/*
NAME
  lex.go

DESCRIPTION
  lex.go provides a lexer for taking RTP HEVC (H265) and lexing into access units.

AUTHORS
  Saxon A. Nelson-Milton <saxon@ausocean.org>

LICENSE
  Copyright (C) 2019 the Australian Ocean Lab (AusOcean).

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License
  in gpl.txt.  If not, see http://www.gnu.org/licenses.
*/

// Package h265 provides an RTP h265 lexer that can extract h265 access units
// from an RTP stream.
package h265

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"io"
	"time"

	"bitbucket.org/ausocean/av/protocol/rtp"
)

// NALU types.
const (
	typeAggregation   = 48
	typeFragmentation = 49
	typePACI          = 50
)

// Buffer sizes.
const (
	maxAUSize  = 100000
	maxRTPSize = 4096
)

// Lexer is an H265 lexer.
type Lexer struct {
	donl bool          // Indicates whether DONL and DOND will be used for the RTP stream.
	buf  *bytes.Buffer // Holds the current access unit.
	frag bool          // Indicates if we're currently dealing with a fragmentation packet.
}

// NewLexer returns a new Lexer.
func NewLexer(donl bool) *Lexer {
	return &Lexer{
		donl: donl,
		buf:  bytes.NewBuffer(make([]byte, 0, maxAUSize)),
	}
}

// Lex continually reads RTP packets from the io.Reader src and lexes into
// access units which are written to the io.Writer dst. Lex expects that for
// each read from src, a single RTP packet is received.
func (l *Lexer) Lex(dst io.Writer, src io.Reader, delay time.Duration) error {
	buf := make([]byte, maxRTPSize)
	for {
		n, err := src.Read(buf)
		switch err {
		case nil: // Do nothing.
		default:
			if err == io.EOF {
				if l.buf.Len() == 0 {
					return io.EOF
				}
				return io.ErrUnexpectedEOF
			}
			return err
		}

		// Get payload from RTP packet.
		payload, err := rtp.Payload(buf[:n])
		if err != nil {
			return fmt.Errorf("could not get rtp payload, failed with err: %w\n", err)
		}
		nalType := (payload[0] >> 1) & 0x3f

		// If not currently fragmented then we ignore current write.
		if l.frag && nalType != typeFragmentation {
			l.buf.Reset()
			l.frag = false
			continue
		}

		switch nalType {
		case typeAggregation:
			l.handleAggregation(payload)
		case typeFragmentation:
			l.handleFragmentation(payload)
		case typePACI:
			l.handlePACI(payload)
		default:
			l.writeWithPrefix(payload)
		}

		markerIsSet, err := rtp.Marker(buf[:n])
		if err != nil {
			return fmt.Errorf("could not get marker bit, failed with err: %w\n", err)
		}

		if markerIsSet {
			_, err := l.buf.WriteTo(dst)
			if err != nil {
				// TODO: work out what to do here.
			}
			l.buf.Reset()
		}
	}
	return nil
}

// handleAggregation parses NAL units from an aggregation packet and writes
// them to the Lexers buffer buf.
func (l *Lexer) handleAggregation(d []byte) {
	idx := 2
	for idx < len(d) {
		if l.donl {
			switch idx {
			case 2:
				idx += 2
			default:
				idx++
			}
		}
		size := int(binary.BigEndian.Uint16(d[idx:]))
		idx += 2
		nalu := d[idx : idx+size]
		idx += size
		l.writeWithPrefix(nalu)
	}
}

// handleFragmentation parses NAL units from fragmentation packets and writes
// them to the Lexer's buf.
func (l *Lexer) handleFragmentation(d []byte) {
	// Get start and end indiciators from FU header.
	start := d[2]&0x80 != 0
	end := d[2]&0x40 != 0

	b1 := (d[0] & 0x81) | ((d[2] & 0x3f) << 1)
	b2 := d[1]
	if start {
		d = d[1:]
		if l.donl {
			d = d[2:]
		}
		d[0] = b1
		d[1] = b2
	} else {
		d = d[3:]
		if l.donl {
			d = d[2:]
		}
	}

	switch {
	case start && !end:
		l.frag = true
		l.writeWithPrefix(d)
	case !start && end:
		l.frag = false
		fallthrough
	case !start && !end:
		l.writeNoPrefix(d)
	default:
		panic("bad fragmentation packet")
	}
}

// handlePACI will handle PACI packets
//
// TODO: complete this
func (l *Lexer) handlePACI(d []byte) {
	panic("unsupported nal type")
}

// write writes a NAL unit to the Lexer's buf in byte stream format using the
// start code.
func (l *Lexer) writeWithPrefix(d []byte) {
	const prefix = "\x00\x00\x00\x01"
	l.buf.Write([]byte(prefix))
	l.buf.Write(d)
}

// writeNoPrefix writes data to the Lexer's buf. This is used for non start
// fragmentations of a NALU.
func (l *Lexer) writeNoPrefix(d []byte) {
	l.buf.Write(d)
}