remove confusing vsdc example, add correct icc field spec (#320)

* remove confusing vsdc example, add correct icc field spec

* update comments for icc fields spec example

examples/icc_test.go

@@ -0,0 +1,95 @@
+package examples
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/moov-io/iso8583/encoding"
+	"github.com/moov-io/iso8583/field"
+	"github.com/moov-io/iso8583/prefix"
+	"github.com/moov-io/iso8583/sort"
+	"github.com/stretchr/testify/require"
+)
+
+//	The sample is for VSDC chip data usage
+
+//		This field 55 VSDC chip data usage contains three subfields after the length subfield.
+//		Positions:
+//			1 2 3 4 ... 255
+//	 Fields
+//		 - Subfield 1: length Byte, a one-byte binary subfield  that contains the number of bytes in this field after the length subfield
+//		 - Subfield 2: dataset ID, a one-byte binary identifier
+//		 - Subfield 3: dataset length, 2-byte binary subfield that contains the total length of all TLV elements that follow.
+//		 - Subfield 4:
+//			Chip Card TLV data elements
+//			Tag Length Value Tag Length Value
+func TestICCField55(t *testing.T) {
+	field55Spec := &field.Spec{
+		Length:      255,
+		Description: "Integrated Circuit Card (ICC) Data",
+		Pref:        prefix.Binary.L,
+		Tag: &field.TagSpec{
+			// We have 1 byte length tag, that in the spec is seen as a Hex string
+			// but will be encoded as a binary byte (ASCIIHexToBytes)
+			// We sort the TLV tags by their hex values, but it's not important
+			// Finally, if we have unknown tag, in order to skip it, we need to know
+			// how long its value is. For this, we need to read the length tag.
+			// To read the length tag, we need to know its length.
+			// Setting PrefUnknownTLV to prefix.Binary.LL will read the length prefix
+			// as a 2-byte binary value.
+			Length:             1,
+			Enc:                encoding.ASCIIHexToBytes,
+			Sort:               sort.StringsByHex,
+			SkipUnknownTLVTags: true,
+			// if we have unknown TLV tags,
+			PrefUnknownTLV: prefix.Binary.LL,
+		},
+		Subfields: map[string]field.Field{
+			"01": field.NewComposite(&field.Spec{
+				Length:      252,
+				Description: "VSDC Data",
+				Pref:        prefix.Binary.LL,
+				Tag: &field.TagSpec{
+					Enc:                encoding.BerTLVTag,
+					Sort:               sort.StringsByHex,
+					SkipUnknownTLVTags: true,
+				},
+				Subfields: map[string]field.Field{
+					"9A": field.NewString(&field.Spec{
+						Description: "Transaction Date",
+						Enc:         encoding.Binary,
+						Pref:        prefix.BerTLV,
+					}),
+					"9F02": field.NewString(&field.Spec{
+						Description: "Amount, Authorized (Numeric)",
+						Enc:         encoding.Binary,
+						Pref:        prefix.BerTLV,
+					}),
+				},
+			}),
+		},
+	}
+
+	type VSDCData struct {
+		TransactionDate string `iso8583:"9A"`
+		Amount          string `iso8583:"9F02"`
+	}
+
+	type ICCData struct {
+		VSDCData *VSDCData `iso8583:"01"`
+	}
+
+	filed55 := field.NewComposite(field55Spec)
+	err := filed55.Marshal(&ICCData{
+		VSDCData: &VSDCData{
+			TransactionDate: "210720",
+			Amount:          "000000000501",
+		},
+	})
+	require.NoError(t, err)
+
+	packed, err := filed55.Pack()
+	require.NoError(t, err)
+
+	require.Equal(t, "1A0100179A063231303732309F020C303030303030303030353031", fmt.Sprintf("%X", packed))
+}