1// Copyright (C) MongoDB, Inc. 2017-present.
2//
3// Licensed under the Apache License, Version 2.0 (the "License"); you may
4// not use this file except in compliance with the License. You may obtain
5// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
6
7package bson
8
9import (
10	"errors"
11	"fmt"
12	"reflect"
13	"sync"
14
15	"go.mongodb.org/mongo-driver/bson/bsoncodec"
16	"go.mongodb.org/mongo-driver/bson/bsonrw"
17)
18
19// ErrDecodeToNil is the error returned when trying to decode to a nil value
20var ErrDecodeToNil = errors.New("cannot Decode to nil value")
21
22// This pool is used to keep the allocations of Decoders down. This is only used for the Marshal*
23// methods and is not consumable from outside of this package. The Decoders retrieved from this pool
24// must have both Reset and SetRegistry called on them.
25var decPool = sync.Pool{
26	New: func() interface{} {
27		return new(Decoder)
28	},
29}
30
31// A Decoder reads and decodes BSON documents from a stream. It reads from a bsonrw.ValueReader as
32// the source of BSON data.
33type Decoder struct {
34	dc bsoncodec.DecodeContext
35	vr bsonrw.ValueReader
36}
37
38// NewDecoder returns a new decoder that uses the DefaultRegistry to read from vr.
39func NewDecoder(vr bsonrw.ValueReader) (*Decoder, error) {
40	if vr == nil {
41		return nil, errors.New("cannot create a new Decoder with a nil ValueReader")
42	}
43
44	return &Decoder{
45		dc: bsoncodec.DecodeContext{Registry: DefaultRegistry},
46		vr: vr,
47	}, nil
48}
49
50// NewDecoderWithContext returns a new decoder that uses DecodeContext dc to read from vr.
51func NewDecoderWithContext(dc bsoncodec.DecodeContext, vr bsonrw.ValueReader) (*Decoder, error) {
52	if dc.Registry == nil {
53		dc.Registry = DefaultRegistry
54	}
55	if vr == nil {
56		return nil, errors.New("cannot create a new Decoder with a nil ValueReader")
57	}
58
59	return &Decoder{
60		dc: dc,
61		vr: vr,
62	}, nil
63}
64
65// Decode reads the next BSON document from the stream and decodes it into the
66// value pointed to by val.
67//
68// The documentation for Unmarshal contains details about of BSON into a Go
69// value.
70func (d *Decoder) Decode(val interface{}) error {
71	if unmarshaler, ok := val.(Unmarshaler); ok {
72		// TODO(skriptble): Reuse a []byte here and use the AppendDocumentBytes method.
73		buf, err := bsonrw.Copier{}.CopyDocumentToBytes(d.vr)
74		if err != nil {
75			return err
76		}
77		return unmarshaler.UnmarshalBSON(buf)
78	}
79
80	rval := reflect.ValueOf(val)
81	switch rval.Kind() {
82	case reflect.Ptr:
83		if rval.IsNil() {
84			return ErrDecodeToNil
85		}
86		rval = rval.Elem()
87	case reflect.Map:
88		if rval.IsNil() {
89			return ErrDecodeToNil
90		}
91	default:
92		return fmt.Errorf("argument to Decode must be a pointer or a map, but got %v", rval)
93	}
94	decoder, err := d.dc.LookupDecoder(rval.Type())
95	if err != nil {
96		return err
97	}
98	return decoder.DecodeValue(d.dc, d.vr, rval)
99}
100
101// Reset will reset the state of the decoder, using the same *DecodeContext used in
102// the original construction but using vr for reading.
103func (d *Decoder) Reset(vr bsonrw.ValueReader) error {
104	d.vr = vr
105	return nil
106}
107
108// SetRegistry replaces the current registry of the decoder with r.
109func (d *Decoder) SetRegistry(r *bsoncodec.Registry) error {
110	d.dc.Registry = r
111	return nil
112}
113
114// SetContext replaces the current registry of the decoder with dc.
115func (d *Decoder) SetContext(dc bsoncodec.DecodeContext) error {
116	d.dc = dc
117	return nil
118}
119