package encoding

import (
	"crypto/md5"
	"encoding/binary"
	"hash/fnv"
	"io"
	"sync"
	"time"

	"golang.org/x/crypto/chacha20poly1305"

	"v2ray.com/core/common"
	"v2ray.com/core/common/bitmask"
	"v2ray.com/core/common/buf"
	"v2ray.com/core/common/crypto"
	"v2ray.com/core/common/net"
	"v2ray.com/core/common/protocol"
	"v2ray.com/core/common/task"
	"v2ray.com/core/proxy/vmess"
)

type sessionId struct {
	user  [16]byte
	key   [16]byte
	nonce [16]byte
}

// SessionHistory keeps track of historical session ids, to prevent replay attacks.
type SessionHistory struct {
	sync.RWMutex
	cache map[sessionId]time.Time
	task  *task.Periodic
}

// NewSessionHistory creates a new SessionHistory object.
func NewSessionHistory() *SessionHistory {
	h := &SessionHistory{
		cache: make(map[sessionId]time.Time, 128),
	}
	h.task = &task.Periodic{
		Interval: time.Second * 30,
		Execute:  h.removeExpiredEntries,
	}
	return h
}

// Close implements common.Closable.
func (h *SessionHistory) Close() error {
	return h.task.Close()
}

func (h *SessionHistory) addIfNotExits(session sessionId) bool {
	h.Lock()

	if expire, found := h.cache[session]; found && expire.After(time.Now()) {
		h.Unlock()
		return false
	}

	h.cache[session] = time.Now().Add(time.Minute * 3)
	h.Unlock()
	common.Must(h.task.Start())
	return true
}

func (h *SessionHistory) removeExpiredEntries() error {
	now := time.Now()

	h.Lock()
	defer h.Unlock()

	if len(h.cache) == 0 {
		return newError("nothing to do")
	}

	for session, expire := range h.cache {
		if expire.Before(now) {
			delete(h.cache, session)
		}
	}

	if len(h.cache) == 0 {
		h.cache = make(map[sessionId]time.Time, 128)
	}

	return nil
}

// ServerSession keeps information for a session in VMess server.
type ServerSession struct {
	userValidator   *vmess.TimedUserValidator
	sessionHistory  *SessionHistory
	requestBodyKey  [16]byte
	requestBodyIV   [16]byte
	responseBodyKey [16]byte
	responseBodyIV  [16]byte
	responseWriter  io.Writer
	responseHeader  byte
}

// NewServerSession creates a new ServerSession, using the given UserValidator.
// The ServerSession instance doesn't take ownership of the validator.
func NewServerSession(validator *vmess.TimedUserValidator, sessionHistory *SessionHistory) *ServerSession {
	return &ServerSession{
		userValidator:  validator,
		sessionHistory: sessionHistory,
	}
}

func parseSecurityType(b byte) protocol.SecurityType {
	if _, f := protocol.SecurityType_name[int32(b)]; f {
		st := protocol.SecurityType(b)
		// For backward compatibility.
		if st == protocol.SecurityType_UNKNOWN {
			st = protocol.SecurityType_LEGACY
		}
		return st
	}
	return protocol.SecurityType_UNKNOWN
}

// DecodeRequestHeader decodes and returns (if successful) a RequestHeader from an input stream.
func (s *ServerSession) DecodeRequestHeader(reader io.Reader) (*protocol.RequestHeader, error) {
	buffer := buf.New()
	defer buffer.Release()

	if _, err := buffer.ReadFullFrom(reader, protocol.IDBytesLen); err != nil {
		return nil, newError("failed to read request header").Base(err)
	}

	user, timestamp, valid := s.userValidator.Get(buffer.Bytes())
	if !valid {
		return nil, newError("invalid user")
	}

	iv := hashTimestamp(md5.New(), timestamp)
	vmessAccount := user.Account.(*vmess.MemoryAccount)

	aesStream := crypto.NewAesDecryptionStream(vmessAccount.ID.CmdKey(), iv[:])
	decryptor := crypto.NewCryptionReader(aesStream, reader)

	buffer.Clear()
	if _, err := buffer.ReadFullFrom(decryptor, 38); err != nil {
		return nil, newError("failed to read request header").Base(err)
	}

	request := &protocol.RequestHeader{
		User:    user,
		Version: buffer.Byte(0),
	}

	copy(s.requestBodyIV[:], buffer.BytesRange(1, 17))   // 16 bytes
	copy(s.requestBodyKey[:], buffer.BytesRange(17, 33)) // 16 bytes
	var sid sessionId
	copy(sid.user[:], vmessAccount.ID.Bytes())
	sid.key = s.requestBodyKey
	sid.nonce = s.requestBodyIV
	if !s.sessionHistory.addIfNotExits(sid) {
		return nil, newError("duplicated session id, possibly under replay attack")
	}

	s.responseHeader = buffer.Byte(33)             // 1 byte
	request.Option = bitmask.Byte(buffer.Byte(34)) // 1 byte
	padingLen := int(buffer.Byte(35) >> 4)
	request.Security = parseSecurityType(buffer.Byte(35) & 0x0F)
	// 1 bytes reserved
	request.Command = protocol.RequestCommand(buffer.Byte(37))

	switch request.Command {
	case protocol.RequestCommandMux:
		request.Address = net.DomainAddress("v1.mux.cool")
		request.Port = 0
	case protocol.RequestCommandTCP, protocol.RequestCommandUDP:
		if addr, port, err := addrParser.ReadAddressPort(buffer, decryptor); err == nil {
			request.Address = addr
			request.Port = port
		}
	}

	if padingLen > 0 {
		if _, err := buffer.ReadFullFrom(decryptor, int32(padingLen)); err != nil {
			return nil, newError("failed to read padding").Base(err)
		}
	}

	if _, err := buffer.ReadFullFrom(decryptor, 4); err != nil {
		return nil, newError("failed to read checksum").Base(err)
	}

	fnv1a := fnv.New32a()
	common.Must2(fnv1a.Write(buffer.BytesTo(-4)))
	actualHash := fnv1a.Sum32()
	expectedHash := binary.BigEndian.Uint32(buffer.BytesFrom(-4))

	if actualHash != expectedHash {
		return nil, newError("invalid auth")
	}

	if request.Address == nil {
		return nil, newError("invalid remote address")
	}

	if request.Security == protocol.SecurityType_UNKNOWN || request.Security == protocol.SecurityType_AUTO {
		return nil, newError("unknown security type: ", request.Security)
	}

	return request, nil
}

// DecodeRequestBody returns Reader from which caller can fetch decrypted body.
func (s *ServerSession) DecodeRequestBody(request *protocol.RequestHeader, reader io.Reader) buf.Reader {
	var sizeParser crypto.ChunkSizeDecoder = crypto.PlainChunkSizeParser{}
	if request.Option.Has(protocol.RequestOptionChunkMasking) {
		sizeParser = NewShakeSizeParser(s.requestBodyIV[:])
	}
	var padding crypto.PaddingLengthGenerator = nil
	if request.Option.Has(protocol.RequestOptionGlobalPadding) {
		padding = sizeParser.(crypto.PaddingLengthGenerator)
	}

	switch request.Security {
	case protocol.SecurityType_NONE:
		if request.Option.Has(protocol.RequestOptionChunkStream) {
			if request.Command.TransferType() == protocol.TransferTypeStream {
				return crypto.NewChunkStreamReader(sizeParser, reader)
			}

			auth := &crypto.AEADAuthenticator{
				AEAD:                    new(NoOpAuthenticator),
				NonceGenerator:          crypto.GenerateEmptyBytes(),
				AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
			}
			return crypto.NewAuthenticationReader(auth, sizeParser, reader, protocol.TransferTypePacket, padding)
		}

		return buf.NewReader(reader)
	case protocol.SecurityType_LEGACY:
		aesStream := crypto.NewAesDecryptionStream(s.requestBodyKey[:], s.requestBodyIV[:])
		cryptionReader := crypto.NewCryptionReader(aesStream, reader)
		if request.Option.Has(protocol.RequestOptionChunkStream) {
			auth := &crypto.AEADAuthenticator{
				AEAD:                    new(FnvAuthenticator),
				NonceGenerator:          crypto.GenerateEmptyBytes(),
				AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
			}
			return crypto.NewAuthenticationReader(auth, sizeParser, cryptionReader, request.Command.TransferType(), padding)
		}

		return buf.NewReader(cryptionReader)
	case protocol.SecurityType_AES128_GCM:
		aead := crypto.NewAesGcm(s.requestBodyKey[:])

		auth := &crypto.AEADAuthenticator{
			AEAD:                    aead,
			NonceGenerator:          GenerateChunkNonce(s.requestBodyIV[:], uint32(aead.NonceSize())),
			AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
		}
		return crypto.NewAuthenticationReader(auth, sizeParser, reader, request.Command.TransferType(), padding)
	case protocol.SecurityType_CHACHA20_POLY1305:
		aead, _ := chacha20poly1305.New(GenerateChacha20Poly1305Key(s.requestBodyKey[:]))

		auth := &crypto.AEADAuthenticator{
			AEAD:                    aead,
			NonceGenerator:          GenerateChunkNonce(s.requestBodyIV[:], uint32(aead.NonceSize())),
			AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
		}
		return crypto.NewAuthenticationReader(auth, sizeParser, reader, request.Command.TransferType(), padding)
	default:
		panic("Unknown security type.")
	}
}

// EncodeResponseHeader writes encoded response header into the given writer.
func (s *ServerSession) EncodeResponseHeader(header *protocol.ResponseHeader, writer io.Writer) {
	s.responseBodyKey = md5.Sum(s.requestBodyKey[:])
	s.responseBodyIV = md5.Sum(s.requestBodyIV[:])

	aesStream := crypto.NewAesEncryptionStream(s.responseBodyKey[:], s.responseBodyIV[:])
	encryptionWriter := crypto.NewCryptionWriter(aesStream, writer)
	s.responseWriter = encryptionWriter

	common.Must2(encryptionWriter.Write([]byte{s.responseHeader, byte(header.Option)}))
	err := MarshalCommand(header.Command, encryptionWriter)
	if err != nil {
		common.Must2(encryptionWriter.Write([]byte{0x00, 0x00}))
	}
}

// EncodeResponseBody returns a Writer that auto-encrypt content written by caller.
func (s *ServerSession) EncodeResponseBody(request *protocol.RequestHeader, writer io.Writer) buf.Writer {
	var sizeParser crypto.ChunkSizeEncoder = crypto.PlainChunkSizeParser{}
	if request.Option.Has(protocol.RequestOptionChunkMasking) {
		sizeParser = NewShakeSizeParser(s.responseBodyIV[:])
	}
	var padding crypto.PaddingLengthGenerator = nil
	if request.Option.Has(protocol.RequestOptionGlobalPadding) {
		padding = sizeParser.(crypto.PaddingLengthGenerator)
	}

	switch request.Security {
	case protocol.SecurityType_NONE:
		if request.Option.Has(protocol.RequestOptionChunkStream) {
			if request.Command.TransferType() == protocol.TransferTypeStream {
				return crypto.NewChunkStreamWriter(sizeParser, writer)
			}

			auth := &crypto.AEADAuthenticator{
				AEAD:                    new(NoOpAuthenticator),
				NonceGenerator:          crypto.GenerateEmptyBytes(),
				AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
			}
			return crypto.NewAuthenticationWriter(auth, sizeParser, writer, protocol.TransferTypePacket, padding)
		}

		return buf.NewWriter(writer)
	case protocol.SecurityType_LEGACY:
		if request.Option.Has(protocol.RequestOptionChunkStream) {
			auth := &crypto.AEADAuthenticator{
				AEAD:                    new(FnvAuthenticator),
				NonceGenerator:          crypto.GenerateEmptyBytes(),
				AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
			}
			return crypto.NewAuthenticationWriter(auth, sizeParser, s.responseWriter, request.Command.TransferType(), padding)
		}

		return &buf.SequentialWriter{Writer: s.responseWriter}
	case protocol.SecurityType_AES128_GCM:
		aead := crypto.NewAesGcm(s.responseBodyKey[:])

		auth := &crypto.AEADAuthenticator{
			AEAD:                    aead,
			NonceGenerator:          GenerateChunkNonce(s.responseBodyIV[:], uint32(aead.NonceSize())),
			AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
		}
		return crypto.NewAuthenticationWriter(auth, sizeParser, writer, request.Command.TransferType(), padding)
	case protocol.SecurityType_CHACHA20_POLY1305:
		aead, _ := chacha20poly1305.New(GenerateChacha20Poly1305Key(s.responseBodyKey[:]))

		auth := &crypto.AEADAuthenticator{
			AEAD:                    aead,
			NonceGenerator:          GenerateChunkNonce(s.responseBodyIV[:], uint32(aead.NonceSize())),
			AdditionalDataGenerator: crypto.GenerateEmptyBytes(),
		}
		return crypto.NewAuthenticationWriter(auth, sizeParser, writer, request.Command.TransferType(), padding)
	default:
		panic("Unknown security type.")
	}
}