Implements approval voting to use as a NposSolver (#13367)

* Implements the approval voting methods in sp_npos_elections

bin/node/runtime/src/lib.rs

@@ -25,7 +25,7 @@
 use codec::{Decode, Encode, MaxEncodedLen};
 use frame_election_provider_support::{
 	onchain, weights::SubstrateWeight, BalancingConfig, ElectionDataProvider, SequentialPhragmen,
-	VoteWeight,
+	VoteWeight, ApprovalVoting,
 };
 use frame_support::{
 	construct_runtime,
@@ -1037,7 +1037,7 @@ impl pallet_elections::Config for Runtime {
 	type MaxVoters = MaxVoters;
 	type MaxVotesPerVoter = MaxVotesPerVoter;
 	type MaxCandidates = MaxCandidates;
-	type ElectionSolver = SequentialPhragmen<Self::AccountId, Perbill>;
+	type ElectionSolver = ApprovalVoting<Self::AccountId, Perbill>;
 	type SolverWeightInfo = SubstrateWeight<Runtime>;
 	type WeightInfo = pallet_elections::weights::SubstrateWeight<Runtime>;
 }

frame/election-provider-support/benchmarking/src/lib.rs

@@ -23,7 +23,7 @@
 
 use codec::Decode;
 use frame_benchmarking::v1::{benchmarks, Vec};
-use frame_election_provider_support::{NposSolver, PhragMMS, SequentialPhragmen};
+use frame_election_provider_support::{ApprovalVoting, NposSolver, PhragMMS, SequentialPhragmen};
 
 pub struct Pallet<T: Config>(frame_system::Pallet<T>);
 pub trait Config: frame_system::Config {}
@@ -88,4 +88,17 @@ benchmarks! {
 				::solve(d as usize, targets, voters).is_ok()
 		);
 	}
+
+	approval_voting {
+		let v in (VOTERS[0]) .. VOTERS[1];
+		let t in (TARGETS[0]) .. TARGETS[1];
+		let d in (VOTES_PER_VOTER[0]) .. VOTES_PER_VOTER[1];
+
+		let (voters, targets) = set_up_voters_targets::<T::AccountId>(v, t, d as usize);
+	}: {
+		assert!(
+			ApprovalVoting::<T::AccountId, sp_runtime::Perbill>
+				::solve(d as usize, targets, voters).is_ok()
+		);
+	}
 }

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

@@ -660,6 +660,29 @@ impl<AccountId: IdentifierT, Accuracy: PerThing128, Balancing: Get<Option<Balanc
 	}
 }
 
+/// A wrapper for [`sp_npos_elections::approval_voting()`] that implements [`NposSolver`]. See the
+/// documentation of [`sp_npos_elections::approval_voting()`] for more info.
+pub struct ApprovalVoting<AccountId, Accuracy>(sp_std::marker::PhantomData<(AccountId, Accuracy)>);
+
+impl<AccountId: IdentifierT, Accuracy: PerThing128> NposSolver
+	for ApprovalVoting<AccountId, Accuracy>
+{
+	type AccountId = AccountId;
+	type Accuracy = Accuracy;
+	type Error = sp_npos_elections::Error;
+	fn solve(
+		winners: usize,
+		targets: Vec<Self::AccountId>,
+		voters: Vec<(Self::AccountId, VoteWeight, impl IntoIterator<Item = Self::AccountId>)>,
+	) -> Result<ElectionResult<Self::AccountId, Self::Accuracy>, Self::Error> {
+		sp_npos_elections::approval_voting(winners, targets, voters)
+	}
+
+	fn weight<T: WeightInfo>(voters: u32, targets: u32, vote_degree: u32) -> Weight {
+		T::approval_voting(voters, targets, vote_degree)
+	}
+}
+
 /// A voter, at the level of abstraction of this crate.
 pub type Voter<AccountId, Bound> = (AccountId, VoteWeight, BoundedVec<AccountId, Bound>);
 

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

@@ -185,7 +185,7 @@ impl<T: Config> ElectionProvider for OnChainExecution<T> {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use crate::{ElectionProvider, PhragMMS, SequentialPhragmen};
+	use crate::{ApprovalVoting, ElectionProvider, PhragMMS, SequentialPhragmen};
 	use frame_support::{assert_noop, parameter_types, traits::ConstU32};
 	use sp_npos_elections::Support;
 	use sp_runtime::Perbill;
@@ -235,6 +235,7 @@ mod tests {
 
 	struct PhragmenParams;
 	struct PhragMMSParams;
+	struct ApprovalVotingParams;
 
 	parameter_types! {
 		pub static MaxWinners: u32 = 10;
@@ -261,6 +262,16 @@ mod tests {
 		type TargetsBound = ConstU32<400>;
 	}
 
+	impl Config for ApprovalVotingParams {
+		type System = Runtime;
+		type Solver = ApprovalVoting<AccountId, Perbill>;
+		type DataProvider = mock_data_provider::DataProvider;
+		type WeightInfo = ();
+		type MaxWinners = MaxWinners;
+		type VotersBound = ConstU32<600>;
+		type TargetsBound = ConstU32<400>;
+	}
+
 	mod mock_data_provider {
 		use frame_support::{bounded_vec, traits::ConstU32};
 
@@ -333,4 +344,21 @@ mod tests {
 			);
 		})
 	}
+
+	#[test]
+	fn onchain_approval_voting_works() {
+		sp_io::TestExternalities::new_empty().execute_with(|| {
+			DesiredTargets::set(3);
+
+			// note that the `OnChainExecution::elect` implementation normalizes the vote weights.
+			assert_eq!(
+				<OnChainExecution::<ApprovalVotingParams> as ElectionProvider>::elect().unwrap(),
+				vec![
+					(10, Support { total: 20, voters: vec![(1, 5), (3, 15)] }),
+					(20, Support { total: 15, voters: vec![(1, 5), (2, 10)] }),
+					(30, Support { total: 25, voters: vec![(2, 10), (3, 15)] })
+				]
+			)
+		})
+	}
 }

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

@@ -47,6 +47,7 @@ use sp_std::marker::PhantomData;
 pub trait WeightInfo {
 	fn phragmen(v: u32, t: u32, d: u32, ) -> Weight;
 	fn phragmms(v: u32, t: u32, d: u32, ) -> Weight;
+	fn approval_voting(v: u32, t: u32, d: u32, ) -> Weight;
 }
 
 /// Weights for pallet_election_provider_support_benchmarking using the Substrate node and recommended hardware.
@@ -70,6 +71,10 @@ impl<T: frame_system::Config> WeightInfo for SubstrateWeight<T> {
 			// Standard Error: 6_649_000
 			.saturating_add(Weight::from_ref_time(1_711_424_000 as u64).saturating_mul(d as u64))
 	}
+
+	fn approval_voting(_v: u32, _t: u32, _d: u32, ) -> Weight {
+        Weight::zero()
+    }
 }
 
 // For backwards compatibility and tests
@@ -92,4 +97,7 @@ impl WeightInfo for () {
 			// Standard Error: 6_649_000
 			.saturating_add(Weight::from_ref_time(1_711_424_000 as u64).saturating_mul(d as u64))
 	}
+    fn approval_voting(_v: u32, _t: u32, _d: u32, ) -> Weight {
+        Weight::zero()
+    }
 }

frame/elections/src/lib.rs

@@ -1285,7 +1285,7 @@ impl<T: Config> ContainsLengthBound for Pallet<T> {
 mod tests {
 	use super::*;
 	use crate as elections;
-	use frame_election_provider_support::{weights::SubstrateWeight, SequentialPhragmen};
+	use frame_election_provider_support::{weights::SubstrateWeight, ApprovalVoting};
 	use frame_support::{
 		assert_noop, assert_ok,
 		dispatch::DispatchResultWithPostInfo,
@@ -1419,7 +1419,7 @@ mod tests {
 		type WeightInfo = ();
 		type MaxVoters = MaxVoters;
 		type MaxCandidates = MaxCandidates;
-		type ElectionSolver = SequentialPhragmen<Self::AccountId, Perbill>;
+		type ElectionSolver = ApprovalVoting<Self::AccountId, Perbill>;
 		type SolverWeightInfo = SubstrateWeight<Test>;
 		type MaxVotesPerVoter = ConstU32<16>;
 	}

primitives/npos-elections/src/approval_voting.rs

@@ -0,0 +1,78 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2023 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Implementation of the approval voting election method.
+//!
+//! This method allows voters to select many candidates and backing each of them with the same
+//! vote weight. The candidates with the most backing are the election winners.
+
+use crate::{setup_inputs, ElectionResult, IdentifierT, PerThing128, VoteWeight};
+use sp_arithmetic::traits::Zero;
+use sp_std::{cmp::Reverse, vec::Vec};
+
+/// Execute an approvals voting election scheme. The return type is a list of winners and a weight
+/// distribution vector of all voters who contribute to the winners.
+///
+/// - The vote assignment distribution for each vote is always 100%, since a voter backs a candidate
+///   with its full stake, regardless of how many candidates are backed by the same stake. However,
+///   the caller may normalize votes on site if required.
+/// - Returning winners are sorted based on desirability. Voters are unsorted.
+/// - The returning winners are zipped with their final backing stake. Yet, to get the exact final
+///   weight distribution from the winner's point of view, one needs to build a support map. See
+///   [`crate::SupportMap`] for more info. Note that this backing stake is computed in
+///   ExtendedBalance and may be slightly different that what will be computed from the support map,
+///   due to accuracy loss.
+///
+/// This can only fail of the normalization fails. This can happen if for any of the resulting
+/// assignments, `assignment.distribution.map(|p| p.deconstruct()).sum()` fails to fit inside
+/// `UpperOf<P>`. A user of this crate may statically assert that this can never happen and safely
+/// `expect` this to return `Ok`.
+pub fn approval_voting<AccountId: IdentifierT, P: PerThing128>(
+	to_elect: usize,
+	candidates: Vec<AccountId>,
+	voters: Vec<(AccountId, VoteWeight, impl IntoIterator<Item = AccountId>)>,
+) -> Result<ElectionResult<AccountId, P>, crate::Error> {
+	let to_elect = to_elect.min(candidates.len());
+
+	let (mut candidates, mut voters) = setup_inputs(candidates, voters);
+
+	candidates.sort_by_key(|c| Reverse(c.borrow().approval_stake));
+
+	let winners = candidates
+		.into_iter()
+		.take(to_elect)
+		.map(|w| {
+			w.borrow_mut().elected = true;
+			w
+		})
+		.map(|w_ptr| (w_ptr.borrow().who.clone(), w_ptr.borrow().approval_stake))
+		.collect();
+
+	for voter in &mut voters {
+		for edge in &mut voter.edges {
+			if edge.candidate.borrow().elected {
+				edge.weight = voter.budget
+			} else {
+				edge.weight = Zero::zero()
+			}
+		}
+	}
+
+	let assignments = voters.into_iter().filter_map(|v| v.into_assignment()).collect::<Vec<_>>();
+
+	Ok(ElectionResult { winners, assignments })
+}

primitives/npos-elections/src/lib.rs

@@ -24,6 +24,8 @@
 //! - [`balance`](balancing::balance): Implements the star balancing algorithm. This iterative
 //!   process can push a solution toward being more "balanced", which in turn can increase its
 //!   score.
+//! - [`approval_voting`](approval_voting::approval_voting): Implements an approval voting electoral
+//!   system where voters can back multiple candidates with the same stake.
 //!
 //! ### Terminology
 //!
@@ -89,6 +91,7 @@ mod mock;
 #[cfg(test)]
 mod tests;
 
+pub mod approval_voting;
 mod assignments;
 pub mod balancing;
 pub mod helpers;
@@ -99,6 +102,7 @@ pub mod pjr;
 pub mod reduce;
 pub mod traits;
 
+pub use approval_voting::*;
 pub use assignments::{Assignment, StakedAssignment};
 pub use balancing::*;
 pub use helpers::*;

primitives/npos-elections/src/phragmen.rs

@@ -57,7 +57,7 @@ const DEN: ExtendedBalance = ExtendedBalance::max_value();
 /// - The returning weight distribution is _normalized_, meaning that it is guaranteed that the sum
 ///   of the ratios in each voter's distribution sums up to exactly `P::one()`.
 ///
-/// This can only fail of the normalization fails. This can happen if for any of the resulting
+/// This can only fail if the normalization fails. This can happen if for any of the resulting
 /// assignments, `assignment.distribution.map(|p| p.deconstruct()).sum()` fails to fit inside
 /// `UpperOf<P>`. A user of this crate may statically assert that this can never happen and safely
 /// `expect` this to return `Ok`.

primitives/npos-elections/src/tests.rs

@@ -18,12 +18,57 @@
 //! Tests for npos-elections.
 
 use crate::{
-	balancing, helpers::*, mock::*, seq_phragmen, seq_phragmen_core, setup_inputs, to_support_map,
-	Assignment, BalancingConfig, ElectionResult, ExtendedBalance, StakedAssignment, Support, Voter,
+	approval_voting::*, balancing, helpers::*, mock::*, seq_phragmen, seq_phragmen_core,
+	setup_inputs, to_support_map, Assignment, BalancingConfig, ElectionResult, ExtendedBalance,
+	StakedAssignment, Support, Voter,
 };
 use sp_arithmetic::{PerU16, Perbill, Percent, Permill};
 use substrate_test_utils::assert_eq_uvec;
 
+#[test]
+fn approval_voting_works() {
+	let candidates = vec![1, 2, 3, 4];
+	let voters = vec![(10, vec![1, 2]), (20, vec![1, 2]), (30, vec![1, 2, 3]), (40, vec![4])];
+	let stake_of = create_stake_of(&[(10, 10), (20, 20), (30, 30), (40, 40)]);
+
+	let voters = voters
+		.iter()
+		.map(|(ref v, ref vs)| (*v, stake_of(v), vs.clone()))
+		.collect::<Vec<_>>();
+
+	let ElectionResult::<_, Perbill> { winners, assignments } =
+		approval_voting(3, candidates, voters).unwrap();
+
+	assert_eq_uvec!(winners, vec![(1, 60), (2, 60), (4, 40)]);
+	assert_eq_uvec!(
+		assignments,
+		vec![
+			Assignment {
+				who: 10u64,
+				distribution: vec![
+					(1, Perbill::from_percent(100)),
+					(2, Perbill::from_percent(100))
+				]
+			},
+			Assignment {
+				who: 20u64,
+				distribution: vec![
+					(1, Perbill::from_percent(100)),
+					(2, Perbill::from_percent(100))
+				]
+			},
+			Assignment {
+				who: 30u64,
+				distribution: vec![
+					(1, Perbill::from_percent(100)),
+					(2, Perbill::from_percent(100))
+				]
+			},
+			Assignment { who: 40u64, distribution: vec![(4, Perbill::from_percent(100))] },
+		]
+	);
+}
+
 #[test]
 fn float_phragmen_poc_works() {
 	let candidates = vec![1, 2, 3];