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Better prime election. (#6939)

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* Better prime election.

* improve docs

* more sensible variable names

* link to Borda count wiki

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
This commit is contained in:
Kian Paimani
2020-08-26 18:26:56 +02:00
committed by GitHub
parent e69d12e76c
commit 314d948687
1 changed files with 24 additions and 9 deletions
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substrate/frame/elections-phragmen/src/lib.rs
+24 -9
View File
@@ -87,7 +87,7 @@ use codec::{Encode, Decode};
use sp_std::prelude::*;
use sp_runtime::{
DispatchError, RuntimeDebug, Perbill,
traits::{Zero, StaticLookup, Convert},
traits::{Zero, StaticLookup, Convert, Saturating},
};
use frame_support::{
decl_storage, decl_event, ensure, decl_module, decl_error,
@@ -904,14 +904,20 @@ impl<T: Trait> Module<T> {
to_votes(Self::locked_stake_of(who))
};
let voters_and_votes = Voting::<T>::iter()
.map(|(voter, (stake, targets))| { (voter, to_votes(stake), targets) })
// used for prime election.
let voters_and_stakes = Voting::<T>::iter()
.map(|(voter, (stake, targets))| { (voter, stake, targets) })
.collect::<Vec<_>>();
// used for phragmen.
let voters_and_votes = voters_and_stakes.iter()
.cloned()
.map(|(voter, stake, targets)| { (voter, to_votes(stake), targets)} )
.collect::<Vec<_>>();
let maybe_phragmen_result = sp_npos_elections::seq_phragmen::<T::AccountId, Perbill>(
num_to_elect,
0,
candidates,
voters_and_votes.clone(),
voters_and_votes,
);
if let Some(ElectionResult { winners, assignments }) = maybe_phragmen_result {
@@ -965,17 +971,26 @@ impl<T: Trait> Module<T> {
// save the members, sorted based on account id.
new_members.sort_by(|i, j| i.0.cmp(&j.0));
let mut prime_votes: Vec<_> = new_members.iter().map(|c| (&c.0, VoteWeight::zero())).collect();
for (_, stake, targets) in voters_and_votes.into_iter() {
for (votes, who) in targets.iter()
// Now we select a prime member using a [Borda count](https://en.wikipedia.org/wiki/Borda_count).
// We weigh everyone's vote for that new member by a multiplier based on the order
// of the votes. i.e. the first person a voter votes for gets a 16x multiplier,
// the next person gets a 15x multiplier, an so on... (assuming `MAXIMUM_VOTE` = 16)
let mut prime_votes: Vec<_> = new_members.iter().map(|c| (&c.0, BalanceOf::<T>::zero())).collect();
for (_, stake, targets) in voters_and_stakes.into_iter() {
for (vote_multiplier, who) in targets.iter()
.enumerate()
.map(|(votes, who)| ((MAXIMUM_VOTE - votes) as u32, who))
.map(|(vote_position, who)| ((MAXIMUM_VOTE - vote_position) as u32, who))
{
if let Ok(i) = prime_votes.binary_search_by_key(&who, |k| k.0) {
prime_votes[i].1 += stake * votes as VoteWeight;
prime_votes[i].1 = prime_votes[i].1.saturating_add(
stake.saturating_mul(vote_multiplier.into())
);
}
}
}
// We then select the new member with the highest weighted stake. In the case of
// a tie, the last person in the list with the tied score is selected. This is
// the person with the "highest" account id based on the sort above.
let prime = prime_votes.into_iter().max_by_key(|x| x.1).map(|x| x.0.clone());
// new_members_ids is sorted by account id.