package usb

import (
	"context"
	"errors"
	"runtime"
	"sync/atomic"
	"testing"
	"time"
)

// mockEnumerator returns predefined snapshots per call number.
// Each call to next() advances the snapshot index.
type mockEnumerator struct {
	snapshots []map[string]string
	idx       atomic.Int32
}

func newMockEnumerator(snapshots ...map[string]string) *mockEnumerator {
	return &mockEnumerator{snapshots: snapshots}
}

func (m *mockEnumerator) next() (map[string]string, error) {
	i := int(m.idx.Add(1)) - 1
	if i >= len(m.snapshots) {
		// Return last snapshot forever after exhaustion.
		i = len(m.snapshots) - 1
	}
	// Deep copy to prevent mutation.
	out := make(map[string]string, len(m.snapshots[i]))
	for k, v := range m.snapshots[i] {
		out[k] = v
	}
	return out, nil
}

// noopSerialOpener is a mock serial opener that does nothing (no hardware needed).
// It returns a mockPort that satisfies the serialPort interface.
func noopSerialOpener(path string, baud int) (serialPort, error) {
	return newMockPort(), nil
}

// --- Test 1: Start with one device → exactly 1 device goroutine spawned ---

func TestManagerStartSpawnsOneGoroutine(t *testing.T) {
	enum := newMockEnumerator(
		map[string]string{"0525:a4a7": "/dev/cu.mock0"}, // device present
	)

	m := newManagerForTest(enum.next, noopSerialOpener, 20*time.Millisecond)

	baseline := runtime.NumGoroutine()
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	m.Start(ctx)
	time.Sleep(60 * time.Millisecond) // allow poll loop and device goroutine to start

	after := runtime.NumGoroutine()
	// We expect at least 1 new goroutine (the device loop) + poll loop goroutine.
	// Baseline can fluctuate ±2 so check at least 1 extra goroutine appeared.
	if after <= baseline {
		t.Errorf("expected goroutine count to increase after Start(); baseline=%d after=%d", baseline, after)
	}

	m.Stop()
}

// --- Test 2: Device disconnect then reconnect emits Disconnected then Connected events ---

func TestManagerDisconnectReconnectEvents(t *testing.T) {
	enum := newMockEnumerator(
		map[string]string{"0525:a4a7": "/dev/cu.mock0"}, // call 1: present
		map[string]string{},                              // call 2: absent
		map[string]string{"0525:a4a7": "/dev/cu.mock0"}, // call 3: present again
	)

	m := newManagerForTest(enum.next, noopSerialOpener, 20*time.Millisecond)

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	m.Start(ctx)
	defer m.Stop()

	var events []DeviceEvent
	done := make(chan struct{})
	go func() {
		defer close(done)
		for e := range m.Events() {
			events = append(events, e)
			if len(events) >= 3 { // Connected, Disconnected, Connected
				return
			}
		}
	}()

	select {
	case <-done:
	case <-time.After(2 * time.Second):
		t.Fatalf("timeout waiting for 3 events; got %d: %v", len(events), events)
	}

	if len(events) < 3 {
		t.Fatalf("expected at least 3 events, got %d: %v", len(events), events)
	}
	if events[0].State != StateConnected {
		t.Errorf("event[0]: expected Connected, got %v", events[0].State)
	}
	if events[1].State != StateDisconnected {
		t.Errorf("event[1]: expected Disconnected, got %v", events[1].State)
	}
	if events[2].State != StateConnected {
		t.Errorf("event[2]: expected Connected, got %v", events[2].State)
	}
}

// --- Test 3: Stop causes all goroutines to exit within 500ms ---

func TestManagerStopGoroutineLeak(t *testing.T) {
	enum := newMockEnumerator(
		map[string]string{"0525:a4a7": "/dev/cu.mock0"},
	)

	m := newManagerForTest(enum.next, noopSerialOpener, 20*time.Millisecond)
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	baseline := runtime.NumGoroutine()
	m.Start(ctx)
	time.Sleep(60 * time.Millisecond) // let goroutines spin up

	m.Stop()

	deadline := time.Now().Add(500 * time.Millisecond)
	for time.Now().Before(deadline) {
		current := runtime.NumGoroutine()
		// Allow ±2 goroutines for runtime background goroutines.
		if current <= baseline+2 {
			return // passed
		}
		time.Sleep(10 * time.Millisecond)
	}

	t.Errorf("goroutine leak: baseline=%d, after Stop()=%d (expected within ±2)",
		baseline, runtime.NumGoroutine())
}

// --- Test 4: Send to absent device returns ErrDeviceNotConnected ---

func TestManagerSendToAbsentDevice(t *testing.T) {
	// Empty snapshot — no devices connected.
	enum := newMockEnumerator(map[string]string{})
	m := newManagerForTest(enum.next, noopSerialOpener, 20*time.Millisecond)

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	m.Start(ctx)
	defer m.Stop()

	time.Sleep(40 * time.Millisecond) // let poll loop run

	replyCh := make(chan error, 1)
	err := m.Send("0525:a4a7", Command{
		Type:  CmdWrite,
		Reply: replyCh,
	})
	if !errors.Is(err, ErrDeviceNotConnected) {
		t.Errorf("expected ErrDeviceNotConnected, got %v", err)
	}
}

// --- Test 5: Goroutine count stable across 5 simulated unplug/replug cycles ---

func TestGoroutineStability(t *testing.T) {
	// Build snapshots: 5 cycles of present/absent/present
	snapshots := []map[string]string{}
	snapshots = append(snapshots, map[string]string{}) // initial empty
	for i := 0; i < 5; i++ {
		snapshots = append(snapshots, map[string]string{"0525:a4a7": "/dev/cu.mock0"}) // plug in
		snapshots = append(snapshots, map[string]string{})                              // unplug
	}
	// Final state: device connected
	snapshots = append(snapshots, map[string]string{"0525:a4a7": "/dev/cu.mock0"})

	enum := newMockEnumerator(snapshots...)
	m := newManagerForTest(enum.next, noopSerialOpener, 20*time.Millisecond)

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	baseline := runtime.NumGoroutine()
	m.Start(ctx)

	// Wait long enough for all snapshots to be polled through.
	// 12 snapshots * 20ms poll interval = ~240ms minimum; give 2x buffer.
	time.Sleep(500 * time.Millisecond)

	m.Stop()

	// Give goroutines up to 500ms to settle.
	deadline := time.Now().Add(500 * time.Millisecond)
	var finalCount int
	for time.Now().Before(deadline) {
		finalCount = runtime.NumGoroutine()
		if finalCount <= baseline+2 {
			return // stable
		}
		time.Sleep(10 * time.Millisecond)
	}

	t.Errorf("goroutine count unstable after 5 replug cycles: baseline=%d, final=%d (max allowed %d)",
		baseline, finalCount, baseline+2)
}