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

// A Decoder manages the receipt of type and data information read from the
// remote side of a connection.
type Decoder struct {
        mutex        sync.Mutex                              // each item must be received atomically
        r            io.Reader                               // source of the data
        buf          bytes.Buffer                            // buffer for more efficient i/o from r
        wireType     map[typeId]*wireType                    // map from remote ID to local description
        decoderCache map[reflect.Type]map[typeId]**decEngine // cache of compiled engines
        ignorerCache map[typeId]**decEngine                  // ditto for ignored objects
        freeList     *decoderState                           // list of free decoderStates; avoids reallocation
        countBuf     []byte                                  // used for decoding integers while parsing messages
        tmp          []byte                                  // temporary storage for i/o; saves reallocating
        err          error
}

// NewDecoder returns a new decoder that reads from the io.Reader.
// If r does not also implement io.ByteReader, it will be wrapped in a
// bufio.Reader.
func NewDecoder(r io.Reader) *Decoder {
        dec := new(Decoder)
        // We use the ability to read bytes as a plausible surrogate for buffering.
        if _, ok := r.(io.ByteReader); !ok {
                r = bufio.NewReader(r)
        }
        dec.r = r
        dec.wireType = make(map[typeId]*wireType)
        dec.decoderCache = make(map[reflect.Type]map[typeId]**decEngine)
        dec.ignorerCache = make(map[typeId]**decEngine)
        dec.countBuf = make([]byte, 9) // counts may be uint64s (unlikely!), require 9 bytes

        return dec
}

// recvType loads the definition of a type.
func (dec *Decoder) recvType(id typeId) {
        // Have we already seen this type?  That's an error
        if id < firstUserId || dec.wireType[id] != nil {
                dec.err = errors.New("gob: duplicate type received")
                return
        }

        // Type:
        wire := new(wireType)
        dec.decodeValue(tWireType, reflect.ValueOf(wire))
        if dec.err != nil {
                return
        }
        // Remember we've seen this type.
        dec.wireType[id] = wire
}

var errBadCount = errors.New("invalid message length")

// recvMessage reads the next count-delimited item from the input. It is the converse
// of Encoder.writeMessage. It returns false on EOF or other error reading the message.
func (dec *Decoder) recvMessage() bool {
        // Read a count.
        nbytes, _, err := decodeUintReader(dec.r, dec.countBuf)
        if err != nil {
                dec.err = err
                return false
        }
        // Upper limit of 1GB, allowing room to grow a little without overflow.
        // TODO: We might want more control over this limit.
        if nbytes >= 1<<30 {
                dec.err = errBadCount
                return false
        }
        dec.readMessage(int(nbytes))
        return dec.err == nil
}

// readMessage reads the next nbytes bytes from the input.
func (dec *Decoder) readMessage(nbytes int) {
        // Allocate the buffer.
        if cap(dec.tmp) < nbytes {
                dec.tmp = make([]byte, nbytes+100) // room to grow
        }
        dec.tmp = dec.tmp[:nbytes]

        // Read the data
        _, dec.err = io.ReadFull(dec.r, dec.tmp)
        if dec.err != nil {
                if dec.err == io.EOF {
                        dec.err = io.ErrUnexpectedEOF
                }
                return
        }
        dec.buf.Write(dec.tmp)
}

// toInt turns an encoded uint64 into an int, according to the marshaling rules.
func toInt(x uint64) int64 {
        i := int64(x >> 1)
        if x&1 != 0 {
                i = ^i
        }
        return i
}

func (dec *Decoder) nextInt() int64 {
        n, _, err := decodeUintReader(&dec.buf, dec.countBuf)
        if err != nil {
                dec.err = err
        }
        return toInt(n)
}

func (dec *Decoder) nextUint() uint64 {
        n, _, err := decodeUintReader(&dec.buf, dec.countBuf)
        if err != nil {
                dec.err = err
        }
        return n
}

// decodeTypeSequence parses:
// TypeSequence
//      (TypeDefinition DelimitedTypeDefinition*)?
// and returns the type id of the next value.  It returns -1 at
// EOF.  Upon return, the remainder of dec.buf is the value to be
// decoded.  If this is an interface value, it can be ignored by
// resetting that buffer.
func (dec *Decoder) decodeTypeSequence(isInterface bool) typeId {
        for dec.err == nil {
                if dec.buf.Len() == 0 {
                        if !dec.recvMessage() {
                                break
                        }
                }
                // Receive a type id.
                id := typeId(dec.nextInt())
                if id >= 0 {
                        // Value follows.
                        return id
                }
                // Type definition for (-id) follows.
                dec.recvType(-id)
                // When decoding an interface, after a type there may be a
                // DelimitedValue still in the buffer.  Skip its count.
                // (Alternatively, the buffer is empty and the byte count
                // will be absorbed by recvMessage.)
                if dec.buf.Len() > 0 {
                        if !isInterface {
                                dec.err = errors.New("extra data in buffer")
                                break
                        }
                        dec.nextUint()
                }
        }
        return -1
}

// Decode reads the next value from the connection and stores
// it in the data represented by the empty interface value.
// If e is nil, the value will be discarded. Otherwise,
// the value underlying e must be a pointer to the
// correct type for the next data item received.
func (dec *Decoder) Decode(e interface{}) error {
        if e == nil {
                return dec.DecodeValue(reflect.Value{})
        }
        value := reflect.ValueOf(e)
        // If e represents a value as opposed to a pointer, the answer won't
        // get back to the caller.  Make sure it's a pointer.
        if value.Type().Kind() != reflect.Ptr {
                dec.err = errors.New("gob: attempt to decode into a non-pointer")
                return dec.err
        }
        return dec.DecodeValue(value)
}

// DecodeValue reads the next value from the connection.
// If v is the zero reflect.Value (v.Kind() == Invalid), DecodeValue discards the value.
// Otherwise, it stores the value into v.  In that case, v must represent
// a non-nil pointer to data or be an assignable reflect.Value (v.CanSet())
func (dec *Decoder) DecodeValue(v reflect.Value) error {
        if v.IsValid() {
                if v.Kind() == reflect.Ptr && !v.IsNil() {
                        // That's okay, we'll store through the pointer.
                } else if !v.CanSet() {
                        return errors.New("gob: DecodeValue of unassignable value")
                }
        }
        // Make sure we're single-threaded through here.
        dec.mutex.Lock()
        defer dec.mutex.Unlock()

        dec.buf.Reset() // In case data lingers from previous invocation.
        dec.err = nil
        id := dec.decodeTypeSequence(false)
        if dec.err == nil {
                dec.decodeValue(id, v)
        }
        return dec.err
}

// If debug.go is compiled into the program , debugFunc prints a human-readable
// representation of the gob data read from r by calling that file's Debug function.
// Otherwise it is nil.
var debugFunc func(io.Reader)