package rtph264

import (
	"encoding/binary"
	"errors"
	"fmt"
	"time"

	"github.com/pion/rtp/v2"

	"github.com/aler9/gortsplib/pkg/rtptimedec"
)

// ErrMorePacketsNeeded is returned when more packets are needed.
var ErrMorePacketsNeeded = errors.New("need more packets")

// ErrNonStartingPacketAndNoPrevious is returned when we decoded a non-starting
// packet of a fragmented NALU and we didn't received anything before.
// It's normal to receive this when we are decoding a stream that has been already
// running for some time.
var ErrNonStartingPacketAndNoPrevious = errors.New(
	"decoded a non-starting fragmented packet without any previous starting packet")

// Decoder is a RTP/H264 decoder.
type Decoder struct {
	timeDecoder            *rtptimedec.Decoder
	startingPacketReceived bool
	isDecodingFragmented   bool
	fragmentedBuffer       []byte

	// for DecodeUntilMarker()
	naluBuffer [][]byte
}

// Init initializes the decoder
func (d *Decoder) Init() {
	d.timeDecoder = rtptimedec.New(90000)
}

// Decode decodes NALUs from a RTP/H264 packet.
func (d *Decoder) Decode(pkt *rtp.Packet) ([][]byte, time.Duration, error) {
	if !d.isDecodingFragmented {
		if len(pkt.Payload) < 1 {
			return nil, 0, fmt.Errorf("payload is too short")
		}

		typ := naluType(pkt.Payload[0] & 0x1F)

		switch typ {
		case naluTypeSTAPA:
			var nalus [][]byte
			payload := pkt.Payload[1:]

			for len(payload) > 0 {
				if len(payload) < 2 {
					return nil, 0, fmt.Errorf("invalid STAP-A packet (invalid size)")
				}

				size := binary.BigEndian.Uint16(payload)
				payload = payload[2:]

				// avoid final padding
				if size == 0 {
					break
				}

				if int(size) > len(payload) {
					return nil, 0, fmt.Errorf("invalid STAP-A packet (invalid size)")
				}

				nalus = append(nalus, payload[:size])
				payload = payload[size:]
			}

			if len(nalus) == 0 {
				return nil, 0, fmt.Errorf("STAP-A packet doesn't contain any NALU")
			}

			d.startingPacketReceived = true
			return nalus, d.timeDecoder.Decode(pkt.Timestamp), nil

		case naluTypeFUA: // first packet of a fragmented NALU
			if len(pkt.Payload) < 2 {
				return nil, 0, fmt.Errorf("invalid FU-A packet (invalid size)")
			}

			start := pkt.Payload[1] >> 7
			if start != 1 {
				if !d.startingPacketReceived {
					return nil, 0, ErrNonStartingPacketAndNoPrevious
				}
				return nil, 0, fmt.Errorf("invalid FU-A packet (non-starting)")
			}

			nri := (pkt.Payload[0] >> 5) & 0x03
			typ := pkt.Payload[1] & 0x1F
			d.fragmentedBuffer = append([]byte{(nri << 5) | typ}, pkt.Payload[2:]...)

			d.isDecodingFragmented = true
			d.startingPacketReceived = true
			return nil, 0, ErrMorePacketsNeeded

		case naluTypeSTAPB, naluTypeMTAP16,
			naluTypeMTAP24, naluTypeFUB:
			return nil, 0, fmt.Errorf("packet type not supported (%v)", typ)
		}

		d.startingPacketReceived = true
		return [][]byte{pkt.Payload}, d.timeDecoder.Decode(pkt.Timestamp), nil
	}

	// we are decoding a fragmented NALU

	if len(pkt.Payload) < 2 {
		d.isDecodingFragmented = false
		return nil, 0, fmt.Errorf("invalid FU-A packet (invalid size)")
	}

	typ := naluType(pkt.Payload[0] & 0x1F)
	if typ != naluTypeFUA {
		d.isDecodingFragmented = false
		return nil, 0, fmt.Errorf("expected FU-A packet, got another type")
	}

	start := pkt.Payload[1] >> 7
	end := (pkt.Payload[1] >> 6) & 0x01

	if start == 1 {
		d.isDecodingFragmented = false
		return nil, 0, fmt.Errorf("invalid FU-A packet (decoded two starting packets in a row)")
	}

	d.fragmentedBuffer = append(d.fragmentedBuffer, pkt.Payload[2:]...)

	if end != 1 {
		return nil, 0, ErrMorePacketsNeeded
	}

	d.isDecodingFragmented = false
	d.startingPacketReceived = true
	return [][]byte{d.fragmentedBuffer}, d.timeDecoder.Decode(pkt.Timestamp), nil
}

// DecodeUntilMarker decodes NALUs from a RTP/H264 packet and puts them in a buffer.
// When a packet has the marker flag (meaning that all the NALUs with the same PTS have
// been received), the buffer is returned.
func (d *Decoder) DecodeUntilMarker(pkt *rtp.Packet) ([][]byte, time.Duration, error) {
	nalus, pts, err := d.Decode(pkt)
	if err != nil {
		return nil, 0, err
	}

	d.naluBuffer = append(d.naluBuffer, nalus...)

	if !pkt.Marker {
		return nil, 0, ErrMorePacketsNeeded
	}

	ret := d.naluBuffer
	d.naluBuffer = d.naluBuffer[:0]

	return ret, pts, nil
}