# Phragmén Election Module

An election module based on sequential phragmen.

## Term and Round

The election happens in _rounds_: every `N` blocks, all previous members are retired and a new set is elected (which may
or may not have an intersection with the previous set). Each round lasts for some number of blocks defined by
`TermDuration` storage item. The words _term_ and _round_ can be used interchangeably in this context.

`TermDuration` might change during a round. This can shorten or extend the length of the round. The next election
round's block number is never stored but rather always checked on the fly. Based on the current block number and
`TermDuration`, the condition `BlockNumber % TermDuration == 0` being satisfied will always trigger a new election
round.

## Voting

Voters can vote for any set of the candidates by providing a list of account ids. Invalid votes (voting for
non-candidates) are ignored during election. Yet, a voter _might_ vote for a future candidate. Voters reserve a bond as
they vote. Each vote defines a `value`. This amount is locked from the account of the voter and indicates the weight of
the vote. Voters can update their votes at any time by calling `vote()` again. This keeps the bond untouched but can
optionally change the locked `value`. After a round, votes are kept and might still be valid for further rounds. A voter
is responsible for calling `remove_voter` once they are done to have their bond back and remove the lock.

Voters also report other voters as being defunct to earn their bond. A voter is defunct once all of the candidates that
they have voted for are neither a valid candidate anymore nor a member. Upon reporting, if the target voter is actually
defunct, the reporter will be rewarded by the voting bond of the target. The target will lose their bond and get
removed. If the target is not defunct, the reporter is slashed and removed. To prevent being reported, voters should
manually submit a `remove_voter()` as soon as they are in the defunct state.

## Candidacy and Members

Candidates also reserve a bond as they submit candidacy. A candidate cannot take their candidacy back. A candidate can
end up in one of the below situations:
  - **Winner**: A winner is kept as a _member_. They must still have a bond in reserve and they are automatically
    counted as a candidate for the next election.
  - **Runner-up**: Runners-up are the best candidates immediately after the winners. The number of runners_up to keep is
    configurable. Runners-up are used, in order that they are elected, as replacements when a candidate is kicked by
    `[remove_member]`, or when an active member renounces their candidacy. Runners are automatically counted as a
    candidate for the next election.
  - **Loser**: Any of the candidate who are not a winner are left as losers. A loser might be an _outgoing member or
    runner_, meaning that they are an active member who failed to keep their spot. An outgoing will always lose their
    bond.

### Renouncing candidacy

All candidates, elected or not, can renounce their candidacy. A call to [`Module::renounce_candidacy`] will always cause
the candidacy bond to be refunded.

Note that with the members being the default candidates for the next round and votes persisting in storage, the election
system is entirely stable given no further input. This means that if the system has a particular set of candidates `C`
and voters `V` that lead to a set of members `M` being elected, as long as `V` and `C` don't remove their candidacy and
votes, `M` will keep being re-elected at the end of each round.

## Module Information

- [`election_sp_phragmen::Config`](https://docs.rs/pallet-elections-phragmen/latest/pallet_elections_phragmen/trait.Config.html)
- [`Call`](https://docs.rs/pallet-elections-phragmen/latest/pallet_elections_phragmen/enum.Call.html)
- [`Module`](https://docs.rs/pallet-elections-phragmen/latest/pallet_elections_phragmen/struct.Module.html)

License: Apache-2.0


## Release

Polkadot SDK stable2409