Implement backwards tie breaking

html/index.html

@@ -105,7 +105,7 @@
 			<label>
 				Ties:
 				<select id="selTies">
-					<option value="backwards_random" selected>Backwards then random (NYI)</option>
+					<option value="backwards_random" selected>Backwards then random</option>
 					<option value="random">Random</option>
 				</select>
 			</label>

html/worker.js

@@ -50,7 +50,7 @@ onmessage = function(evt) {
 	}
 	
 	if (evt.data.options['ties'] === 'backwards_random') {
-		counter.options['ties'] = [py.pyRCV2.ties.TiesBackwards(), py.pyRCV2.ties.TiesRandom(evt.data.seed)];
+		counter.options['ties'] = [py.pyRCV2.ties.TiesBackwards(counter), py.pyRCV2.ties.TiesRandom(evt.data.seed)];
 	} else if (evt.data.options['ties'] === 'random') {
 		counter.options['ties'] = [py.pyRCV2.ties.TiesRandom(evt.data.seed)];
 	}

pyRCV2/cli/stv.py

@@ -91,7 +91,7 @@ def main(args):
 	counter.options['ties'] = []
 	for t in args.ties:
 		if t == 'backwards':
-			counter.options['ties'].append(TiesBackwards())
+			counter.options['ties'].append(TiesBackwards(counter))
 		elif t == 'prompt':
 			counter.options['ties'].append(TiesPrompt())
 		elif t == 'random':

pyRCV2/method/base_stv.py

@@ -62,7 +62,8 @@ class BaseSTVCounter:
 	def reset(self):
 		"""
 		Public function:
-		Reset the count and perform the first step
+		Perform the first step (distribute first preferences)
+		Does not reset the states of candidates, etc.
 		"""
 		
 		# Distribute first preferences
@@ -83,7 +84,7 @@ class BaseSTVCounter:
 		self.elect_meeting_quota()
 		
 		__pragma__('opov')
-		return CountStepResult(
+		result = CountStepResult(
 			'First preferences',
 			self.candidates,
 			self.exhausted,
@@ -93,6 +94,9 @@ class BaseSTVCounter:
 		)
 		__pragma__('noopov')
 		
+		self.step_results = [result]
+		return result
+	
 	def step(self):
 		"""
 		Public function:
@@ -151,7 +155,7 @@ class BaseSTVCounter:
 					self.num_elected += 1
 			
 			__pragma__('opov')
-			return CountStepResult(
+			result = CountStepResult(
 				'Bulk election',
 				self.candidates,
 				self.exhausted,
@@ -161,6 +165,9 @@ class BaseSTVCounter:
 			)
 			__pragma__('noopov')
 			
+			self.step_results.append(result)
+			return result
+	
 	def distribute_surpluses(self):
 		"""
 		Distribute surpluses, if any
@@ -200,7 +207,7 @@ class BaseSTVCounter:
 			self.elect_meeting_quota()
 			
 			__pragma__('opov')
-			return CountStepResult(
+			result = CountStepResult(
 				'Surplus of ' + candidate_surplus.name,
 				self.candidates,
 				self.exhausted,
@@ -210,6 +217,9 @@ class BaseSTVCounter:
 			)
 			__pragma__('noopov')
 			
+			self.step_results.append(result)
+			return result
+	
 	def do_surplus(self, candidate_surplus, count_card, surplus):
 		"""
 		Transfer the surplus of the given candidate
@@ -237,7 +247,7 @@ class BaseSTVCounter:
 		self.elect_meeting_quota()
 		
 		__pragma__('opov')
-		return CountStepResult(
+		result = CountStepResult(
 			'Exclusion of ' + candidate_excluded.name,
 			self.candidates,
 			self.exhausted,
@@ -247,6 +257,9 @@ class BaseSTVCounter:
 		)
 		__pragma__('noopov')
 		
+		self.step_results.append(result)
+		return result
+	
 	def candidate_to_exclude(self):
 		"""
 		Determine the candidate to exclude

pyRCV2/model.py

@@ -40,6 +40,9 @@ class Ballot:
 		self.value = value
 		self.preferences = preferences
 	
+	def clone(self):
+		return Ballot(self.value, self.preferences)
+
 class Election:
 	"""
 	Represents a BLT election
@@ -74,6 +77,16 @@ class CountCard:
 		self.orig_votes = self.votes
 		self.transfers = Num('0')
 	
+	def clone(self):
+		"""Return a clone of this count card (including cloning ballots) as a record of this stage"""
+		result = CountCard()
+		result.orig_votes = self.orig_votes
+		result.transfers = self.transfers
+		result.ballots = [b.clone() for b in self.ballots]
+		result.state = self.state
+		result.order_elected = self.order_elected
+		return result
+
 class CountCompleted:
 	pass
 
@@ -87,3 +100,14 @@ class CountStepResult:
 		
 		self.total = total
 		self.quota = quota
+	
+	def clone(self):
+		"""Return a clone of this result as a record of this stage"""
+		
+		candidates = SafeDict()
+		for c, cc in self.candidates.items():
+			__pragma__('opov')
+			candidates[c] = cc.clone()
+			__pragma__('noopov')
+		
+		return CountStepResult(self.comment, candidates, self.exhausted.clone(), self.loss_fraction.clone(), self.total, self.quota)

pyRCV2/ties.py

@@ -65,14 +65,40 @@ class TiesPrompt:
 	def choose_highest(self, l):
 		return self.choose_lowest(l)
 
+# FIXME: This is untested!
 class TiesBackwards:
+	"""
+	Break ties based on the candidate who had the highest/lowest total at the end
+	of the most recent stage where one candidate had a higher/lower total than
+	all other tied candidates, if such a stage exists
+	"""
+	
+	def __init__(self, counter):
+		self.counter = counter
+	
 	def choose_lowest(self, l):
-		raise Exception('Not yet implemented')
+		for result in reversed(self.counter.step_results):
+			__pragma__('opov')
+			l2 = [(x, result.candidates[x[0]].votes) for x in l]
+			l2.sort(key=lambda x: x[1])
+			if l2[0][1] < l2[1][1]: # Did one candidate have fewer votes than the others?
+				return l2[0][0]
+			__pragma__('noopov')
+		return None
 	
 	def choose_highest(self, l):
-		raise Exception('Not yet implemented')
+		for result in reversed(self.counter.step_results):
+			__pragma__('opov')
+			l2 = [(x, result.candidates[x[0]].votes) for x in l]
+			l2.sort(key=lambda x: x[1], reverse=True)
+			if l2[0][1] > l2[1][1]: # Did one candidate have more votes than the others?
+				return l2[0][0]
+			__pragma__('noopov')
+		return None
 
 class TiesRandom:
+	"""Break ties randomly, using the SHARandom deterministic RNG"""
+	
 	def __init__(self, seed):
 		self.random = SHARandom(seed)