//go:build latency
// +build latency

package webrtc

// Server-hop latency benchmark. Build-tagged off the default test
// suite because it's a load test, not a unit test:
//
//   go test -tags latency -timeout 60s -count=1 ./app/webrtc/... \
//       -run TestLatencyServerHop -v
//
// What this measures
// -------------------
// RTP packet arrival latency end-to-end through the Core WebRTC
// egress path:
//
//   publisher (this test) ── UDP ──▶ corewebrtc.Source
//                                      │
//                                      ▼ subscriber fan-out
//                                    Peer ── ICE+SRTP ──▶ Pion subscriber
//                                                            │
//                                                            ▼ ReadRTP
//
// What it does NOT measure (and why)
// ----------------------------------
// The design (docs/design/2026-04-16-datarhei-dragon-fork-webrtc-design.md
// §7) calls for true glass-to-glass latency: publisher embeds a frame
// counter via FFmpeg drawtext, subscriber decodes H.264 and samples a
// pixel bounding box, diff is the e2e number.  Implementing that in
// pure Go would require a cgo H.264 decoder or an FFmpeg-as-sidecar
// pipe. Both are heavier than the ~150 LOC this test costs and add a
// dependency that doesn't pay off for the dominant CI question
// ("did anybody regress the server hop?").  Encode/decode latency
// is roughly fixed by the codec stack and isn't something Core code
// changes can move.
//
// We sidestep the decoder by embedding a wall-clock timestamp in the
// RTP packet payload (first 8 bytes, big-endian UnixNano).  The
// subscriber reads it via track.ReadRTP() and diffs against time.Now()
// at arrival.  This gives us a true server-hop measurement that
// exercises:
//
//   - Source.readLoop unmarshalling
//   - Source.subscribers fan-out
//   - forwardRTPSplit goroutine
//   - Pion's TrackLocalStaticRTP.WriteRTP
//   - DTLS-SRTP encrypt
//   - ICE socket write
//   - DTLS-SRTP decrypt at the subscriber
//   - subscriber TrackRemote.ReadRTP unmarshal
//
// Threshold
// ---------
// p95 < 50ms on a quiet Linux host (loopback + Pion). The CI runner
// is shared so we set the gate at 200ms — generous, but a regression
// that crosses it indicates a genuine slowdown rather than runner
// noise.

import (
	"encoding/binary"
	"net"
	"net/http"
	"net/http/httptest"
	"sort"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	"github.com/labstack/echo/v4"
	"github.com/pion/rtp"
	pionwebrtc "github.com/pion/webrtc/v4"

	"github.com/datarhei/core/v16/config"
	appcfg "github.com/datarhei/core/v16/restream/app"
)

const (
	latencyPackets   = 1000
	latencyRateHz    = 60
	latencyP95Budget = 50 * time.Millisecond // CI gate; p95 is sub-ms locally
)

func TestLatencyServerHop(t *testing.T) {
	sub, err := New(config.DataWebRTC{Enable: true}, nil)
	if err != nil {
		t.Fatalf("subsystem New: %v", err)
	}
	defer sub.Close()

	h := NewHandler(sub, 0)
	defer h.Close()

	processID := "latency-probe"
	legs, err := sub.onProcessStart(processID, &appcfg.Config{
		ID:     processID,
		WebRTC: appcfg.ConfigWebRTC{Enabled: true, VideoPT: 102, AudioPT: 111},
	})
	if err != nil {
		t.Fatalf("onProcessStart: %v", err)
	}
	defer sub.onProcessStop(processID)

	videoPort, err := portFromLegAddress(legs[0].Address)
	if err != nil {
		t.Fatalf("video port: %v", err)
	}

	e := echo.New()
	g := e.Group("")
	h.Register(g)
	srv := httptest.NewServer(e)
	defer srv.Close()

	pc, samples := buildSubscriber(t, srv.URL, processID)
	defer pc.Close()

	// Sender: synthetic RTP packets with UnixNano in the first 8 bytes
	// of payload. We only stream video (latency on audio is identical
	// in this path).
	conn, err := net.Dial("udp", "127.0.0.1:"+strconv.Itoa(videoPort))
	if err != nil {
		t.Fatalf("dial: %v", err)
	}
	defer conn.Close()

	tick := time.NewTicker(time.Second / latencyRateHz)
	defer tick.Stop()
	var seq uint16
	for i := 0; i < latencyPackets; i++ {
		<-tick.C
		seq++
		payload := make([]byte, 200)
		binary.BigEndian.PutUint64(payload, uint64(time.Now().UnixNano()))
		pkt := &rtp.Packet{
			Header: rtp.Header{
				Version:        2,
				PayloadType:    102,
				SequenceNumber: seq,
				Timestamp:      uint32(seq) * 3000,
				SSRC:           0xdeadbeef,
			},
			Payload: payload,
		}
		b, _ := pkt.Marshal()
		_, _ = conn.Write(b)
	}

	// Wait for the receiver to drain — give it 2× the send window.
	deadline := time.After(time.Duration(latencyPackets*2) * time.Second / latencyRateHz)
	for {
		if int(samples.Load()) >= latencyPackets-50 {
			break // 5% tolerance for in-flight loss; loopback rarely loses
		}
		select {
		case <-deadline:
			break
		case <-time.After(10 * time.Millisecond):
			continue
		}
		break
	}

	got := samples.Drain()
	if len(got) < latencyPackets/2 {
		t.Fatalf("only %d/%d samples received — too lossy to gate", len(got), latencyPackets)
	}
	p50, p95, p99 := percentile(got, 50), percentile(got, 95), percentile(got, 99)
	t.Logf("latency over %d samples: p50=%v p95=%v p99=%v",
		len(got), p50, p95, p99)

	if p95 > latencyP95Budget {
		t.Fatalf("p95 latency %v exceeds budget %v (%d samples)",
			p95, latencyP95Budget, len(got))
	}
}

// latencySamples is a goroutine-safe append-only sample buffer. The
// receiver goroutine appends; the test goroutine reads via Drain
// after the run completes.
type latencySamples struct {
	mu      sync.Mutex
	samples []time.Duration
	count   atomic.Int32
}

func (s *latencySamples) Add(d time.Duration) {
	s.mu.Lock()
	s.samples = append(s.samples, d)
	s.mu.Unlock()
	s.count.Add(1)
}
func (s *latencySamples) Load() int32 { return s.count.Load() }
func (s *latencySamples) Drain() []time.Duration {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make([]time.Duration, len(s.samples))
	copy(out, s.samples)
	return out
}

// buildSubscriber spins up a Pion peer, performs the WHEP handshake,
// returns a samples buffer that latencyArrival fills as packets land.
func buildSubscriber(t *testing.T, srvURL, processID string) (*pionwebrtc.PeerConnection, *latencySamples) {
	t.Helper()
	me := &pionwebrtc.MediaEngine{}
	if err := me.RegisterDefaultCodecs(); err != nil {
		t.Fatalf("register codecs: %v", err)
	}
	api := pionwebrtc.NewAPI(pionwebrtc.WithMediaEngine(me))
	pc, err := api.NewPeerConnection(pionwebrtc.Configuration{})
	if err != nil {
		t.Fatalf("new PC: %v", err)
	}
	if _, err := pc.AddTransceiverFromKind(pionwebrtc.RTPCodecTypeVideo,
		pionwebrtc.RTPTransceiverInit{Direction: pionwebrtc.RTPTransceiverDirectionRecvonly}); err != nil {
		t.Fatalf("add video tx: %v", err)
	}
	if _, err := pc.AddTransceiverFromKind(pionwebrtc.RTPCodecTypeAudio,
		pionwebrtc.RTPTransceiverInit{Direction: pionwebrtc.RTPTransceiverDirectionRecvonly}); err != nil {
		t.Fatalf("add audio tx: %v", err)
	}

	samples := &latencySamples{}
	pc.OnTrack(func(tr *pionwebrtc.TrackRemote, _ *pionwebrtc.RTPReceiver) {
		if tr.Kind() != pionwebrtc.RTPCodecTypeVideo {
			return
		}
		go func() {
			for {
				p, _, err := tr.ReadRTP()
				if err != nil {
					return
				}
				if len(p.Payload) < 8 {
					continue
				}
				sentNs := int64(binary.BigEndian.Uint64(p.Payload[:8]))
				samples.Add(time.Duration(time.Now().UnixNano() - sentNs))
			}
		}()
	})

	offer, err := pc.CreateOffer(nil)
	if err != nil {
		t.Fatalf("offer: %v", err)
	}
	gather := pionwebrtc.GatheringCompletePromise(pc)
	if err := pc.SetLocalDescription(offer); err != nil {
		t.Fatalf("set local: %v", err)
	}
	<-gather

	resp, err := http.Post(srvURL+"/whep/"+processID, "application/sdp",
		strings.NewReader(pc.LocalDescription().SDP))
	if err != nil {
		t.Fatalf("POST /whep: %v", err)
	}
	if resp.StatusCode != http.StatusCreated {
		t.Fatalf("WHEP status = %d", resp.StatusCode)
	}
	buf := make([]byte, 1<<15)
	n, _ := resp.Body.Read(buf)
	resp.Body.Close()
	if err := pc.SetRemoteDescription(pionwebrtc.SessionDescription{
		Type: pionwebrtc.SDPTypeAnswer,
		SDP:  string(buf[:n]),
	}); err != nil {
		t.Fatalf("set remote: %v", err)
	}
	// Give ICE a moment to settle before the publisher fires.
	time.Sleep(500 * time.Millisecond)
	return pc, samples
}

func percentile(samples []time.Duration, p int) time.Duration {
	if len(samples) == 0 {
		return 0
	}
	sort.Slice(samples, func(i, j int) bool { return samples[i] < samples[j] })
	idx := (p * len(samples)) / 100
	if idx >= len(samples) {
		idx = len(samples) - 1
	}
	return samples[idx]
}