//! Tests for explicit preemption via `smarm::check!()`.

use smarm::{run, spawn};
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::Arc;

#[test]
fn check_yields_when_timeslice_expired() {
    // A single actor that drives the timeslice clock to zero manually,
    // then calls check!() and expects to yield. The scheduler has nothing
    // else to run, so it just re-queues us. To prove we actually yielded,
    // observe the run counter on the slot... we don't have one. So
    // instead: spawn a second actor that increments a counter and joins
    // it; verify both actors made progress in interleaved order under
    // forced timeslice expiry.
    let order: Arc<std::sync::Mutex<Vec<u8>>> = Arc::new(std::sync::Mutex::new(Vec::new()));
    let o1 = order.clone();
    let o2 = order.clone();

    run(move || {
        let a = spawn(move || {
            o1.lock().unwrap().push(b'A');
            // Force the timeslice to be considered expired.
            smarm::preempt::expire_timeslice_for_test();
            smarm::check!();
            o1.lock().unwrap().push(b'a');
        });
        let b = spawn(move || {
            o2.lock().unwrap().push(b'B');
            smarm::preempt::expire_timeslice_for_test();
            smarm::check!();
            o2.lock().unwrap().push(b'b');
        });
        a.join().unwrap();
        b.join().unwrap();
    });

    // FIFO scheduling + forced preemption: A starts, expires, yields to B;
    // B starts, expires, yields to A; A finishes, B finishes.
    // Required: both uppercase letters appear before either lowercase.
    let v = order.lock().unwrap();
    let pos_big_a = v.iter().position(|&c| c == b'A').unwrap();
    let pos_big_b = v.iter().position(|&c| c == b'B').unwrap();
    let pos_lit_a = v.iter().position(|&c| c == b'a').unwrap();
    let pos_lit_b = v.iter().position(|&c| c == b'b').unwrap();
    assert!(pos_big_a < pos_lit_a, "A's tail ran before B's head: {:?}", *v);
    assert!(pos_big_b < pos_lit_b, "B's tail ran before A's head: {:?}", *v);
    assert!(pos_big_a.max(pos_big_b) < pos_lit_a.min(pos_lit_b),
        "preemption didn't interleave: {:?}", *v);
}

#[test]
fn check_is_a_noop_when_timeslice_not_expired() {
    // After a fresh resume, check!() should be cheap and not yield. Run
    // a single actor that calls check!() many times; it should complete
    // promptly.
    let count = Arc::new(AtomicU64::new(0));
    let c = count.clone();
    run(move || {
        for _ in 0..1_000 {
            smarm::check!();
            c.fetch_add(1, Ordering::Relaxed);
        }
    });
    assert_eq!(count.load(Ordering::Relaxed), 1_000);
}