///|
/// A replicated state machine: it consumes committed commands in log order.
/// Implementors decide what each command means. Keeping this pluggable lets the
/// same consensus core drive a key-value store, a config registry or anything
/// else.
pub(open) trait StateMachine {
  fn apply(Self, Bytes) -> Unit
}

///|
/// Apply every committed-but-unapplied entry to `sm` in index order and
/// advance `last_applied` (Raft §5.3). This is how a committed log turns into
/// application state; calling it again once caught up does nothing.
pub fn Node::apply_committed(self : Node, sm : &StateMachine) -> Unit {
  while self.last_applied < self.commit_index {
    let next = self.last_applied + 1
    sm.apply(self.log[(next - 1).to_int()].command)
    self.last_applied = next
  }
}