493 lines
15 KiB
Python
493 lines
15 KiB
Python
# pyRCV2: Preferential vote counting
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# Copyright © 2020 Lee Yingtong Li (RunasSudo)
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#
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# This program is free software: you can redistribute it and/or modify
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# it under the terms of the GNU Affero General Public License as published by
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# the Free Software Foundation, either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU Affero General Public License for more details.
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#
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# You should have received a copy of the GNU Affero General Public License
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# along with this program. If not, see <https://www.gnu.org/licenses/>.
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__pragma__ = lambda x: None
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from pyRCV2.model import CandidateState, CountCard, CountCompleted, CountStepResult
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from pyRCV2.numbers import Num, Rational
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from pyRCV2.safedict import SafeDict
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from pyRCV2.ties import TiesBackwards, TiesPrompt, TiesRandom
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class STVException(Exception):
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pass
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class BaseSTVCounter:
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"""
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Basic STV counter for various different variations
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"""
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def __init__(self, election, options=None):
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self.election = election
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self.cls_ballot_value = Num # Need to use Rational in unweighted inclusive Gregory
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# Default options
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self.options = {
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'prog_quota': False, # Progressively reducing quota?
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'quota': 'droop', # 'droop', 'droop_exact', 'hare' or 'hare_exact'
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'quota_criterion': 'geq', # 'geq' or 'gt'
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'surplus_order': 'size', # 'size' or 'order'
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'ties': [TiesBackwards(), TiesRandom()]
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}
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if options is not None:
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self.options.update(options)
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self.candidates = SafeDict([(c, CountCard()) for c in self.election.candidates])
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self.exhausted = CountCard()
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self.loss_fraction = CountCard()
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self.total_orig = sum((b.value for b in self.election.ballots), Num('0'))
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self.num_elected = 0
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# Withdraw candidates
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for candidate in self.election.withdrawn:
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__pragma__('opov')
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self.candidates[candidate].state = CandidateState.WITHDRAWN
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__pragma__('noopov')
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def reset(self):
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"""
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Public function:
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Reset the count and perform the first step
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"""
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# Distribute first preferences
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for ballot in self.election.ballots:
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__pragma__('opov')
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candidate = next((c for c in ballot.preferences if self.candidates[c].state == CandidateState.HOPEFUL), None)
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if candidate is not None:
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self.candidates[candidate].transfers += ballot.value
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self.candidates[candidate].ballots.append((ballot, self.cls_ballot_value(ballot.value)))
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else:
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self.exhausted.transfers += ballot.value
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self.exhausted.ballots.append((ballot, self.cls_ballot_value(ballot.value)))
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__pragma__('noopov')
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self.quota = None
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self.compute_quota()
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self.elect_meeting_quota()
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__pragma__('opov')
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return CountStepResult(
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'First preferences',
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self.candidates,
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self.exhausted,
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self.loss_fraction,
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self.total + self.exhausted.votes + self.loss_fraction.votes,
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self.quota
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)
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__pragma__('noopov')
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def step(self):
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"""
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Public function:
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Perform one step of the STV count
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"""
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# Step count cards
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self.step_count_cards()
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# Check if done
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result = self.check_if_done()
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if result:
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return result
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# Distribute surpluses
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result = self.distribute_surpluses()
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if result:
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return result
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# Insufficient winners and no surpluses to distribute
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# Exclude the lowest ranked hopeful
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result = self.exclude_candidate()
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if result:
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return result
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raise STVException('Unable to complete step')
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def step_count_cards(self):
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"""
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Reset the count cards for the beginning of a new step
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"""
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for candidate, count_card in self.candidates.items():
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count_card.step()
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self.exhausted.step()
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self.loss_fraction.step()
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def check_if_done(self):
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"""
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Check if the count can be completed
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"""
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# Have sufficient candidates been elected?
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if self.num_elected >= self.election.seats:
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return CountCompleted()
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# Are there just enough candidates to fill all the seats
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if self.num_elected + sum(1 for c, cc in self.candidates.items() if cc.state == CandidateState.HOPEFUL) <= self.election.seats:
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# TODO: Sort by size
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# Declare elected all remaining candidates
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for candidate, count_card in self.candidates.items():
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if count_card.state == CandidateState.HOPEFUL:
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count_card.state = CandidateState.PROVISIONALLY_ELECTED
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count_card.order_elected = self.num_elected
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self.num_elected += 1
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__pragma__('opov')
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return CountStepResult(
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'Bulk election',
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self.candidates,
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self.exhausted,
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self.loss_fraction,
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self.total + self.exhausted.votes + self.loss_fraction.votes,
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self.quota
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)
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__pragma__('noopov')
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def distribute_surpluses(self):
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"""
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Distribute surpluses, if any
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"""
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# Do surpluses need to be distributed?
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__pragma__('opov')
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has_surplus = [(c, cc) for c, cc in self.candidates.items() if cc.state == CandidateState.PROVISIONALLY_ELECTED and cc.votes > self.quota]
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__pragma__('noopov')
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if len(has_surplus) > 0:
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# Distribute surpluses in specified order
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if self.options['surplus_order'] == 'size':
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has_surplus.sort(key=lambda x: x[1].votes, reverse=True)
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candidate_surplus, count_card = self.choose_highest(has_surplus) # May need to break ties
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elif self.options['surplus_order'] == 'order':
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has_surplus.sort(key=lambda x: x[1].order_elected)
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candidate_surplus, count_card = has_surplus[0] # Ties were already broken when these were assigned
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else:
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raise STVException('Invalid surplus order option')
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count_card.state = CandidateState.ELECTED
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__pragma__('opov')
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surplus = count_card.votes - self.quota
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__pragma__('noopov')
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# Transfer surplus
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self.do_surplus(candidate_surplus, count_card, surplus)
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__pragma__('opov')
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count_card.transfers -= surplus
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__pragma__('noopov')
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# Declare elected any candidates meeting the quota as a result of surpluses
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self.compute_quota()
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self.elect_meeting_quota()
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__pragma__('opov')
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return CountStepResult(
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'Surplus of ' + candidate_surplus.name,
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self.candidates,
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self.exhausted,
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self.loss_fraction,
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self.total + self.exhausted.votes + self.loss_fraction.votes,
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self.quota
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)
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__pragma__('noopov')
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def do_surplus(self, candidate_surplus, count_card, surplus):
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"""
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Transfer the surplus of the given candidate
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Subclasses must override this function
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"""
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raise NotImplementedError('Method not implemented')
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def exclude_candidate(self):
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"""
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Exclude the lowest ranked hopeful
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"""
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candidate_excluded, count_card = self.candidate_to_exclude()
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count_card.state = CandidateState.EXCLUDED
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# Exclude this candidate
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self.do_exclusion(candidate_excluded, count_card)
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__pragma__('opov')
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count_card.transfers -= count_card.votes
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__pragma__('noopov')
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# Declare any candidates meeting the quota as a result of exclusion
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self.compute_quota()
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self.elect_meeting_quota()
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__pragma__('opov')
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return CountStepResult(
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'Exclusion of ' + candidate_excluded.name,
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self.candidates,
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self.exhausted,
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self.loss_fraction,
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self.total + self.exhausted.votes + self.loss_fraction.votes,
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self.quota
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)
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__pragma__('noopov')
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def candidate_to_exclude(self):
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"""
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Determine the candidate to exclude
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"""
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hopefuls = [(c, cc) for c, cc in self.candidates.items() if cc.state == CandidateState.HOPEFUL]
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hopefuls.sort(key=lambda x: x[1].votes)
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candidate_excluded, count_card = self.choose_lowest(hopefuls)
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return candidate_excluded, count_card
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def do_exclusion(self, candidate_excluded, count_card):
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"""
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Exclude the given candidate and transfer the votes
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Subclasses must override this function
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"""
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raise NotImplementedError('Method not implemented')
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def compute_quota(self):
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"""
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Recount total votes and (if applicable) recalculate the quota
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"""
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__pragma__('opov')
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self.total = sum((cc.votes for c, cc in self.candidates.items()), Num('0'))
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self.loss_fraction.transfers += (self.total_orig - self.total - self.exhausted.votes) - self.loss_fraction.votes
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if self.quota is None or self.options['prog_quota']:
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if self.options['quota'] == 'droop':
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self.quota = (self.total / Num(self.election.seats + 1)).__floor__() + Num('1')
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elif self.options['quota'] == 'droop_exact':
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self.quota = self.total / Num(self.election.seats + 1)
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elif self.options['quota'] == 'hare':
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self.quota = (self.total / Num(self.election.seats)).__floor__() + Num('1')
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elif self.options['quota'] == 'hare_exact':
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self.quota = self.total / Num(self.election.seats)
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else:
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raise STVException('Invalid quota option')
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__pragma__('noopov')
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def meets_quota(self, count_card):
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"""
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Determine if the given candidate meets the quota
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"""
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if self.options['quota_criterion'] == 'geq':
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__pragma__('opov')
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return count_card.votes >= self.quota
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__pragma__('noopov')
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elif self.options['quota_criterion'] == 'gt':
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__pragma__('opov')
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return count_card.votes > self.quota
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__pragma__('noopov')
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else:
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raise STVException('Invalid quota criterion')
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def elect_meeting_quota(self):
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"""
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Elect all candidates meeting the quota
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"""
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# Does a candidate meet the quota?
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meets_quota = [(c, cc) for c, cc in self.candidates.items() if cc.state == CandidateState.HOPEFUL and self.meets_quota(cc)]
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if len(meets_quota) > 0:
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meets_quota.sort(key=lambda x: x[1].votes, reverse=True)
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# Declare elected any candidate who meets the quota
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while len(meets_quota) > 0:
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x = self.choose_highest(meets_quota)
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candidate, count_card = x[0], x[1]
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count_card.state = CandidateState.PROVISIONALLY_ELECTED
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count_card.order_elected = self.num_elected
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self.num_elected += 1
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meets_quota.remove(x)
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def choose_lowest(self, l):
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"""
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Provided a list of tuples (Candidate, CountCard), sorted in ASCENDING order of votes, choose the tuple with the fewest votes, breaking ties appropriately
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"""
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if len(l) == 1:
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return l[0]
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tied = [(c, cc) for c, cc in l if cc.votes == l[0][1].votes]
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if len(tied) == 1:
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return tied[0]
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# A tie exists
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for tie in self.options['ties']:
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result = tie.choose_lowest(tied)
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if result is not None:
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return result
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raise Exception('Unable to resolve tie')
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def choose_highest(self, l):
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"""
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Provided a list of tuples (Candidate, CountCard), sorted in DESCENDING order of votes, choose the tuple with the most votes, breaking ties appropriately
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"""
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if len(l) == 1:
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return l[0]
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tied = [(c, cc) for c, cc in l if cc.votes == l[0][1].votes]
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if len(tied) == 1:
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return tied[0]
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# A tie exists
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for tie in self.options['ties']:
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result = tie.choose_highest(tied)
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if result is not None:
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return result
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raise Exception('Unable to resolve tie')
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class BaseWIGSTVCounter(BaseSTVCounter):
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"""
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Basic weighted inclusive Gregory STV counter
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"""
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def do_surplus(self, candidate_surplus, count_card, surplus):
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for ballot, ballot_value in count_card.ballots:
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__pragma__('opov')
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new_value = (ballot_value * surplus) / count_card.votes # Multiply first to avoid rounding errors
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candidate = next((c for c in ballot.preferences if self.candidates[c].state == CandidateState.HOPEFUL), None)
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if candidate is not None:
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self.candidates[candidate].transfers += new_value
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self.candidates[candidate].ballots.append((ballot, new_value))
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else:
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self.exhausted.transfers += new_value
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self.exhausted.ballots.append((ballot, new_value))
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__pragma__('noopov')
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def do_exclusion(self, candidate_excluded, count_card):
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for ballot, ballot_value in count_card.ballots:
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__pragma__('opov')
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candidate = next((c for c in ballot.preferences if self.candidates[c].state == CandidateState.HOPEFUL), None)
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if candidate is not None:
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self.candidates[candidate].transfers += ballot_value
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self.candidates[candidate].ballots.append((ballot, ballot_value))
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else:
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self.exhausted.transfers += ballot_value
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self.exhausted.ballots.append((ballot, ballot_value))
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__pragma__('noopov')
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class BaseUIGSTVCounter(BaseSTVCounter):
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"""
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Basic unweighted inclusive Gregory STV counter
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"""
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def __init__(self, *args, **kwargs):
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BaseSTVCounter.__init__(self, *args, **kwargs)
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# Need to use Rational for ballot value internally, as Num may be set to integers only
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self.cls_ballot_value = Rational
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def do_surplus(self, candidate_surplus, count_card, surplus):
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# FIXME: Is it okay to use native int's here?
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next_preferences = SafeDict([(c, []) for c, cc in self.candidates.items()])
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next_exhausted = []
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total_ballots = Num('0')
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# Count next preferences
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for ballot, ballot_value in count_card.ballots:
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__pragma__('opov')
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total_ballots += ballot.value
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candidate = next((c for c in ballot.preferences if self.candidates[c].state == CandidateState.HOPEFUL), None)
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__pragma__('noopov')
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if candidate is not None:
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__pragma__('opov')
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next_preferences[candidate].append(ballot)
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__pragma__('noopov')
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else:
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next_exhausted.append(ballot)
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# Make transfers
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for candidate, cand_ballots in next_preferences.items():
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num_ballots = sum((b.value for b in cand_ballots), Num('0'))
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__pragma__('opov')
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self.candidates[candidate].transfers += (num_ballots * surplus) / total_ballots # Multiply first to avoid rounding errors
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__pragma__('noopov')
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for ballot in cand_ballots:
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__pragma__('opov')
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new_value = (ballot.value.to_rational() * surplus.to_rational()) / total_ballots.to_rational()
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self.candidates[candidate].ballots.append((ballot, new_value))
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__pragma__('noopov')
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num_exhausted = sum((b.value for b in next_exhausted), Num('0'))
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__pragma__('opov')
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self.exhausted.transfers += (num_exhausted * surplus) / total_ballots
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__pragma__('noopov')
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for ballot in next_exhausted:
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__pragma__('opov')
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new_value = (ballot.value.to_rational() * surplus.to_rational()) / total_ballots.to_rational()
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__pragma__('noopov')
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self.exhausted.ballots.append((ballot, new_value))
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def do_exclusion(self, candidate_excluded, count_card):
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next_preferences = SafeDict([(c, Rational('0')) for c, cc in self.candidates.items()])
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next_exhausted = Rational('0')
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total_votes = Rational('0')
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# Count and transfer next preferences
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for ballot, ballot_value in count_card.ballots:
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__pragma__('opov')
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total_votes += ballot_value
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candidate = next((c for c in ballot.preferences if self.candidates[c].state == CandidateState.HOPEFUL), None)
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__pragma__('noopov')
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if candidate is not None:
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__pragma__('opov')
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next_preferences[candidate] += ballot_value
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self.candidates[candidate].ballots.append((ballot, ballot_value))
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__pragma__('noopov')
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else:
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__pragma__('opov')
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next_exhausted += ballot_value
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__pragma__('noopov')
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self.exhausted.ballots.append((ballot, ballot_value))
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# Credit votes
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for candidate, cand_votes in next_preferences.items():
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__pragma__('opov')
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self.candidates[candidate].transfers += cand_votes.to_num()
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__pragma__('noopov')
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__pragma__('opov')
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self.exhausted.transfers += next_exhausted.to_num()
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__pragma__('noopov')
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