fix: added nil and struct type check on NewMapper function, and dereference it if neccessary

Author: ralvarezdev

mapper/errors.go

@@ -10,4 +10,7 @@ var (
 	ErrProtobufTagNotFound     = "missing protobuf tag: %s"
 	ErrProtobufTagNameNotFound = "missing protobuf tag name: %s"
 	ErrEmptyJSONTag            = "empty json tag: %s"
+	ErrNilStructInstance      = errors.New("struct instance cannot be nil")
+	ErrStructInstanceNotStruct = errors.New("struct instance must be a struct")
+	ErrInvalidStructInstance = errors.New("invalid struct instance")
 )

mapper/json.go

@@ -59,7 +59,10 @@ func (j JSONGenerator) NewMapper(structInstance any) (
 	structTypeName := reflectedType.Name()
 
 	// Initialize the root map of fields and the map of nested mappers
-	rootMapper := NewMapper(structInstance)
+	rootMapper, err := NewMapper(structInstance)
+	if err != nil {
+		return nil, err
+	}
 
 	// Reflection of the type of data
 	var jsonTag string

mapper/mapper.go

@@ -30,8 +30,32 @@ type (
 // Returns:
 //
 //   - *Mapper: instance of the mapper
-func NewMapper(structInstance any) *Mapper {
-	return &Mapper{structInstance: structInstance}
+//   - error: error if the struct instance is nil
+func NewMapper(structInstance any) (*Mapper, error) {
+	// Check if the struct instance is nil
+	if structInstance == nil {
+		return nil, ErrNilStructInstance
+	}
+
+	// Get the type of the struct instance
+	structInstanceType := reflect.TypeOf(structInstance)
+	structInstanceValue := reflect.ValueOf(structInstance)
+
+	// Dereference pointer if necessary
+	if structInstanceType.Kind() == reflect.Ptr {
+		structInstanceType = structInstanceType.Elem()
+		structInstanceValue = structInstanceValue.Elem()
+	}
+
+	// Ensure it's a struct
+	if structInstanceType.Kind() != reflect.Struct {
+		return nil, ErrInvalidStructInstance
+	}
+
+	// Use the dereferenced value (as interface{})
+	dereferencedStructInstance := structInstanceValue.Interface()
+
+	return &Mapper{structInstance: dereferencedStructInstance}, nil
 }
 
 // GetStructInstance returns the instance of the struct

mapper/protobuf.go

@@ -51,15 +51,18 @@ func NewProtobufGenerator(logger *slog.Logger) *ProtobufGenerator {
 func (p ProtobufGenerator) NewMapper(structInstance any) (
 	*Mapper,
 	error,
-) {
+) {	
 	// Reflection of data
 	reflectedType := goreflect.GetDereferencedType(structInstance)
 
 	// Get the struct type name
 	structTypeName := goreflect.GetTypeName(reflectedType)
 
 	// Initialize the root map of fields and the map of nested mappers
-	rootMapper := NewMapper(structInstance)
+	rootMapper, err := NewMapper(structInstance)
+	if err != nil {
+		return nil, err
+	}
 
 	// Reflection of the type of data
 	for i := 0; i < reflectedType.NumField(); i++ {