Abstracts elections-phragmen pallet to use NposSolver (#12588)

* Abstracts elections-phragmen pallet to use NposSolver

* Update frame/elections-phragmen/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/elections-phragmen/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* changes the name of the pallet; adds changelog

* update changelog

* Adds weight testing

* Adds log macro_rules

* renames elections-phragment dir to elections

* weights rename

* fixes typo in cargo toml

* pre/post solve weight scafolding

* refactor do_post_election

* refactors into pre and post election solve for independent benchmarking

* deconstructs PreElectionResults struct

* updates benchmarking pre and post election solve; mock weights

* Update frame/elections/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/elections/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* addresses PR comments

* adds pre_solve and post_sove weights

* Adds comments on election pallet id param name change

* ".git/.scripts/bench-bot.sh" pallet dev pallet_elections

* Finishes pre-post solve weights

* Update frame/elections/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/elections/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Addresses PR comments: no panic in on_init path; nits

* Fixes node build

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

* Implements the approval voting methods in sp_npos_elections

* fmt

* remove unecessary file

* comment clarification

* re-run weights

* fix typo

* updates MaxVoters in tests for integrity_tests to pass

* Refactors election provider support benchmarks outside its own crate (#13431)

* Refactors election provider support benchmarks outside its own crate
---------

Co-authored-by: command-bot <>

---------

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: parity-processbot <>
Co-authored-by: Ross Bulat <ross@parity.io>

substrate/primitives/npos-elections/src/approval_voting.rs

@@ -0,0 +1,79 @@
+// 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. The weight
+/// vector of all voters who contribute to the winners, which for this scheme is always 100% per
+/// vote.
+///
+/// - 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 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`.
+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 })
+}

substrate/primitives/npos-elections/src/lib.rs

@@ -25,6 +25,8 @@
 //! - [`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
 //!
@@ -90,6 +92,7 @@ mod mock;
 #[cfg(test)]
 mod tests;
 
+pub mod approval_voting;
 mod assignments;
 pub mod balancing;
 pub mod helpers;
@@ -100,6 +103,7 @@ pub mod pjr;
 pub mod reduce;
 pub mod traits;
 
+pub use approval_voting::*;
 pub use assignments::{Assignment, StakedAssignment};
 pub use balancing::*;
 pub use helpers::*;

substrate/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`.

substrate/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];