clean the interface of supports map

frame/election-provider-multi-phase/src/lib.rs

@@ -1398,17 +1398,8 @@ impl<T: Config> Pallet<T> {
 		let target_at = helpers::target_at_fn::<T>(&snapshot_targets);
 		let voter_index = helpers::voter_index_fn_usize::<T>(&cache);
 
-		// First, make sure that all the winners are sane.
-		// OPTIMIZATION: we could first build the assignments, and then extract the winners directly
-		// from that, as that would eliminate a little bit of duplicate work. For now, we keep them
-		// separate: First extract winners separately from solution, and then assignments. This is
-		// also better, because we can reject solutions that don't meet `desired_targets` early on.
-		let winners = winners
-			.into_iter()
-			.map(|i| target_at(i).ok_or(FeasibilityError::InvalidWinner))
-			.collect::<Result<Vec<T::AccountId>, FeasibilityError>>()?;
-
-		// Then convert solution -> assignment. This will fail if any of the indices are gibberish.
+		// Then convert solution -> assignment. This will fail if any of the indices are gibberish,
+		// namely any of the voters or targets.
 		let assignments = solution
 			.into_assignment(voter_at, target_at)
 			.map_err::<FeasibilityError, _>(Into::into)?;
@@ -1444,14 +1435,10 @@ impl<T: Config> Pallet<T> {
 		// This might fail if the normalization fails. Very unlikely. See `integrity_test`.
 		let staked_assignments = assignment_ratio_to_staked_normalized(assignments, stake_of)
 			.map_err::<FeasibilityError, _>(Into::into)?;
-
-		// This might fail if one of the voter edges is pointing to a non-winner, which is not
-		// really possible anymore because all the winners come from the same `solution`.
-		let supports = sp_npos_elections::to_supports(&winners, &staked_assignments)
-			.map_err::<FeasibilityError, _>(Into::into)?;
+		let supports = sp_npos_elections::to_supports(&staked_assignments);
 
 		// Finally, check that the claimed score was indeed correct.
-		let known_score = (&supports).evaluate();
+		let known_score = supports.evaluate();
 		ensure!(known_score == score, FeasibilityError::InvalidScore);
 
 		Ok(ReadySolution { supports, compute, score })

frame/election-provider-multi-phase/src/mock.rs

@@ -30,8 +30,8 @@ use sp_core::{
 	H256,
 };
 use sp_npos_elections::{
-	assignment_ratio_to_staked_normalized, seq_phragmen, to_supports, to_without_backing,
-	ElectionResult, EvaluateSupport, NposSolution,
+	assignment_ratio_to_staked_normalized, seq_phragmen, to_supports, ElectionResult,
+	EvaluateSupport, NposSolution,
 };
 use sp_runtime::{
 	testing::Header,
@@ -157,25 +157,24 @@ pub fn raw_solution() -> RawSolution<SolutionOf<Runtime>> {
 	let RoundSnapshot { voters, targets } = MultiPhase::snapshot().unwrap();
 	let desired_targets = MultiPhase::desired_targets().unwrap();
 
-	let ElectionResult { winners, assignments } = seq_phragmen::<_, SolutionAccuracyOf<Runtime>>(
-		desired_targets as usize,
-		targets.clone(),
-		voters.clone(),
-		None,
-	)
-	.unwrap();
+	let ElectionResult { winners: _, assignments } =
+		seq_phragmen::<_, SolutionAccuracyOf<Runtime>>(
+			desired_targets as usize,
+			targets.clone(),
+			voters.clone(),
+			None,
+		)
+		.unwrap();
 
 	// closures
 	let cache = helpers::generate_voter_cache::<Runtime>(&voters);
 	let voter_index = helpers::voter_index_fn_linear::<Runtime>(&voters);
 	let target_index = helpers::target_index_fn_linear::<Runtime>(&targets);
 	let stake_of = helpers::stake_of_fn::<Runtime>(&voters, &cache);
 
-	let winners = to_without_backing(winners);
-
 	let score = {
 		let staked = assignment_ratio_to_staked_normalized(assignments.clone(), &stake_of).unwrap();
-		to_supports(&winners, &staked).unwrap().evaluate()
+		to_supports(&staked).evaluate()
 	};
 	let solution =
 		<SolutionOf<Runtime>>::from_assignment(&assignments, &voter_index, &target_index).unwrap();

frame/election-provider-multi-phase/src/unsigned.rs

@@ -312,7 +312,7 @@ impl<T: Config> Pallet<T> {
 			SolutionOf::<T>::try_from(assignments).map(|s| s.encoded_size())
 		};
 
-		let ElectionResult { assignments, winners } = election_result;
+		let ElectionResult { assignments, winners: _ } = election_result;
 
 		// Reduce (requires round-trip to staked form)
 		let sorted_assignments = {
@@ -371,8 +371,7 @@ impl<T: Config> Pallet<T> {
 		let solution = SolutionOf::<T>::try_from(&index_assignments)?;
 
 		// re-calc score.
-		let winners = sp_npos_elections::to_without_backing(winners);
-		let score = solution.clone().score(&winners, stake_of, voter_at, target_at)?;
+		let score = solution.clone().score(stake_of, voter_at, target_at)?;
 
 		let round = Self::round();
 		Ok((RawSolution { solution, score, round }, size))

frame/election-provider-support/src/onchain.rs

@@ -82,14 +82,13 @@ impl<T: Config> ElectionProvider<T::AccountId, T::BlockNumber> for OnChainSequen
 		let stake_of =
 			|w: &T::AccountId| -> VoteWeight { stake_map.get(w).cloned().unwrap_or_default() };
 
-		let ElectionResult { winners, assignments } =
+		let ElectionResult { winners: _, assignments } =
 			seq_phragmen::<_, T::Accuracy>(desired_targets as usize, targets, voters, None)
 				.map_err(Error::from)?;
 
 		let staked = assignment_ratio_to_staked_normalized(assignments, &stake_of)?;
-		let winners = to_without_backing(winners);
 
-		to_supports(&winners, &staked).map_err(Error::from)
+		Ok(to_supports(&staked))
 	}
 }
 

primitives/npos-elections/fuzzer/src/phragmen_balancing.rs

@@ -24,7 +24,7 @@ use honggfuzz::fuzz;
 use rand::{self, SeedableRng};
 use sp_npos_elections::{
 	assignment_ratio_to_staked_normalized, is_score_better, seq_phragmen, to_supports,
-	to_without_backing, EvaluateSupport, VoteWeight,
+	EvaluateSupport, VoteWeight,
 };
 use sp_runtime::Perbill;
 
@@ -58,8 +58,7 @@ fn main() {
 					&stake_of,
 				)
 				.unwrap();
-				let winners = to_without_backing(unbalanced.winners.clone());
-				let score = to_supports(winners.as_ref(), staked.as_ref()).unwrap().evaluate();
+				let score = to_supports(staked.as_ref()).evaluate();
 
 				if score[0] == 0 {
 					// such cases cannot be improved by balancing.
@@ -83,8 +82,7 @@ fn main() {
 						&stake_of,
 					)
 					.unwrap();
-					let winners = to_without_backing(balanced.winners);
-					to_supports(winners.as_ref(), staked.as_ref()).unwrap().evaluate()
+					to_supports(staked.as_ref()).evaluate()
 				};
 
 				let enhance = is_score_better(balanced_score, unbalanced_score, Perbill::zero());

primitives/npos-elections/fuzzer/src/phragmms_balancing.rs

@@ -23,8 +23,8 @@ use common::*;
 use honggfuzz::fuzz;
 use rand::{self, SeedableRng};
 use sp_npos_elections::{
-	assignment_ratio_to_staked_normalized, is_score_better, phragmms, to_supports,
-	to_without_backing, EvaluateSupport, VoteWeight,
+	assignment_ratio_to_staked_normalized, is_score_better, phragmms, to_supports, EvaluateSupport,
+	VoteWeight,
 };
 use sp_runtime::Perbill;
 
@@ -58,8 +58,7 @@ fn main() {
 					&stake_of,
 				)
 				.unwrap();
-				let winners = to_without_backing(unbalanced.winners.clone());
-				let score = to_supports(&winners, &staked).unwrap().evaluate();
+				let score = to_supports(&staked).evaluate();
 
 				if score[0] == 0 {
 					// such cases cannot be improved by balancing.
@@ -80,8 +79,7 @@ fn main() {
 				let staked =
 					assignment_ratio_to_staked_normalized(balanced.assignments.clone(), &stake_of)
 						.unwrap();
-				let winners = to_without_backing(balanced.winners);
-				to_supports(winners.as_ref(), staked.as_ref()).unwrap().evaluate()
+				to_supports(staked.as_ref()).evaluate()
 			};
 
 			let enhance = is_score_better(balanced_score, unbalanced_score, Perbill::zero());

primitives/npos-elections/fuzzer/src/reduce.rs

@@ -104,13 +104,11 @@ fn generate_random_phragmen_assignment(
 }
 
 fn assert_assignments_equal(
-	winners: &Vec<AccountId>,
 	ass1: &Vec<StakedAssignment<AccountId>>,
 	ass2: &Vec<StakedAssignment<AccountId>>,
 ) {
-	let support_1 = to_support_map::<AccountId>(winners, ass1).unwrap();
-	let support_2 = to_support_map::<AccountId>(winners, ass2).unwrap();
-
+	let support_1 = to_support_map::<AccountId>(ass1);
+	let support_2 = to_support_map::<AccountId>(ass2);
 	for (who, support) in support_1.iter() {
 		assert_eq!(support.total, support_2.get(who).unwrap().total);
 	}
@@ -134,7 +132,7 @@ fn reduce_and_compare(assignment: &Vec<StakedAssignment<AccountId>>, winners: &V
 		num_changed,
 	);
 
-	assert_assignments_equal(winners, &assignment, &altered_assignment);
+	assert_assignments_equal(&assignment, &altered_assignment);
 }
 
 fn assignment_len(assignments: &[StakedAssignment<AccountId>]) -> u32 {

primitives/npos-elections/src/lib.rs

@@ -361,107 +361,50 @@ pub type Supports<A> = Vec<(A, Support<A>)>;
 /// This is more helpful than a normal [`Supports`] as it allows faster error checking.
 pub type SupportMap<A> = BTreeMap<A, Support<A>>;
 
-/// Helper trait to convert from a support map to a flat support vector.
-pub trait FlattenSupportMap<A> {
-	/// Flatten the support.
-	fn flatten(self) -> Supports<A>;
-}
-
-impl<A> FlattenSupportMap<A> for SupportMap<A> {
-	fn flatten(self) -> Supports<A> {
-		self.into_iter().collect::<Vec<_>>()
-	}
-}
-
-/// Build the support map from the winners and assignments.
-///
-/// The list of winners is basically a redundancy for error checking only; It ensures that all the
-/// targets pointed to by the [`Assignment`] are present in the `winners`.
+/// Build the support map from the assignments.
 pub fn to_support_map<AccountId: IdentifierT>(
-	winners: &[AccountId],
 	assignments: &[StakedAssignment<AccountId>],
-) -> Result<SupportMap<AccountId>, Error> {
-	// Initialize the support of each candidate.
-	let mut supports = <SupportMap<AccountId>>::new();
-	winners.iter().for_each(|e| {
-		supports.insert(e.clone(), Default::default());
-	});
+) -> SupportMap<AccountId> {
+	let mut supports = <BTreeMap<AccountId, Support<AccountId>>>::new();
 
 	// build support struct.
-	for StakedAssignment { who, distribution } in assignments.iter() {
-		for (c, weight_extended) in distribution.iter() {
-			if let Some(support) = supports.get_mut(c) {
-				support.total = support.total.saturating_add(*weight_extended);
-				support.voters.push((who.clone(), *weight_extended));
-			} else {
-				return Err(Error::InvalidSupportEdge)
-			}
+	for StakedAssignment { who, distribution } in assignments.into_iter() {
+		for (c, weight_extended) in distribution.into_iter() {
+			let mut support = supports.entry(c.clone()).or_default();
+			support.total = support.total.saturating_add(*weight_extended);
+			support.voters.push((who.clone(), *weight_extended));
 		}
 	}
-	Ok(supports)
+
+	supports
 }
 
-/// Same as [`to_support_map`] except it calls `FlattenSupportMap` on top of the result to return a
+/// Same as [`to_support_map`] except it returns a
 /// flat vector.
-///
-/// Similar to [`to_support_map`], `winners` is used for error checking.
 pub fn to_supports<AccountId: IdentifierT>(
-	winners: &[AccountId],
 	assignments: &[StakedAssignment<AccountId>],
-) -> Result<Supports<AccountId>, Error> {
-	to_support_map(winners, assignments).map(FlattenSupportMap::flatten)
+) -> Supports<AccountId> {
+	to_support_map(assignments).into_iter().collect()
 }
 
 /// Extension trait for evaluating a support map or vector.
-pub trait EvaluateSupport<K> {
+pub trait EvaluateSupport {
 	/// Evaluate a support map. The returned tuple contains:
 	///
 	/// - Minimum support. This value must be **maximized**.
 	/// - Sum of all supports. This value must be **maximized**.
 	/// - Sum of all supports squared. This value must be **minimized**.
-	fn evaluate(self) -> ElectionScore;
-}
-
-/// A common wrapper trait for both (&A, &B) and &(A, B).
-///
-/// This allows us to implemented something for both `Vec<_>` and `BTreeMap<_>`, such as
-/// [`EvaluateSupport`].
-pub trait TupleRef<K, V> {
-	fn extract(&self) -> (&K, &V);
-}
-
-impl<K, V> TupleRef<K, V> for &(K, V) {
-	fn extract(&self) -> (&K, &V) {
-		(&self.0, &self.1)
-	}
-}
-
-impl<K, V> TupleRef<K, V> for (K, V) {
-	fn extract(&self) -> (&K, &V) {
-		(&self.0, &self.1)
-	}
-}
-
-impl<K, V> TupleRef<K, V> for (&K, &V) {
-	fn extract(&self) -> (&K, &V) {
-		(self.0, self.1)
-	}
+	fn evaluate(&self) -> ElectionScore;
 }
 
-impl<A, C, I> EvaluateSupport<A> for C
-where
-	C: IntoIterator<Item = I>,
-	I: TupleRef<A, Support<A>>,
-	A: IdentifierT,
-{
-	fn evaluate(self) -> ElectionScore {
+impl<AccountId: IdentifierT> EvaluateSupport for Supports<AccountId> {
+	fn evaluate(&self) -> ElectionScore {
 		let mut min_support = ExtendedBalance::max_value();
 		let mut sum: ExtendedBalance = Zero::zero();
 		// NOTE: The third element might saturate but fine for now since this will run on-chain and
 		// need to be fast.
 		let mut sum_squared: ExtendedBalance = Zero::zero();
-		for item in self {
-			let (_, support) = item.extract();
+		for (_, support) in self {
 			sum = sum.saturating_add(support.total);
 			let squared = support.total.saturating_mul(support.total);
 			sum_squared = sum_squared.saturating_add(squared);