Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
103 lines (91 sloc) 3.76 KB
// Copyright 2017-2019 Parity Technologies (UK) Ltd.
// This file is part of Substrate.
// Substrate is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
// Substrate is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with Substrate. If not, see <http://www.gnu.org/licenses/>.
//! Voting thresholds.
#[cfg(feature = "std")]
use serde::{Serialize, Deserialize};
use codec::{Encode, Decode};
use sr_primitives::traits::{Zero, IntegerSquareRoot};
use rstd::ops::{Add, Mul, Div, Rem};
/// A means of determining if a vote is past pass threshold.
#[derive(Clone, Copy, PartialEq, Eq, Encode, Decode)]
#[cfg_attr(feature = "std", derive(Serialize, Deserialize, Debug))]
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 `approve` votes for and `against` votes against from a total electorate size of
/// `electorate` (`electorate - (approve + against)` are abstainers), then returns true if the
/// overall outcome is in favor of approval.
fn approved(&self, approve: Balance, against: Balance, voters: 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 {
/// Given `approve` votes for and `against` votes against from a total electorate size of
/// `electorate` of whom `voters` voted (`electorate - voters` are abstainers) then returns true if the
/// overall outcome is in favor of approval.
///
/// We assume each *voter* may cast more than one *vote*, hence `voters` is not necessarily equal to
/// `approve + against`.
fn approved(&self, approve: Balance, against: Balance, voters: Balance, electorate: Balance) -> bool {
let sqrt_voters = voters.integer_sqrt();
let sqrt_electorate = electorate.integer_sqrt();
if sqrt_voters.is_zero() { return false; }
match *self {
VoteThreshold::SuperMajorityApprove =>
compare_rationals(against, sqrt_voters, approve, sqrt_electorate),
VoteThreshold::SuperMajorityAgainst =>
compare_rationals(against, sqrt_electorate, approve, sqrt_voters),
VoteThreshold::SimpleMajority => approve > against,
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn should_work() {
assert_eq!(VoteThreshold::SuperMajorityApprove.approved(60, 50, 110, 210), false);
assert_eq!(VoteThreshold::SuperMajorityApprove.approved(100, 50, 150, 210), true);
}
}
You can’t perform that action at this time.