package main

import (
	"context"
	"sync"
	"testing"
	"time"
)

func TestMessageQueue_FIFO(t *testing.T) {
	q := newMessageQueue()
	q.enqueue("s", "first")
	q.enqueue("s", "second")
	q.enqueue("s", "third")

	want := []string{"first", "second", "third"}
	for _, w := range want {
		got, ok := q.dequeue("s")
		if !ok || got != w {
			t.Fatalf("dequeue = %q,%v want %q,true", got, ok, w)
		}
	}
	if _, ok := q.dequeue("s"); ok {
		t.Fatal("dequeue on drained queue should return ok=false")
	}
}

func TestMessageQueue_RequeueFront(t *testing.T) {
	q := newMessageQueue()
	q.enqueue("s", "a")
	q.enqueue("s", "b")
	// Pop "a", then push it back to the front; "a" must come out before "b".
	a, _ := q.dequeue("s")
	q.requeueFront("s", a)
	got, _ := q.dequeue("s")
	if got != "a" {
		t.Fatalf("after requeueFront, dequeue = %q want %q", got, "a")
	}
	got2, _ := q.dequeue("s")
	if got2 != "b" {
		t.Fatalf("next dequeue = %q want %q", got2, "b")
	}
}

func TestMessageQueue_IsolatedPerSession(t *testing.T) {
	q := newMessageQueue()
	q.enqueue("s1", "one")
	q.enqueue("s2", "two")
	if got, _ := q.dequeue("s1"); got != "one" {
		t.Fatalf("s1 = %q want one", got)
	}
	if got, _ := q.dequeue("s2"); got != "two" {
		t.Fatalf("s2 = %q want two", got)
	}
	if q.peek("s1") != 0 || q.peek("s2") != 0 {
		t.Fatal("both sessions should be drained")
	}
}

func TestMessageQueue_Concurrent(t *testing.T) {
	q := newMessageQueue()
	const n = maxQueuedPerSession // stay under the cap so every enqueue lands
	var wg sync.WaitGroup
	for i := 0; i < n; i++ {
		wg.Add(1)
		go func(i int) {
			defer wg.Done()
			q.enqueue("s", "m")
		}(i)
	}
	wg.Wait()
	if q.peek("s") != n {
		t.Fatalf("peek = %d want %d (all enqueues must be counted)", q.peek("s"), n)
	}
	seen := 0
	for {
		if _, ok := q.dequeue("s"); !ok {
			break
		}
		seen++
	}
	if seen != n {
		t.Fatalf("drained %d want %d", seen, n)
	}
}

func TestMessageQueue_CapsOverflow(t *testing.T) {
	q := newMessageQueue()
	for i := 0; i < maxQueuedPerSession; i++ {
		if !q.enqueue("s", "m") {
			t.Fatalf("enqueue #%d within cap should succeed", i)
		}
	}
	if q.enqueue("s", "overflow") {
		t.Fatal("enqueue past the cap should return false (dropped)")
	}
	if got := q.peek("s"); got != maxQueuedPerSession {
		t.Fatalf("peek = %d want %d (overflow must not append)", got, maxQueuedPerSession)
	}
}

// drainQueued on an empty queue must be a no-op: it returns immediately and
// never touches the gate (so the session stays free for the next turn).
func TestDrainQueued_NoOpOnEmpty(t *testing.T) {
	a := &agent{gate: newTurnGate(), queue: newMessageQueue()}
	a.drainQueued(context.Background(), "s")
	if !a.gate.acquire("s", 0) {
		t.Fatal("gate should be free after a no-op drain (drain must not hold it)")
	}
	a.gate.release("s")
}

// With a queued message but the gate held by another turn, drainQueued must
// re-queue the message and return WITHOUT running a turn (no store/provider → a
// real run would panic). This is the "never stack" property: a busy gate
// defers to the holder's own release-drain.
func TestDrainQueued_RequeuesWhenBusy(t *testing.T) {
	prev := drainAcquireWait
	drainAcquireWait = 10 * time.Millisecond
	t.Cleanup(func() { drainAcquireWait = prev })

	a := &agent{gate: newTurnGate(), queue: newMessageQueue()}
	if !a.gate.acquire("s", 0) {
		t.Fatal("precondition: hold the gate")
	}
	a.queue.enqueue("s", "queued-msg")

	done := make(chan struct{})
	go func() {
		a.drainQueued(context.Background(), "s") // must not panic; must requeue
		close(done)
	}()
	select {
	case <-done:
	case <-time.After(time.Second):
		t.Fatal("drainQueued did not return promptly while the gate was busy")
	}
	if got := a.queue.peek("s"); got != 1 {
		t.Fatalf("message should be re-queued while busy; peek = %d want 1", got)
	}
	a.gate.release("s")
}