package rtmp

import (
	"net"
	"time"
	"encoding/binary"
)

// data intake object, only needed since most of the work is
// just keeping track of the previous values for higher chunk formats to reuse
type ChunkStream struct {
	timestamp uint32
	last_msg_strm_id uint32
	last_msg_len uint32
	last_msg_type uint8
	timedelta uint32
}

// actual message object, metadata from the chunk header
// data either spans the chunk or split across multiple with format 3
type Message struct {
	data []byte
	curr_bytes_read uint32
	timestamp uint32
	msg_type uint8
	msg_len uint32
}

// just here because I got tired of constantly assigning byte buffers and didn't want to
// abstract it, plus helped figuring out how to use pointers and memory allocation on
// restricted memory
type ChunkBuffers struct {
	time []byte
	msg_len []byte
	msg_typeid []byte
	msg_streamid []byte
	fmt_csid_byte []byte
	csid_true []byte
}

// reads the initial variable size header that defines the chunk's format and chunkstream id
// 5.3.1.1
func read_basic_header(conn net.Conn, chunk_bufs_ptr *ChunkBuffers) (format uint8, csid uint32, err error) {
	if _, err = conn.Read(chunk_bufs_ptr.fmt_csid_byte); err != nil {
		return
	}
	format = uint8(chunk_bufs_ptr.fmt_csid_byte[0] >> 6) // get first 2 bits for format 0-3
	switch chunk_bufs_ptr.fmt_csid_byte[0] & 0x3f { // last 6 bits 0-63
	case 0: // csid 0 is invalid, means true csid is the next byte, + 64 for the 6 bits prior
		if _, err = conn.Read(chunk_bufs_ptr.csid_true[1:]); err != nil {
			return
		}
		csid = uint32(chunk_bufs_ptr.csid_true[1]) + 64
	case 1: // csid 1 is invalid, means true csid is in the next 2 bytes, reverse order (little endian) and the 64, reconstruct
		if _, err = conn.Read(chunk_bufs_ptr.csid_true); err != nil {
			return
		}
		csid = uint32(binary.LittleEndian.Uint16(chunk_bufs_ptr.csid_true)) + 64
	default: // by default true csid was read
		csid = uint32(chunk_bufs_ptr.fmt_csid_byte[0] & 0x3f)
	}
	return
}

func read_time(conn net.Conn, chunk_bufs_ptr *ChunkBuffers) (time uint32, extended_time bool, err error) {
	chunk_bufs_ptr.time[0] = 0
	if _, err = conn.Read(chunk_bufs_ptr.time[1:]); err != nil {
		return
	}
	time = binary.BigEndian.Uint32(chunk_bufs_ptr.time)
	if time ^ 0xffffff == 0 {
		extended_time = true
	}
	return
}

func read_msg_len(conn net.Conn, chunk_bufs_ptr *ChunkBuffers) (msg_len uint32, err error) {
	if _, err = conn.Read(chunk_bufs_ptr.msg_len[1:]); err != nil {
		return
	}
	msg_len = binary.BigEndian.Uint32(chunk_bufs_ptr.msg_len)
	return
}

func read_msg_typeid(conn net.Conn, chunk_bufs_ptr *ChunkBuffers) (msg_type uint8, err error) {
	if _, err = conn.Read(chunk_bufs_ptr.msg_typeid); err != nil {
		return
	}
	msg_type = chunk_bufs_ptr.msg_typeid[0]
	return
}

func read_msg_streamid(conn net.Conn, chunk_bufs_ptr *ChunkBuffers) (msg_streamid uint32, err error) {
	if _, err = conn.Read(chunk_bufs_ptr.msg_streamid); err != nil {
		return
	}
	msg_streamid = binary.LittleEndian.Uint32(chunk_bufs_ptr.msg_streamid)
	return
}

func read_time_extd(conn net.Conn, chunk_bufs_ptr *ChunkBuffers) (time uint32, err error) {
	if _, err = conn.Read(chunk_bufs_ptr.time); err != nil {
		return
	}
	time = binary.BigEndian.Uint32(chunk_bufs_ptr.time)
	return
}

// msg header format 0, all data, typically for the start of a new chunkstream or reset, or all data changes
// 5.3.1.2.1
func read_message_header_0(conn net.Conn, chnk_stream_ptr *ChunkStream, msg_ptr *Message, chunk_bufs_ptr *ChunkBuffers) (error) {
	var extended_time bool
	var err error
	// first bit of data is time in the first 3 bytes, if max value, defined as extended time
	// true time value is at end of header (technically after but its still effectively in the header)
	chnk_stream_ptr.timestamp, extended_time, err = read_time(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}

	msg_ptr.msg_len, err = read_msg_len(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}
	msg_ptr.data = make([]byte, msg_ptr.msg_len)
	chnk_stream_ptr.last_msg_len = msg_ptr.msg_len // assign data for different formats

	msg_ptr.msg_type, err = read_msg_typeid(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}
	chnk_stream_ptr.last_msg_type = msg_ptr.msg_type

	chnk_stream_ptr.last_msg_strm_id, err = read_msg_streamid(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}

	if extended_time {
		chnk_stream_ptr.timestamp, err = read_time_extd(conn, chunk_bufs_ptr)
		if err != nil {
			return err
		}
	}
	msg_ptr.timestamp = chnk_stream_ptr.timestamp

	return nil
}

// format 1, typically for the 2nd message in a previously init chunkstream with most other data changed
// no longer time but timedelta. timestamp for the message is computed from the stored timestamp when the stream was opened
// otherwise the same except the msg_stream_id is reused (unchanged)
// 5.3.1.2.2
func read_message_header_1(conn net.Conn, chnk_stream_ptr *ChunkStream, msg_ptr *Message, chunk_bufs_ptr *ChunkBuffers) (error) {
	var extended_time bool
	var err error

	chnk_stream_ptr.timedelta, extended_time, err = read_time(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}
	msg_ptr.msg_len, err = read_msg_len(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}
	msg_ptr.data = make([]byte, msg_ptr.msg_len)
	chnk_stream_ptr.last_msg_len = msg_ptr.msg_len
		
	msg_ptr.msg_type, err = read_msg_typeid(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}
	chnk_stream_ptr.last_msg_type = msg_ptr.msg_type

	if extended_time {
		chnk_stream_ptr.timedelta, err = read_time_extd(conn, chunk_bufs_ptr)
		if err != nil {
			return err
		}
	}
	chnk_stream_ptr.timestamp += chnk_stream_ptr.timedelta
	msg_ptr.timestamp = chnk_stream_ptr.timestamp
	
	return nil
}

// format 2, typically for when format 1 can be used but even the message len and type are identical
// 5.3.1.2.3
func read_message_header_2(conn net.Conn, chnk_stream_ptr *ChunkStream, msg_ptr *Message, chunk_bufs_ptr *ChunkBuffers) (error) {
	var extended_time bool
	var err error

	chnk_stream_ptr.timedelta, extended_time, err = read_time(conn, chunk_bufs_ptr)
	if err != nil {
		return err
	}
	
	msg_ptr.msg_len = chnk_stream_ptr.last_msg_len
	msg_ptr.data = make([]byte, msg_ptr.msg_len)

	msg_ptr.msg_type = chnk_stream_ptr.last_msg_type

	if extended_time {
		chnk_stream_ptr.timedelta, err = read_time_extd(conn, chunk_bufs_ptr)
		if err != nil {
			return err
		}
	}
	chnk_stream_ptr.timestamp += chnk_stream_ptr.timedelta
	msg_ptr.timestamp = chnk_stream_ptr.timestamp

	return nil
}

// read the actual chunkdata given the space for the message that has been constructed using the chunk headers and
// configured peer chunk size
func read_chunk_data(conn net.Conn, msg_ptr *Message, chnk_size uint32) (uint32, error) {
	bytes_left := msg_ptr.msg_len - msg_ptr.curr_bytes_read
	var buffer_end uint32
	if bytes_left < chnk_size {
		buffer_end = bytes_left + msg_ptr.curr_bytes_read
	} else {
		buffer_end = chnk_size + msg_ptr.curr_bytes_read
	}
	n, err := conn.Read(msg_ptr.data[msg_ptr.curr_bytes_read:buffer_end])
	if err != nil {
		return 0, err
	}
	msg_ptr.curr_bytes_read += uint32(n)
	return uint32(n), nil

}

// wrapper function to handle all formats and chunkstreams returns a pointer to a Message (can be nil if not completed)
// n_read was there to handle ack win, but doesn't really seem to matter at all
func ReadChunk(conn net.Conn, open_chnkstrms map[uint32]*ChunkStream, open_msgs map[uint32]*Message, chnk_size uint32, chunk_bufs_ptr *ChunkBuffers, n_read *uint32) (*Message, error){
	conn.SetDeadline(time.Now().Add(10 * time.Second))

	var chnkstream_ptr *ChunkStream
	var msg_ptr *Message

	format, csid, err := read_basic_header(conn, chunk_bufs_ptr)
	if err != nil {
		return nil, err
	}
	if csid < 64 {
		*n_read += 1
	} else if csid < 320 {
		*n_read += 2
	} else {
		*n_read += 3
	}

	switch format {
	case 0:
		// format 0, assume new chunkstream which will override and reset any previous chunkstream regardless
		chnkstream_ptr = new(ChunkStream)
		open_chnkstrms[csid] = chnkstream_ptr // assign the newly made chunkstream to the map of those currently open
		msg_ptr = new(Message) // cannot be a continued message since the stream just started

		if err := read_message_header_0(conn, chnkstream_ptr, msg_ptr, chunk_bufs_ptr); err != nil {
			return nil, err
		}
		*n_read += 11
		if msg_ptr.timestamp > 0xffffff {
			*n_read += 4
		}
	case 1:
		// format 1, chunkstream with this csid MUST have been opened before, but message is just starting since
		// message metadata is being declared
		chnkstream_ptr = open_chnkstrms[csid]
		msg_ptr = new(Message)

		if err := read_message_header_1(conn, chnkstream_ptr, msg_ptr, chunk_bufs_ptr); err != nil {
			return nil, err
		}
		*n_read += 7
		if msg_ptr.timestamp > 0xffffff {
			*n_read += 4
		}
	case 2:
		// format 2, same as 1
		chnkstream_ptr = open_chnkstrms[csid]
		msg_ptr = new(Message)

		if err := read_message_header_2(conn, chnkstream_ptr, msg_ptr, chunk_bufs_ptr); err != nil {
			return nil, err
		}
		*n_read += 3
		if msg_ptr.timestamp > 0xffffff {
			*n_read += 4
		}
	case 3:
		// format 3 has no header, only happens when either a previous message over the csid was too large for the chunksize
		// or a new message with the same exact metadata including timedelta is being sent.
		chnkstream_ptr = open_chnkstrms[csid]
		msg_prev, ok := open_msgs[chnkstream_ptr.last_msg_strm_id]
		if !ok {
			msg_ptr = new(Message)
			msg_ptr.msg_type = chnkstream_ptr.last_msg_type
			msg_ptr.msg_len = chnkstream_ptr.last_msg_len
			msg_ptr.data = make([]byte, msg_ptr.msg_len)

			chnkstream_ptr.timestamp += chnkstream_ptr.timedelta
			msg_ptr.timestamp = chnkstream_ptr.timestamp
		} else {
			msg_ptr = msg_prev
		}
	}

	n, err := read_chunk_data(conn, msg_ptr, chnk_size)
	if err != nil {
		return nil, err
	}
	*n_read += n

	conn.SetDeadline(time.Time{})

	if msg_ptr.curr_bytes_read < msg_ptr.msg_len { // if the full message was not read, save it as an open message for the next chunk
		open_msgs[chnkstream_ptr.last_msg_strm_id] = msg_ptr
		return nil, nil
	} else { // else a full message was read, remove it from the list of open messages if it was there at all and return it
		delete(open_msgs, chnkstream_ptr.last_msg_strm_id)
		return msg_ptr, nil
	}
}