/*
NAME
  Client.go

DESCRIPTION
  Client.go provides an implemntation of a basic RTCP Client that will send
  receiver reports, and receive sender reports to parse relevant statistics.

AUTHORS
  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 rtcp

import (
	"encoding/binary"
	"errors"
	"fmt"
	"math"
	"net"
	"sync"
	"time"

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

const (
	senderSSRC        = 1 // Any non-zero value will do.
	defaultClientName = "Client"
	delayUnit         = 1.0 / 65536.0
	pkg               = "rtcp: "
	rtcpVer           = 2
	receiverBufSize   = 200
)

type Log func(lvl int8, msg string, args ...interface{})

// Client is an RTCP Client that will handle receiving SenderReports from a server
// and sending out ReceiverReports.
type Client struct {
	cAddr       *net.UDPAddr          // Address of client.
	sAddr       *net.UDPAddr          // Address of RTSP server.
	name        string                // Name of the client for source description purposes.
	sourceSSRC  uint32                // Source identifier of this client.
	mu          sync.Mutex            // Will be used to change parameters during operation safely.
	seq         uint32                // Last RTP sequence number.
	senderTs    [8]byte               // The timestamp of the last sender report.
	interval    time.Duration         // Interval between sender report and receiver report.
	receiveTime time.Time             // Time last sender report was received.
	buf         [receiverBufSize]byte // Buf used to store the receiver report and source descriptions.
	conn        *net.UDPConn          // The UDP connection used for receiving and sending RTSP packets.
	wg          sync.WaitGroup        // This is used to wait for send and recv routines to stop when Client is stopped.
	quit        chan struct{}         // Channel used to communicate quit signal to send and recv routines.
	log         Log                   // Used to log any messages.

	err chan error // Client will send any errors through this chan. Can be accessed by Err().
}

// NewClient returns a pointer to a new Client.
func NewClient(clientAddress, serverAddress, name string, sendInterval time.Duration, rtpSSRC uint32, l Log) (*Client, error) {
	if name == "" {
		name = defaultClientName
	}

	c := &Client{
		name:       name,
		err:        make(chan error),
		quit:       make(chan struct{}),
		interval:   sendInterval,
		sourceSSRC: rtpSSRC,
		log:        l,
	}

	var err error
	c.cAddr, err = net.ResolveUDPAddr("udp", clientAddress)
	if err != nil {
		return nil, errors.New(fmt.Sprintf("can't resolve Client address, failed with error: %v\n", err))
	}

	c.sAddr, err = net.ResolveUDPAddr("udp", serverAddress)
	if err != nil {
		return nil, errors.New(fmt.Sprintf("can't resolve server address, failed with error: %v\n", err))
	}

	c.conn, err = net.DialUDP("udp", c.cAddr, c.sAddr)
	if err != nil {
		return nil, errors.New(fmt.Sprintf("can't dial, failed with error: %v\n", err))
	}
	return c, nil
}

// Start starts the listen and send routines. This will start the process of
// receiving and parsing sender reports, and the process of sending receiver
// reports to the server.
func (c *Client) Start() {
	c.log(logger.Debug, pkg+"Client is starting")
	c.wg.Add(2)
	go c.recv()
	go c.send()
}

// Stop sends a quit signal to the send and receive routines and closes the
// UDP connection. It will wait until both routines have returned.
func (c *Client) Stop() {
	c.log(logger.Debug, pkg+"Client is stopping")
	close(c.quit)
	c.conn.Close()
	c.wg.Wait()
}

// Err provides read access to the Client err channel. This must be checked
// otherwise the client will block if an error encountered.
func (c *Client) Err() <-chan error {
	return c.err
}

// recv reads from the UDP connection and parses SenderReports.
func (c *Client) recv() {
	defer c.wg.Done()
	c.log(logger.Debug, pkg+"Client is receiving")
	buf := make([]byte, 4096)
	for {
		select {
		case <-c.quit:
			return
		default:
			n, _, err := c.conn.ReadFromUDP(buf)
			if err != nil {
				c.err <- err
				continue
			}
			c.log(logger.Debug, pkg+"sender report received", "report", buf[:n])
			c.parse(buf[:n])
		}
	}
}

// send writes receiver reports to the server.
func (c *Client) send() {
	defer c.wg.Done()
	c.log(logger.Debug, pkg+"Client is sending")
	for {
		select {
		case <-c.quit:
			return
		default:
			time.Sleep(c.interval)

			report := ReceiverReport{
				Header: Header{
					Version:     rtcpVer,
					Padding:     false,
					ReportCount: 1,
					Type:        typeReceiverReport,
				},
				SenderSSRC: senderSSRC,
				Blocks: []ReportBlock{
					ReportBlock{
						SourceIdentifier:  c.sourceSSRC,
						FractionLost:      0,
						PacketsLost:       math.MaxUint32,
						HighestSequence:   c.sequence(),
						Jitter:            c.jitter(),
						SenderReportTs:    c.lastSenderTs(),
						SenderReportDelay: c.delay(),
					},
				},
				Extensions: nil,
			}

			description := Description{
				Header: Header{
					Version:     rtcpVer,
					Padding:     false,
					ReportCount: 1,
					Type:        typeDescription,
				},
				Chunks: []Chunk{
					Chunk{
						SSRC: senderSSRC,
						Items: []SDESItem{
							SDESItem{
								Type: typeCName,
								Text: []byte(c.name),
							},
						},
					},
				},
			}

			c.log(logger.Debug, pkg+"sending receiver report")
			_, err := c.conn.Write(c.formPayload(&report, &description))
			if err != nil {
				c.err <- err
			}
		}
	}
}

// formPayload takes a pointer to a ReceiverReport and a pointer to a
// Source Description and calls Bytes on both, writing to the underlying Client
// buf. A slice to the combined writtem memory is returned.
func (c *Client) formPayload(r *ReceiverReport, d *Description) []byte {
	rl := len(r.Bytes(c.buf[:]))
	dl := len(d.Bytes(c.buf[rl:]))
	t := rl + dl
	if t > cap(c.buf) {
		panic("Client buf not big enough")
	}
	return c.buf[:t]
}

// parse will read important statistics from sender reports.
func (c *Client) parse(buf []byte) {
	c.markReceivedTime()
	t, err := ParseTimestamp(buf)
	if err != nil {
		c.err <- fmt.Errorf("could not get timestamp from sender report, failed with error: %v", err)
	}
	c.setSenderTs(t)
}

// SetSequence will allow updating of the highest sequence number received
// through an RTP stream.
func (c *Client) SetSequence(s uint32) {
	c.mu.Lock()
	c.seq = s
	c.mu.Unlock()
}

// sequence will return the highest sequence number received through RTP.
func (c *Client) sequence() uint32 {
	c.mu.Lock()
	s := c.seq
	c.mu.Unlock()
	return s
}

// jitter returns the interarrival jitter as described by RTCP specifications:
// https://tools.ietf.org/html/rfc3550
// TODO(saxon): complete this.
func (c *Client) jitter() uint32 {
	return 0
}

// setSenderTs allows us to safely set the current sender report timestamp.
func (c *Client) setSenderTs(t Timestamp) {
	c.mu.Lock()
	binary.BigEndian.PutUint32(c.senderTs[:], t.Seconds)
	binary.BigEndian.PutUint32(c.senderTs[4:], t.Fraction)
	c.mu.Unlock()
}

// lastSenderTs returns the timestamp of the most recent sender report.
func (c *Client) lastSenderTs() uint32 {
	c.mu.Lock()
	t := binary.BigEndian.Uint32(c.senderTs[2:])
	c.mu.Unlock()
	return t
}

// delay returns the duration between the receive time of the last sender report
// and now. This is called when forming a receiver report.
func (c *Client) delay() uint32 {
	c.mu.Lock()
	t := c.receiveTime
	c.mu.Unlock()
	return uint32(time.Now().Sub(t).Seconds() / delayUnit)
}

// markReceivedTime is called when a sender report is received to mark the receive time.
func (c *Client) markReceivedTime() {
	c.mu.Lock()
	c.receiveTime = time.Now()
	c.mu.Unlock()
}