fix: properly support negative numerator for CBOR rational numbers (#793)

Author: agaffney

cbor/tags.go

@@ -15,6 +15,7 @@
 package cbor
 
 import (
+	"fmt"
 	"math/big"
 	"reflect"
 
@@ -93,29 +94,57 @@ type Rat struct {
 }
 
 func (r *Rat) UnmarshalCBOR(cborData []byte) error {
-	tmpRat := []uint64{}
+	tmpRat := []any{}
 	if _, err := Decode(cborData, &tmpRat); err != nil {
 		return err
 	}
-	// Convert numerator and denominator to big.Int
-	// It's necessary to do this to support num/denom larger than int64 (up to uint64)
+	// Convert numerator to big.Int
 	tmpNum := new(big.Int)
-	tmpNum.SetUint64(tmpRat[0])
+	switch v := tmpRat[0].(type) {
+	case int64:
+		tmpNum.SetInt64(v)
+	case uint64:
+		tmpNum.SetUint64(v)
+	default:
+		return fmt.Errorf("unsupported numerator type for cbor.Rat: %T", v)
+	}
+	// Convert denominator to big.Int
 	tmpDenom := new(big.Int)
-	tmpDenom.SetUint64(tmpRat[1])
+	switch v := tmpRat[1].(type) {
+	case int64:
+		tmpDenom.SetInt64(v)
+	case uint64:
+		tmpDenom.SetUint64(v)
+	default:
+		return fmt.Errorf("unsupported demoninator type for cbor.Rat: %T", v)
+	}
 	// Create new big.Rat with num/denom set to big.Int values above
 	r.Rat = new(big.Rat)
 	r.Rat.SetFrac(tmpNum, tmpDenom)
 	return nil
 }
 
 func (r *Rat) MarshalCBOR() ([]byte, error) {
+	tmpContent := make([]any, 2)
+	// Numerator
+	if r.Num().IsUint64() {
+		tmpContent[0] = r.Num().Uint64()
+	} else if r.Num().IsInt64() {
+		tmpContent[0] = r.Num().Int64()
+	} else {
+		return nil, fmt.Errorf("numerator cannot be represented at int64/uint64")
+	}
+	// Denominator
+	if r.Denom().IsUint64() {
+		tmpContent[1] = r.Denom().Uint64()
+	} else if r.Denom().IsInt64() {
+		tmpContent[1] = r.Denom().Int64()
+	} else {
+		return nil, fmt.Errorf("numerator cannot be represented at int64/uint64")
+	}
 	tmpData := _cbor.Tag{
-		Number: CborTagRational,
-		Content: []uint64{
-			r.Num().Uint64(),
-			r.Denom().Uint64(),
-		},
+		Number:  CborTagRational,
+		Content: tmpContent,
 	}
 	return Encode(&tmpData)
 }

cbor/tags_test.go

@@ -54,6 +54,7 @@ var tagsTestDefs = []struct {
 			},
 		),
 	},
+	// 30([9223372036854775809, 10000000000000000000])
 	{
 		cborHex: "d81e821b80000000000000011b8ac7230489e80000",
 		object: cbor.Rat{
@@ -63,6 +64,13 @@ var tagsTestDefs = []struct {
 			),
 		},
 	},
+	// 30([-1, 2])
+	{
+		cborHex: "d81e822002",
+		object: cbor.Rat{
+			Rat: big.NewRat(-1, 2),
+		},
+	},
 }
 
 func TestTagsDecode(t *testing.T) {