# 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::Trait`](https://docs.rs/pallet-elections-phragmen/latest/pallet_elections_phragmen/trait.Trait.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
