Subversion Repositories Remote Hare Voting

## Compute Hare Ballot

##

## Tim Korb (jtk@cs.purdue.edu), May 2007

##

## A few definitions and notes...

##

## tally: dictionary in which the keys are candidate names and the

## values are lists of ballots currently assigned to that candidate

##

## ballot: list of candidates in the order determined by the voter

##

## winners: list of candidates that have reached the quota of ballots

## or have remained in the running long enough to be declared a winner

##

## losers: list of candidates that have been eliminated from the running

##

## Note that plurals are generally used to indicate lists of other

## items, e.g., ballots is a list of ballot items.

##

## To download the complete source distribution, use Subversion:

##

##      % svn co http://www.bikmort.com/hare

##

## $Id: tally.py 64 2007-10-15 15:48:23Z jtkorb $

import sys

import math 

import copy

import random

fdOut = None

traceLevel = 0	# generate trace output if trace >= trace parameter

fParallel = 0	# track parallel universes instead of randomization if possible

class Universe:

    id = 0                              # id of last created Universe

    pendingUniverses = []

    results = {}

    dropped = 0

    def __reset(self):

	Universe.id = 0

 	Universe.pendingUniverses = []

	Universe.results = {}

	Universe.dropped = 0

    def __init__(self, p, nWinners, nBallots, quota):

	if p == 1.0:

	    self.__reset()

        Universe.id += 1

        self.id = Universe.id

        self.p = p

	self.nWinners = nWinners

        self.nBallots = nBallots

        self.quota = quota

        self.winners = []

        self.losers = []

        self.tally = {}

	self.pendingItem = None

	self.pendingType = None

	self.pendingData = None

    def forkPendingWinners(self, lWin):

	return self.__forkPendingList(lWin, "winner")

    def forkPendingLosers(self, lMin):

	return self.__forkPendingList(lMin, "loser")

    def forkPendingRedist(self, lRedist, winner):

	return self.__forkPendingList(lRedist, "redist", winner)

    def __forkPendingList(self, lItems, type, data=None):

	global fParallel

        if len(lItems) == 1:

	    return lItems[0]

	if not fParallel:

	    trace(1, "\tmaking random choice among %d alternatives" % 

		len(lItems))

	    return random.choice(lItems)

	# Split into parallel universes unless too many with small probability

	#

	if Universe.id > 10000 and self.p/len(lItems) < 1.0/10000:

	    trace(1, "\tpassing on %d universes (p = %f)" % 

		(len(lItems)-1, self.p/len(lItems)))

	    trace(1, "\tmaking random choice among %d alternatives" % 

		len(lItems))

	    Universe.dropped += len(lItems)-1

	    return random.choice(lItems)

	self.p /= len(lItems)

	trace(1, "\tcreating %d universes (p = %f)" % (len(lItems)-1, self.p))

	for item in lItems[1:]:

	    u = Universe(self.p, self.nWinners, self.nBallots, self.quota)

	    u.winners = self.winners[:]

	    u.losers = self.losers[:]

	    u.tally = copy.deepcopy(self.tally)

	    u.pendingItem = item

	    u.pendingType = type

	    u.pendingData = data

	    Universe.pendingUniverses.append(u)

	return lItems[0]

def combinations(l, n):

    if n == 0:

	return [[]]

    else:

        lResult = []

	for i in range(len(l)):

	    for lsub in combinations(l[i+1:], n-1):

		lResult.append([l[i]] + lsub)

        return lResult

def trace(t, s):

    global traceLevel, fdOut

    if traceLevel >= t: print >>fdOut, s

    return

def findNextWinner(u):

    cWin = u.quota	# number of votes for highest winner

    lWin = []		# list of candidates with highest votes

    for c in u.tally.keys():

        if c not in u.losers and c not in u.winners and len(u.tally[c]) >= cWin:

            if len(u.tally[c]) == cWin:

                lWin.append(c)

            else:

                lWin = [c]

                cWin = len(u.tally[c])

    if len(lWin) > 0:

        return u.forkPendingWinners(lWin)

    # Check to see if only enough candidates remain

    ## TODO: sort by len(tally[c]) to choose larger winners first

    ## create multiple universes if some have equal votes

    n = 0

    last = ""

    for c in u.tally.keys():

        if c not in u.winners and c not in u.losers:

            last = c

            n = n + 1

    if u.nWinners - len(u.winners) >= n:

        trace(1, "\tremaining winner(s) have fewer than quota (%d) ballots" %

            u.quota)

        return last

    return

def redistributeWinner(winner, u):

    excess = len(u.tally[winner]) - u.quota

    if excess <= 0:

        trace(1, "\tno excess ballots to redistribute")

    else:

        lEffective = gatherEffectiveBallots(winner, u)

        nRedist = min(excess, len(lEffective))

	trace(1, "\tredistributing %d effective ballots, %d at a time" % 

	    (len(lEffective), nRedist))

	for ballot in u.forkPendingRedist(combinations(lEffective, nRedist), winner):

            u.tally[winner].remove(ballot)

            redistributeBallot(ballot, u)

    traceTally(1, u)

def gatherEffectiveBallots(winner, u):

    lEffective = []

    for ballot in u.tally[winner]:

        for candidateTo in ballot:

           if candidateTo not in u.winners and candidateTo not in u.losers:

                lEffective.append(ballot)

                break

    return lEffective

def redistributeBallot(ballot, u):

    for candidateTo in ballot:

        if candidateTo not in u.winners and candidateTo not in u.losers:

            trace(1, "\tto %s: %s" % (candidateTo, ballot))

            if not u.tally.has_key(candidateTo): u.tally[candidateTo] = []

            u.tally[candidateTo].append(ballot)

            ballot = ""

            break

    if ballot:

        trace(1, "\tineffective ballot dropped: %s" % ballot)

def findNextLoser(u):

    cMin = sys.maxint  # least number of votes for candidate loser

    lMin = []          # list of candidates with least votes

    for c in u.tally.keys():

        if c not in u.losers and c not in u.winners and len(u.tally[c]) <= cMin:

            if len(u.tally[c]) == cMin:

                lMin.append(c)

            else:

                lMin = [c]

                cMin = len(u.tally[c])

    if len(lMin) == 0:

        return None

    else: 

        return u.forkPendingLosers(lMin)

def redistributeLoser(loser, u):

    excess = len(u.tally[loser])

    if excess <= 0:

        trace(1, "\tno ballots to redistribute")

    else:

        trace(1, "\tredistributing %d ballot(s) from %s" % (excess, loser))

        while len(u.tally[loser]) > 0:

            ballot = u.tally[loser][0]

            u.tally[loser] = u.tally[loser][1:]

            redistributeBallot(ballot, u)

    traceTally(1, u)

    return

def traceTally(t, u):

    global traceLevel

    if traceLevel < t: return

    trace(t, "\nCURRENT ASSIGNMENT OF BALLOTS (%d needed to win)" % u.quota)

    for candidate in u.tally.keys():

        trace(t, "\t%s:" % candidate)

        for ballot in u.tally[candidate]:

            trace(t, "\t\t%s" % ballot)

    return

# The basic Single Transferable Vote algorithm with Hare quota

#

# while winners < nWinners:

#   if a candidate has more than quota votes:

#       redistribute excess votes to next priority candidate

#   else:

#       eliminate lowest ranking candidate

#       redistribute wasted votes to next priority candidate

#

def dotally(nWinners, ballots):

    global fParallel

    nBallots=len(ballots)

    quota=int(math.ceil((nBallots + 1.0)/(nWinners + 1)))

    u = Universe(1.0, nWinners, nBallots, quota)

    trace(0, "INPUT SUMMARY")

    trace(0, "\t%d ballots" % u.nBallots)

    trace(0, "\tChoosing %s winners" % u.nWinners)

    trace(0, "\tNeed ceil((%d + 1)/(%d + 1)) = %d ballots to win" %

          (u.nBallots, u.nWinners, u.quota))

    # Create initial tally and statistics

    #

    for ballot in ballots:

        candidate = ballot[0]

        if not u.tally.has_key(candidate):

            u.tally[candidate] = []

        u.tally[candidate].append(ballot)

    total1st = {}

    total2nd = {}

    totalMrk = {}

    for ballot in ballots:

        for c in ballot:

            total1st[c] = 0

            total2nd[c] = 0

            totalMrk[c] = 0

    for ballot in ballots:

        total1st[ballot[0]] += 1

        if len(ballot) > 1:

            total2nd[ballot[1]] += 1

        for c in ballot:

            totalMrk[c] += 1

    trace(0, "\n\tBALLOT MARKS\t1ST\t2ND\tNONE")

    candidates = totalMrk.keys()

    candidates.sort()

    for c in candidates:

        trace(0, "\t%-16s%3d\t%3d\t%4d" % 

	    (c, total1st[c], total2nd[c], len(ballots)-totalMrk[c]))

    traceTally(0, u)

    Universe.pendingUniverses.append(u)

    while len(Universe.pendingUniverses) > 0:

    	u = Universe.pendingUniverses.pop()

	trace(1, "POPPED UNIVERSE %d (p = %f, %d pending):" % 

	    (u.id, u.p, len(Universe.pendingUniverses)))

	if u.pendingType == "redist":

	    trace(1, "\tredistributing %d ballots" % len(u.pendingItem))

	    winner = u.pendingData

	    assert(winner in u.winners)

	    for ballot in u.pendingItem:

                u.tally[winner].remove(ballot)

                redistributeBallot(ballot, u)

        elif u.pendingType == "winner":

	    trace(1, "\tfinishing pending winner %s" % u.pendingItem)

	    winner = u.pendingItem

	    assert(winner not in u.winners)

            u.winners.append(winner)

            trace(1, "\nSELECTION #%d: %s" % (len(u.winners), winner))

            redistributeWinner(winner, u)

	elif u.pendingType == "loser":

	    trace(1, "\tfinishing pending loser %s" % u.pendingItem)

	    loser = u.pendingItem

            u.losers.append(loser)

            trace(1, "\nELIMINATED: %s" % loser)

            redistributeLoser(loser, u)

        u.pendingType = None

        while len(u.winners) < u.nWinners:

            winner = findNextWinner(u)

            if winner:

                u.winners.append(winner)

                trace(1, "\nSELECTION #%d: %s" % (len(u.winners), winner))

                redistributeWinner(winner, u)

            else:

	        loser = findNextLoser(u)

                if loser:

                    u.losers.append(loser)

                    trace(1, "\nELIMINATED: %s" % loser)

                    redistributeLoser(loser, u)

                else:

                    trace(1, "Not enough chosen candidates to fill all positions")

                    break

	winners = str(u.winners)

	if fParallel:

	    trace(1, "RESULTS FOR UNIVERSE %d (p = %f): %s" % (u.id, u.p, winners))

	if winners not in Universe.results: Universe.results[winners] = 0.0

	Universe.results[winners] += u.p

    if fParallel:

	trace(0, "\nTracked %d alternative decisions (skipped %d with low probability)" %

	    (Universe.id, Universe.dropped))

    trace(0, "\nFINAL RESULTS")

    lWinners = []

    for winner in Universe.results.keys():

	if fParallel:

	    lWinners.append("%f: %s" % (Universe.results[winner], winner))

	else:

	    lWinners.append("%s" % winner)

    lWinners.sort()

    lWinners.reverse()

    for winner in lWinners:

	trace(0, winner)

    return lWinners