chore(bigint): write with Writer for fewer allocations

big/int.go

@@ -218,6 +218,40 @@ func (bi *Int) UnmarshalJSON(b []byte) error {
 	return nil
 }
 
+func (bi *Int) WriteBytes(out io.Writer) (int, error) {
+	if bi.Int == nil {
+		return 0, fmt.Errorf("failed to convert to bytes, big is nil")
+	}
+
+	var wrote int
+	switch {
+	case bi.Sign() > 0:
+		w, err := out.Write([]byte{0})
+		if err != nil {
+			return wrote, err
+		}
+		wrote += w
+		w, err = out.Write(bi.Int.Bytes())
+		if err != nil {
+			return wrote, err
+		}
+		wrote += w
+	case bi.Sign() < 0:
+		w, err := out.Write([]byte{1})
+		if err != nil {
+			return wrote, err
+		}
+		wrote += w
+		w, err = out.Write(bi.Int.Bytes())
+		if err != nil {
+			return wrote, err
+		}
+		wrote += w
+	default: //  bi.Sign() == 0:
+	}
+	return wrote, nil
+}
+
 func (bi *Int) Bytes() ([]byte, error) {
 	if bi.Int == nil {
 		return []byte{}, fmt.Errorf("failed to convert to bytes, big is nil")
@@ -233,6 +267,14 @@ func (bi *Int) Bytes() ([]byte, error) {
 	}
 }
 
+func (bi *Int) byteLength() int {
+	if bi.Int == nil || bi.Sign() == 0 {
+		return 0
+	}
+
+	return 1 + (bi.Int.BitLen()+7)/8
+}
+
 func FromBytes(buf []byte) (Int, error) {
 	if len(buf) == 0 {
 		return NewInt(0), nil
@@ -281,12 +323,7 @@ func (bi *Int) MarshalCBOR(w io.Writer) error {
 		return zero.MarshalCBOR(w)
 	}
 
-	enc, err := bi.Bytes()
-	if err != nil {
-		return err
-	}
-
-	encLen := len(enc)
+	encLen := bi.byteLength()
 	if encLen > BigIntMaxSerializedLen {
 		return fmt.Errorf("big integer byte array too long (%d bytes)", encLen)
 	}
@@ -296,10 +333,11 @@ func (bi *Int) MarshalCBOR(w io.Writer) error {
 		return err
 	}
 
-	if _, err := w.Write(enc); err != nil {
+	if wrote, err := bi.WriteBytes(w); err != nil {
 		return err
+	} else if wrote != encLen {
+		return fmt.Errorf("failed to write full big int byte array (%d bytes written, %d expected)", wrote, encLen)
 	}
-
 	return nil
 }