tlv: add MakeBigSizeRecord to save space

This commit adds a pair of encoder/decoder to take the advantage of the
BigSize format when encoding an uint64 and possibly an uint32.

Often the time an uint64 value is not big enough to fill all the 8
bytes, thus using BigSize can save extra bytes when save it to db. And
for uint32, if we know most of the values do not exceed 65536, we can
also save at least 1 byte using BigSize format.

This commit introduces `MakeBigSizeRecord` that can be used optionally
where db space is a concern.

tlv/primitive.go

@@ -307,3 +307,58 @@ func DVarBytes(r io.Reader, val interface{}, _ *[8]byte, l uint64) error {
 	}
 	return NewTypeForDecodingErr(val, "[]byte", l, l)
 }
+
+// EBigSize encodes an uint32 or an uint64 using BigSize format. An error is
+// returned if val is not either *uint32 or *uint64.
+func EBigSize(w io.Writer, val interface{}, buf *[8]byte) error {
+	if i, ok := val.(*uint32); ok {
+		return WriteVarInt(w, uint64(*i), buf)
+	}
+
+	if i, ok := val.(*uint64); ok {
+		return WriteVarInt(w, uint64(*i), buf)
+	}
+
+	return NewTypeForEncodingErr(val, "BigSize")
+}
+
+// DBigSize decodes an uint32 or an uint64 using BigSize format. An error is
+// returned if val is not either *uint32 or *uint64.
+func DBigSize(r io.Reader, val interface{}, buf *[8]byte, l uint64) error {
+	if i, ok := val.(*uint32); ok {
+		v, err := ReadVarInt(r, buf)
+		if err != nil {
+			return err
+		}
+		*i = uint32(v)
+		return nil
+	}
+
+	if i, ok := val.(*uint64); ok {
+		v, err := ReadVarInt(r, buf)
+		if err != nil {
+			return err
+		}
+		*i = v
+		return nil
+	}
+
+	return NewTypeForDecodingErr(val, "BigSize", l, 8)
+}
+
+// SizeBigSize returns a SizeFunc that can compute the length of BigSize.
+func SizeBigSize(val interface{}) SizeFunc {
+	var size uint64
+
+	if i, ok := val.(*uint32); ok {
+		size = VarIntSize(uint64(*i))
+	}
+
+	if i, ok := val.(*uint64); ok {
+		size = VarIntSize(uint64(*i))
+	}
+
+	return func() uint64 {
+		return size
+	}
+}