stackitem: allow to (de-)serialize items to JSON

This commit implements behavior identical to that of C# `System.Json.*` interops.

pkg/vm/stackitem/json.go

@@ -0,0 +1,202 @@
+package stackitem
+
+import (
+	"bytes"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	gio "io"
+	"math"
+	"math/big"
+
+	"github.com/nspcc-dev/neo-go/pkg/io"
+)
+
+// decoder is a wrapper around json.Decoder helping to mimic C# json decoder behaviour.
+type decoder struct {
+	json.Decoder
+
+	depth int
+}
+
+// MaxAllowedInteger is the maximum integer allowed to be encoded.
+const MaxAllowedInteger = 2<<53 - 1
+
+// maxJSONDepth is a maximum allowed depth-level of decoded JSON.
+const maxJSONDepth = 10
+
+// ToJSON encodes Item to JSON.
+// It behaves as following:
+//   ByteArray -> base64 string
+//   BigInteger -> number
+//   Bool -> bool
+//   Null -> null
+//   Array, Struct -> array
+//   Map -> map with keys as UTF-8 bytes
+func ToJSON(item Item) ([]byte, error) {
+	buf := io.NewBufBinWriter()
+	toJSON(buf, item)
+	if buf.Err != nil {
+		return nil, buf.Err
+	}
+	return buf.Bytes(), nil
+}
+
+func toJSON(buf *io.BufBinWriter, item Item) {
+	w := buf.BinWriter
+	if w.Err != nil {
+		return
+	} else if buf.Len() > MaxSize {
+		w.Err = errors.New("item is too big")
+	}
+	switch it := item.(type) {
+	case *Array, *Struct:
+		w.WriteB('[')
+		items := it.Value().([]Item)
+		for i, v := range items {
+			toJSON(buf, v)
+			if i < len(items)-1 {
+				w.WriteB(',')
+			}
+		}
+		w.WriteB(']')
+	case *Map:
+		w.WriteB('{')
+		for i := range it.value {
+			bs, _ := it.value[i].Key.TryBytes() // map key can always be converted to []byte
+			w.WriteB('"')
+			w.WriteBytes(bs)
+			w.WriteBytes([]byte(`":`))
+			toJSON(buf, it.value[i].Value)
+			if i < len(it.value)-1 {
+				w.WriteB(',')
+			}
+		}
+		w.WriteB('}')
+	case *BigInteger:
+		if it.value.CmpAbs(big.NewInt(MaxAllowedInteger)) == 1 {
+			w.Err = errors.New("too big integer")
+			return
+		}
+		w.WriteBytes([]byte(it.value.String()))
+	case *ByteArray:
+		w.WriteB('"')
+		val := it.Value().([]byte)
+		b := make([]byte, base64.StdEncoding.EncodedLen(len(val)))
+		base64.StdEncoding.Encode(b, val)
+		w.WriteBytes(b)
+		w.WriteB('"')
+	case *Bool:
+		if it.value {
+			w.WriteBytes([]byte("true"))
+		} else {
+			w.WriteBytes([]byte("false"))
+		}
+	case Null:
+		w.WriteBytes([]byte("null"))
+	default:
+		w.Err = fmt.Errorf("invalid item: %s", it.String())
+		return
+	}
+	if w.Err == nil && buf.Len() > MaxSize {
+		w.Err = errors.New("item is too big")
+	}
+}
+
+// FromJSON decodes Item from JSON.
+// It behaves as following:
+//   string -> ByteArray from base64
+//   number -> BigInteger
+//   bool -> Bool
+//   null -> Null
+//   array -> Array
+//   map -> Map, keys are UTF-8
+func FromJSON(data []byte) (Item, error) {
+	d := decoder{Decoder: *json.NewDecoder(bytes.NewReader(data))}
+	if item, err := d.decode(); err != nil {
+		return nil, err
+	} else if _, err := d.Token(); err != gio.EOF {
+		return nil, errors.New("unexpected items")
+	} else {
+		return item, nil
+	}
+}
+
+func (d *decoder) decode() (Item, error) {
+	tok, err := d.Token()
+	if err != nil {
+		return nil, err
+	}
+	switch t := tok.(type) {
+	case json.Delim:
+		switch t {
+		case json.Delim('{'), json.Delim('['):
+			if d.depth == maxJSONDepth {
+				return nil, errors.New("JSON depth limit exceeded")
+			}
+			d.depth++
+			var item Item
+			if t == json.Delim('{') {
+				item, err = d.decodeMap()
+			} else {
+				item, err = d.decodeArray()
+			}
+			d.depth--
+			return item, err
+		default:
+			// no error above means corresponding closing token
+			// was encountered for map or array respectively
+			return nil, nil
+		}
+	case string:
+		b, err := base64.StdEncoding.DecodeString(t)
+		if err != nil {
+			return nil, err
+		}
+		return NewByteArray(b), nil
+	case float64:
+		if math.Floor(t) != t {
+			return nil, fmt.Errorf("real value is not allowed: %v", t)
+		}
+		return NewBigInteger(big.NewInt(int64(t))), nil
+	case bool:
+		return NewBool(t), nil
+	default:
+		// it can be only `nil`
+		return Null{}, nil
+	}
+}
+
+func (d *decoder) decodeArray() (*Array, error) {
+	items := []Item{}
+	for {
+		item, err := d.decode()
+		if err != nil {
+			return nil, err
+		}
+		if item == nil {
+			return NewArray(items), nil
+		}
+		items = append(items, item)
+	}
+}
+
+func (d *decoder) decodeMap() (*Map, error) {
+	m := NewMap()
+	for {
+		key, err := d.Token()
+		if err != nil {
+			return nil, err
+		}
+		k, ok := key.(string)
+		if !ok {
+			return m, nil
+		}
+		val, err := d.decode()
+		if err != nil {
+			return nil, err
+		}
+		m.Add(NewByteArray([]byte(k)), val)
+	}
+}