func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, config *Config) (err error) {
	overwrite := config.Overwrite
	typeCheck := config.TypeCheck
	overwriteWithEmptySrc := config.overwriteWithEmptyValue
	overwriteSliceWithEmptySrc := config.overwriteSliceWithEmptyValue
	sliceDeepCopy := config.sliceDeepCopy
	if !src.IsValid() {
		return
	}
	if dst.CanAddr() {
		addr := dst.UnsafeAddr()
		h := 17 * addr
		seen := visited[h]
		typ := dst.Type()
		for p := seen; p != nil; p = p.next {
			if p.ptr == addr && p.typ == typ {
				return nil
			}
		}
		visited[h] = &visit{addr, typ, seen}
	}
	if config.Transformers != nil && !isEmptyValue(dst) {
		if fn := config.Transformers.Transformer(dst.Type()); fn != nil {
			err = fn(dst, src)
			return
		}
	}
	switch dst.Kind() {
	case reflect.Struct:
		if hasMergeableFields(dst) {
			for i, n := 0, dst.NumField(); i < n; i++ {
				if err = deepMerge(dst.Field(i), src.Field(i), visited, depth+1, config); err != nil {
					return
				}
			}
		} else {
			if (isReflectNil(dst) || overwrite) && (!isEmptyValue(src) || overwriteWithEmptySrc) {
				dst.Set(src)
			}
		}
	case reflect.Map:
		if dst.IsNil() && !src.IsNil() {
			dst.Set(reflect.MakeMap(dst.Type()))
		}
		if src.Kind() != reflect.Map {
			if overwrite {
				dst.Set(src)
			}
			return
		}
		for _, key := range src.MapKeys() {
			srcElement := src.MapIndex(key)
			if !srcElement.IsValid() {
				continue
			}
			dstElement := dst.MapIndex(key)
			switch srcElement.Kind() {
			case reflect.Chan, reflect.Func, reflect.Map, reflect.Interface, reflect.Slice:
				if srcElement.IsNil() {
					if overwrite {
						dst.SetMapIndex(key, srcElement)
					}
					continue
				}
				fallthrough
			default:
				if !srcElement.CanInterface() {
					continue
				}
				switch reflect.TypeOf(srcElement.Interface()).Kind() {
				case reflect.Struct:
					fallthrough
				case reflect.Ptr:
					fallthrough
				case reflect.Map:
					srcMapElm := srcElement
					dstMapElm := dstElement
					if srcMapElm.CanInterface() {
						srcMapElm = reflect.ValueOf(srcMapElm.Interface())
						if dstMapElm.IsValid() {
							dstMapElm = reflect.ValueOf(dstMapElm.Interface())
						}
					}
					if err = deepMerge(dstMapElm, srcMapElm, visited, depth+1, config); err != nil {
						return
					}
				case reflect.Slice:
					srcSlice := reflect.ValueOf(srcElement.Interface())
					var dstSlice reflect.Value
					if !dstElement.IsValid() || dstElement.IsNil() {
						dstSlice = reflect.MakeSlice(srcSlice.Type(), 0, srcSlice.Len())
					} else {
						dstSlice = reflect.ValueOf(dstElement.Interface())
					}
					if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice && !sliceDeepCopy {
						if typeCheck && srcSlice.Type() != dstSlice.Type() {
							return fmt.Errorf("cannot override two slices with different type (%s, %s)", srcSlice.Type(), dstSlice.Type())
						}
						dstSlice = srcSlice
					} else if config.AppendSlice {
						if srcSlice.Type() != dstSlice.Type() {
							return fmt.Errorf("cannot append two slices with different type (%s, %s)", srcSlice.Type(), dstSlice.Type())
						}
						dstSlice = reflect.AppendSlice(dstSlice, srcSlice)
					} else if sliceDeepCopy {
						i := 0
						for ; i < srcSlice.Len() && i < dstSlice.Len(); i++ {
							srcElement := srcSlice.Index(i)
							dstElement := dstSlice.Index(i)
							if srcElement.CanInterface() {
								srcElement = reflect.ValueOf(srcElement.Interface())
							}
							if dstElement.CanInterface() {
								dstElement = reflect.ValueOf(dstElement.Interface())
							}
							if err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
								return
							}
						}
					}
					dst.SetMapIndex(key, dstSlice)
				}
			}
			if dstElement.IsValid() && !isEmptyValue(dstElement) && (reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Map || reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Slice) {
				continue
			}
			if srcElement.IsValid() && ((srcElement.Kind() != reflect.Ptr && overwrite) || !dstElement.IsValid() || isEmptyValue(dstElement)) {
				if dst.IsNil() {
					dst.Set(reflect.MakeMap(dst.Type()))
				}
				dst.SetMapIndex(key, srcElement)
			}
		}
	case reflect.Slice:
		if !dst.CanSet() {
			break
		}
		if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice && !sliceDeepCopy {
			dst.Set(src)
		} else if config.AppendSlice {
			if src.Type() != dst.Type() {
				return fmt.Errorf("cannot append two slice with different type (%s, %s)", src.Type(), dst.Type())
			}
			dst.Set(reflect.AppendSlice(dst, src))
		} else if sliceDeepCopy {
			for i := 0; i < src.Len() && i < dst.Len(); i++ {
				srcElement := src.Index(i)
				dstElement := dst.Index(i)
				if srcElement.CanInterface() {
					srcElement = reflect.ValueOf(srcElement.Interface())
				}
				if dstElement.CanInterface() {
					dstElement = reflect.ValueOf(dstElement.Interface())
				}
				if err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
					return
				}
			}
		}
	case reflect.Ptr:
		fallthrough
	case reflect.Interface:
		if isReflectNil(src) {
			if overwriteWithEmptySrc && dst.CanSet() && src.Type().AssignableTo(dst.Type()) {
				dst.Set(src)
			}
			break
		}
		if src.Kind() != reflect.Interface {
			if dst.IsNil() || (src.Kind() != reflect.Ptr && overwrite) {
				if dst.CanSet() && (overwrite || isEmptyValue(dst)) {
					dst.Set(src)
				}
			} else if src.Kind() == reflect.Ptr {
				if err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
					return
				}
			} else if dst.Elem().Type() == src.Type() {
				if err = deepMerge(dst.Elem(), src, visited, depth+1, config); err != nil {
					return
				}
			} else {
				return ErrDifferentArgumentsTypes
			}
			break
		}
		if dst.IsNil() || overwrite {
			if dst.CanSet() && (overwrite || isEmptyValue(dst)) {
				dst.Set(src)
			}
			break
		}
		if dst.Elem().Kind() == src.Elem().Kind() {
			if err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
				return
			}
			break
		}
	default:
		mustSet := (isEmptyValue(dst) || overwrite) && (!isEmptyValue(src) || overwriteWithEmptySrc)
		if mustSet {
			if dst.CanSet() {
				dst.Set(src)
			} else {
				dst = src
			}
		}
	}
	return
}
Traverses recursively both values, assigning src's fields values to dst.
The map argument tracks comparisons that have already been seen, which allows
short circuiting on recursive types.
package mergo
import (
	"fmt"
	"reflect"
)
func hasMergeableFields(dst reflect.Value) (exported bool) {
	for i, n := 0, dst.NumField(); i < n; i++ {
		field := dst.Type().Field(i)
		if field.Anonymous && dst.Field(i).Kind() == reflect.Struct {
			exported = exported || hasMergeableFields(dst.Field(i))
		} else if isExportedComponent(&field) {
			exported = exported || len(field.PkgPath) == 0
		}
	}
	return
}
func isExportedComponent(field *reflect.StructField) bool {
	pkgPath := field.PkgPath
	if len(pkgPath) > 0 {
		return false
	}
	c := field.Name[0]
	if 'a' <= c && c <= 'z' || c == '_' {
		return false
	}
	return true
}
type Config struct {
	Overwrite                    bool
	AppendSlice                  bool
	TypeCheck                    bool
	Transformers                 Transformers
	overwriteWithEmptyValue      bool
	overwriteSliceWithEmptyValue bool
	sliceDeepCopy                bool
	debug                        bool
}
type Transformers interface {
	Transformer(reflect.Type) func(dst, src reflect.Value) error
}