encode.go

Documentation: encoding/gob

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:generate go run encgen.go -output enc_helpers.go
     6  
     7  package gob
     8  
     9  import (
    10  	"encoding"
    11  	"encoding/binary"
    12  	"math"
    13  	"math/bits"
    14  	"reflect"
    15  	"sync"
    16  )
    17  
    18  const uint64Size = 8
    19  
    20  type encHelper func(state *encoderState, v reflect.Value) bool
    21  
    22  // encoderState is the global execution state of an instance of the encoder.
    23  // Field numbers are delta encoded and always increase. The field
    24  // number is initialized to -1 so 0 comes out as delta(1). A delta of
    25  // 0 terminates the structure.
    26  type encoderState struct {
    27  	enc      *Encoder
    28  	b        *encBuffer
    29  	sendZero bool                 // encoding an array element or map key/value pair; send zero values
    30  	fieldnum int                  // the last field number written.
    31  	buf      [1 + uint64Size]byte // buffer used by the encoder; here to avoid allocation.
    32  	next     *encoderState        // for free list
    33  }
    34  
    35  // encBuffer is an extremely simple, fast implementation of a write-only byte buffer.
    36  // It never returns a non-nil error, but Write returns an error value so it matches io.Writer.
    37  type encBuffer struct {
    38  	data    []byte
    39  	scratch [64]byte
    40  }
    41  
    42  var encBufferPool = sync.Pool{
    43  	New: func() interface{} {
    44  		e := new(encBuffer)
    45  		e.data = e.scratch[0:0]
    46  		return e
    47  	},
    48  }
    49  
    50  func (e *encBuffer) writeByte(c byte) {
    51  	e.data = append(e.data, c)
    52  }
    53  
    54  func (e *encBuffer) Write(p []byte) (int, error) {
    55  	e.data = append(e.data, p...)
    56  	return len(p), nil
    57  }
    58  
    59  func (e *encBuffer) WriteString(s string) {
    60  	e.data = append(e.data, s...)
    61  }
    62  
    63  func (e *encBuffer) Len() int {
    64  	return len(e.data)
    65  }
    66  
    67  func (e *encBuffer) Bytes() []byte {
    68  	return e.data
    69  }
    70  
    71  func (e *encBuffer) Reset() {
    72  	if len(e.data) >= tooBig {
    73  		e.data = e.scratch[0:0]
    74  	} else {
    75  		e.data = e.data[0:0]
    76  	}
    77  }
    78  
    79  func (enc *Encoder) newEncoderState(b *encBuffer) *encoderState {
    80  	e := enc.freeList
    81  	if e == nil {
    82  		e = new(encoderState)
    83  		e.enc = enc
    84  	} else {
    85  		enc.freeList = e.next
    86  	}
    87  	e.sendZero = false
    88  	e.fieldnum = 0
    89  	e.b = b
    90  	if len(b.data) == 0 {
    91  		b.data = b.scratch[0:0]
    92  	}
    93  	return e
    94  }
    95  
    96  func (enc *Encoder) freeEncoderState(e *encoderState) {
    97  	e.next = enc.freeList
    98  	enc.freeList = e
    99  }
   100  
   101  // Unsigned integers have a two-state encoding. If the number is less
   102  // than 128 (0 through 0x7F), its value is written directly.
   103  // Otherwise the value is written in big-endian byte order preceded
   104  // by the byte length, negated.
   105  
   106  // encodeUint writes an encoded unsigned integer to state.b.
   107  func (state *encoderState) encodeUint(x uint64) {
   108  	if x <= 0x7F {
   109  		state.b.writeByte(uint8(x))
   110  		return
   111  	}
   112  
   113  	binary.BigEndian.PutUint64(state.buf[1:], x)
   114  	bc := bits.LeadingZeros64(x) >> 3      // 8 - bytelen(x)
   115  	state.buf[bc] = uint8(bc - uint64Size) // and then we subtract 8 to get -bytelen(x)
   116  
   117  	state.b.Write(state.buf[bc : uint64Size+1])
   118  }
   119  
   120  // encodeInt writes an encoded signed integer to state.w.
   121  // The low bit of the encoding says whether to bit complement the (other bits of the)
   122  // uint to recover the int.
   123  func (state *encoderState) encodeInt(i int64) {
   124  	var x uint64
   125  	if i < 0 {
   126  		x = uint64(^i<<1) | 1
   127  	} else {
   128  		x = uint64(i << 1)
   129  	}
   130  	state.encodeUint(x)
   131  }
   132  
   133  // encOp is the signature of an encoding operator for a given type.
   134  type encOp func(i *encInstr, state *encoderState, v reflect.Value)
   135  
   136  // The 'instructions' of the encoding machine
   137  type encInstr struct {
   138  	op    encOp
   139  	field int   // field number in input
   140  	index []int // struct index
   141  	indir int   // how many pointer indirections to reach the value in the struct
   142  }
   143  
   144  // update emits a field number and updates the state to record its value for delta encoding.
   145  // If the instruction pointer is nil, it does nothing
   146  func (state *encoderState) update(instr *encInstr) {
   147  	if instr != nil {
   148  		state.encodeUint(uint64(instr.field - state.fieldnum))
   149  		state.fieldnum = instr.field
   150  	}
   151  }
   152  
   153  // Each encoder for a composite is responsible for handling any
   154  // indirections associated with the elements of the data structure.
   155  // If any pointer so reached is nil, no bytes are written. If the
   156  // data item is zero, no bytes are written. Single values - ints,
   157  // strings etc. - are indirected before calling their encoders.
   158  // Otherwise, the output (for a scalar) is the field number, as an
   159  // encoded integer, followed by the field data in its appropriate
   160  // format.
   161  
   162  // encIndirect dereferences pv indir times and returns the result.
   163  func encIndirect(pv reflect.Value, indir int) reflect.Value {
   164  	for ; indir > 0; indir-- {
   165  		if pv.IsNil() {
   166  			break
   167  		}
   168  		pv = pv.Elem()
   169  	}
   170  	return pv
   171  }
   172  
   173  // encBool encodes the bool referenced by v as an unsigned 0 or 1.
   174  func encBool(i *encInstr, state *encoderState, v reflect.Value) {
   175  	b := v.Bool()
   176  	if b || state.sendZero {
   177  		state.update(i)
   178  		if b {
   179  			state.encodeUint(1)
   180  		} else {
   181  			state.encodeUint(0)
   182  		}
   183  	}
   184  }
   185  
   186  // encInt encodes the signed integer (int int8 int16 int32 int64) referenced by v.
   187  func encInt(i *encInstr, state *encoderState, v reflect.Value) {
   188  	value := v.Int()
   189  	if value != 0 || state.sendZero {
   190  		state.update(i)
   191  		state.encodeInt(value)
   192  	}
   193  }
   194  
   195  // encUint encodes the unsigned integer (uint uint8 uint16 uint32 uint64 uintptr) referenced by v.
   196  func encUint(i *encInstr, state *encoderState, v reflect.Value) {
   197  	value := v.Uint()
   198  	if value != 0 || state.sendZero {
   199  		state.update(i)
   200  		state.encodeUint(value)
   201  	}
   202  }
   203  
   204  // floatBits returns a uint64 holding the bits of a floating-point number.
   205  // Floating-point numbers are transmitted as uint64s holding the bits
   206  // of the underlying representation. They are sent byte-reversed, with
   207  // the exponent end coming out first, so integer floating point numbers
   208  // (for example) transmit more compactly. This routine does the
   209  // swizzling.
   210  func floatBits(f float64) uint64 {
   211  	u := math.Float64bits(f)
   212  	return bits.ReverseBytes64(u)
   213  }
   214  
   215  // encFloat encodes the floating point value (float32 float64) referenced by v.
   216  func encFloat(i *encInstr, state *encoderState, v reflect.Value) {
   217  	f := v.Float()
   218  	if f != 0 || state.sendZero {
   219  		bits := floatBits(f)
   220  		state.update(i)
   221  		state.encodeUint(bits)
   222  	}
   223  }
   224  
   225  // encComplex encodes the complex value (complex64 complex128) referenced by v.
   226  // Complex numbers are just a pair of floating-point numbers, real part first.
   227  func encComplex(i *encInstr, state *encoderState, v reflect.Value) {
   228  	c := v.Complex()
   229  	if c != 0+0i || state.sendZero {
   230  		rpart := floatBits(real(c))
   231  		ipart := floatBits(imag(c))
   232  		state.update(i)
   233  		state.encodeUint(rpart)
   234  		state.encodeUint(ipart)
   235  	}
   236  }
   237  
   238  // encUint8Array encodes the byte array referenced by v.
   239  // Byte arrays are encoded as an unsigned count followed by the raw bytes.
   240  func encUint8Array(i *encInstr, state *encoderState, v reflect.Value) {
   241  	b := v.Bytes()
   242  	if len(b) > 0 || state.sendZero {
   243  		state.update(i)
   244  		state.encodeUint(uint64(len(b)))
   245  		state.b.Write(b)
   246  	}
   247  }
   248  
   249  // encString encodes the string referenced by v.
   250  // Strings are encoded as an unsigned count followed by the raw bytes.
   251  func encString(i *encInstr, state *encoderState, v reflect.Value) {
   252  	s := v.String()
   253  	if len(s) > 0 || state.sendZero {
   254  		state.update(i)
   255  		state.encodeUint(uint64(len(s)))
   256  		state.b.WriteString(s)
   257  	}
   258  }
   259  
   260  // encStructTerminator encodes the end of an encoded struct
   261  // as delta field number of 0.
   262  func encStructTerminator(i *encInstr, state *encoderState, v reflect.Value) {
   263  	state.encodeUint(0)
   264  }
   265  
   266  // Execution engine
   267  
   268  // encEngine an array of instructions indexed by field number of the encoding
   269  // data, typically a struct. It is executed top to bottom, walking the struct.
   270  type encEngine struct {
   271  	instr []encInstr
   272  }
   273  
   274  const singletonField = 0
   275  
   276  // valid reports whether the value is valid and a non-nil pointer.
   277  // (Slices, maps, and chans take care of themselves.)
   278  func valid(v reflect.Value) bool {
   279  	switch v.Kind() {
   280  	case reflect.Invalid:
   281  		return false
   282  	case reflect.Ptr:
   283  		return !v.IsNil()
   284  	}
   285  	return true
   286  }
   287  
   288  // encodeSingle encodes a single top-level non-struct value.
   289  func (enc *Encoder) encodeSingle(b *encBuffer, engine *encEngine, value reflect.Value) {
   290  	state := enc.newEncoderState(b)
   291  	defer enc.freeEncoderState(state)
   292  	state.fieldnum = singletonField
   293  	// There is no surrounding struct to frame the transmission, so we must
   294  	// generate data even if the item is zero. To do this, set sendZero.
   295  	state.sendZero = true
   296  	instr := &engine.instr[singletonField]
   297  	if instr.indir > 0 {
   298  		value = encIndirect(value, instr.indir)
   299  	}
   300  	if valid(value) {
   301  		instr.op(instr, state, value)
   302  	}
   303  }
   304  
   305  // encodeStruct encodes a single struct value.
   306  func (enc *Encoder) encodeStruct(b *encBuffer, engine *encEngine, value reflect.Value) {
   307  	if !valid(value) {
   308  		return
   309  	}
   310  	state := enc.newEncoderState(b)
   311  	defer enc.freeEncoderState(state)
   312  	state.fieldnum = -1
   313  	for i := 0; i < len(engine.instr); i++ {
   314  		instr := &engine.instr[i]
   315  		if i >= value.NumField() {
   316  			// encStructTerminator
   317  			instr.op(instr, state, reflect.Value{})
   318  			break
   319  		}
   320  		field := value.FieldByIndex(instr.index)
   321  		if instr.indir > 0 {
   322  			field = encIndirect(field, instr.indir)
   323  			// TODO: Is field guaranteed valid? If so we could avoid this check.
   324  			if !valid(field) {
   325  				continue
   326  			}
   327  		}
   328  		instr.op(instr, state, field)
   329  	}
   330  }
   331  
   332  // encodeArray encodes an array.
   333  func (enc *Encoder) encodeArray(b *encBuffer, value reflect.Value, op encOp, elemIndir int, length int, helper encHelper) {
   334  	state := enc.newEncoderState(b)
   335  	defer enc.freeEncoderState(state)
   336  	state.fieldnum = -1
   337  	state.sendZero = true
   338  	state.encodeUint(uint64(length))
   339  	if helper != nil && helper(state, value) {
   340  		return
   341  	}
   342  	for i := 0; i < length; i++ {
   343  		elem := value.Index(i)
   344  		if elemIndir > 0 {
   345  			elem = encIndirect(elem, elemIndir)
   346  			// TODO: Is elem guaranteed valid? If so we could avoid this check.
   347  			if !valid(elem) {
   348  				errorf("encodeArray: nil element")
   349  			}
   350  		}
   351  		op(nil, state, elem)
   352  	}
   353  }
   354  
   355  // encodeReflectValue is a helper for maps. It encodes the value v.
   356  func encodeReflectValue(state *encoderState, v reflect.Value, op encOp, indir int) {
   357  	for i := 0; i < indir && v.IsValid(); i++ {
   358  		v = reflect.Indirect(v)
   359  	}
   360  	if !v.IsValid() {
   361  		errorf("encodeReflectValue: nil element")
   362  	}
   363  	op(nil, state, v)
   364  }
   365  
   366  // encodeMap encodes a map as unsigned count followed by key:value pairs.
   367  func (enc *Encoder) encodeMap(b *encBuffer, mv reflect.Value, keyOp, elemOp encOp, keyIndir, elemIndir int) {
   368  	state := enc.newEncoderState(b)
   369  	state.fieldnum = -1
   370  	state.sendZero = true
   371  	keys := mv.MapKeys()
   372  	state.encodeUint(uint64(len(keys)))
   373  	for _, key := range keys {
   374  		encodeReflectValue(state, key, keyOp, keyIndir)
   375  		encodeReflectValue(state, mv.MapIndex(key), elemOp, elemIndir)
   376  	}
   377  	enc.freeEncoderState(state)
   378  }
   379  
   380  // encodeInterface encodes the interface value iv.
   381  // To send an interface, we send a string identifying the concrete type, followed
   382  // by the type identifier (which might require defining that type right now), followed
   383  // by the concrete value. A nil value gets sent as the empty string for the name,
   384  // followed by no value.
   385  func (enc *Encoder) encodeInterface(b *encBuffer, iv reflect.Value) {
   386  	// Gobs can encode nil interface values but not typed interface
   387  	// values holding nil pointers, since nil pointers point to no value.
   388  	elem := iv.Elem()
   389  	if elem.Kind() == reflect.Ptr && elem.IsNil() {
   390  		errorf("gob: cannot encode nil pointer of type %s inside interface", iv.Elem().Type())
   391  	}
   392  	state := enc.newEncoderState(b)
   393  	state.fieldnum = -1
   394  	state.sendZero = true
   395  	if iv.IsNil() {
   396  		state.encodeUint(0)
   397  		return
   398  	}
   399  
   400  	ut := userType(iv.Elem().Type())
   401  	namei, ok := concreteTypeToName.Load(ut.base)
   402  	if !ok {
   403  		errorf("type not registered for interface: %s", ut.base)
   404  	}
   405  	name := namei.(string)
   406  
   407  	// Send the name.
   408  	state.encodeUint(uint64(len(name)))
   409  	state.b.WriteString(name)
   410  	// Define the type id if necessary.
   411  	enc.sendTypeDescriptor(enc.writer(), state, ut)
   412  	// Send the type id.
   413  	enc.sendTypeId(state, ut)
   414  	// Encode the value into a new buffer. Any nested type definitions
   415  	// should be written to b, before the encoded value.
   416  	enc.pushWriter(b)
   417  	data := encBufferPool.Get().(*encBuffer)
   418  	data.Write(spaceForLength)
   419  	enc.encode(data, elem, ut)
   420  	if enc.err != nil {
   421  		error_(enc.err)
   422  	}
   423  	enc.popWriter()
   424  	enc.writeMessage(b, data)
   425  	data.Reset()
   426  	encBufferPool.Put(data)
   427  	if enc.err != nil {
   428  		error_(enc.err)
   429  	}
   430  	enc.freeEncoderState(state)
   431  }
   432  
   433  // isZero reports whether the value is the zero of its type.
   434  func isZero(val reflect.Value) bool {
   435  	switch val.Kind() {
   436  	case reflect.Array:
   437  		for i := 0; i < val.Len(); i++ {
   438  			if !isZero(val.Index(i)) {
   439  				return false
   440  			}
   441  		}
   442  		return true
   443  	case reflect.Map, reflect.Slice, reflect.String:
   444  		return val.Len() == 0
   445  	case reflect.Bool:
   446  		return !val.Bool()
   447  	case reflect.Complex64, reflect.Complex128:
   448  		return val.Complex() == 0
   449  	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Ptr:
   450  		return val.IsNil()
   451  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   452  		return val.Int() == 0
   453  	case reflect.Float32, reflect.Float64:
   454  		return val.Float() == 0
   455  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   456  		return val.Uint() == 0
   457  	case reflect.Struct:
   458  		for i := 0; i < val.NumField(); i++ {
   459  			if !isZero(val.Field(i)) {
   460  				return false
   461  			}
   462  		}
   463  		return true
   464  	}
   465  	panic("unknown type in isZero " + val.Type().String())
   466  }
   467  
   468  // encGobEncoder encodes a value that implements the GobEncoder interface.
   469  // The data is sent as a byte array.
   470  func (enc *Encoder) encodeGobEncoder(b *encBuffer, ut *userTypeInfo, v reflect.Value) {
   471  	// TODO: should we catch panics from the called method?
   472  
   473  	var data []byte
   474  	var err error
   475  	// We know it's one of these.
   476  	switch ut.externalEnc {
   477  	case xGob:
   478  		data, err = v.Interface().(GobEncoder).GobEncode()
   479  	case xBinary:
   480  		data, err = v.Interface().(encoding.BinaryMarshaler).MarshalBinary()
   481  	case xText:
   482  		data, err = v.Interface().(encoding.TextMarshaler).MarshalText()
   483  	}
   484  	if err != nil {
   485  		error_(err)
   486  	}
   487  	state := enc.newEncoderState(b)
   488  	state.fieldnum = -1
   489  	state.encodeUint(uint64(len(data)))
   490  	state.b.Write(data)
   491  	enc.freeEncoderState(state)
   492  }
   493  
   494  var encOpTable = [...]encOp{
   495  	reflect.Bool:       encBool,
   496  	reflect.Int:        encInt,
   497  	reflect.Int8:       encInt,
   498  	reflect.Int16:      encInt,
   499  	reflect.Int32:      encInt,
   500  	reflect.Int64:      encInt,
   501  	reflect.Uint:       encUint,
   502  	reflect.Uint8:      encUint,
   503  	reflect.Uint16:     encUint,
   504  	reflect.Uint32:     encUint,
   505  	reflect.Uint64:     encUint,
   506  	reflect.Uintptr:    encUint,
   507  	reflect.Float32:    encFloat,
   508  	reflect.Float64:    encFloat,
   509  	reflect.Complex64:  encComplex,
   510  	reflect.Complex128: encComplex,
   511  	reflect.String:     encString,
   512  }
   513  
   514  // encOpFor returns (a pointer to) the encoding op for the base type under rt and
   515  // the indirection count to reach it.
   516  func encOpFor(rt reflect.Type, inProgress map[reflect.Type]*encOp, building map[*typeInfo]bool) (*encOp, int) {
   517  	ut := userType(rt)
   518  	// If the type implements GobEncoder, we handle it without further processing.
   519  	if ut.externalEnc != 0 {
   520  		return gobEncodeOpFor(ut)
   521  	}
   522  	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
   523  	// Return the pointer to the op we're already building.
   524  	if opPtr := inProgress[rt]; opPtr != nil {
   525  		return opPtr, ut.indir
   526  	}
   527  	typ := ut.base
   528  	indir := ut.indir
   529  	k := typ.Kind()
   530  	var op encOp
   531  	if int(k) < len(encOpTable) {
   532  		op = encOpTable[k]
   533  	}
   534  	if op == nil {
   535  		inProgress[rt] = &op
   536  		// Special cases
   537  		switch t := typ; t.Kind() {
   538  		case reflect.Slice:
   539  			if t.Elem().Kind() == reflect.Uint8 {
   540  				op = encUint8Array
   541  				break
   542  			}
   543  			// Slices have a header; we decode it to find the underlying array.
   544  			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
   545  			helper := encSliceHelper[t.Elem().Kind()]
   546  			op = func(i *encInstr, state *encoderState, slice reflect.Value) {
   547  				if !state.sendZero && slice.Len() == 0 {
   548  					return
   549  				}
   550  				state.update(i)
   551  				state.enc.encodeArray(state.b, slice, *elemOp, elemIndir, slice.Len(), helper)
   552  			}
   553  		case reflect.Array:
   554  			// True arrays have size in the type.
   555  			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
   556  			helper := encArrayHelper[t.Elem().Kind()]
   557  			op = func(i *encInstr, state *encoderState, array reflect.Value) {
   558  				state.update(i)
   559  				state.enc.encodeArray(state.b, array, *elemOp, elemIndir, array.Len(), helper)
   560  			}
   561  		case reflect.Map:
   562  			keyOp, keyIndir := encOpFor(t.Key(), inProgress, building)
   563  			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
   564  			op = func(i *encInstr, state *encoderState, mv reflect.Value) {
   565  				// We send zero-length (but non-nil) maps because the
   566  				// receiver might want to use the map.  (Maps don't use append.)
   567  				if !state.sendZero && mv.IsNil() {
   568  					return
   569  				}
   570  				state.update(i)
   571  				state.enc.encodeMap(state.b, mv, *keyOp, *elemOp, keyIndir, elemIndir)
   572  			}
   573  		case reflect.Struct:
   574  			// Generate a closure that calls out to the engine for the nested type.
   575  			getEncEngine(userType(typ), building)
   576  			info := mustGetTypeInfo(typ)
   577  			op = func(i *encInstr, state *encoderState, sv reflect.Value) {
   578  				state.update(i)
   579  				// indirect through info to delay evaluation for recursive structs
   580  				enc := info.encoder.Load().(*encEngine)
   581  				state.enc.encodeStruct(state.b, enc, sv)
   582  			}
   583  		case reflect.Interface:
   584  			op = func(i *encInstr, state *encoderState, iv reflect.Value) {
   585  				if !state.sendZero && (!iv.IsValid() || iv.IsNil()) {
   586  					return
   587  				}
   588  				state.update(i)
   589  				state.enc.encodeInterface(state.b, iv)
   590  			}
   591  		}
   592  	}
   593  	if op == nil {
   594  		errorf("can't happen: encode type %s", rt)
   595  	}
   596  	return &op, indir
   597  }
   598  
   599  // gobEncodeOpFor returns the op for a type that is known to implement GobEncoder.
   600  func gobEncodeOpFor(ut *userTypeInfo) (*encOp, int) {
   601  	rt := ut.user
   602  	if ut.encIndir == -1 {
   603  		rt = reflect.PtrTo(rt)
   604  	} else if ut.encIndir > 0 {
   605  		for i := int8(0); i < ut.encIndir; i++ {
   606  			rt = rt.Elem()
   607  		}
   608  	}
   609  	var op encOp
   610  	op = func(i *encInstr, state *encoderState, v reflect.Value) {
   611  		if ut.encIndir == -1 {
   612  			// Need to climb up one level to turn value into pointer.
   613  			if !v.CanAddr() {
   614  				errorf("unaddressable value of type %s", rt)
   615  			}
   616  			v = v.Addr()
   617  		}
   618  		if !state.sendZero && isZero(v) {
   619  			return
   620  		}
   621  		state.update(i)
   622  		state.enc.encodeGobEncoder(state.b, ut, v)
   623  	}
   624  	return &op, int(ut.encIndir) // encIndir: op will get called with p == address of receiver.
   625  }
   626  
   627  // compileEnc returns the engine to compile the type.
   628  func compileEnc(ut *userTypeInfo, building map[*typeInfo]bool) *encEngine {
   629  	srt := ut.base
   630  	engine := new(encEngine)
   631  	seen := make(map[reflect.Type]*encOp)
   632  	rt := ut.base
   633  	if ut.externalEnc != 0 {
   634  		rt = ut.user
   635  	}
   636  	if ut.externalEnc == 0 && srt.Kind() == reflect.Struct {
   637  		for fieldNum, wireFieldNum := 0, 0; fieldNum < srt.NumField(); fieldNum++ {
   638  			f := srt.Field(fieldNum)
   639  			if !isSent(&f) {
   640  				continue
   641  			}
   642  			op, indir := encOpFor(f.Type, seen, building)
   643  			engine.instr = append(engine.instr, encInstr{*op, wireFieldNum, f.Index, indir})
   644  			wireFieldNum++
   645  		}
   646  		if srt.NumField() > 0 && len(engine.instr) == 0 {
   647  			errorf("type %s has no exported fields", rt)
   648  		}
   649  		engine.instr = append(engine.instr, encInstr{encStructTerminator, 0, nil, 0})
   650  	} else {
   651  		engine.instr = make([]encInstr, 1)
   652  		op, indir := encOpFor(rt, seen, building)
   653  		engine.instr[0] = encInstr{*op, singletonField, nil, indir}
   654  	}
   655  	return engine
   656  }
   657  
   658  // getEncEngine returns the engine to compile the type.
   659  func getEncEngine(ut *userTypeInfo, building map[*typeInfo]bool) *encEngine {
   660  	info, err := getTypeInfo(ut)
   661  	if err != nil {
   662  		error_(err)
   663  	}
   664  	enc, ok := info.encoder.Load().(*encEngine)
   665  	if !ok {
   666  		enc = buildEncEngine(info, ut, building)
   667  	}
   668  	return enc
   669  }
   670  
   671  func buildEncEngine(info *typeInfo, ut *userTypeInfo, building map[*typeInfo]bool) *encEngine {
   672  	// Check for recursive types.
   673  	if building != nil && building[info] {
   674  		return nil
   675  	}
   676  	info.encInit.Lock()
   677  	defer info.encInit.Unlock()
   678  	enc, ok := info.encoder.Load().(*encEngine)
   679  	if !ok {
   680  		if building == nil {
   681  			building = make(map[*typeInfo]bool)
   682  		}
   683  		building[info] = true
   684  		enc = compileEnc(ut, building)
   685  		info.encoder.Store(enc)
   686  	}
   687  	return enc
   688  }
   689  
   690  func (enc *Encoder) encode(b *encBuffer, value reflect.Value, ut *userTypeInfo) {
   691  	defer catchError(&enc.err)
   692  	engine := getEncEngine(ut, nil)
   693  	indir := ut.indir
   694  	if ut.externalEnc != 0 {
   695  		indir = int(ut.encIndir)
   696  	}
   697  	for i := 0; i < indir; i++ {
   698  		value = reflect.Indirect(value)
   699  	}
   700  	if ut.externalEnc == 0 && value.Type().Kind() == reflect.Struct {
   701  		enc.encodeStruct(b, engine, value)
   702  	} else {
   703  		enc.encodeSingle(b, engine, value)
   704  	}
   705  }
   706
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