// Package memcached provides a memcached binary protocol client.
package memcached

import (
	"encoding/binary"
	"fmt"
	"github.com/couchbase/gomemcached"
	"github.com/couchbase/goutils/logging"
	"github.com/couchbase/goutils/scramsha"
	"github.com/pkg/errors"
	"io"
	"math"
	"net"
	"strings"
	"sync"
	"sync/atomic"
	"time"
)

type ClientIface interface {
	Add(vb uint16, key string, flags int, exp int, body []byte) (*gomemcached.MCResponse, error)
	Append(vb uint16, key string, data []byte) (*gomemcached.MCResponse, error)
	Auth(user, pass string) (*gomemcached.MCResponse, error)
	AuthList() (*gomemcached.MCResponse, error)
	AuthPlain(user, pass string) (*gomemcached.MCResponse, error)
	AuthScramSha(user, pass string) (*gomemcached.MCResponse, error)
	CASNext(vb uint16, k string, exp int, state *CASState) bool
	CAS(vb uint16, k string, f CasFunc, initexp int) (*gomemcached.MCResponse, error)
	Close() error
	Decr(vb uint16, key string, amt, def uint64, exp int) (uint64, error)
	Del(vb uint16, key string) (*gomemcached.MCResponse, error)
	EnableMutationToken() (*gomemcached.MCResponse, error)
	Get(vb uint16, key string) (*gomemcached.MCResponse, error)
	GetSubdoc(vb uint16, key string, subPaths []string) (*gomemcached.MCResponse, error)
	GetAndTouch(vb uint16, key string, exp int) (*gomemcached.MCResponse, error)
	GetBulk(vb uint16, keys []string, rv map[string]*gomemcached.MCResponse, subPaths []string) error
	GetMeta(vb uint16, key string) (*gomemcached.MCResponse, error)
	GetRandomDoc() (*gomemcached.MCResponse, error)
	Hijack() io.ReadWriteCloser
	Incr(vb uint16, key string, amt, def uint64, exp int) (uint64, error)
	Observe(vb uint16, key string) (result ObserveResult, err error)
	ObserveSeq(vb uint16, vbuuid uint64) (result *ObserveSeqResult, err error)
	Receive() (*gomemcached.MCResponse, error)
	ReceiveWithDeadline(deadline time.Time) (*gomemcached.MCResponse, error)
	Send(req *gomemcached.MCRequest) (rv *gomemcached.MCResponse, err error)
	Set(vb uint16, key string, flags int, exp int, body []byte) (*gomemcached.MCResponse, error)
	SetKeepAliveOptions(interval time.Duration)
	SetReadDeadline(t time.Time)
	SetDeadline(t time.Time)
	SelectBucket(bucket string) (*gomemcached.MCResponse, error)
	SetCas(vb uint16, key string, flags int, exp int, cas uint64, body []byte) (*gomemcached.MCResponse, error)
	Stats(key string) ([]StatValue, error)
	StatsMap(key string) (map[string]string, error)
	StatsMapForSpecifiedStats(key string, statsMap map[string]string) error
	Transmit(req *gomemcached.MCRequest) error
	TransmitWithDeadline(req *gomemcached.MCRequest, deadline time.Time) error
	TransmitResponse(res *gomemcached.MCResponse) error

	// UprFeed Related
	NewUprFeed() (*UprFeed, error)
	NewUprFeedIface() (UprFeedIface, error)
	NewUprFeedWithConfig(ackByClient bool) (*UprFeed, error)
	NewUprFeedWithConfigIface(ackByClient bool) (UprFeedIface, error)
	UprGetFailoverLog(vb []uint16) (map[uint16]*FailoverLog, error)
}

const bufsize = 1024

var UnHealthy uint32 = 0
var Healthy uint32 = 1

type Features []Feature
type Feature uint16

const FeatureMutationToken = Feature(0x04)
const FeatureXattr = Feature(0x06)
const FeatureDataType = Feature(0x0b)

// The Client itself.
type Client struct {
	conn io.ReadWriteCloser
	// use uint32 type so that it can be accessed through atomic APIs
	healthy uint32
	opaque  uint32

	hdrBuf []byte
}

var (
	DefaultDialTimeout = time.Duration(0) // No timeout

	DefaultWriteTimeout = time.Duration(0) // No timeout

	dialFun = func(prot, dest string) (net.Conn, error) {
		return net.DialTimeout(prot, dest, DefaultDialTimeout)
	}
)

// Connect to a memcached server.
func Connect(prot, dest string) (rv *Client, err error) {
	conn, err := dialFun(prot, dest)
	if err != nil {
		return nil, err
	}
	return Wrap(conn)
}

func SetDefaultTimeouts(dial, read, write time.Duration) {
	DefaultDialTimeout = dial
	DefaultWriteTimeout = write
}

func SetDefaultDialTimeout(dial time.Duration) {
	DefaultDialTimeout = dial
}

func (c *Client) SetKeepAliveOptions(interval time.Duration) {
	c.conn.(*net.TCPConn).SetKeepAlive(true)
	c.conn.(*net.TCPConn).SetKeepAlivePeriod(interval)
}

func (c *Client) SetReadDeadline(t time.Time) {
	c.conn.(*net.TCPConn).SetReadDeadline(t)
}

func (c *Client) SetDeadline(t time.Time) {
	c.conn.(*net.TCPConn).SetDeadline(t)
}

// Wrap an existing transport.
func Wrap(rwc io.ReadWriteCloser) (rv *Client, err error) {
	client := &Client{
		conn:   rwc,
		hdrBuf: make([]byte, gomemcached.HDR_LEN),
		opaque: uint32(1),
	}
	client.setHealthy(true)
	return client, nil
}

// Close the connection when you're done.
func (c *Client) Close() error {
	return c.conn.Close()
}

// IsHealthy returns true unless the client is belived to have
// difficulty communicating to its server.
//
// This is useful for connection pools where we want to
// non-destructively determine that a connection may be reused.
func (c Client) IsHealthy() bool {
	healthyState := atomic.LoadUint32(&c.healthy)
	return healthyState == Healthy
}

// Send a custom request and get the response.
func (c *Client) Send(req *gomemcached.MCRequest) (rv *gomemcached.MCResponse, err error) {
	err = c.Transmit(req)
	if err != nil {
		return
	}
	resp, _, err := getResponse(c.conn, c.hdrBuf)
	c.setHealthy(!gomemcached.IsFatal(err))
	return resp, err
}

// Transmit send a request, but does not wait for a response.
func (c *Client) Transmit(req *gomemcached.MCRequest) error {
	if DefaultWriteTimeout > 0 {
		c.conn.(net.Conn).SetWriteDeadline(time.Now().Add(DefaultWriteTimeout))
	}
	_, err := transmitRequest(c.conn, req)
	// clear write deadline to avoid interference with future write operations
	if DefaultWriteTimeout > 0 {
		c.conn.(net.Conn).SetWriteDeadline(time.Time{})
	}
	if err != nil {
		c.setHealthy(false)
	}
	return err
}

func (c *Client) TransmitWithDeadline(req *gomemcached.MCRequest, deadline time.Time) error {
	c.conn.(net.Conn).SetWriteDeadline(deadline)

	_, err := transmitRequest(c.conn, req)

	// clear write deadline to avoid interference with future write operations
	c.conn.(net.Conn).SetWriteDeadline(time.Time{})

	if err != nil {
		c.setHealthy(false)
	}
	return err
}

// TransmitResponse send a response, does not wait.
func (c *Client) TransmitResponse(res *gomemcached.MCResponse) error {
	if DefaultWriteTimeout > 0 {
		c.conn.(net.Conn).SetWriteDeadline(time.Now().Add(DefaultWriteTimeout))
	}
	_, err := transmitResponse(c.conn, res)
	// clear write deadline to avoid interference with future write operations
	if DefaultWriteTimeout > 0 {
		c.conn.(net.Conn).SetWriteDeadline(time.Time{})
	}
	if err != nil {
		c.setHealthy(false)
	}
	return err
}

// Receive a response
func (c *Client) Receive() (*gomemcached.MCResponse, error) {
	resp, _, err := getResponse(c.conn, c.hdrBuf)
	if err != nil && resp.Status != gomemcached.KEY_ENOENT && resp.Status != gomemcached.EBUSY {
		c.setHealthy(false)
	}
	return resp, err
}

func (c *Client) ReceiveWithDeadline(deadline time.Time) (*gomemcached.MCResponse, error) {
	c.conn.(net.Conn).SetReadDeadline(deadline)

	resp, _, err := getResponse(c.conn, c.hdrBuf)

	// Clear read deadline to avoid interference with future read operations.
	c.conn.(net.Conn).SetReadDeadline(time.Time{})

	if err != nil && resp.Status != gomemcached.KEY_ENOENT && resp.Status != gomemcached.EBUSY {
		c.setHealthy(false)
	}
	return resp, err
}

func appendMutationToken(bytes []byte) []byte {
	bytes = append(bytes, 0, 0)
	binary.BigEndian.PutUint16(bytes[len(bytes)-2:], uint16(0x04))
	return bytes
}

//Send a hello command to enable MutationTokens
func (c *Client) EnableMutationToken() (*gomemcached.MCResponse, error) {
	var payload []byte
	payload = appendMutationToken(payload)

	return c.Send(&gomemcached.MCRequest{
		Opcode: gomemcached.HELLO,
		Key:    []byte("GoMemcached"),
		Body:   payload,
	})

}

//Send a hello command to enable specific features
func (c *Client) EnableFeatures(features Features) (*gomemcached.MCResponse, error) {
	var payload []byte

	for _, feature := range features {
		payload = append(payload, 0, 0)
		binary.BigEndian.PutUint16(payload[len(payload)-2:], uint16(feature))
	}

	return c.Send(&gomemcached.MCRequest{
		Opcode: gomemcached.HELLO,
		Key:    []byte("GoMemcached"),
		Body:   payload,
	})

}

// Get the value for a key.
func (c *Client) Get(vb uint16, key string) (*gomemcached.MCResponse, error) {
	return c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.GET,
		VBucket: vb,
		Key:     []byte(key),
	})
}

// Get the xattrs, doc value for the input key
func (c *Client) GetSubdoc(vb uint16, key string, subPaths []string) (*gomemcached.MCResponse, error) {

	extraBuf, valueBuf := GetSubDocVal(subPaths)
	res, err := c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.SUBDOC_MULTI_LOOKUP,
		VBucket: vb,
		Key:     []byte(key),
		Extras:  extraBuf,
		Body:    valueBuf,
	})

	if err != nil && IfResStatusError(res) {
		return res, err
	}
	return res, nil
}

// Get the value for a key, and update expiry
func (c *Client) GetAndTouch(vb uint16, key string, exp int) (*gomemcached.MCResponse, error) {
	extraBuf := make([]byte, 4)
	binary.BigEndian.PutUint32(extraBuf[0:], uint32(exp))
	return c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.GAT,
		VBucket: vb,
		Key:     []byte(key),
		Extras:  extraBuf,
	})
}

// Get metadata for a key
func (c *Client) GetMeta(vb uint16, key string) (*gomemcached.MCResponse, error) {
	return c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.GET_META,
		VBucket: vb,
		Key:     []byte(key),
	})
}

// Del deletes a key.
func (c *Client) Del(vb uint16, key string) (*gomemcached.MCResponse, error) {
	return c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.DELETE,
		VBucket: vb,
		Key:     []byte(key)})
}

// Get a random document
func (c *Client) GetRandomDoc() (*gomemcached.MCResponse, error) {
	return c.Send(&gomemcached.MCRequest{
		Opcode: 0xB6,
	})
}

// AuthList lists SASL auth mechanisms.
func (c *Client) AuthList() (*gomemcached.MCResponse, error) {
	return c.Send(&gomemcached.MCRequest{
		Opcode: gomemcached.SASL_LIST_MECHS})
}

// Auth performs SASL PLAIN authentication against the server.
func (c *Client) Auth(user, pass string) (*gomemcached.MCResponse, error) {
	res, err := c.AuthList()

	if err != nil {
		return res, err
	}

	authMech := string(res.Body)
	if strings.Index(authMech, "PLAIN") != -1 {
		return c.AuthPlain(user, pass)
	}
	return nil, fmt.Errorf("auth mechanism PLAIN not supported")
}

// AuthScramSha performs SCRAM-SHA authentication against the server.
func (c *Client) AuthScramSha(user, pass string) (*gomemcached.MCResponse, error) {
	res, err := c.AuthList()
	if err != nil {
		return nil, errors.Wrap(err, "Unable to obtain list of methods.")
	}

	methods := string(res.Body)
	method, err := scramsha.BestMethod(methods)
	if err != nil {
		return nil, errors.Wrap(err,
			"Unable to select SCRAM-SHA method.")
	}

	s, err := scramsha.NewScramSha(method)
	if err != nil {
		return nil, errors.Wrap(err, "Unable to initialize scramsha.")
	}

	logging.Infof("Using %v authentication for user %v%v%v", method, gomemcached.UdTagBegin, user, gomemcached.UdTagEnd)

	message, err := s.GetStartRequest(user)
	if err != nil {
		return nil, errors.Wrapf(err,
			"Error building start request for user %s.", user)
	}

	startRequest := &gomemcached.MCRequest{
		Opcode: 0x21,
		Key:    []byte(method),
		Body:   []byte(message)}

	startResponse, err := c.Send(startRequest)
	if err != nil {
		return nil, errors.Wrap(err, "Error sending start request.")
	}

	err = s.HandleStartResponse(string(startResponse.Body))
	if err != nil {
		return nil, errors.Wrap(err, "Error handling start response.")
	}

	message = s.GetFinalRequest(pass)

	// send step request
	finalRequest := &gomemcached.MCRequest{
		Opcode: 0x22,
		Key:    []byte(method),
		Body:   []byte(message)}
	finalResponse, err := c.Send(finalRequest)
	if err != nil {
		return nil, errors.Wrap(err, "Error sending final request.")
	}

	err = s.HandleFinalResponse(string(finalResponse.Body))
	if err != nil {
		return nil, errors.Wrap(err, "Error handling final response.")
	}

	return finalResponse, nil
}

func (c *Client) AuthPlain(user, pass string) (*gomemcached.MCResponse, error) {
	logging.Infof("Using plain authentication for user %v%v%v", gomemcached.UdTagBegin, user, gomemcached.UdTagEnd)
	return c.Send(&gomemcached.MCRequest{
		Opcode: gomemcached.SASL_AUTH,
		Key:    []byte("PLAIN"),
		Body:   []byte(fmt.Sprintf("\x00%s\x00%s", user, pass))})
}

// select bucket
func (c *Client) SelectBucket(bucket string) (*gomemcached.MCResponse, error) {

	return c.Send(&gomemcached.MCRequest{
		Opcode: gomemcached.SELECT_BUCKET,
		Key:    []byte(fmt.Sprintf("%s", bucket))})
}

func (c *Client) store(opcode gomemcached.CommandCode, vb uint16,
	key string, flags int, exp int, body []byte) (*gomemcached.MCResponse, error) {

	req := &gomemcached.MCRequest{
		Opcode:  opcode,
		VBucket: vb,
		Key:     []byte(key),
		Cas:     0,
		Opaque:  0,
		Extras:  []byte{0, 0, 0, 0, 0, 0, 0, 0},
		Body:    body}

	binary.BigEndian.PutUint64(req.Extras, uint64(flags)<<32|uint64(exp))
	return c.Send(req)
}

func (c *Client) storeCas(opcode gomemcached.CommandCode, vb uint16,
	key string, flags int, exp int, cas uint64, body []byte) (*gomemcached.MCResponse, error) {

	req := &gomemcached.MCRequest{
		Opcode:  opcode,
		VBucket: vb,
		Key:     []byte(key),
		Cas:     cas,
		Opaque:  0,
		Extras:  []byte{0, 0, 0, 0, 0, 0, 0, 0},
		Body:    body}

	binary.BigEndian.PutUint64(req.Extras, uint64(flags)<<32|uint64(exp))
	return c.Send(req)
}

// Incr increments the value at the given key.
func (c *Client) Incr(vb uint16, key string,
	amt, def uint64, exp int) (uint64, error) {

	req := &gomemcached.MCRequest{
		Opcode:  gomemcached.INCREMENT,
		VBucket: vb,
		Key:     []byte(key),
		Extras:  make([]byte, 8+8+4),
	}
	binary.BigEndian.PutUint64(req.Extras[:8], amt)
	binary.BigEndian.PutUint64(req.Extras[8:16], def)
	binary.BigEndian.PutUint32(req.Extras[16:20], uint32(exp))

	resp, err := c.Send(req)
	if err != nil {
		return 0, err
	}

	return binary.BigEndian.Uint64(resp.Body), nil
}

// Decr decrements the value at the given key.
func (c *Client) Decr(vb uint16, key string,
	amt, def uint64, exp int) (uint64, error) {

	req := &gomemcached.MCRequest{
		Opcode:  gomemcached.DECREMENT,
		VBucket: vb,
		Key:     []byte(key),
		Extras:  make([]byte, 8+8+4),
	}
	binary.BigEndian.PutUint64(req.Extras[:8], amt)
	binary.BigEndian.PutUint64(req.Extras[8:16], def)
	binary.BigEndian.PutUint32(req.Extras[16:20], uint32(exp))

	resp, err := c.Send(req)
	if err != nil {
		return 0, err
	}

	return binary.BigEndian.Uint64(resp.Body), nil
}

// Add a value for a key (store if not exists).
func (c *Client) Add(vb uint16, key string, flags int, exp int,
	body []byte) (*gomemcached.MCResponse, error) {
	return c.store(gomemcached.ADD, vb, key, flags, exp, body)
}

// Set the value for a key.
func (c *Client) Set(vb uint16, key string, flags int, exp int,
	body []byte) (*gomemcached.MCResponse, error) {
	return c.store(gomemcached.SET, vb, key, flags, exp, body)
}

// SetCas set the value for a key with cas
func (c *Client) SetCas(vb uint16, key string, flags int, exp int, cas uint64,
	body []byte) (*gomemcached.MCResponse, error) {
	return c.storeCas(gomemcached.SET, vb, key, flags, exp, cas, body)
}

// Append data to the value of a key.
func (c *Client) Append(vb uint16, key string, data []byte) (*gomemcached.MCResponse, error) {
	req := &gomemcached.MCRequest{
		Opcode:  gomemcached.APPEND,
		VBucket: vb,
		Key:     []byte(key),
		Cas:     0,
		Opaque:  0,
		Body:    data}

	return c.Send(req)
}

// GetBulk gets keys in bulk
func (c *Client) GetBulk(vb uint16, keys []string, rv map[string]*gomemcached.MCResponse, subPaths []string) error {
	stopch := make(chan bool)
	var wg sync.WaitGroup

	defer func() {
		close(stopch)
		wg.Wait()
	}()

	if (math.MaxInt32 - c.opaque) < (uint32(len(keys)) + 1) {
		c.opaque = uint32(1)
	}

	opStart := c.opaque

	errch := make(chan error, 2)

	wg.Add(1)
	go func() {
		defer func() {
			if r := recover(); r != nil {
				logging.Infof("Recovered in f %v", r)
			}
			errch <- nil
			wg.Done()
		}()

		ok := true
		for ok {

			select {
			case <-stopch:
				return
			default:
				res, err := c.Receive()

				if err != nil && IfResStatusError(res) {
					if res == nil || res.Status != gomemcached.KEY_ENOENT {
						errch <- err
						return
					}
					// continue receiving in case of KEY_ENOENT
				} else if res.Opcode == gomemcached.GET ||
					res.Opcode == gomemcached.SUBDOC_GET ||
					res.Opcode == gomemcached.SUBDOC_MULTI_LOOKUP {
					opaque := res.Opaque - opStart
					if opaque < 0 || opaque >= uint32(len(keys)) {
						// Every now and then we seem to be seeing an invalid opaque
						// value returned from the server. When this happens log the error
						// and the calling function will retry the bulkGet. MB-15140
						logging.Errorf(" Invalid opaque Value. Debug info : Res.opaque : %v(%v), Keys %v, Response received %v \n key list %v this key %v", res.Opaque, opaque, len(keys), res, keys, string(res.Body))
						errch <- fmt.Errorf("Out of Bounds error")
						return
					}

					rv[keys[opaque]] = res
				}

				if res.Opcode == gomemcached.NOOP {
					ok = false
				}
			}
		}
	}()

	memcachedReqPkt := &gomemcached.MCRequest{
		Opcode:  gomemcached.GET,
		VBucket: vb,
	}

	if len(subPaths) > 0 {
		extraBuf, valueBuf := GetSubDocVal(subPaths)
		memcachedReqPkt.Opcode = gomemcached.SUBDOC_MULTI_LOOKUP
		memcachedReqPkt.Extras = extraBuf
		memcachedReqPkt.Body = valueBuf
	}

	for _, k := range keys { // Start of Get request
		memcachedReqPkt.Key = []byte(k)
		memcachedReqPkt.Opaque = c.opaque

		err := c.Transmit(memcachedReqPkt)
		if err != nil {
			logging.Errorf(" Transmit failed in GetBulkAll %v", err)
			return err
		}
		c.opaque++
	} // End of Get request

	// finally transmit a NOOP
	err := c.Transmit(&gomemcached.MCRequest{
		Opcode:  gomemcached.NOOP,
		VBucket: vb,
		Opaque:  c.opaque,
	})

	if err != nil {
		logging.Errorf(" Transmit of NOOP failed  %v", err)
		return err
	}
	c.opaque++

	return <-errch
}

func GetSubDocVal(subPaths []string) (extraBuf, valueBuf []byte) {

	var ops []string
	totalBytesLen := 0
	num := 1

	for _, v := range subPaths {
		totalBytesLen = totalBytesLen + len([]byte(v))
		ops = append(ops, v)
		num = num + 1
	}

	// Xattr retrieval - subdoc multi get
	extraBuf = append(extraBuf, uint8(0x04))

	valueBuf = make([]byte, num*4+totalBytesLen)

	//opcode for subdoc get
	op := gomemcached.SUBDOC_GET

	// Calculate path total bytes
	// There are 2 ops - get xattrs - both input and $document and get whole doc
	valIter := 0

	for _, v := range ops {
		pathBytes := []byte(v)
		valueBuf[valIter+0] = uint8(op)

		// SubdocFlagXattrPath indicates that the path refers to
		// an Xattr rather than the document body.
		valueBuf[valIter+1] = uint8(gomemcached.SUBDOC_FLAG_XATTR)

		// 2 byte key
		binary.BigEndian.PutUint16(valueBuf[valIter+2:], uint16(len(pathBytes)))

		// Then n bytes path
		copy(valueBuf[valIter+4:], pathBytes)
		valIter = valIter + 4 + len(pathBytes)
	}

	return
}

// ObservedStatus is the type reported by the Observe method
type ObservedStatus uint8

// Observation status values.
const (
	ObservedNotPersisted     = ObservedStatus(0x00) // found, not persisted
	ObservedPersisted        = ObservedStatus(0x01) // found, persisted
	ObservedNotFound         = ObservedStatus(0x80) // not found (or a persisted delete)
	ObservedLogicallyDeleted = ObservedStatus(0x81) // pending deletion (not persisted yet)
)

// ObserveResult represents the data obtained by an Observe call
type ObserveResult struct {
	Status          ObservedStatus // Whether the value has been persisted/deleted
	Cas             uint64         // Current value's CAS
	PersistenceTime time.Duration  // Node's average time to persist a value
	ReplicationTime time.Duration  // Node's average time to replicate a value
}

// Observe gets the persistence/replication/CAS state of a key
func (c *Client) Observe(vb uint16, key string) (result ObserveResult, err error) {
	// http://www.couchbase.com/wiki/display/couchbase/Observe
	body := make([]byte, 4+len(key))
	binary.BigEndian.PutUint16(body[0:2], vb)
	binary.BigEndian.PutUint16(body[2:4], uint16(len(key)))
	copy(body[4:4+len(key)], key)

	res, err := c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.OBSERVE,
		VBucket: vb,
		Body:    body,
	})
	if err != nil {
		return
	}

	// Parse the response data from the body:
	if len(res.Body) < 2+2+1 {
		err = io.ErrUnexpectedEOF
		return
	}
	outVb := binary.BigEndian.Uint16(res.Body[0:2])
	keyLen := binary.BigEndian.Uint16(res.Body[2:4])
	if len(res.Body) < 2+2+int(keyLen)+1+8 {
		err = io.ErrUnexpectedEOF
		return
	}
	outKey := string(res.Body[4 : 4+keyLen])
	if outVb != vb || outKey != key {
		err = fmt.Errorf("observe returned wrong vbucket/key: %d/%q", outVb, outKey)
		return
	}
	result.Status = ObservedStatus(res.Body[4+keyLen])
	result.Cas = binary.BigEndian.Uint64(res.Body[5+keyLen:])
	// The response reuses the Cas field to store time statistics:
	result.PersistenceTime = time.Duration(res.Cas>>32) * time.Millisecond
	result.ReplicationTime = time.Duration(res.Cas&math.MaxUint32) * time.Millisecond
	return
}

// CheckPersistence checks whether a stored value has been persisted to disk yet.
func (result ObserveResult) CheckPersistence(cas uint64, deletion bool) (persisted bool, overwritten bool) {
	switch {
	case result.Status == ObservedNotFound && deletion:
		persisted = true
	case result.Cas != cas:
		overwritten = true
	case result.Status == ObservedPersisted:
		persisted = true
	}
	return
}

// Sequence number based Observe Implementation
type ObserveSeqResult struct {
	Failover           uint8  // Set to 1 if a failover took place
	VbId               uint16 // vbucket id
	Vbuuid             uint64 // vucket uuid
	LastPersistedSeqNo uint64 // last persisted sequence number
	CurrentSeqNo       uint64 // current sequence number
	OldVbuuid          uint64 // Old bucket vbuuid
	LastSeqNo          uint64 // last sequence number received before failover
}

func (c *Client) ObserveSeq(vb uint16, vbuuid uint64) (result *ObserveSeqResult, err error) {
	// http://www.couchbase.com/wiki/display/couchbase/Observe
	body := make([]byte, 8)
	binary.BigEndian.PutUint64(body[0:8], vbuuid)

	res, err := c.Send(&gomemcached.MCRequest{
		Opcode:  gomemcached.OBSERVE_SEQNO,
		VBucket: vb,
		Body:    body,
		Opaque:  0x01,
	})
	if err != nil {
		return
	}

	if res.Status != gomemcached.SUCCESS {
		return nil, fmt.Errorf(" Observe returned error %v", res.Status)
	}

	// Parse the response data from the body:
	if len(res.Body) < (1 + 2 + 8 + 8 + 8) {
		err = io.ErrUnexpectedEOF
		return
	}

	result = &ObserveSeqResult{}
	result.Failover = res.Body[0]
	result.VbId = binary.BigEndian.Uint16(res.Body[1:3])
	result.Vbuuid = binary.BigEndian.Uint64(res.Body[3:11])
	result.LastPersistedSeqNo = binary.BigEndian.Uint64(res.Body[11:19])
	result.CurrentSeqNo = binary.BigEndian.Uint64(res.Body[19:27])

	// in case of failover processing we can have old vbuuid and the last persisted seq number
	if result.Failover == 1 && len(res.Body) >= (1+2+8+8+8+8+8) {
		result.OldVbuuid = binary.BigEndian.Uint64(res.Body[27:35])
		result.LastSeqNo = binary.BigEndian.Uint64(res.Body[35:43])
	}

	return
}

// CasOp is the type of operation to perform on this CAS loop.
type CasOp uint8

const (
	// CASStore instructs the server to store the new value normally
	CASStore = CasOp(iota)
	// CASQuit instructs the client to stop attempting to CAS, leaving value untouched
	CASQuit
	// CASDelete instructs the server to delete the current value
	CASDelete
)

// User specified termination is returned as an error.
func (c CasOp) Error() string {
	switch c {
	case CASStore:
		return "CAS store"
	case CASQuit:
		return "CAS quit"
	case CASDelete:
		return "CAS delete"
	}
	panic("Unhandled value")
}

//////// CAS TRANSFORM

// CASState tracks the state of CAS over several operations.
//
// This is used directly by CASNext and indirectly by CAS
type CASState struct {
	initialized bool   // false on the first call to CASNext, then true
	Value       []byte // Current value of key; update in place to new value
	Cas         uint64 // Current CAS value of key
	Exists      bool   // Does a value exist for the key? (If not, Value will be nil)
	Err         error  // Error, if any, after CASNext returns false
	resp        *gomemcached.MCResponse
}

// CASNext is a non-callback, loop-based version of CAS method.
//
//  Usage is like this:
//
// var state memcached.CASState
// for client.CASNext(vb, key, exp, &state) {
//     state.Value = some_mutation(state.Value)
// }
// if state.Err != nil { ... }
func (c *Client) CASNext(vb uint16, k string, exp int, state *CASState) bool {
	if state.initialized {
		if !state.Exists {
			// Adding a new key:
			if state.Value == nil {
				state.Cas = 0
				return false // no-op (delete of non-existent value)
			}
			state.resp, state.Err = c.Add(vb, k, 0, exp, state.Value)
		} else {
			// Updating / deleting a key:
			req := &gomemcached.MCRequest{
				Opcode:  gomemcached.DELETE,
				VBucket: vb,
				Key:     []byte(k),
				Cas:     state.Cas}
			if state.Value != nil {
				req.Opcode = gomemcached.SET
				req.Opaque = 0
				req.Extras = []byte{0, 0, 0, 0, 0, 0, 0, 0}
				req.Body = state.Value

				flags := 0
				binary.BigEndian.PutUint64(req.Extras, uint64(flags)<<32|uint64(exp))
			}
			state.resp, state.Err = c.Send(req)
		}

		// If the response status is KEY_EEXISTS or NOT_STORED there's a conflict and we'll need to
		// get the new value (below). Otherwise, we're done (either success or failure) so return:
		if !(state.resp != nil && (state.resp.Status == gomemcached.KEY_EEXISTS ||
			state.resp.Status == gomemcached.NOT_STORED)) {
			state.Cas = state.resp.Cas
			return false // either success or fatal error
		}
	}

	// Initial call, or after a conflict: GET the current value and CAS and return them:
	state.initialized = true
	if state.resp, state.Err = c.Get(vb, k); state.Err == nil {
		state.Exists = true
		state.Value = state.resp.Body
		state.Cas = state.resp.Cas
	} else if state.resp != nil && state.resp.Status == gomemcached.KEY_ENOENT {
		state.Err = nil
		state.Exists = false
		state.Value = nil
		state.Cas = 0
	} else {
		return false // fatal error
	}
	return true // keep going...
}

// CasFunc is type type of function to perform a CAS transform.
//
// Input is the current value, or nil if no value exists.
// The function should return the new value (if any) to set, and the store/quit/delete operation.
type CasFunc func(current []byte) ([]byte, CasOp)

// CAS performs a CAS transform with the given function.
//
// If the value does not exist, a nil current value will be sent to f.
func (c *Client) CAS(vb uint16, k string, f CasFunc,
	initexp int) (*gomemcached.MCResponse, error) {
	var state CASState
	for c.CASNext(vb, k, initexp, &state) {
		newValue, operation := f(state.Value)
		if operation == CASQuit || (operation == CASDelete && state.Value == nil) {
			return nil, operation
		}
		state.Value = newValue
	}
	return state.resp, state.Err
}

// StatValue is one of the stats returned from the Stats method.
type StatValue struct {
	// The stat key
	Key string
	// The stat value
	Val string
}

// Stats requests server-side stats.
//
// Use "" as the stat key for toplevel stats.
func (c *Client) Stats(key string) ([]StatValue, error) {
	rv := make([]StatValue, 0, 128)

	req := &gomemcached.MCRequest{
		Opcode: gomemcached.STAT,
		Key:    []byte(key),
		Opaque: 918494,
	}

	err := c.Transmit(req)
	if err != nil {
		return rv, err
	}

	for {
		res, _, err := getResponse(c.conn, c.hdrBuf)
		if err != nil {
			return rv, err
		}
		k := string(res.Key)
		if k == "" {
			break
		}
		rv = append(rv, StatValue{
			Key: k,
			Val: string(res.Body),
		})
	}
	return rv, nil
}

// StatsMap requests server-side stats similarly to Stats, but returns
// them as a map.
//
// Use "" as the stat key for toplevel stats.
func (c *Client) StatsMap(key string) (map[string]string, error) {
	rv := make(map[string]string)

	req := &gomemcached.MCRequest{
		Opcode: gomemcached.STAT,
		Key:    []byte(key),
		Opaque: 918494,
	}

	err := c.Transmit(req)
	if err != nil {
		return rv, err
	}

	for {
		res, _, err := getResponse(c.conn, c.hdrBuf)
		if err != nil {
			return rv, err
		}
		k := string(res.Key)
		if k == "" {
			break
		}
		rv[k] = string(res.Body)
	}

	return rv, nil
}

// instead of returning a new statsMap, simply populate passed in statsMap, which contains all the keys
// for which stats needs to be retrieved
func (c *Client) StatsMapForSpecifiedStats(key string, statsMap map[string]string) error {

	// clear statsMap
	for key, _ := range statsMap {
		statsMap[key] = ""
	}

	req := &gomemcached.MCRequest{
		Opcode: gomemcached.STAT,
		Key:    []byte(key),
		Opaque: 918494,
	}

	err := c.Transmit(req)
	if err != nil {
		return err
	}

	for {
		res, _, err := getResponse(c.conn, c.hdrBuf)
		if err != nil {
			return err
		}
		k := string(res.Key)
		if k == "" {
			break
		}
		if _, ok := statsMap[k]; ok {
			statsMap[k] = string(res.Body)
		}
	}

	return nil
}

// Hijack exposes the underlying connection from this client.
//
// It also marks the connection as unhealthy since the client will
// have lost control over the connection and can't otherwise verify
// things are in good shape for connection pools.
func (c *Client) Hijack() io.ReadWriteCloser {
	c.setHealthy(false)
	return c.conn
}

func (c *Client) setHealthy(healthy bool) {
	healthyState := UnHealthy
	if healthy {
		healthyState = Healthy
	}
	atomic.StoreUint32(&c.healthy, healthyState)
}

func IfResStatusError(response *gomemcached.MCResponse) bool {
	return response == nil ||
		(response.Status != gomemcached.SUBDOC_BAD_MULTI &&
			response.Status != gomemcached.SUBDOC_PATH_NOT_FOUND &&
			response.Status != gomemcached.SUBDOC_MULTI_PATH_FAILURE_DELETED)
}
