Tiny fixes (#331)

* do not skip fields in composite if keepzero is set

* fix tag parsing

* allow to use field name and tag options together

in case when you don't want to duplicate the field name in the
iso8583 tag, you can just skip it and set the rest of the options like
this:

```go
F2 *field.String `iso8583:",keepzero"`
```

then the field id will be derived from the field name and keepzero
option will be used as well

* add test for resetting field values

* remove unnecessary test

field/composite.go

@@ -234,17 +234,23 @@ func (f *Composite) Marshal(v interface{}) error {
 	defer f.mu.Unlock()
 
 	rv := reflect.ValueOf(v)
-	if rv.Kind() != reflect.Ptr || rv.IsNil() {
-		return errors.New("data is not a pointer or nil")
+	if rv.Kind() != reflect.Ptr {
+		return errors.New("data is not a pointer")
 	}
 
-	// get the struct from the pointer
-	dataStruct := rv.Elem()
+	elemType := rv.Type().Elem()
+	if elemType.Kind() != reflect.Struct {
+		return errors.New("data must be a pointer to struct")
+	}
 
-	if dataStruct.Kind() != reflect.Struct {
-		return errors.New("data is not a struct")
+	// If nil, create a new instance of the struct
+	if rv.IsNil() {
+		rv = reflect.New(elemType)
 	}
 
+	// get the struct from the pointer
+	dataStruct := rv.Elem()
+
 	// iterate over struct fields
 	for i := 0; i < dataStruct.NumField(); i++ {
 		indexTag := NewIndexTag(dataStruct.Type().Field(i))
@@ -258,7 +264,7 @@ func (f *Composite) Marshal(v interface{}) error {
 		}
 
 		dataField := dataStruct.Field(i)
-		if dataField.IsZero() {
+		if dataField.IsZero() && !indexTag.KeepZero {
 			continue
 		}
 

field/composite_test.go

@@ -351,10 +351,23 @@ type SubConstructedTLVTestData struct {
 }
 
 func TestCompositeField_Marshal(t *testing.T) {
+	t.Run("Marshal returns error on nil value", func(t *testing.T) {
+		composite := NewComposite(compositeTestSpec)
+		err := composite.Marshal(nil)
+		require.EqualError(t, err, "data is not a pointer")
+	})
+
+	t.Run("Marshal doesn't return an error when nil pointer is of struct type", func(t *testing.T) {
+		composite := NewComposite(compositeTestSpec)
+		var data *CompositeTestData
+		err := composite.Marshal(data)
+		require.NoError(t, err)
+	})
+
 	t.Run("Marshal returns an error on provision of primitive type data", func(t *testing.T) {
 		composite := NewComposite(compositeTestSpec)
 		err := composite.Marshal("primitive str")
-		require.EqualError(t, err, "data is not a pointer or nil")
+		require.EqualError(t, err, "data is not a pointer")
 	})
 
 	t.Run("Marshal skips fields without index tag", func(t *testing.T) {

field/index_tag.go

@@ -3,6 +3,7 @@ package field
 import (
 	"reflect"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -20,6 +21,18 @@ type IndexTag struct {
 }
 
 func NewIndexTag(field reflect.StructField) IndexTag {
+	indexTag := extractTagInfo(field)
+
+	if indexTag.Tag == "" {
+		id, tag := extractIdAndTagFromName(field.Name)
+		indexTag.ID = id
+		indexTag.Tag = tag
+	}
+
+	return indexTag
+}
+
+func extractTagInfo(field reflect.StructField) IndexTag {
 	// value of the key "index" in the tag
 	var value string
 
@@ -34,66 +47,52 @@ func NewIndexTag(field reflect.StructField) IndexTag {
 	// format of the value is "id[,keep_zero_value]"
 	// id is the id of the field
 	// let's parse it
-	if value != "" {
-		tag, opts := parseTag(value)
-
-		id, err := strconv.Atoi(tag)
-		if err != nil {
-			id = -1
-		}
-
+	if value == "" {
 		return IndexTag{
-			ID:       id,
-			Tag:      tag,
-			KeepZero: opts.Contains("keepzero"),
+			ID: -1,
 		}
 	}
 
-	dataFieldName := field.Name
-	if len(dataFieldName) > 0 && fieldNameIndexRe.MatchString(dataFieldName) {
-		indexStr := dataFieldName[1:]
+	tag, opts := parseTag(value)
+	id, err := strconv.Atoi(tag)
+	if err != nil {
+		id = -1
+	}
+
+	return IndexTag{
+		ID:       id,
+		Tag:      tag,
+		KeepZero: opts.Contains("keepzero"),
+	}
+}
+
+func extractIdAndTagFromName(fieldName string) (int, string) {
+	if len(fieldName) > 0 && fieldNameIndexRe.MatchString(fieldName) {
+		indexStr := fieldName[1:]
 		fieldIndex, err := strconv.Atoi(indexStr)
 		if err != nil {
-			return IndexTag{
-				ID:  -1,
-				Tag: indexStr,
-			}
+			return -1, indexStr
 		}
 
-		return IndexTag{
-			ID:  fieldIndex,
-			Tag: indexStr,
-		}
+		return fieldIndex, indexStr
 	}
 
-	return IndexTag{
-		ID: -1,
-	}
+	return -1, ""
 }
 
-type tagOptions string
+type tagOptions []string
 
 // parseTag splits a struct field's index tag into its id and
 // comma-separated options.
 func parseTag(tag string) (string, tagOptions) {
 	tag, opt, _ := strings.Cut(tag, ",")
-	return tag, tagOptions(opt)
+
+	return tag, tagOptions(strings.Split(opt, ","))
 }
 
 // Contains reports whether a comma-separated list of options
 // contains a particular substr flag. substr must be surrounded by a
 // string boundary or commas.
 func (o tagOptions) Contains(optionName string) bool {
-	if len(o) == 0 {
-		return false
-	}
-	s := string(o)
-	for s != "" {
-		var name string
-		name, s, _ = strings.Cut(s, ",")
-		if name == optionName {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(o, optionName)
 }

message_test.go

@@ -367,6 +367,79 @@ func TestMessage(t *testing.T) {
 		wantMsg := []byte("01007000000000000000164242424242424242123456000000000100")
 		require.Equal(t, wantMsg, rawMsg)
 	})
+
+	t.Run("Clone, set zero values and reset fields", func(t *testing.T) {
+		type TestISOF3Data struct {
+			F1 *field.String
+			F2 *field.String
+			F3 *field.String
+		}
+
+		type ISO87Data struct {
+			F0 *field.String
+			F2 *field.String
+			F3 *TestISOF3Data
+			F4 *field.String
+		}
+
+		message := NewMessage(spec)
+		err := message.Marshal(&ISO87Data{
+			F0: field.NewStringValue("0100"),
+			F2: field.NewStringValue("4242424242424242"),
+			F3: &TestISOF3Data{
+				F1: field.NewStringValue("12"),
+				F2: field.NewStringValue("34"),
+				F3: field.NewStringValue("56"),
+			},
+			F4: field.NewStringValue("100"),
+		})
+		require.NoError(t, err)
+
+		// clone the message and reset some fields
+		clone, err := message.Clone()
+		require.NoError(t, err)
+
+		// reset the fields
+		// first, check that the fields are set
+		data := &ISO87Data{}
+		require.NoError(t, clone.Unmarshal(data))
+
+		require.Equal(t, "0100", data.F0.Value())
+		require.Equal(t, "4242424242424242", data.F2.Value())
+		require.Equal(t, "12", data.F3.F1.Value())
+		require.Equal(t, "34", data.F3.F2.Value())
+		require.Equal(t, "56", data.F3.F3.Value())
+		require.Equal(t, "100", data.F4.Value())
+
+		// reset the fields
+		err = clone.Marshal(&struct {
+			F2 *field.String `iso8583:",keepzero"`
+			F3 *struct {
+				F2 *field.String `iso8583:",keepzero"`
+			} `iso8583:",keepzero"`
+		}{})
+		require.NoError(t, err)
+
+		// check that the field values are set to zero values
+		data = &ISO87Data{}
+		require.NoError(t, clone.Unmarshal(data))
+
+		// check that fields are set
+		require.NotNil(t, data.F2)
+		require.NotNil(t, data.F3)
+		require.NotNil(t, data.F3.F2)
+
+		// check the zero values
+		require.Equal(t, "", data.F2.Value())
+		require.Equal(t, "", data.F3.F2.Value())
+
+		// check the reset fields in the message
+		require.Equal(t, "0100", data.F0.Value())
+		require.Equal(t, "12", data.F3.F1.Value())
+		require.Equal(t, "56", data.F3.F3.Value())
+		require.Equal(t, "100", data.F4.Value())
+	})
+
 }
 
 func TestPackUnpack(t *testing.T) {