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// Copyright 2009 The Go 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 gob

import (
        "bytes"
        "errors"
        "io"
        "reflect"
        "sync"
)

// An Encoder manages the transmission of type and data information to the
// other side of a connection.
type Encoder struct {
        mutex      sync.Mutex              // each item must be sent atomically
        w          []io.Writer             // where to send the data
        sent       map[reflect.Type]typeId // which types we've already sent
        countState *encoderState           // stage for writing counts
        freeList   *encoderState           // list of free encoderStates; avoids reallocation
        byteBuf    bytes.Buffer            // buffer for top-level encoderState
        err        error
}

// Before we encode a message, we reserve space at the head of the
// buffer in which to encode its length. This means we can use the
// buffer to assemble the message without another allocation.
const maxLength = 9 // Maximum size of an encoded length.
var spaceForLength = make([]byte, maxLength)

// NewEncoder returns a new encoder that will transmit on the io.Writer.
func NewEncoder(w io.Writer) *Encoder {
        enc := new(Encoder)
        enc.w = []io.Writer{w}
        enc.sent = make(map[reflect.Type]typeId)
        enc.countState = enc.newEncoderState(new(bytes.Buffer))
        return enc
}

// writer() returns the innermost writer the encoder is using
func (enc *Encoder) writer() io.Writer {
        return enc.w[len(enc.w)-1]
}

// pushWriter adds a writer to the encoder.
func (enc *Encoder) pushWriter(w io.Writer) {
        enc.w = append(enc.w, w)
}

// popWriter pops the innermost writer.
func (enc *Encoder) popWriter() {
        enc.w = enc.w[0 : len(enc.w)-1]
}

func (enc *Encoder) badType(rt reflect.Type) {
        enc.setError(errors.New("gob: can't encode type " + rt.String()))
}

func (enc *Encoder) setError(err error) {
        if enc.err == nil { // remember the first.
                enc.err = err
        }
}

// writeMessage sends the data item preceded by a unsigned count of its length.
func (enc *Encoder) writeMessage(w io.Writer, b *bytes.Buffer) {
        // Space has been reserved for the length at the head of the message.
        // This is a little dirty: we grab the slice from the bytes.Buffer and massage
        // it by hand.
        message := b.Bytes()
        messageLen := len(message) - maxLength
        // Encode the length.
        enc.countState.b.Reset()
        enc.countState.encodeUint(uint64(messageLen))
        // Copy the length to be a prefix of the message.
        offset := maxLength - enc.countState.b.Len()
        copy(message[offset:], enc.countState.b.Bytes())
        // Write the data.
        _, err := w.Write(message[offset:])
        // Drain the buffer and restore the space at the front for the count of the next message.
        b.Reset()
        b.Write(spaceForLength)
        if err != nil {
                enc.setError(err)
        }
}

// sendActualType sends the requested type, without further investigation, unless
// it's been sent before.
func (enc *Encoder) sendActualType(w io.Writer, state *encoderState, ut *userTypeInfo, actual reflect.Type) (sent bool) {
        if _, alreadySent := enc.sent[actual]; alreadySent {
                return false
        }
        typeLock.Lock()
        info, err := getTypeInfo(ut)
        typeLock.Unlock()
        if err != nil {
                enc.setError(err)
                return
        }
        // Send the pair (-id, type)
        // Id:
        state.encodeInt(-int64(info.id))
        // Type:
        enc.encode(state.b, reflect.ValueOf(info.wire), wireTypeUserInfo)
        enc.writeMessage(w, state.b)
        if enc.err != nil {
                return
        }

        // Remember we've sent this type, both what the user gave us and the base type.
        enc.sent[ut.base] = info.id
        if ut.user != ut.base {
                enc.sent[ut.user] = info.id
        }
        // Now send the inner types
        switch st := actual; st.Kind() {
        case reflect.Struct:
                for i := 0; i < st.NumField(); i++ {
                        if isExported(st.Field(i).Name) {
                                enc.sendType(w, state, st.Field(i).Type)
                        }
                }
        case reflect.Array, reflect.Slice:
                enc.sendType(w, state, st.Elem())
        case reflect.Map:
                enc.sendType(w, state, st.Key())
                enc.sendType(w, state, st.Elem())
        }
        return true
}

// sendType sends the type info to the other side, if necessary. 
func (enc *Encoder) sendType(w io.Writer, state *encoderState, origt reflect.Type) (sent bool) {
        ut := userType(origt)
        if ut.isGobEncoder {
                // The rules are different: regardless of the underlying type's representation,
                // we need to tell the other side that this exact type is a GobEncoder.
                return enc.sendActualType(w, state, ut, ut.user)
        }

        // It's a concrete value, so drill down to the base type.
        switch rt := ut.base; rt.Kind() {
        default:
                // Basic types and interfaces do not need to be described.
                return
        case reflect.Slice:
                // If it's []uint8, don't send; it's considered basic.
                if rt.Elem().Kind() == reflect.Uint8 {
                        return
                }
                // Otherwise we do send.
                break
        case reflect.Array:
                // arrays must be sent so we know their lengths and element types.
                break
        case reflect.Map:
                // maps must be sent so we know their lengths and key/value types.
                break
        case reflect.Struct:
                // structs must be sent so we know their fields.
                break
        case reflect.Chan, reflect.Func:
                // Probably a bad field in a struct.
                enc.badType(rt)
                return
        }

        return enc.sendActualType(w, state, ut, ut.base)
}

// Encode transmits the data item represented by the empty interface value,
// guaranteeing that all necessary type information has been transmitted first.
func (enc *Encoder) Encode(e interface{}) error {
        return enc.EncodeValue(reflect.ValueOf(e))
}

// sendTypeDescriptor makes sure the remote side knows about this type.
// It will send a descriptor if this is the first time the type has been
// sent.
func (enc *Encoder) sendTypeDescriptor(w io.Writer, state *encoderState, ut *userTypeInfo) {
        // Make sure the type is known to the other side.
        // First, have we already sent this type?
        rt := ut.base
        if ut.isGobEncoder {
                rt = ut.user
        }
        if _, alreadySent := enc.sent[rt]; !alreadySent {
                // No, so send it.
                sent := enc.sendType(w, state, rt)
                if enc.err != nil {
                        return
                }
                // If the type info has still not been transmitted, it means we have
                // a singleton basic type (int, []byte etc.) at top level.  We don't
                // need to send the type info but we do need to update enc.sent.
                if !sent {
                        typeLock.Lock()
                        info, err := getTypeInfo(ut)
                        typeLock.Unlock()
                        if err != nil {
                                enc.setError(err)
                                return
                        }
                        enc.sent[rt] = info.id
                }
        }
}

// sendTypeId sends the id, which must have already been defined.
func (enc *Encoder) sendTypeId(state *encoderState, ut *userTypeInfo) {
        // Identify the type of this top-level value.
        state.encodeInt(int64(enc.sent[ut.base]))
}

// EncodeValue transmits the data item represented by the reflection value,
// guaranteeing that all necessary type information has been transmitted first.
func (enc *Encoder) EncodeValue(value reflect.Value) error {
        // Make sure we're single-threaded through here, so multiple
        // goroutines can share an encoder.
        enc.mutex.Lock()
        defer enc.mutex.Unlock()

        // Remove any nested writers remaining due to previous errors.
        enc.w = enc.w[0:1]

        ut, err := validUserType(value.Type())
        if err != nil {
                return err
        }

        enc.err = nil
        enc.byteBuf.Reset()
        enc.byteBuf.Write(spaceForLength)
        state := enc.newEncoderState(&enc.byteBuf)

        enc.sendTypeDescriptor(enc.writer(), state, ut)
        enc.sendTypeId(state, ut)
        if enc.err != nil {
                return enc.err
        }

        // Encode the object.
        enc.encode(state.b, value, ut)
        if enc.err == nil {
                enc.writeMessage(enc.writer(), state.b)
        }

        enc.freeEncoderState(state)
        return enc.err
}