feat(loader): Adding a loading method that loads the diff of two stru…

…cts into one (#10)

* Adding a diff loader

* Adding tests

loader.go

@@ -0,0 +1,87 @@
+package patcher
+
+import (
+	"errors"
+	"reflect"
+)
+
+var (
+	// ErrInvalidType is returned when the provided type is not a pointer to a struct
+	ErrInvalidType = errors.New("invalid type")
+
+	// ErrInvalidFieldType is returned when the provided field type is not a struct
+	ErrInvalidFieldType = errors.New("invalid field type")
+)
+
+// LoadDiff inserts the fields provided in the new object into the old object and returns the result.
+//
+// This can be if you are inserting a patch into an existing object but require a new object to be returned with
+// all fields.
+func LoadDiff[T any](old T, newT T) error {
+	return loadDiff(old, newT)
+}
+
+func loadDiff[T any](old T, newT T) error {
+	orv := reflect.ValueOf(old)
+	if orv.Kind() != reflect.Ptr || orv.IsNil() {
+		return ErrInvalidType
+	}
+
+	nrv := reflect.ValueOf(newT)
+	if nrv.Kind() != reflect.Ptr || nrv.IsNil() {
+		return ErrInvalidType
+	}
+
+	oElem := orv.Elem()
+	nElem := nrv.Elem()
+
+	if oElem.Kind() != reflect.Struct || nElem.Kind() != reflect.Struct {
+		return ErrInvalidFieldType
+	}
+
+	for i := 0; i < oElem.NumField(); i++ {
+		// Include only exported fields
+		if !oElem.Field(i).CanSet() || !nElem.Field(i).CanSet() {
+			continue
+		}
+
+		// Handle embedded structs (Anonymous fields)
+		if oElem.Type().Field(i).Anonymous {
+			// If the embedded field is a pointer, dereference it
+			if oElem.Field(i).Kind() == reflect.Ptr {
+				if !oElem.Field(i).IsNil() && !nElem.Field(i).IsNil() {
+					if err := loadDiff(oElem.Field(i).Interface(), nElem.Field(i).Interface()); err != nil {
+						return err
+					}
+				} else if nElem.Field(i).IsValid() && !nElem.Field(i).IsNil() {
+					oElem.Field(i).Set(nElem.Field(i))
+				}
+
+				continue
+			}
+
+			if err := loadDiff(oElem.Field(i).Addr().Interface(), nElem.Field(i).Addr().Interface()); err != nil {
+				return err
+			}
+			continue
+		}
+
+		// If the field is a struct, we need to recursively call LoadDiff
+		if oElem.Field(i).Kind() == reflect.Struct {
+			if err := loadDiff(oElem.Field(i).Addr().Interface(), nElem.Field(i).Addr().Interface()); err != nil {
+				return err
+			}
+			continue
+		}
+
+		// Compare the old and new fields.
+		//
+		// New fields take priority over old fields if they are provided. We ignore zero values as they are not
+		// provided in the new object.
+		if !nElem.Field(i).IsZero() {
+			oElem.Field(i).Set(nElem.Field(i))
+		}
+	}
+
+	return nil
+}