/*
NAME
  client.go

DESCRIPTION
  client.go provides an RTP client, Client, that will receive RTP on a UDP
  connection, perform an operation on the packets and then store in a ringBuffer
  that can be read from Read, an implemntation of io.Reader.

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

LICENSE
  This is 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 rtp

import (
	"io"
	"net"
	"sync"

	"bitbucket.org/ausocean/utils/ring"
)

// Misc consts.
const (
	chanSize = 10
)

// RingBuffer consts.
const (
	ringBufferSize        = 10
	ringBufferElementSize = 4096
)

// Client describes an RTP client that can receive an RTP stream and implements
// io.Reader.
type Client struct {
	conn    *net.UDPConn                 // The UDP connection RTP packets will be read from.
	wg      sync.WaitGroup               // Used to wait for recv routine to finish.
	done    chan struct{}                // Used to terminate the recv routine.
	ring    *ring.Buffer                 // Processed data from RTP packets will be stored here.
	op      func([]byte) ([]byte, error) // The operation to perform on received RTP packets before storing.
	ErrChan chan error                   // Errors encountered during recv will be sent to this chan.
}

// NewClient returns a pointer to a new Client.
//
// addr is the address of form <ip>:<port> that we expect to receive
// RTP at. op is a function, if non nil, that will be used to perform an
// operation on each received RTP packet. For example, the op func may parse
// out the RTP payload. The result of the operation is then what is stored
// in the ringBuffer for reading.
func NewClient(addr string, op func([]byte) ([]byte, error)) (*Client, error) {
	c := &Client{
		done: make(chan struct{}, chanSize),
		ring: ring.NewBuffer(ringBufferSize, ringBufferElementSize, 0),
		op:   op,
	}

	a, err := net.ResolveUDPAddr("udp", addr)
	if err != nil {
		return nil, err
	}

	c.conn, err = net.ListenUDP("udp", a)
	if err != nil {
		return nil, err
	}

	return c, nil
}

// Start will start the recv routine.
func (c *Client) Start() {
	c.wg.Add(1)
	go c.recv()
}

// recv will read RTP packets from the UDP connection, perform the operation
// on them given at creation of the Client and then store the result in the
// ringBuffer for Reading.
func (c *Client) recv() {
	defer c.wg.Done()
	buf := make([]byte, ringBufferElementSize)
	for {
		select {
		case <-c.done:
			return
		default:
			n, _, err := c.conn.ReadFromUDP(buf)
			if err != nil {
				c.ErrChan <- err
				continue
			}

			var _buf []byte
			switch c.op {
			case nil:
				_buf = buf[:n]
			default:
				_buf, err = c.op(buf[:n])
				if err != nil {
					c.ErrChan <- err
					continue
				}
			}

			_, err = c.ring.Write(_buf)
			c.ring.Flush()
			if err != nil {
				c.ErrChan <- err
				continue
			}
		}
	}
}

// Stop will send a done signal to the receive routine, and also close the
// connection.
func (c *Client) Stop() {
	close(c.done)
	c.conn.Close()
	c.wg.Wait()
}

// Read implements io.Reader.
//
// Read will get the next chunk from the ringBuffer and copy the bytes to p.
func (c *Client) Read(p []byte) (int, error) {
	chunk, err := c.ring.Next(0)
	if err != nil {
		return 0, io.EOF
	}
	n := copy(p, chunk.Bytes())
	chunk.Close()
	chunk = nil
	return n, nil
}