split("")} + 2.times {vote_array << "DABC".split("")} + 2.times {vote_array << "DBCA".split("")} + 2.times {vote_array << "CBDA".split("")} + + print_winner( CloneproofSSDVote.new(vote_array).result ) +end + + +def borda_test1 + puts "USING BORDA..." + puts "The winner should be: B" + + vote_array = Array.new + 3.times {vote_array << "ABC".split("")} + 3.times {vote_array << "CBA".split("")} + 2.times {vote_array << "BAC".split("")} + + print_winner( BordaVote.new(vote_array).result ) +end + +def plurality_test1 + puts "USING PLURALITY..." + puts "The winner should be: C" + + vote_array = "ABCABCABCCCBBAAABABABCCCCCCCCCCCCCA".split("") + + print_winner( PluralityVote.new(vote_array).result ) +end + + +def approval_test1 + puts "USING APPROVAL..." + puts "The winner should be: A" + + vote_array = Array.new + 10.times {vote_array << "AB".split("")} + 10.times {vote_array << "CB".split("")} + 11.times {vote_array << "AC".split("")} + 5.times {vote_array << "A".split("")} + + print_winner( ApprovalVote.new(vote_array).result ) +end + +condorcet_test1() +ssd_test1() +ssd_test2() +ssd_test3() +borda_test1() +plurality_test1() +approval_test1() +