package webrtc

import (
	"fmt"
	"net"

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

// ingestStream captures the two loopback UDP ports that FFmpeg binds
// for WHIP ingest — video on videoPort, audio on audioPort.
// The WHIPHandler writes received WebRTC RTP to these ports.
type ingestStream struct {
	id        string
	videoPort int
	audioPort int
}

// onWHIPProcessStart is registered as the restream OnInputStart hook.
// It fires just before FFmpeg starts, holding the restream write lock.
//
// When the per-process WHIPIngest config is disabled it returns (nil, nil)
// so FFmpeg starts normally. When enabled it:
//
//  1. Allocates two adjacent loopback UDP ports (video on V, audio on V+1)
//     using the same retry strategy as the WHEP egress allocator.
//  2. Registers the pair under the process ID in whipIngests so the
//     WHIPHandler can route incoming WebRTC RTP to them.
//  3. Returns two RTP ConfigIO input legs that FFmpeg will open as UDP
//     listeners. The restream manager prepends them to cfg.Input and
//     rebuilds the FFmpeg command before Start().
func (s *Subsystem) onWHIPProcessStart(id string, cfg *appcfg.Config) ([]appcfg.ConfigIO, error) {
	if cfg == nil || !cfg.WHIPIngest.Enabled {
		return nil, nil
	}

	// Normalize PTs — zero values mean "use defaults".
	wcfg := cfg.WHIPIngest
	if wcfg.VideoPT == 0 {
		wcfg.VideoPT = defaultVideoPT
	}
	if wcfg.AudioPT == 0 {
		wcfg.AudioPT = defaultAudioPT
	}

	// Refuse to re-register.
	s.mu.Lock()
	if _, exists := s.whipIngests[id]; exists {
		s.mu.Unlock()
		return nil, fmt.Errorf("webrtc: whip: process %q already has an active ingest", id)
	}
	s.mu.Unlock()

	// Find an adjacent pair (V, V+1). The same retry logic used by
	// the WHEP egress allocator (allocAdjacentPair) except we only
	// need port numbers, not Source objects.
	videoPort, err := allocAdjacentPortPair()
	if err != nil {
		return nil, fmt.Errorf("webrtc: whip: allocate port pair for process %q: %w", id, err)
	}
	audioPort := videoPort + 1

	s.mu.Lock()
	s.whipIngests[id] = &ingestStream{
		id:        id,
		videoPort: videoPort,
		audioPort: audioPort,
	}
	s.mu.Unlock()

	s.logger.WithFields(map[string]interface{}{
		"id":         id,
		"video_port": videoPort,
		"audio_port": audioPort,
		"video_pt":   wcfg.VideoPT,
		"audio_pt":   wcfg.AudioPT,
	}).Info().Log("WebRTC WHIP ingest registered for process")

	return buildWHIPInputLegs(wcfg, videoPort), nil
}

// onWHIPProcessStop is registered as the restream OnInputStop hook.
// It fires just after FFmpeg has been stopped. It removes the port
// allocation and signals the WHIPHandler to close any active publisher.
func (s *Subsystem) onWHIPProcessStop(id string) {
	s.mu.Lock()
	_, ok := s.whipIngests[id]
	teardown := s.whipTeardown
	if ok {
		delete(s.whipIngests, id)
	}
	s.mu.Unlock()

	if !ok {
		return
	}

	if teardown != nil {
		teardown(id)
	}

	s.logger.WithField("id", id).Info().Log("WebRTC WHIP ingest torn down for process")
}

// allocAdjacentPortPair finds two consecutive free loopback UDP ports
// (V, V+1) and returns V. It retries up to allocAttempts times because
// the kernel may hand us a port whose +1 neighbor is already taken.
//
// The caller owns the returned port numbers; FFmpeg will bind them
// immediately on process start via the ConfigIO input legs.
func allocAdjacentPortPair() (int, error) {
	var lastErr error
	for attempt := 0; attempt < allocAttempts; attempt++ {
		videoPort, err := Alloc()
		if err != nil {
			lastErr = err
			continue
		}
		audioPort := videoPort + 1
		if audioPort > 65535 {
			lastErr = fmt.Errorf("video port %d would push audio port above 65535", videoPort)
			continue
		}
		// Verify the audio port (videoPort+1) is also free by attempting
		// a momentary bind. TOCTOU race is accepted; FFmpeg will fail-fast
		// on the actual bind and the process will restart cleanly.
		if err := checkPortFree(audioPort); err != nil {
			lastErr = err
			continue
		}
		return videoPort, nil
	}
	if lastErr == nil {
		lastErr = fmt.Errorf("unknown allocation failure")
	}
	return 0, fmt.Errorf("after %d attempts: %w", allocAttempts, lastErr)
}

// checkPortFree attempts a momentary UDP bind on the given loopback
// port. Returns nil if the port appears available, non-nil otherwise.
func checkPortFree(port int) error {
	c, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1), Port: port})
	if err != nil {
		return fmt.Errorf("port %d not free: %w", port, err)
	}
	_ = c.Close()
	return nil
}

// buildWHIPInputLegs produces the two FFmpeg input ConfigIO legs for
// WHIP ingest. FFmpeg opens each as a UDP RTP listener:
//
//	-i udp://127.0.0.1:V?overrun_nonfatal=1&fifo_size=50000000
//	-i udp://127.0.0.1:A?overrun_nonfatal=1&fifo_size=50000000
//
// The IngestPeer.forwardTrack goroutine writes received WebRTC RTP to
// these ports once the WHIP publisher connects.
func buildWHIPInputLegs(cfg appcfg.ConfigWHIPIngest, videoPort int) []appcfg.ConfigIO {
	audioPort := videoPort + 1
	return []appcfg.ConfigIO{
		{
			ID:      "whip:video",
			Address: fmt.Sprintf("udp://127.0.0.1:%d?overrun_nonfatal=1&fifo_size=50000000", videoPort),
			Options: []string{
				"-re",
				"-protocol_whitelist", "udp,rtp",
				"-fflags", "+genpts",
				"-payload_type", fmt.Sprint(cfg.VideoPT),
				"-codec:v", "copy",
			},
		},
		{
			ID:      "whip:audio",
			Address: fmt.Sprintf("udp://127.0.0.1:%d?overrun_nonfatal=1&fifo_size=50000000", audioPort),
			Options: []string{
				"-re",
				"-protocol_whitelist", "udp,rtp",
				"-fflags", "+genpts",
				"-payload_type", fmt.Sprint(cfg.AudioPT),
				"-codec:a", "copy",
			},
		},
	}
}