// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package derp

import (
	"bufio"
	"crypto/rand"
	"encoding/json"
	"fmt"
	"io"
	"net"
	"time"

	"golang.org/x/crypto/curve25519"
	"golang.org/x/crypto/nacl/box"
	"tailscale.com/types/logger"
)

type Client struct {
	serverKey  [32]byte
	privateKey [32]byte // TODO(crawshaw): make this wgcfg.PrivateKey?
	publicKey  [32]byte
	logf       logger.Logf
	netConn    net.Conn
	conn       *bufio.ReadWriter
}

func NewClient(privateKey [32]byte, netConn net.Conn, conn *bufio.ReadWriter, logf logger.Logf) (*Client, error) {
	c := &Client{
		privateKey: privateKey,
		logf:       logf,
		netConn:    netConn,
		conn:       conn,
	}
	curve25519.ScalarBaseMult(&c.publicKey, &c.privateKey)

	if err := c.recvServerKey(); err != nil {
		return nil, fmt.Errorf("derp.Client: failed to receive server key: %v", err)
	}
	if err := c.sendClientKey(); err != nil {
		return nil, fmt.Errorf("derp.Client: failed to send client key: %v", err)
	}
	_, err := c.recvServerInfo()
	if err != nil {
		return nil, fmt.Errorf("derp.Client: failed to receive server info: %v", err)
	}

	return c, nil
}

func (c *Client) recvServerKey() error {
	gotMagic, err := readUint32(c.conn, 0xffffffff)
	if err != nil {
		return err
	}
	if gotMagic != magic {
		return fmt.Errorf("bad magic %x, want %x", gotMagic, magic)
	}
	if err := readType(c.conn.Reader, typeServerKey); err != nil {
		return err
	}
	if _, err := io.ReadFull(c.conn, c.serverKey[:]); err != nil {
		return err
	}
	return nil
}

func (c *Client) recvServerInfo() (*serverInfo, error) {
	if err := readType(c.conn.Reader, typeServerInfo); err != nil {
		return nil, err
	}
	var nonce [24]byte
	if _, err := io.ReadFull(c.conn, nonce[:]); err != nil {
		return nil, fmt.Errorf("nonce: %v", err)
	}
	msgLen, err := readUint32(c.conn, oneMB)
	if err != nil {
		return nil, fmt.Errorf("msglen: %v", err)
	}
	msgbox := make([]byte, msgLen)
	if _, err := io.ReadFull(c.conn, msgbox); err != nil {
		return nil, fmt.Errorf("msgbox: %v", err)
	}
	msg, ok := box.Open(nil, msgbox, &nonce, &c.serverKey, &c.privateKey)
	if !ok {
		return nil, fmt.Errorf("msgbox: cannot open len=%d with server key %x", msgLen, c.serverKey[:])
	}
	info := new(serverInfo)
	if err := json.Unmarshal(msg, info); err != nil {
		return nil, fmt.Errorf("msg: %v", err)
	}
	return info, nil
}

func (c *Client) sendClientKey() error {
	var nonce [24]byte
	if _, err := rand.Read(nonce[:]); err != nil {
		return err
	}
	msg := []byte("{}") // no clientInfo for now
	msgbox := box.Seal(nil, msg, &nonce, &c.serverKey, &c.privateKey)

	if _, err := c.conn.Write(c.publicKey[:]); err != nil {
		return err
	}
	if _, err := c.conn.Write(nonce[:]); err != nil {
		return err
	}
	if err := putUint32(c.conn.Writer, uint32(len(msgbox))); err != nil {
		return err
	}
	if _, err := c.conn.Write(msgbox); err != nil {
		return err
	}
	return c.conn.Flush()
}

func (c *Client) Send(dstKey [32]byte, msg []byte) (err error) {
	defer func() {
		if err != nil {
			err = fmt.Errorf("derp.Send: %v", err)
		}
	}()

	if err := c.conn.WriteByte(typeSendPacket); err != nil {
		return err
	}
	if _, err := c.conn.Write(dstKey[:]); err != nil {
		return err
	}
	msgLen := uint32(len(msg))
	if int(msgLen) != len(msg) {
		return fmt.Errorf("packet too big: %d", len(msg))
	}
	if err := putUint32(c.conn.Writer, msgLen); err != nil {
		return err
	}
	if _, err := c.conn.Write(msg); err != nil {
		return err
	}
	return c.conn.Flush()
}

func (c *Client) Recv(b []byte) (n int, err error) {
	defer func() {
		if err != nil {
			err = fmt.Errorf("derp.Recv: %v", err)
		}
	}()

loop:
	for {
		c.netConn.SetReadDeadline(time.Now().Add(120 * time.Second))
		packetType, err := c.conn.ReadByte()
		if err != nil {
			return 0, err
		}
		switch packetType {
		case typeKeepAlive:
			continue
		case typeRecvPacket:
			break loop
		default:
			return 0, fmt.Errorf("derp.Recv: unknown packet type %d", packetType)
		}
	}

	packetLen, err := readUint32(c.conn.Reader, oneMB)
	if err != nil {
		return 0, err
	}
	if int(packetLen) > len(b) {
		// TODO(crawshaw): discard the packet
		return 0, io.ErrShortBuffer
	}
	b = b[:packetLen]
	if _, err := io.ReadFull(c.conn, b); err != nil {
		return 0, err
	}
	return int(packetLen), nil
}