Roughly speaking, hopper is a concurrent deque with two cooperating finite state machines layered on top. We'll start in with the deque, defined in src/deque.rs.
The discussion of hopper that follows lists almost all of its source code. We'll call out the few instances where the reader will need to refer to the listing in the book's source repository.
To be totally clear, a deque is a data structure that allows for queuing and dequeuing at either end of the queue. Rust's stdlib has VecDeque<T>, which is very useful. Hopper is unable to use it, however, as one of its design goals is to allow for parallel sending and receiving against the hopper queue and VecDeque is not thread-safe. Also, while there are concurrent deque implementations ...
