Source: merge.go in package github.com/imdario/mergo

Based on src/pkg/reflect/deepequal.go from official golang's stdlib.


package mergo

import (
	"fmt"
	"reflect"
	"unsafe"
)

func hasExportedField(dst reflect.Value) (exported bool) {
	for i, n := 0, dst.NumField(); i < n; i++ {
		field := dst.Type().Field(i)
		if isExportedComponent(&field) {
			return true
		}
	}
	return
}

func isExportedComponent(field *reflect.StructField) bool {
	name := field.Name
	pkgPath := field.PkgPath
	if len(pkgPath) > 0 {
		return false
	}
	c := 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
}

type Transformers interface {
	Transformer(reflect.Type) func(dst, src reflect.Value) error
}

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.

func deepMerge(dstIn, src reflect.Value, visited map[uintptr]*visit, depth int, config *Config) (dst reflect.Value, err error) {
	dst = dstIn
	overwrite := config.Overwrite
	typeCheck := config.TypeCheck
	overwriteWithEmptySrc := config.overwriteWithEmptyValue
	overwriteSliceWithEmptySrc := config.overwriteSliceWithEmptyValue

	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 dst, nil
			}

Remember, remember...

		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
		}
	}

	if dst.IsValid() && src.IsValid() && src.Type() != dst.Type() {
		err = fmt.Errorf("cannot append two different types (%s, %s)", src.Kind(), dst.Kind())
		return
	}

	switch dst.Kind() {
	case reflect.Struct:
		if hasExportedField(dst) {
			dstCp := reflect.New(dst.Type()).Elem()
			for i, n := 0, dst.NumField(); i < n; i++ {
				dstField := dst.Field(i)

copy un-exported struct fields

				if !isExportedComponent(&structField) {
					rf := dstCp.Field(i)
					rf = reflect.NewAt(rf.Type(), unsafe.Pointer(rf.UnsafeAddr())).Elem() //nolint:gosec
					dstRF := dst.Field(i)
					if !dst.Field(i).CanAddr() {
						continue
					}

					dstRF = reflect.NewAt(dstRF.Type(), unsafe.Pointer(dstRF.UnsafeAddr())).Elem() //nolint:gosec
					rf.Set(dstRF)
					continue
				}
				dstField, err = deepMerge(dstField, src.Field(i), visited, depth+1, config)
				if err != nil {
					return
				}
				dstCp.Field(i).Set(dstField)
			}

			if dst.CanSet() {
				dst.Set(dstCp)
			} else {
				dst = dstCp
			}
			return
		} else {
			if (isReflectNil(dst) || overwrite) && (!isEmptyValue(src) || overwriteWithEmptySrc) {
				dst = src
			}
		}

	case reflect.Map:
		if dst.IsNil() && !src.IsNil() {
			if dst.CanSet() {
				dst.Set(reflect.MakeMap(dst.Type()))
			} else {
				dst = src
				return
			}
		}
		for _, key := range src.MapKeys() {
			srcElement := src.MapIndex(key)
			dstElement := dst.MapIndex(key)
			if !srcElement.IsValid() {
				continue
			}
			if dst.MapIndex(key).IsValid() {
				k := dstElement.Interface()
				dstElement = reflect.ValueOf(k)
			}
			if isReflectNil(srcElement) {
				if overwrite || isReflectNil(dstElement) {
					dst.SetMapIndex(key, srcElement)
				}
				continue
			}
			if !srcElement.CanInterface() {
				continue
			}

			if srcElement.CanInterface() {
				srcElement = reflect.ValueOf(srcElement.Interface())
				if dstElement.IsValid() {
					dstElement = reflect.ValueOf(dstElement.Interface())
				}
			}
			dstElement, err = deepMerge(dstElement, srcElement, visited, depth+1, config)
			if err != nil {
				return
			}
			dst.SetMapIndex(key, dstElement)

		}
	case reflect.Slice:
		newSlice := dst
		if (!isEmptyValue(src) || overwriteWithEmptySrc || overwriteSliceWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
			if typeCheck && src.Type() != dst.Type() {
				return dst, fmt.Errorf("cannot override two slices with different type (%s, %s)", src.Type(), dst.Type())
			}
			newSlice = src
		} else if config.AppendSlice {
			if typeCheck && src.Type() != dst.Type() {
				err = fmt.Errorf("cannot append two slice with different type (%s, %s)", src.Type(), dst.Type())
				return
			}
			newSlice = reflect.AppendSlice(dst, src)
		}
		if dst.CanSet() {
			dst.Set(newSlice)
		} else {
			dst = newSlice
		}
	case reflect.Ptr, reflect.Interface:
		if isReflectNil(src) {
			break
		}

		if dst.Kind() != reflect.Ptr && src.Type().AssignableTo(dst.Type()) {
			if dst.IsNil() || overwrite {
				if overwrite || isEmptyValue(dst) {
					if dst.CanSet() {
						dst.Set(src)
					} else {
						dst = 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 dst, err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
					return
				}
				dst = dst.Addr()
			} else if dst.Elem().Type() == src.Type() {
				if dst, err = deepMerge(dst.Elem(), src, visited, depth+1, config); err != nil {
					return
				}
			} else {
				return dst, ErrDifferentArgumentsTypes
			}
			break
		}
		if dst.IsNil() || overwrite {
			if (overwrite || isEmptyValue(dst)) && (overwriteWithEmptySrc || !isEmptyValue(src)) {
				if dst.CanSet() {
					dst.Set(src)
				} else {
					dst = src
				}
			}
		} else if _, err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
			return
		}
	default:
		overwriteFull := (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst))
		if overwriteFull {
			if dst.CanSet() {
				dst.Set(src)
			} else {
				dst = src
			}
		}
	}

	return
}

Merge will fill any empty for value type attributes on the dst struct using corresponding src attributes if they themselves are not empty. dst and src must be valid same-type structs and dst must be a pointer to struct. It won't merge unexported (private) fields and will do recursively any exported field.

func Merge(dst, src interface{}, opts ...func(*Config)) error {
	return merge(dst, src, opts...)
}

MergeWithOverwrite will do the same as Merge except that non-empty dst attributes will be overridden by non-empty src attribute values. Deprecated: use Merge(…) with WithOverride

func MergeWithOverwrite(dst, src interface{}, opts ...func(*Config)) error {
	return merge(dst, src, append(opts, WithOverride)...)
}

WithTransformers adds transformers to merge, allowing to customize the merging of some types.

func WithTransformers(transformers Transformers) func(*Config) {
	return func(config *Config) {
		config.Transformers = transformers
	}
}

WithOverride will make merge override non-empty dst attributes with non-empty src attributes values.

func WithOverride(config *Config) {
	config.Overwrite = true
}

WithOverwriteWithEmptyValue will make merge override non empty dst attributes with empty src attributes values.

func WithOverwriteWithEmptyValue(config *Config) {
	config.overwriteWithEmptyValue = true
}

WithOverrideEmptySlice will make merge override empty dst slice with empty src slice.

func WithOverrideEmptySlice(config *Config) {
	config.overwriteSliceWithEmptyValue = true
}

WithAppendSlice will make merge append slices instead of overwriting it.

func WithAppendSlice(config *Config) {
	config.AppendSlice = true
}

WithTypeCheck will make merge check types while overwriting it (must be used with WithOverride).

func WithTypeCheck(config *Config) {
	config.TypeCheck = true
}

func merge(dst, src interface{}, opts ...func(*Config)) error {
	var (
		vDst, vSrc reflect.Value
		err        error
	)

	config := &Config{}

	for _, opt := range opts {
		opt(config)
	}

	if vDst, vSrc, err = resolveValues(dst, src); err != nil {
		return err
	}
	if !vDst.CanSet() {
		return fmt.Errorf("cannot set dst, needs reference")
	}
	if vDst.Type() != vSrc.Type() {
		return ErrDifferentArgumentsTypes
	}
	_, err = deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, config)
	return err
}

IsReflectNil is the reflect value provided nil

func isReflectNil(v reflect.Value) bool {
	k := v.Kind()
	switch k {

Both interface and slice are nil if first word is 0. Both are always bigger than a word; assume flagIndir.

		return v.IsNil()
	default:
		return false
	}