add comments to ASCIIHexToBytes to clarify its usage (#329)

encoding/hex.go

@@ -11,8 +11,13 @@ import (
 
 // HEX to ASCII encoder
 var (
-	_               Encoder = (*hexToASCIIEncoder)(nil)
-	BytesToASCIIHex         = &hexToASCIIEncoder{}
+	_ Encoder = (*hexToASCIIEncoder)(nil)
+	// BytesToASCIIHex is an encoder that converts bytes into their ASCII
+	// representation.  On success, the ASCII representation bytes are returned
+	// Don't use this encoder with String, Numeric or Binary fields as packing and
+	// unpacking in these fields uses length of value/bytes, so only Pack will be
+	// able to write the value correctly.
+	BytesToASCIIHex = &hexToASCIIEncoder{}
 )
 
 type hexToASCIIEncoder struct{}
@@ -56,8 +61,14 @@ func (e hexToASCIIEncoder) Decode(data []byte, length int) ([]byte, int, error)
 
 // ASCII To HEX encoder
 var (
-	_               Encoder = (*asciiToHexEncoder)(nil)
-	ASCIIHexToBytes         = &asciiToHexEncoder{}
+	_ Encoder = (*asciiToHexEncoder)(nil)
+	// ASCIIHexToBytes is an encoder that converts ASCII Hex-digits into a byte slice
+	// This encoder is used in TagSpec, BerTLVTag and similar.
+	// It shouldn't be used with String, Numeric or Binary fields as packing and unpacking
+	// in these fields uses length of value/bytes, so only Unpack will be able to read
+	// the value correctly.
+	// If you are looking for a way to work with HEX strings, use Hex field instead.
+	ASCIIHexToBytes = &asciiToHexEncoder{}
 )
 
 type asciiToHexEncoder struct{}

field/hex.go

@@ -20,7 +20,7 @@ var (
 // field. It's convenient to use when you need to work with hex strings, but
 // don't want to deal with converting them to bytes manually.
 // If provided value is not a valid hex string, it will return an error during
-// packing.
+// packing. For the Hex field, the Binary encoding shoud be used in the Spec.
 type Hex struct {
 	value string
 	spec  *Spec
@@ -85,43 +85,24 @@ func (f *Hex) Pack() ([]byte, error) {
 		return nil, utils.NewSafeErrorf(err, "converting hex field into bytes")
 	}
 
-	if f.spec.Pad != nil {
-		data = f.spec.Pad.Pad(data, f.spec.Length)
-	}
-
-	packed, err := f.spec.Enc.Encode(data)
-	if err != nil {
-		return nil, fmt.Errorf("failed to encode content: %w", err)
-	}
+	packer := f.spec.getPacker()
 
-	packedLength, err := f.spec.Pref.EncodeLength(f.spec.Length, len(data))
-	if err != nil {
-		return nil, fmt.Errorf("failed to encode length: %w", err)
-	}
-
-	return append(packedLength, packed...), nil
+	return packer.Pack(data, f.spec)
 }
 
 func (f *Hex) Unpack(data []byte) (int, error) {
-	dataLen, prefBytes, err := f.spec.Pref.DecodeLength(f.spec.Length, data)
-	if err != nil {
-		return 0, fmt.Errorf("failed to decode length: %w", err)
-	}
+	unpacker := f.spec.getUnpacker()
 
-	raw, read, err := f.spec.Enc.Decode(data[prefBytes:], dataLen)
+	raw, bytesRead, err := unpacker.Unpack(data, f.spec)
 	if err != nil {
-		return 0, fmt.Errorf("failed to decode content: %w", err)
-	}
-
-	if f.spec.Pad != nil {
-		raw = f.spec.Pad.Unpad(raw)
+		return 0, err
 	}
 
 	if err := f.SetBytes(raw); err != nil {
 		return 0, fmt.Errorf("failed to set bytes: %w", err)
 	}
 
-	return read + prefBytes, nil
+	return bytesRead, nil
 }
 
 // Deprecated. Use Marshal instead

field/hex_test.go

@@ -38,6 +38,31 @@ func TestHexField(t *testing.T) {
 		require.Equal(t, "AABBCCDDEE", f.Value())
 	})
 
+	t.Run("packing and unpacking with variable length", func(t *testing.T) {
+		spec := &Spec{
+			Length:      5, // 5 bytes, 10 hex chars
+			Description: "Field",
+			Enc:         encoding.Binary,
+			Pref:        prefix.Binary.LL,
+		}
+
+		f := NewHexValue("AABBCCDDEE")
+		f.SetSpec(spec)
+
+		packed, err := f.Pack()
+
+		require.NoError(t, err)
+		require.Equal(t, []byte{0x00, 0x05, 0xaa, 0xbb, 0xcc, 0xdd, 0xee}, packed)
+
+		f = NewHex(spec)
+		read, err := f.Unpack(packed)
+
+		require.NoError(t, err)
+		require.Equal(t, 7, read)
+		require.Equal(t, "AABBCCDDEE", f.Value())
+
+	})
+
 	t.Run("marshaling", func(t *testing.T) {
 		f := NewHexValue("AABBCCDDEE")
 		f2 := &Hex{}

field/packer_unpacker.go

@@ -1,6 +1,8 @@
 package field
 
-import "fmt"
+import (
+	"fmt"
+)
 
 type defaultPacker struct{}