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// This file is part of Substrate.

// Copyright (C) 2020 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.

//! An implementation of [`ElectionProvider`] that does an on-chain sequential phragmen.

use crate::{ElectionDataProvider, ElectionProvider};
use frame_support::{traits::Get, weights::Weight};
use sp_npos_elections::*;
use sp_std::{collections::btree_map::BTreeMap, marker::PhantomData, prelude::*};

/// Errors of the on-chain election.
#[derive(Eq, PartialEq, Debug)]
pub enum Error {
	/// An internal error in the NPoS elections crate.
	NposElections(sp_npos_elections::Error),
	/// Errors from the data provider.
	DataProvider(&'static str),
}

impl From<sp_npos_elections::Error> for Error {
	fn from(e: sp_npos_elections::Error) -> Self {
		Error::NposElections(e)
	}
}

/// A simple on-chain implementation of the election provider trait.
///
/// This will accept voting data on the fly and produce the results immediately.
///
/// ### Warning
///
/// This can be very expensive to run frequently on-chain. Use with care. Moreover, this
/// implementation ignores the additional data of the election data provider and gives no insight on
/// how much weight was consumed.
///
/// Finally, this implementation does not impose any limits on the number of voters and targets that
/// are provided.
pub struct OnChainSequentialPhragmen<T: Config>(PhantomData<T>);

/// Configuration trait of [`OnChainSequentialPhragmen`].
///
/// Note that this is similar to a pallet traits, but [`OnChainSequentialPhragmen`] is not a pallet.
pub trait Config {
	/// The block limits.
	type BlockWeights: Get<frame_system::limits::BlockWeights>;
	/// The account identifier type.
	type AccountId: IdentifierT;
	/// The block number type.
	type BlockNumber;
	/// The accuracy used to compute the election:
	type Accuracy: PerThing128;
	/// Something that provides the data for election.
	type DataProvider: ElectionDataProvider<Self::AccountId, Self::BlockNumber>;
}

impl<T: Config> ElectionProvider<T::AccountId, T::BlockNumber> for OnChainSequentialPhragmen<T> {
	type Error = Error;
	type DataProvider = T::DataProvider;

	fn elect() -> Result<(Supports<T::AccountId>, Weight), Self::Error> {
		let (voters, _) = Self::DataProvider::voters(None).map_err(Error::DataProvider)?;
		let (targets, _) = Self::DataProvider::targets(None).map_err(Error::DataProvider)?;
		let (desired_targets, _) =
			Self::DataProvider::desired_targets().map_err(Error::DataProvider)?;

		let mut stake_map: BTreeMap<T::AccountId, VoteWeight> = BTreeMap::new();

		voters.iter().for_each(|(v, s, _)| {
			stake_map.insert(v.clone(), *s);
		});

		let stake_of =
			|w: &T::AccountId| -> VoteWeight { stake_map.get(w).cloned().unwrap_or_default() };

		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)
			.map(|s| (s, T::BlockWeights::get().max_block))
	}
}

#[cfg(test)]
mod tests {
	use super::*;
	use frame_support::weights::Weight;
	use sp_npos_elections::Support;
	use sp_runtime::Perbill;

	type AccountId = u64;
	type BlockNumber = u32;
	struct Runtime;
	impl Config for Runtime {
		type BlockWeights = ();
		type AccountId = AccountId;
		type BlockNumber = BlockNumber;
		type Accuracy = Perbill;
		type DataProvider = mock_data_provider::DataProvider;
	}

	type OnChainPhragmen = OnChainSequentialPhragmen<Runtime>;

	mod mock_data_provider {
		use super::*;
		use crate::data_provider;

		pub struct DataProvider;

		impl ElectionDataProvider<AccountId, BlockNumber> for DataProvider {
			const MAXIMUM_VOTES_PER_VOTER: u32 = 2;
			fn voters(
				_: Option<usize>,
			) -> data_provider::Result<(Vec<(AccountId, VoteWeight, Vec<AccountId>)>, Weight)> {
				Ok((vec![(1, 10, vec![10, 20]), (2, 20, vec![30, 20]), (3, 30, vec![10, 30])], 0))
			}

			fn targets(_: Option<usize>) -> data_provider::Result<(Vec<AccountId>, Weight)> {
				Ok((vec![10, 20, 30], 0))
			}

			fn desired_targets() -> data_provider::Result<(u32, Weight)> {
				Ok((2, 0))
			}

			fn next_election_prediction(_: BlockNumber) -> BlockNumber {
				0
			}
		}
	}

	#[test]
	fn onchain_seq_phragmen_works() {
		assert_eq!(
			OnChainPhragmen::elect().unwrap().0,
			vec![
				(10, Support { total: 25, voters: vec![(1, 10), (3, 15)] }),
				(30, Support { total: 35, voters: vec![(2, 20), (3, 15)] })
			]
		);
	}
}