mirror of https://github.com/poanetwork/hbbft.git
270 lines
10 KiB
Rust
270 lines
10 KiB
Rust
//! Binary Byzantine agreement protocol from a common coin protocol.
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use itertools::Itertools;
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use std::collections::{BTreeSet, HashMap, VecDeque};
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use std::hash::Hash;
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/// Type of output from the Agreement message handler. The first component is
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/// the value on which the Agreement has decided, also called "output" in the
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/// HoneyadgerBFT paper. The second component is a queue of messages to be sent
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/// to remote nodes as a result of handling the incomming message.
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type AgreementOutput = (Option<bool>, VecDeque<AgreementMessage>);
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/// Messages sent during the binary Byzantine agreement stage.
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#[cfg_attr(feature = "serialization-serde", derive(Serialize))]
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#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
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pub enum AgreementMessage {
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/// BVAL message with an epoch.
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BVal(u32, bool),
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/// AUX message with an epoch.
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Aux(u32, bool),
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}
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/// Binary Agreement instance.
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pub struct Agreement<NodeUid> {
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/// The UID of the corresponding proposer node.
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uid: NodeUid,
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num_nodes: usize,
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num_faulty_nodes: usize,
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epoch: u32,
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/// Bin values. Reset on every epoch update.
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bin_values: BTreeSet<bool>,
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/// Values received in BVAL messages. Reset on every epoch update.
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received_bval: HashMap<NodeUid, BTreeSet<bool>>,
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/// Sent BVAL values. Reset on every epoch update.
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sent_bval: BTreeSet<bool>,
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/// Values received in AUX messages. Reset on every epoch update.
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received_aux: HashMap<NodeUid, bool>,
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/// The estimate of the decision value in the current epoch.
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estimated: Option<bool>,
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/// The value output by the agreement instance. It is set once to `Some(b)`
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/// and then never changed. That is, no instance of Binary Agreement can
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/// decide on two different values of output.
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output: Option<bool>,
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/// Termination flag. The Agreement instance doesn't terminate immediately
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/// upon deciding on the agreed value. This is done in order to help other
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/// nodes decide despite asynchrony of communication. Once the instance
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/// determines that all the remote nodes have reached agreement, it sets the
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/// `terminated` flag and accepts no more incoming messages.
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terminated: bool,
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}
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impl<NodeUid: Clone + Eq + Hash> Agreement<NodeUid> {
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pub fn new(uid: NodeUid, num_nodes: usize) -> Self {
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let num_faulty_nodes = (num_nodes - 1) / 3;
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Agreement {
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uid,
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num_nodes,
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num_faulty_nodes,
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epoch: 0,
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bin_values: BTreeSet::new(),
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received_bval: HashMap::new(),
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sent_bval: BTreeSet::new(),
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received_aux: HashMap::new(),
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estimated: None,
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output: None,
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terminated: false,
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}
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}
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pub fn our_id(&self) -> &NodeUid {
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&self.uid
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}
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/// Algorithm has terminated.
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pub fn terminated(&self) -> bool {
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self.terminated
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}
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/// Sets the input value for agreement.
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pub fn set_input(&mut self, input: bool) -> Result<AgreementMessage, Error> {
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if self.epoch != 0 {
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return Err(Error::InputNotAccepted);
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}
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// Set the initial estimated value to the input value.
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self.estimated = Some(input);
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// Record the input value as sent.
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self.sent_bval.insert(input);
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// Receive the BVAL message locally.
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self.received_bval
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.entry(self.uid.clone())
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.or_insert_with(BTreeSet::new)
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.insert(input);
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// Multicast BVAL
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Ok(AgreementMessage::BVal(self.epoch, input))
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}
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/// Acceptance check to be performed before setting the input value.
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pub fn accepts_input(&self) -> bool {
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self.epoch == 0 && self.estimated.is_none()
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}
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/// Receive input from a remote node.
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///
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/// Outputs an optional agreement result and a queue of agreement messages
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/// to remote nodes. There can be up to 2 messages.
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pub fn handle_agreement_message(
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&mut self,
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sender_id: &NodeUid,
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message: &AgreementMessage,
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) -> Result<AgreementOutput, Error> {
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match *message {
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// The algorithm instance has already terminated.
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_ if self.terminated => Err(Error::Terminated),
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AgreementMessage::BVal(epoch, b) if epoch == self.epoch => {
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self.handle_bval(sender_id, b)
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}
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AgreementMessage::Aux(epoch, b) if epoch == self.epoch => self.handle_aux(sender_id, b),
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// Epoch does not match. Ignore the message.
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_ => Ok((None, VecDeque::new())),
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}
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}
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fn handle_bval(&mut self, sender_id: &NodeUid, b: bool) -> Result<AgreementOutput, Error> {
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let mut outgoing = VecDeque::new();
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self.received_bval
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.entry(sender_id.clone())
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.or_insert_with(BTreeSet::new)
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.insert(b);
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let count_bval = self.received_bval
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.values()
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.filter(|values| values.contains(&b))
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.count();
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// upon receiving BVAL_r(b) messages from 2f + 1 nodes,
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// bin_values_r := bin_values_r ∪ {b}
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if count_bval == 2 * self.num_faulty_nodes + 1 {
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self.bin_values.insert(b);
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// wait until bin_values_r != 0, then multicast AUX_r(w)
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// where w ∈ bin_values_r
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if self.bin_values.len() == 1 {
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// Send an AUX message at most once per epoch.
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outgoing.push_back(AgreementMessage::Aux(self.epoch, b));
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// Receive the AUX message locally.
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self.received_aux.insert(self.uid.clone(), b);
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}
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let (decision, maybe_message) = self.try_coin();
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outgoing.extend(maybe_message);
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Ok((decision, outgoing))
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}
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// upon receiving BVAL_r(b) messages from f + 1 nodes, if
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// BVAL_r(b) has not been sent, multicast BVAL_r(b)
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else if count_bval == self.num_faulty_nodes + 1 && !self.sent_bval.contains(&b) {
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// Record the value `b` as sent.
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self.sent_bval.insert(b);
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// Receive the BVAL message locally.
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self.received_bval
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.entry(self.uid.clone())
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.or_insert_with(BTreeSet::new)
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.insert(b);
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// Multicast BVAL.
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outgoing.push_back(AgreementMessage::BVal(self.epoch, b));
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Ok((None, outgoing))
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} else {
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Ok((None, outgoing))
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}
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}
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fn handle_aux(&mut self, sender_id: &NodeUid, b: bool) -> Result<AgreementOutput, Error> {
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self.received_aux.insert(sender_id.clone(), b);
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let mut outgoing = VecDeque::new();
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if !self.bin_values.is_empty() {
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let (decision, maybe_message) = self.try_coin();
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outgoing.extend(maybe_message);
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Ok((decision, outgoing))
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} else {
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Ok((None, outgoing))
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}
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}
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/// AUX_r messages such that the set of values carried by those messages is
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/// a subset of bin_values_r. Outputs this subset.
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///
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/// FIXME: Clarify whether the values of AUX messages should be the same or
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/// not. It is assumed in `count_aux` that they can differ.
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///
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/// In general, we can't expect every good node to send the same AUX value,
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/// so waiting for N - f agreeing messages would not always terminate. We
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/// can, however, expect every good node to send an AUX value that will
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/// eventually end up in our bin_values.
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fn count_aux(&self) -> (usize, BTreeSet<bool>) {
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let (vals_cnt, vals) = self.received_aux
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.values()
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.filter(|b| self.bin_values.contains(b))
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.tee();
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(vals_cnt.count(), vals.cloned().collect())
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}
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/// Waits until at least (N − f) AUX_r messages have been received, such that
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/// the set of values carried by these messages, vals, are a subset of
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/// bin_values_r (note that bin_values_r may continue to change as BVAL_r
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/// messages are received, thus this condition may be triggered upon arrival
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/// of either an AUX_r or a BVAL_r message).
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///
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/// Once the (N - f) messages are received, gets a common coin and uses it
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/// to compute the next decision estimate and outputs the optional decision
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/// value. The function may start the next epoch. In that case, it also
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/// returns a message for broadcast.
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fn try_coin(&mut self) -> (Option<bool>, Vec<AgreementMessage>) {
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let (count_aux, vals) = self.count_aux();
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if count_aux < self.num_nodes - self.num_faulty_nodes {
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// Continue waiting for the (N - f) AUX messages.
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return (None, Vec::new());
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}
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// FIXME: Implement the Common Coin algorithm. At the moment the
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// coin value is common across different nodes but not random.
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let coin = (self.epoch % 2) == 0;
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// Check the termination condition: "continue looping until both a
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// value b is output in some round r, and the value Coin_r' = b for
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// some round r' > r."
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self.terminated = self.terminated || self.output == Some(coin);
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// Start the next epoch.
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self.bin_values.clear();
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self.sent_bval.clear();
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self.received_aux.clear();
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self.sent_bval.clear();
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self.epoch += 1;
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let decision = if vals.len() != 1 {
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self.estimated = Some(coin);
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None
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} else {
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// NOTE: `vals` has exactly one element due to `vals.len() == 1`
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let v: Vec<bool> = vals.into_iter().collect();
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let b = v[0];
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self.estimated = Some(b);
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// Outputting a value is allowed only once.
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if self.output.is_none() && b == coin {
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// Output the agreement value.
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self.output = Some(b);
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self.output
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} else {
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None
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}
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};
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let b = self.estimated.unwrap();
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self.sent_bval.insert(b);
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let bval_msg = AgreementMessage::BVal(self.epoch, b);
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(decision, vec![bval_msg])
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}
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}
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#[derive(Clone, Debug)]
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pub enum Error {
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Terminated,
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InputNotAccepted,
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}
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