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 60 2007-09-22 13:58:59Z jtkorb $

import sys

import math 

import random

fdOut = None

fTrace = 1

randcalls = 0

class Universe:

    id = 0                              # id of last created Universe

    pendingUniverses = []

    completedUniverses = []

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

        Universe.id += 1

        self.id = Universe.id

        self.p = p

        self.nBallots = nBallots

	self.nWinners = nWinners

        self.quota = quota

        self.winners = []

        self.losers = []

        self.tally = {}

    def forkPendingWinners(self, lWin):

	return self.__forkPendingList(lWin, "winner")

    def forkPendingLosers(self, lMin):

	return self.__forkPendingList(lMin, "loser")

    def forkPendingRedist(self, lRedist):

	return self.__forkPendingList(lRedist, "redist")

    def __forkPendingList(self, lItems, pendingType):

	self.p /= len(lItems)

	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 = self.tally.copy()

	    u.pending = item

	    u.pendingType = pendingType

	    Universe.pendingUniverses.append(u)

	return lItems[0]

def combinations(l, n):

    assert(n >= 0)

    if n == 0:

	yield []

    else:

	for i in range(len(l)):

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

		yield [l[i]] + lsub

def trace(s):

    global fTrace, fdOut

    if fTrace: print >>fdOut, s

    return

def findNextWinner(u):

    global randcalls

    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

    ## randomize and count if some with 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("\tremaining winner(s) have fewer than quota (%d) ballots" %

            u.quota)

        return last

    return

def redistributeWinner(winner, u):

    global randcalls

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

    if excess <= 0:

        trace("\tno excess ballots to redistribute")

    else:

        lEffective = gatherEffectiveBallots(winner, u)

        nRedistribute = min(excess, len(lEffective))

        trace("\tredistributing %d effective of %d excess ballot(s) at random from %s" %

              (nRedistribute, excess, winner))

## LEFT OFF HERE...

## 	Need to create (len(lEffective) choose nRedistribute)-1 universes.

##  	One solution is to set self.nRedistribute, then forkPendingRedist the lEffective ballots.

## 	Must wind up with nRedistribute ballots redistributed in current universe.

## 	u.forkPendingRedist(lEffective)

        for ballot in random.sample(lEffective, nRedistribute):

            randcalls += 1

            trace("\trandom choice = %s" % ballot)

            u.tally[winner].remove(ballot)

            redistributeBallot(ballot, u)

            nRedistribute -= 1

    traceTally(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("\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("\tineffective ballot dropped: %s" % ballot)

def findNextLoser(u):

    global randcalls

    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("\tno ballots to redistribute")

    else:

        trace("\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(u)

    return

def traceTally(u):

    global fTrace

    if not fTrace: return

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

    for candidate in u.tally.keys():

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

        for ballot in u.tally[candidate]:

            trace("\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 randcalls

    randcalls = 0

    nBallots=len(ballots)

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

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

    trace("INPUT SUMMARY AND RUN TRACE")

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

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

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

    candidates = totalMrk.keys()

    candidates.sort()

    for c in candidates:

        trace("\t%-16s%3d\t%3d\t%4d" % (c, total1st[c], total2nd[c], len(ballots)-totalMrk[c]))

    traceTally(u)

    continueTally(u)

    return u.winners

def continueTally(u):

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

        winner = findNextWinner(u)

        if winner:

            u.winners.append(winner)

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

            redistributeWinner(winner, u)

        else:

            loser = findNextLoser(u)

            if loser:

                u.losers.append(loser)

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

                redistributeLoser(loser, u)

            else:

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

                break

    trace("\nNumber of random choices made: %d" % randcalls)