// This file is part of Substrate.

// Copyright (C) 2017-2020 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.

//! Voting thresholds.

#[cfg(feature = "std")]
use serde::{Serialize, Deserialize};
use codec::{Encode, Decode};
use sp_runtime::traits::{Zero, IntegerSquareRoot};
use sp_std::ops::{Add, Mul, Div, Rem};
use crate::Tally;

/// A means of determining if a vote is past pass threshold.
#[derive(Clone, Copy, PartialEq, Eq, Encode, Decode, sp_runtime::RuntimeDebug)]
#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
pub enum VoteThreshold {
	/// A supermajority of approvals is needed to pass this vote.
	SuperMajorityApprove,
	/// A supermajority of rejects is needed to fail this vote.
	SuperMajorityAgainst,
	/// A simple majority of approvals is needed to pass this vote.
	SimpleMajority,
}

pub trait Approved<Balance> {
	/// Given a `tally` of votes and a total size of `electorate`, this returns `true` if the
	/// overall outcome is in favor of approval according to `self`'s threshold method.
	fn approved(&self, tally: Tally<Balance>, electorate: Balance) -> bool;
}

/// Return `true` iff `n1 / d1 < n2 / d2`. `d1` and `d2` may not be zero.
fn compare_rationals<T: Zero + Mul<T, Output = T> + Div<T, Output = T> + Rem<T, Output = T> + Ord + Copy>(mut n1: T, mut d1: T, mut n2: T, mut d2: T) -> bool {
	// Uses a continued fractional representation for a non-overflowing compare.
	// Detailed at https://janmr.com/blog/2014/05/comparing-rational-numbers-without-overflow/.
	loop {
		let q1 = n1 / d1;
		let q2 = n2 / d2;
		if q1 < q2 {
			return true;
		}
		if q2 < q1 {
			return false;
		}
		let r1 = n1 % d1;
		let r2 = n2 % d2;
		if r2.is_zero() {
			return false;
		}
		if r1.is_zero() {
			return true;
		}
		n1 = d2;
		n2 = d1;
		d1 = r2;
		d2 = r1;
	}
}

impl<
	Balance: IntegerSquareRoot + Zero + Ord + Add<Balance, Output = Balance>
		+ Mul<Balance, Output = Balance> + Div<Balance, Output = Balance>
		+ Rem<Balance, Output = Balance> + Copy,
> Approved<Balance> for VoteThreshold {
	fn approved(&self, tally: Tally<Balance>, electorate: Balance) -> bool {
		let sqrt_voters = tally.turnout.integer_sqrt();
		let sqrt_electorate = electorate.integer_sqrt();
		if sqrt_voters.is_zero() { return false; }
		match *self {
			VoteThreshold::SuperMajorityApprove =>
				compare_rationals(tally.nays, sqrt_voters, tally.ayes, sqrt_electorate),
			VoteThreshold::SuperMajorityAgainst =>
				compare_rationals(tally.nays, sqrt_electorate, tally.ayes, sqrt_voters),
			VoteThreshold::SimpleMajority => tally.ayes > tally.nays,
		}
	}
}

#[cfg(test)]
mod tests {
	use super::*;

	#[test]
	fn should_work() {
		assert!(!VoteThreshold::SuperMajorityApprove.approved(Tally{ayes: 60, nays: 50, turnout: 110}, 210));
		assert!(VoteThreshold::SuperMajorityApprove.approved(Tally{ayes: 100, nays: 50, turnout: 150}, 210));
	}
}
