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//! Specification of the sequence flow of the event-driven [`implementation`](super::implementation)
//! of the HotStuff subprotocol.
//!
//! # Event handlers
//!
//! ## Enter View
//!
//! ```ignore
//! fn enter_view(view: ViewNum) {
//! // 1. Create a NewView message for the current view and send it to the next leader(s).
//! let new_view = NewView {
//! chain_id,
//! view: current_view,
//! highest_pc: block_tree.highest_pc(),
//! };
//!
//! for leader in new_view_recipients(&new_view, block_tree.validator_sets_state()) {
//! network.send(leader, new_view);
//! }
//!
//! // 2. Update the HotStuff subprotocol's copy of the current view.
//! current_view = view;
//!
//! // 3. Replace the existing vote collectors with new ones for the current view.
//! vote_collectors = VoteCollector::new(chain_id, current_view, block_tree.validator_sets_state());
//!
//! // 4. If I am a proposer for the newly-entered view, then broadcast a `Proposal` or a `Nudge`.
//! if is_proposer(
//! keypair.verifying(),
//! view,
//! block_tree.validator_sets_state(),
//! ) {
//!
//! // 4.1. If a chain of consecutive views justifying a validator set updating block has been broken,
//! // re-propose the validator set updating block.
//! if let Some(block) = block_tree.repropose_block(view) {
//! let proposal = Proposal {
//! chain_id,
//! view,
//! block,
//! }
//!
//! network.broadcast(proposal);
//! }
//!
//! // 4.2. Otherwise, decide whether to broadcast a new proposal, or a new nudge, according to phase of the highest PC.
//! else {
//! match block_tree.highest_pc().phase {
//!
//! // 4.2.1. If the phase of the highest PC is Generic or Decide, create a new Proposal and broadcast it.
//! Phase::Generic | Phase::Decide => {
//! let block = app.produce_block(&block_tree, block_tree.highest_pc());
//! let proposal = Proposal {
//! chain_id,
//! view,
//! block,
//! }
//!
//! network.broadcast(proposal);
//! },
//!
//! // 4.2.2. If the phase of the highest PC is Prepare, Precommit, or Commit, create a new Nudge and broadcast it.
//! Phase::Prepare | Phase::Precommit | Phase::Commit => {
//! let nudge = Nudge {
//! chain_id,
//! view,
//! justify: block_tree.highest_pc(),
//! }
//!
//! network.broadcast(nudge);
//! }
//! }
//! }
//! }
//! }
//! ```
//!
//! ## On Receive Proposal
//!
//! ```ignore
//! fn on_receive_proposal(proposal: Proposal, origin: VerifyingKey) {
//! // 1. Confirm that `origin` really is a proposer in the current view.
//! if is_proposer(origin, current_view, block_tree.validator_set_state()) {
//!
//! // 2. Confirm that `proposal.block` is safe according to the rules of the block tree.
//! if block_tree.safe_block(&proposal.block, chain_id) {
//!
//! // 3. Confirm that `proposal.block` is valid according to the rules of the app.
//! if let Ok((app_state_updates, validator_set_updates)) = app.validate_block(&block_tree) {
//!
//! // 4. Insert `proposal.block` into the block tree.
//! block_tree.insert(proposal.block, app_state_updates, validator_set_updates);
//!
//! // 5. Update the block tree using `proposal.block.justify`.
//! block_tree.update(&proposal.block.justify);
//!
//! // 6. Tell the vote collectors to start collecting votes according to the new validator sets state (which
//! // may or may not have been changed in the block tree update in the previous step).
//! vote_collectors.update_validator_sets(block_tree.validator_sets_state());
//!
//! // 7. If the local replica's votes can become part of PCs that directly extend `proposal.block.justify`,
//! // vote for `proposal`.
//! if is_voter(
//! keypair.public(),
//! block_tree.validator_sets_state(),
//! &proposal.block.justify,
//! ) {
//! // 7.1. Compute the phase to vote in: if `proposal.block` updates the validator set, then vote in the
//! // `Prepare` phase. Otherwise, vote in the `Generic` phase.
//! let vote_phase = if validator_set_updates.is_some() {
//! Phase::Prepare
//! } else {
//! Phase::Generic
//! }
//! let vote = Vote::new(
//! keypair,
//! chain_id,
//! current_view,
//! proposal.block.hash,
//! vote_phase,
//! );
//!
//! // 7.2. Send the vote to the leader that should receive it.
//! network.send(vote, vote_recipient(&vote, block_tree.validator_sets_state()));
//! }
//! }
//! }
//! }
//! }
//! ```
//!
//! ## On Receive Nudge
//!
//! ```ignore
//! fn on_receive_nudge(nudge: Nudge, origin: VerifyingKey) {
//! // 1. Confirm that `origin` really is a proposer in the current view.
//! if is_proposer(origin, current_view, block_tree.validator_set_state()) {
//!
//! // 2. Confirm that `nudge` is safe according to the rules of the block tree.
//! if block_tree.safe_nudge(&nudge, current_view, chain_id) {
//!
//! // 3. Update the block tree using `nudge.justify`.
//! block_tree.update(&nudge.justify);
//!
//! // 4. Tell the vote collectors to start collecting votes according to the new validator sets state (which
//! // may or may not have been changed in the block tree update in the previous step).
//! vote_collectors.update_validator_sets(block_tree.validator_sets_state());
//!
//! // 5. If the local replica's votes can become part of PCs that directly extend `nudge.justify`, vote for
//! // `nudge`.
//! if is_voter(
//! keypair.public(),
//! block_tree.validator_sets_state(),
//! &nudge.justify,
//! ) {
//! // 5.1. Compute the phase to vote in: this will be the phase that follows `nudge.justify.phase`.
//! let vote_phase = match nudge.justify.phase {
//! Phase::Prepare => Phase::Precommit,
//! Phase::Precommit => Phase::Commit,
//! Phase::Commit => Phase::Decide,
//! _ => unreachable!("`safe_nudge` should have ensured that `nudge.justify.phase` is neither `Generic` or `Decide`"),
//! };
//! let vote = Vote::new(
//! keypair,
//! chain_id,
//! current_view,
//! proposal
//! )
//!
//! // 5.2. Send the vote to the leader that should receive it.
//! network.send(vote, vote_recipient(&vote, block_tree.validator_sets_state()))
//! }
//! }
//! }
//! }
//! ```
//!
//! # On Receive Phase Vote
//!
//! ```ignore
//! fn on_receive_phase_vote(phase_vote: Vote, origin: VerifyingKey) {
//! // 1. Confirm that `phase_vote` was signed by `origin`.
//! if phase_vote.is_correct(origin) {
//!
//! // 2. Collect `phase_vote` using the phase vote collectors.
//! let new_pc = phase_vote_collectors.collect(phase_vote, origin);
//!
//! // 3. If sufficient votes were collected to form a `new_pc`, use `new_pc` to update the block tree.
//! if let Some(new_pc) = new_pc {
//! // 3.1. Confirm that `new_pc` is safe according to the rules of the block tree.
//! //
//! // Note (TODO): I can think of at least three ways this check can fail:
//! // 1. A quorum of replicas are byzantine and form a PC with an illegal phase, that is:
//! // 1. A Generic PC that justifies a VSU-block.
//! // 2. A non-Generic PC that justifies a non-VSU-block.
//! // 2. We forgot to create a new vote collector with a higher view in `enter_view` (library bug).
//! // 3. We collected a PC for a block that isn't in the block tree yet (block sync may help).
//! if block_tree.safe_pc(new_pc) {
//!
//! // 3.2. Update the block tree using `new_pc`.
//! block_tree.update(new_pc);
//!
//! // 3.3. Tell the vote collectors to start collecting votes according to the new validator sets state (which
//! // may or may not have been changed in the block tree update in the previous step).
//! phase_vote_collectors.update_validator_sets(block_tree.validator_set_state());
//! }
//! }
//! }
//! }
//! ```
//!
//! ## On Receive New View
//!
//! ```ignore
//! fn on_receive_new_view(new_view: NewView, origin: VerifyingKey) {
//! // 1. Confirm that `new_pc` is safe according to the rules of the block tree.
//! if block_tree.safe_pc(&new_view.highest_pc) {
//!
//! // 2. Update the block tree using `new_view.highest_pc`.
//! block_tree.update(new_view.highest_pc);
//!
//! // 3. Tell the vote collectors to start collecting votes according to the new validator sets state (which
//! // may or may not have been changed in the block tree update in the previous step).
//! vote_collectors.update_validator_sets(block_tree.validator_set_state());
//! }
//! }
//! ```