Function par_unfold 

pub fn par_unfold<Item, State, P, F, Fut>(
    params: P,
    init: State,
    f: F,
) -> RecvStream<'static, Item>
where
    P: Into<ParParams>,
    F: 'static + FnMut(usize, State) -> Fut + Send + Clone,
    Fut: 'static + Future<Output = Option<(Item, State)>> + Send,
    Item: 'static + Send,
    State: 'static + Send + Clone,

Produce stream elements from a parallel asynchronous task.

This function spawns a set of parallel workers. Each worker produces and places items to an output buffer. The worker pool size and buffer size is determined by params.

Each worker receives a copy of initialized state init, then iteratively calls the function f(worker_index, State) -> Fut. The Fut is a future that returns Option<(output, State)>. The future updates the state and produces an output item.

If a worker receives a None, the worker with that worker index will halt, but it does not halt the other workers. The output stream terminates after every worker halts.

use futures::prelude::*;
use par_stream::prelude::*;
use std::sync::{
    atomic::{AtomicUsize, Ordering::*},
    Arc,
};

let mut vec: Vec<_> = par_stream::par_unfold(
    None,
    Arc::new(AtomicUsize::new(0)),
    |_, counter| async move {
        let output = counter.fetch_add(1, SeqCst);
        (output < 1000).then(|| (output, counter))
    },
)
.collect()
.await;

vec.sort();
itertools::assert_equal(vec, 0..1000);