priorityq/README.md

# priorityq - generic prioritized queues in Go

This module was inspired by [a reddit post][reddit] wherein /u/zandery23 asked
how to implement a prioritized message queue in Go. A fantastic solution was
[provided by /u/Ploobers][sol]. That's probably right for 99 out of 100 use
cases, but it's not completely precise.

Particularly, the second select block does not guarantee that an item from the
prioritized queue will be taken if there is also an item in the regular queue.

```go
select {
case job := <-mq.priorityQueue:
    // ...
case job := <-mq.regularQueue:
    // ...
// ...
}
```

From the [Go Language Specification][go_select]:

> If one or more of the communications can proceed, a single one that can
> proceed is chosen via a uniform pseudo-random selection.

Thus, it is possible for the second case to be chosen even if the first case is
also ready.

The `mq` package in this module implements a concurrent, prioritized message
queue that guarantees receipt of a high-priority items before low-priority
ones. This is primarily a fun exercise, I cannot recommend that anyone
actually use this in a real project.

Additionally, the `pq` package implements a concurrent priority queue, using a
binary max-heap. This is more general than `mq`, because it allows multiple
levels of priority, instead of just "high" and "low". This, of course, also
makes operations slower.

[reddit]: https://www.reddit.com/r/golang/comments/11drc17/worker_pool_reading_from_two_channels_one_chan/
[sol]: https://www.reddit.com/r/golang/comments/11drc17/worker_pool_reading_from_two_channels_one_chan/jabfvkh/
[go_select]: https://go.dev/ref/spec#Select_statements