add test / example for the custom packer / unpacker

field/packer_unpacker_test.go

@@ -0,0 +1,106 @@
+package field_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/moov-io/iso8583/encoding"
+	"github.com/moov-io/iso8583/field"
+	"github.com/moov-io/iso8583/prefix"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCustomPackerAndUnpacker(t *testing.T) {
+	// let's say we have a requirements that the lentgh prefix should
+	// contain the length of the data we pass, not the length of the field
+	// value. This is the case when you use BCD or HEX encoding, where the
+	// length of the field value is not the same as the length of the
+	// encoded field value.
+
+	// Here is an example of such requirement:
+	// - the max length of the field is 9
+	// - the field value should be encoded using BCD encoding
+	// - the lenth prefix is L (1 byte) and should contain the length of the
+	//   the data in the field we pass.
+	// - the field value is "123"
+
+	// let's see the default behavior of the Numeric field
+	fd := field.NewNumeric(&field.Spec{
+		Length:      9, // the max length of the field is 9 digits
+		Description: "Amount",
+		Enc:         encoding.BCD,
+		Pref:        prefix.Binary.L,
+	})
+
+	fd.SetValue(123)
+
+	packed, err := fd.Pack()
+	require.NoError(t, err)
+
+	// we expect the length to be 2 bytes, as 123 encoded in BCD is 0x01, 0x23
+	// by the default behavior, the length prefix will contain the length of the
+	// field value, which is 3 digits, so the length prefix as you can see is 0x03
+	require.Equal(t, []byte{0x03, 0x01, 0x23}, packed)
+
+	// now let's create a custom packer and unpacker for the Numeric field
+	// that will pack the field value as BCD and the length prefix as the length
+	// of the encoded field value.
+	fc := field.NewNumeric(&field.Spec{
+		Length:      5, // now, this field indicates the max length of the encoded field value 9/2+1 = 5
+		Description: "Amount",
+		Enc:         encoding.BCD,
+		Pref:        prefix.Binary.L,
+		// we define a custom packer here, which will encode the length of the packed data
+		Packer: field.PackerFunc(func(data []byte, spec *field.Spec) ([]byte, error) {
+			if spec.Pad != nil {
+				data = spec.Pad.Pad(data, spec.Length)
+			}
+
+			packed, err := spec.Enc.Encode(data)
+			if err != nil {
+				return nil, fmt.Errorf("failed to encode content: %w", err)
+			}
+
+			// here is where we encode the length of the packed data, not the length of the value
+			packedLength, err := spec.Pref.EncodeLength(spec.Length, len(packed))
+			if err != nil {
+				return nil, fmt.Errorf("failed to encode length: %w", err)
+			}
+
+			return append(packedLength, packed...), nil
+		}),
+		// we define a custom unpacker here, which will decode the length of the packed data
+		Unpacker: field.UnpackerFunc(func(data []byte, spec *field.Spec) ([]byte, int, error) {
+			dataLen, prefBytes, err := spec.Pref.DecodeLength(spec.Length, data)
+			if err != nil {
+				return nil, 0, fmt.Errorf("failed to decode length: %w", err)
+			}
+
+			// dataLen here is the length of the packed data, not the length of the value
+			// as we use BCD decoding, we have to multiply it by 2, as each BCD byte
+			// represents 2 digits. If the number of digits is even, it will be prepended
+			// with a 0. As the type of the field is Numeric, leading 0 will be removed
+			// so we will have exactly the number of digits we need.
+			raw, read, err := spec.Enc.Decode(data[prefBytes:], dataLen*2)
+			if err != nil {
+				return nil, 0, fmt.Errorf("failed to decode content: %w", err)
+			}
+
+			if spec.Pad != nil {
+				raw = spec.Pad.Unpad(raw)
+			}
+
+			return raw, read + prefBytes, nil
+		}),
+	})
+
+	fc.SetValue(123)
+
+	packed, err = fc.Pack()
+	require.NoError(t, err)
+
+	// we expect the length to be 2 bytes, as 123 encoded in BCD is 0x01, 0x23
+	// so, you can see that the length prefix is 0x02, as the length of the packed
+	// data is 2 bytes.
+	require.Equal(t, []byte{0x02, 0x01, 0x23}, packed)
+}