Merge 2 generic structs values

merge.go

package structs

import "reflect"

// obj1 := struct{ ID string Name string }
// obj2 := struct{ ID string Name string }{ ID: "1", Name: "Jhon" }
// 
// merged := merge.Structs(obj1, obj2)
//
// merge => struct{ ID string Name string }{ ID: "1", Name: "Jhon" }
//
func Structs(baseStruct interface{}, fromValues interface{}) interface{} {
	fieldNames := getFieldsName(baseStruct)

	pointerBaseStruct := interfacePointer(baseStruct)
	fromPtr := interfacePointer(fromValues)

	for parentIndex, fieldName := range fieldNames {
		keyIndex := hasKey(fieldName, fromValues)

		if keyIndex != -1 {
			setInterfaceValue(
				reflect.Indirect(pointerBaseStruct).Field(parentIndex),
				reflect.Indirect(fromPtr).Field(keyIndex),
			)
		}
	}

	return pointerBaseStruct.Elem().Interface()
}

func getFieldsName(a interface{}) ( fields []string ) {
	val := interfacePointer(a)
	elem := val.Elem()
	for i:=0; i<elem.NumField();i++{
		fields = append(fields, elem.Type().Field(i).Name)
	}

	return fields
}

// https://stackoverflow.com/a/14162161/2138322
func interfacePointer(data interface{}) reflect.Value {
	var ptr reflect.Value
	value := reflect.ValueOf(data)
	if value.Type().Kind() == reflect.Ptr {
		ptr = value
		value = ptr.Elem() // acquire value referenced by pointer
	} else {
		ptr = reflect.New(reflect.TypeOf(data)) // create new pointer
		temp := ptr.Elem() // create variable to value of pointer
		temp.Set(value) // set value of variable to our passed in value
	}

	return ptr
}


func setInterfaceValue(baseStruct reflect.Value, fromStruct reflect.Value) {
	baseStruct.Set(fromStruct)
}

func hasKey(key string, object interface{}) int {
	fields := getFieldsName(object)

	for index, fieldName := range fields {
		if fieldName == key {
			return index
		}
	}

	return -1
}