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///|
/// A point-in-time snapshot of the state machine that stands in for the log
/// prefix up to and including `last_index`. Raft uses snapshots to bound log
/// growth and to bring a badly lagging follower up to date in one shot (ยง7).
pub(all) struct Snapshot {
  last_index : UInt64
  last_term : UInt64
  data : Bytes
  // The membership as of the snapshot (etcd's SnapshotMetadata.ConfState), so a
  // node restoring from it rebuilds its voter/learner sets rather than losing
  // them. Empty for a snapshot written without a recorded configuration.
  conf_state : ConfState
} derive(Eq)

///|
/// The empty snapshot: no state captured, baseline at index 0.
pub fn Snapshot::empty() -> Snapshot {
  { last_index: 0, last_term: 0, data: b"", conf_state: ConfState::empty() }
}

///|
/// Whether this snapshot actually captures any state. A baseline index of 0 is
/// the empty snapshot a fresh node starts with.
pub fn Snapshot::is_empty(self : Snapshot) -> Bool {
  self.last_index == 0
}