Implement Meek STV

README.md

@@ -15,6 +15,7 @@ pyRCV2 accepts data in the [BLT file format](http://www.dia.govt.nz/diawebsite.N
 * weighted inclusive Gregory STV (e.g. [Scottish STV](https://www.opavote.com/methods/scottish-stv-rules))
 * unweighted inclusive Gregory STV (e.g. [Australian Senate STV](https://www.legislation.gov.au/Details/C2020C00400/Html/Text#_Toc59107700))
 * exclusive Gregory STV (e.g. [PRSA 1977](https://www.prsa.org.au/rule1977.htm) and [ERS97](https://www.electoral-reform.org.uk/latest-news-and-research/publications/how-to-conduct-an-election-by-the-single-transferable-vote-3rd-edition/))
+* [Meek STV](http://www.dia.govt.nz/diawebsite.NSF/Files/meekm/%24file/meekm.pdf)
 * [Wright STV](https://www.aph.gov.au/Parliamentary_Business/Committees/House_of_Representatives_Committees?url=em/elect07/subs/sub051.1.pdf)
 
 pyRCV2 is highly customisable, including options for:

docs/options.md

@@ -6,6 +6,7 @@ The preset dropdown allows you to choose from a hardcoded list of preloaded STV
 
 * [Scottish STV](https://www.opavote.com/methods/scottish-stv-rules)
 * [Australian Senate STV](https://www.legislation.gov.au/Details/C2020C00400/Html/Text#_Toc59107700)
+* [Meek STV](http://www.dia.govt.nz/diawebsite.NSF/Files/meekm/%24file/meekm.pdf)
 * [Wright STV](https://www.aph.gov.au/Parliamentary_Business/Committees/House_of_Representatives_Committees?url=em/elect07/subs/sub051.1.pdf)
 * [PRSA 1977](https://www.prsa.org.au/rule1977.htm) – with counting performed to the thousandth of a vote, and results reported in raw votes to 3 decimal places
 * [ERS97](https://www.electoral-reform.org.uk/latest-news-and-research/publications/how-to-conduct-an-election-by-the-single-transferable-vote-3rd-edition/)
@@ -91,9 +92,10 @@ Some STV counting rules provide, for example, that ‘no surplus shall be transf
 
 ## Method (-m/--method)
 
-This dropdown allows you to select how ballots are transferred during surplus transfers or exclusions. The recommended method is:
+This dropdown allows you to select how ballots are transferred during surplus transfers or exclusions. The recommended methods are:
 
 * *Weighted inclusive Gregory* (default): During surplus transfers, all applicable ballot papers of the transferring candidate are examined. Transfers are weighted according to the weights of the ballot papers.
+* *Meek STV*: Transfers are computed as described at <http://www.dia.govt.nz/diawebsite.NSF/Files/meekm/%24file/meekm.pdf>.
 
 Other methods are supported, but not recommended:
 

html/index.html

@@ -33,6 +33,7 @@
 					<option value="scottish" selected>Scottish STV</option>
 					<!--<option value="stvc">pyRCV STV-C</option>-->
 					<option value="senate">Australian Senate STV</option>
+					<option value="meek">Meek STV</option>
 					<option value="wright">Wright STV</option>
 					<option value="prsa77">PRSA 1977</option>
 					<option value="ers97">ERS97</option>
@@ -86,6 +87,7 @@
 							<option value="wig" selected>Weighted inclusive Gregory</option>
 							<option value="uig">Unweighted inclusive Gregory</option>
 							<option value="eg">Exclusive Gregory (last bundle)</option>
+							<option value="meek">Meek method</option>
 						</select>
 					</label>
 					<label>

html/index.js

@@ -86,6 +86,25 @@ function changePreset() {
 		document.getElementById('selPapers').value = 'both';
 		document.getElementById('selExclusion').value = 'by_value';
 		document.getElementById('selTies').value = 'backwards_random';
+	} else if (document.getElementById('selPreset').value === 'meek') {
+		document.getElementById('selQuotaCriterion').value = 'gt';
+		document.getElementById('selQuota').value = 'droop_exact';
+		document.getElementById('selQuotaMode').value = 'progressive';
+		document.getElementById('chkBulkElection').checked = true;
+		document.getElementById('chkBulkExclusion').checked = false;
+		document.getElementById('chkDeferSurpluses').checked = false;
+		document.getElementById('selNumbers').value = 'fixed';
+		document.getElementById('txtDP').value = '9';
+		document.getElementById('txtPPDP').value = '2';
+		document.getElementById('chkRoundQuota').checked = false;
+		document.getElementById('chkRoundVotes').checked = false;
+		document.getElementById('chkRoundTVs').checked = false;
+		document.getElementById('chkRoundWeights').checked = false;
+		document.getElementById('selSurplus').value = 'size';
+		document.getElementById('selTransfers').value = 'meek';
+		document.getElementById('selPapers').value = 'both';
+		document.getElementById('selExclusion').value = 'one_round';
+		document.getElementById('selTies').value = 'random';
 	} else if (document.getElementById('selPreset').value === 'wright') {
 		document.getElementById('selQuotaCriterion').value = 'geq';
 		document.getElementById('selQuota').value = 'droop';

html/worker.js

@@ -46,6 +46,8 @@ onmessage = function(evt) {
 			counter = py.pyRCV2.method.base_stv.UIGSTVCounter(election, evt.data.data.options);
 		} else if (evt.data.data.transfers === 'eg') {
 			counter = py.pyRCV2.method.base_stv.EGSTVCounter(election, evt.data.data.options);
+		} else if (evt.data.data.transfers === 'meek') {
+			counter = py.pyRCV2.method.meek.MeekSTVCounter(election, evt.data.data.options);
 		} else {
 			counter = py.pyRCV2.method.base_stv.WIGSTVCounter(election, evt.data.data.options);
 		}
@@ -63,9 +65,19 @@ onmessage = function(evt) {
 		
 		// Reset
 		stage = 1;
-		result = counter.reset();
+		try {
+			result = counter.reset();
+		} catch (ex) {
+			handleException(ex);
+			return;
+		}
 		
-		postMessage({'type': 'result', 'result': resultToJS(result)});
+		if (py.isinstance(result, py.pyRCV2.model.CountCompleted)) {
+			postMessage({'type': 'done', 'result': resultToJS(result)});
+			return;
+		} else {
+			postMessage({'type': 'result', 'result': resultToJS(result)});
+		}
 		
 		stepElection();
 	} else if (evt.data.type === 'require_input') {
@@ -79,22 +91,13 @@ function stepElection() {
 	while (true) {
 		try {
 			result = counter.step();
-			stage += 1;
 		} catch (ex) {
-			if (py.isinstance(ex, py.pyRCV2.ties.RequireInput)) {
+			handleException(ex);
-				// Signals we require input to break a tie
+			return;
-				postMessage({'type': 'require_input', 'message': ex.message});
-				break;
-			} else if (py.isinstance(ex, py.pyRCV2.method.base_stv.STVException)) {
-				console.error(ex);
-				postMessage({'type': 'stv_exception', 'message': ex.message});
-				break;
-			} else {
-				console.error(ex);
-				throw ex;
-			}
 		}
 		
+		stage += 1;
+		
 		if (py.isinstance(result, py.pyRCV2.model.CountCompleted)) {
 			postMessage({'type': 'done', 'result': resultToJS(result)});
 			break;
@@ -104,6 +107,19 @@ function stepElection() {
 	}
 }
 
+function handleException(ex) {
+	if (py.isinstance(ex, py.pyRCV2.ties.RequireInput)) {
+		// Signals we require input to break a tie
+		postMessage({'type': 'require_input', 'message': ex.message});
+	} else if (py.isinstance(ex, py.pyRCV2.method.base_stv.STVException)) {
+		console.error(ex);
+		postMessage({'type': 'stv_exception', 'message': ex.message});
+	} else {
+		console.error(ex);
+		throw ex;
+	}
+}
+
 function resultToJS(result) {
 	return {
 		'stage': stage,

pyRCV2/cli/stv.py

@@ -19,6 +19,7 @@ import pyRCV2.model
 import pyRCV2.numbers
 
 from pyRCV2.method.base_stv import UIGSTVCounter, WIGSTVCounter, EGSTVCounter
+from pyRCV2.method.meek import MeekSTVCounter
 from pyRCV2.ties import TiesBackwards, TiesForwards, TiesPrompt, TiesRandom
 
 import sys
@@ -41,7 +42,7 @@ def add_parser(subparsers):
 	parser.add_argument('--round-tvs', type=int, help='round transfer values to specified decimal places')
 	parser.add_argument('--round-weights', type=int, help='round ballot weights to specified decimal places')
 	parser.add_argument('--surplus-order', '-s', choices=['size', 'order'], default='size', help='whether to distribute surpluses by size or by order of election (default: size)')
-	parser.add_argument('--method', '-m', choices=['wig', 'uig', 'eg'], default='wig', help='method of transferring surpluses (default: wig - weighted inclusive Gregory)')
+	parser.add_argument('--method', '-m', choices=['wig', 'uig', 'eg', 'meek'], default='wig', help='method of transferring surpluses (default: wig - weighted inclusive Gregory)')
 	parser.add_argument('--transferable-only', action='store_true', help='examine only transferable papers during surplus distributions')
 	parser.add_argument('--exclusion', choices=['one_round', 'parcels_by_order', 'by_value', 'wright'], default='one_round', help='how to perform exclusions (default: one_round)')
 	parser.add_argument('--ties', '-t', action='append', choices=['backwards', 'forwards', 'prompt', 'random'], default=None, help='how to resolve ties (default: backwards then random)')
@@ -104,6 +105,8 @@ def main(args):
 		counter = UIGSTVCounter(election, vars(args))
 	elif args.method == 'eg':
 		counter = EGSTVCounter(election, vars(args))
+	elif args.method == 'meek':
+		counter = MeekSTVCounter(election, vars(args))
 	else:
 		counter = WIGSTVCounter(election, vars(args))
 	

pyRCV2/method/base_stv.py

@@ -88,6 +88,7 @@ class BaseSTVCounter:
 		
 		self.total_orig = sum((b.value for b in self.election.ballots), Num('0'))
 		
+		self.step_results = []
 		self.num_elected = 0
 		self.num_excluded = 0
 		

pyRCV2/method/meek.py

@@ -0,0 +1,207 @@
+#   pyRCV2: Preferential vote counting
+#   Copyright © 2020–2021  Lee Yingtong Li (RunasSudo)
+#
+#   This program is free software: you can redistribute it and/or modify
+#   it under the terms of the GNU Affero General Public License as published by
+#   the Free Software Foundation, either version 3 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 Affero General Public License for more details.
+#
+#   You should have received a copy of the GNU Affero General Public License
+#   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+__pragma__ = lambda x: None
+
+from pyRCV2.method.base_stv import BaseSTVCounter, STVException
+from pyRCV2.model import CandidateState, CountCard, CountStepResult
+from pyRCV2.numbers import Num
+from pyRCV2.safedict import SafeDict
+
+class MeekCountCard(CountCard):
+	def __init__(self, *args):
+		CountCard.__init__(self, *args)
+		self.keep_value = Num(1) # Not read by the count algorithm, but can be used for auditing
+	
+	def clone(self):
+		"""Overrides CountCard.clone"""
+		result = MeekCountCard()
+		result.orig_votes = self.orig_votes
+		result.transfers = self.transfers
+		result.parcels = [[(b[0].clone(), b[1]) for b in p] for p in self.parcels]
+		result.state = self.state
+		result.order_elected = self.order_elected
+		result.keep_value = keep_value
+		return result
+
+class MeekSTVCounter(BaseSTVCounter):
+	def __init__(self, *args):
+		BaseSTVCounter.__init__(self, *args)
+		self.candidates = SafeDict([(c, MeekCountCard()) for c in self.election.candidates])
+		
+		self._quota_tolerance_ub = Num('1.0001')
+		self._quota_tolerance_lb = Num('0.9999')
+	
+	def reset(self):
+		if self.options['quota_mode'] != 'progressive':
+			raise STVException('Meek method requires --quota-mode progressive')
+		if self.options['bulk_exclude']:
+			raise STVException('Meek method is incompatible with --bulk_exclude')
+		if self.options['defer_surpluses']:
+			raise STVException('Meek method is incompatible with --defer-surpluses')
+		if self.options['papers'] != 'both':
+			raise STVException('Meek method is incompatible with --transferable-only')
+		if self.options['exclusion'] != 'one_round':
+			raise STVException('Meek method requires --exclusion one_round')
+		if self.options['round_tvs'] is not None:
+			raise STVException('Meek method is incompatible with --round-tvs')
+		return BaseSTVCounter.reset(self)
+	
+	def distribute_first_preferences(self):
+		"""
+		Overrides BaseSTVCounter.distribute_first_preferences
+		Unlike in other STV methods, this is called not only as part of reset() but also at other stages
+		"""
+		
+		# Reset the count
+		# Carry over candidate states, keep values, etc.
+		new_candidates = SafeDict()
+		for candidate, count_card in self.candidates.items():
+			new_count_card = MeekCountCard()
+			new_count_card.state = count_card.state
+			new_count_card.keep_value = count_card.keep_value
+			new_count_card.order_elected = count_card.order_elected
+			
+			__pragma__('opov')
+			new_candidates[candidate] = new_count_card
+			__pragma__('noopov')
+		
+		self.candidates = new_candidates
+		self.exhausted = CountCard()
+		self.loss_fraction = CountCard()
+		
+		# Distribute votes
+		for ballot in self.election.ballots:
+			remaining_value = Num(ballot.value) # Clone the value so we don't update ballot.value
+			for candidate in ballot.preferences:
+				__pragma__('opov')
+				count_card = self.candidates[candidate]
+				__pragma__('noopov')
+				
+				if count_card.state == CandidateState.HOPEFUL:
+					# Hopeful candidate has keep value 1, so transfer entire remaining value
+					__pragma__('opov')
+					count_card.transfers += remaining_value
+					__pragma__('noopov')
+					remaining_value = Num(0)
+					break
+				elif count_card.state == CandidateState.EXCLUDED:
+					# Excluded candidate has keep value 0, so skip over this candidate
+					pass
+				elif count_card.state == CandidateState.ELECTED:
+					# Transfer according to elected candidate's keep value
+					__pragma__('opov')
+					count_card.transfers += remaining_value * count_card.keep_value
+					remaining_value *= (Num(1) - count_card.keep_value)
+					__pragma__('noopov')
+				else:
+					raise STVException('Unexpected candidate state')
+			
+			# Credit exhausted votes
+			__pragma__('opov')
+			self.exhausted.transfers += remaining_value
+			__pragma__('noopov')
+		
+		# Recompute transfers
+		if len(self.step_results) > 0:
+			last_result = self.step_results[len(self.step_results)-1]
+			__pragma__('opov')
+			for candidate, count_card in self.candidates.items():
+				count_card.continue_from(last_result.candidates[candidate])
+			self.exhausted.continue_from(last_result.exhausted)
+			self.loss_fraction.continue_from(last_result.loss_fraction)
+			__pragma__('noopov')
+	
+	def distribute_surpluses(self):
+		"""
+		Overrides BaseSTVCounter.distribute_surpluses
+		Surpluses are distributed in Meek STV by recomputing the keep values, and redistributing all votes
+		"""
+		
+		# Do surpluses need to be distributed?
+		__pragma__('opov')
+		has_surplus = [(c, cc) for c, cc in self.candidates.items() if cc.state == CandidateState.ELECTED and cc.votes / self.quota > self._quota_tolerance_ub]
+		__pragma__('noopov')
+		
+		if len(has_surplus) > 0:
+			while len(has_surplus) > 0:
+				# Recompute keep values
+				for candidate, count_card in has_surplus:
+					__pragma__('opov')
+					count_card.keep_value *= self.quota / count_card.votes
+					__pragma__('noopov')
+				
+				# Redistribute votes
+				self.distribute_first_preferences()
+				
+				# Recompute quota if more ballots have become exhausted
+				self.compute_quota()
+				
+				__pragma__('opov')
+				has_surplus = [(c, cc) for c, cc in self.candidates.items() if cc.state == CandidateState.ELECTED and cc.votes / self.quota > self._quota_tolerance_ub]
+				__pragma__('noopov')
+			
+			# Declare elected any candidates meeting the quota as a result of surpluses
+			# NB: We could do this earlier, but this shows the flow of the election more clearly in the count sheet
+			self.elect_meeting_quota()
+			
+			__pragma__('opov')
+			result = CountStepResult(
+				'Surpluses distributed',
+				self.candidates,
+				self.exhausted,
+				self.loss_fraction,
+				self.total + self.exhausted.votes + self.loss_fraction.votes,
+				self.quota,
+				self.vote_required_election,
+			)
+			__pragma__('noopov')
+			
+			self.step_results.append(result)
+			return result
+	
+	def do_exclusion(self, candidates_excluded):
+		"""
+		Overrides BaseSTVCounter.do_exclusion
+		"""
+		for candidate, count_card in candidates_excluded:
+			count_card.state = CandidateState.EXCLUDED
+		
+		# Redistribute votes
+		self.distribute_first_preferences()
+	
+	def elect_meeting_quota(self):
+		"""
+		Overrides BaseSTVCounter.elect_meeting_quota
+		Skip directly to CandidateState.ELECTED
+		"""
+		
+		# Does a candidate meet the quota?
+		meets_quota = [(c, cc) for c, cc in self.candidates.items() if cc.state == CandidateState.HOPEFUL and self.meets_quota(cc)]
+		
+		if len(meets_quota) > 0:
+			meets_quota.sort(key=lambda x: x[1].votes, reverse=True)
+			
+			# Declare elected any candidate who meets the quota
+			while len(meets_quota) > 0:
+				x = self.choose_highest(meets_quota)
+				candidate, count_card = x[0], x[1]
+				
+				count_card.state = CandidateState.ELECTED
+				self.num_elected += 1
+				count_card.order_elected = self.num_elected
+				
+				meets_quota.remove(x)

pyRCV2/model.py

@@ -94,6 +94,14 @@ class CountCard:
 		result.order_elected = self.order_elected
 		return result
 	
+	def continue_from(self, previous):
+		"""Adjust this count card's transfers, etc. so its total votes continue on from the previous values"""
+		votes = self.votes
+		self.orig_votes = previous.votes
+		__pragma__('opov')
+		self.transfers = votes - self.orig_votes
+		__pragma__('noopov')
+
 class CountStepResult:
 	def __init__(self, comment, candidates, exhausted, loss_fraction, total, quota, vote_required_election):
 		self.comment = comment

pyRCV2/transcrypt.py

@@ -16,7 +16,7 @@
 
 import pyRCV2.blt
 import pyRCV2.model
-import pyRCV2.method, pyRCV2.method.base_stv
+import pyRCV2.method, pyRCV2.method.base_stv, pyRCV2.method.meek
 import pyRCV2.numbers
 import pyRCV2.random
 import pyRCV2.ties