[#1444] pilorama: Optimize internal encoding/decoding

```
name                      old time/op    new time/op    delta
ApplySequential/bbolt-8     55.5µs ± 4%    55.5µs ± 3%     ~     (p=1.000 n=10+7)
ApplyReorderLast/bbolt-8     108µs ± 6%     112µs ± 8%     ~     (p=0.077 n=9+9)

name                      old alloc/op   new alloc/op   delta
ApplySequential/bbolt-8     28.8kB ± 3%    27.7kB ± 6%   -3.79%  (p=0.005 n=10+10)
ApplyReorderLast/bbolt-8    41.4kB ± 5%    38.9kB ± 5%   -6.19%  (p=0.001 n=10+9)

name                      old allocs/op  new allocs/op  delta
ApplySequential/bbolt-8        262 ± 2%       235 ±10%  -10.41%  (p=0.000 n=10+10)
ApplyReorderLast/bbolt-8       684 ± 6%       616 ± 7%  -10.04%  (p=0.000 n=10+9)
```

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/pilorama/boltdb.go

@@ -231,9 +231,13 @@ func (t *boltForest) applyOperation(logBucket, treeBucket *bbolt.Bucket, m *Move
 
 	key, value := c.Last()
 
+	b := bytes.NewReader(nil)
+	r := io.NewBinReaderFromIO(b)
+
 	// 1. Undo up until the desired timestamp is here.
 	for len(key) == 8 && binary.BigEndian.Uint64(key) > m.Time {
-		if err := t.logFromBytes(&tmp, value); err != nil {
+		b.Reset(value)
+		if err := t.logFromBytes(&tmp, r); err != nil {
 			return nil, err
 		}
 		if err := t.undo(&tmp.Move, &tmp, treeBucket, cKey[:]); err != nil {
@@ -253,7 +257,8 @@ func (t *boltForest) applyOperation(logBucket, treeBucket *bbolt.Bucket, m *Move
 
 	// 3. Re-apply all other operations.
 	for len(key) == 8 {
-		if err := t.logFromBytes(&tmp, value); err != nil {
+		b.Reset(value)
+		if err := t.logFromBytes(&tmp, r); err != nil {
 			return nil, err
 		}
 		if err := t.do(logBucket, treeBucket, cKey[:], &tmp); err != nil {
@@ -559,40 +564,37 @@ func (t *boltForest) moveFromBytes(m *Move, data []byte) error {
 	r := io.NewBinReaderFromBuf(data)
 	m.Child = r.ReadU64LE()
 	m.Parent = r.ReadU64LE()
-	if err := m.Meta.FromBytes(r.ReadVarBytes()); err != nil {
-		return err
-	}
+	m.Meta.DecodeBinary(r)
 	return r.Err
 }
 
-func (t *boltForest) logFromBytes(lm *LogMove, data []byte) error {
-	r := io.NewBinReaderFromBuf(data)
+func (t *boltForest) logFromBytes(lm *LogMove, r *io.BinReader) error {
 	lm.Child = r.ReadU64LE()
 	lm.Parent = r.ReadU64LE()
-	if err := lm.Meta.FromBytes(r.ReadVarBytes()); err != nil {
-		return err
-	}
-
+	lm.Meta.DecodeBinary(r)
 	lm.HasOld = r.ReadBool()
 	if lm.HasOld {
 		lm.Old.Parent = r.ReadU64LE()
-		if err := lm.Old.Meta.FromBytes(r.ReadVarBytes()); err != nil {
-			return err
+		lm.Old.Meta.DecodeBinary(r)
 	}
-	}
-
 	return r.Err
 }
 
 func (t *boltForest) logToBytes(lm *LogMove) []byte {
 	w := io.NewBufBinWriter()
+	size := 8 + 8 + lm.Meta.Size() + 1
+	if lm.HasOld {
+		size += 8 + lm.Old.Meta.Size()
+	}
+
+	w.Grow(size)
 	w.WriteU64LE(lm.Child)
 	w.WriteU64LE(lm.Parent)
-	w.WriteVarBytes(lm.Meta.Bytes())
+	lm.Meta.EncodeBinary(w.BinWriter)
 	w.WriteBool(lm.HasOld)
 	if lm.HasOld {
 		w.WriteU64LE(lm.Old.Parent)
-		w.WriteVarBytes(lm.Old.Meta.Bytes())
+		lm.Old.Meta.EncodeBinary(w.BinWriter)
 	}
 	return w.Bytes()
 }

pkg/local_object_storage/pilorama/meta.go

@@ -10,31 +10,13 @@ func (x *Meta) FromBytes(data []byte) error {
 	}
 
 	r := io.NewBinReaderFromBuf(data)
-	ts := r.ReadVarUint()
-	size := r.ReadVarUint()
-	m := make([]KeyValue, size)
-	for i := range m {
-		m[i].Key = r.ReadString()
-		m[i].Value = r.ReadVarBytes()
-	}
-	if r.Err != nil {
+	x.DecodeBinary(r)
 	return r.Err
 }
 
-	x.Time = ts
-	x.Items = m
-	return nil
-}
-
 func (x Meta) Bytes() []byte {
 	w := io.NewBufBinWriter()
-	w.WriteVarUint(x.Time)
-	w.WriteVarUint(uint64(len(x.Items)))
-	for _, e := range x.Items {
-		w.WriteString(e.Key)
-		w.WriteVarBytes(e.Value)
-	}
-
+	x.EncodeBinary(w.BinWriter)
 	return w.Bytes()
 }
 
@@ -46,3 +28,59 @@ func (x Meta) GetAttr(name string) []byte {
 	}
 	return nil
 }
+
+// DecodeBinary implements the io.Serializable interface.
+func (x *Meta) DecodeBinary(r *io.BinReader) {
+	ts := r.ReadVarUint()
+	size := r.ReadVarUint()
+	m := make([]KeyValue, size)
+	for i := range m {
+		m[i].Key = r.ReadString()
+		m[i].Value = r.ReadVarBytes()
+	}
+	if r.Err != nil {
+		return
+	}
+
+	x.Time = ts
+	x.Items = m
+}
+
+// EncodeBinary implements the io.Serializable interface.
+func (x Meta) EncodeBinary(w *io.BinWriter) {
+	w.WriteVarUint(x.Time)
+	w.WriteVarUint(uint64(len(x.Items)))
+	for _, e := range x.Items {
+		w.WriteString(e.Key)
+		w.WriteVarBytes(e.Value)
+	}
+}
+
+// Size returns size of x in bytes.
+func (x Meta) Size() int {
+	size := getVarIntSize(x.Time)
+	size += getVarIntSize(uint64(len(x.Items)))
+	for i := range x.Items {
+		ln := len(x.Items[i].Key)
+		size += getVarIntSize(uint64(ln)) + ln
+
+		ln = len(x.Items[i].Value)
+		size += getVarIntSize(uint64(ln)) + ln
+	}
+	return size
+}
+
+// getVarIntSize returns the size in number of bytes of a variable integer.
+// (reference: GetVarSize(int value),  https://github.com/neo-project/neo/blob/master/neo/IO/Helper.cs)
+func getVarIntSize(value uint64) int {
+	var size int
+
+	if value < 0xFD {
+		size = 1 // unit8
+	} else if value <= 0xFFFF {
+		size = 3 // byte + uint16
+	} else {
+		size = 5 // byte + uint32
+	}
+	return size
+}