Improve performance of realise_equal_rankings

src/election.rs

@@ -80,13 +80,18 @@ impl<N: Number> Election<N> {
 	
 	/// Convert ballots with equal rankings to strict-preference "minivoters"
 	pub fn realise_equal_rankings(&mut self) {
-		// Record total_votes so loss by fraction can be calculated
-		self.total_votes = Some(self.ballots.iter().fold(N::new(), |acc, b| acc + &b.orig_value));
+		if !self.ballots.iter().any(|b| b.has_equal_rankings()) {
+			// No equal rankings
+			return;
+		}
 		
-		let mut realised_ballots = Vec::new();
-		for ballot in self.ballots.iter() {
-			let mut b = ballot.realise_equal_rankings();
-			realised_ballots.append(&mut b);
+		// Record total_votes so loss by fraction can be calculated
+		// See crate::stv::gregory::distribute_first_preferences, etc.
+		self.total_votes = Some(self.ballots.iter().fold(N::new(), |mut acc, b| { acc += &b.orig_value; acc }));
+		
+		let mut realised_ballots = Vec::with_capacity(self.ballots.len());
+		for ballot in self.ballots.drain(..) {
+			ballot.realise_equal_rankings_into(&mut realised_ballots);
 		}
 		self.ballots = realised_ballots;
 	}
@@ -509,8 +514,18 @@ pub struct Ballot<N> {
 }
 
 impl<N: Number> Ballot<N> {
+	/// Check if this ballot has any equal rankings
+	pub fn has_equal_rankings(&self) -> bool {
+		return self.preferences.iter().any(|p| p.len() > 1);
+	}
+	
 	/// Convert ballot with equal rankings to strict-preference "minivoters"
-	pub fn realise_equal_rankings(&self) -> Vec<Ballot<N>> {
+	pub fn realise_equal_rankings_into(self, dest: &mut Vec<Ballot<N>>) {
+		if !self.has_equal_rankings() {
+			dest.push(self);
+			return;
+		}
+		
 		// Preferences for each minivoter
 		let mut minivoters = vec![Vec::new()];
 		
@@ -541,10 +556,13 @@ impl<N: Number> Ballot<N> {
 		}
 		
 		let weight_each = self.orig_value.clone() / N::from(minivoters.len());
-		let ballots = minivoters.into_iter()
-			.map(|p| Ballot { orig_value: weight_each.clone(), preferences: p })
-			.collect();
-		return ballots;
+		
+		for minivoter in minivoters {
+			dest.push(Ballot {
+				orig_value: weight_each.clone(),
+				preferences: minivoter
+			});
+		}
 	}
 }