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

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


package mergo

import (
	"fmt"
	"reflect"
	"unicode"
	"unicode/utf8"
)

func changeInitialCase(s string, mapper func(rune) rune) string {
	if s == "" {
		return s
	}
	r, n := utf8.DecodeRuneInString(s)
	return string(mapper(r)) + s[n:]
}

func isExported(field reflect.StructField) bool {
	r, _ := utf8.DecodeRuneInString(field.Name)
	return r >= 'A' && r <= 'Z'
}

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 deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, config *Config) (err error) {
	overwrite := config.Overwrite
	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
			}

Remember, remember...

		visited[h] = &visit{addr, typ, seen}
	}
	zeroValue := reflect.Value{}
	switch dst.Kind() {
	case reflect.Map:
		dstMap := dst.Interface().(map[string]interface{})
		for i, n := 0, src.NumField(); i < n; i++ {
			srcType := src.Type()
			field := srcType.Field(i)
			if !isExported(field) {
				continue
			}
			fieldName := field.Name
			fieldName = changeInitialCase(fieldName, unicode.ToLower)
			if v, ok := dstMap[fieldName]; !ok || (isEmptyValue(reflect.ValueOf(v)) || overwrite) {
				dstMap[fieldName] = src.Field(i).Interface()
			}
		}
	case reflect.Ptr:
		if dst.IsNil() {
			v := reflect.New(dst.Type().Elem())
			dst.Set(v)
		}
		dst = dst.Elem()
		fallthrough
	case reflect.Struct:
		srcMap := src.Interface().(map[string]interface{})
		for key := range srcMap {
			config.overwriteWithEmptyValue = true
			srcValue := srcMap[key]
			fieldName := changeInitialCase(key, unicode.ToUpper)
			dstElement := dst.FieldByName(fieldName)

We discard it because the field doesn't exist.

				continue
			}
			srcElement := reflect.ValueOf(srcValue)
			dstKind := dstElement.Kind()
			srcKind := srcElement.Kind()
			if srcKind == reflect.Ptr && dstKind != reflect.Ptr {
				srcElement = srcElement.Elem()
				srcKind = reflect.TypeOf(srcElement.Interface()).Kind()

Can this work? I guess it can't.

				if srcKind != reflect.Ptr && srcElement.CanAddr() {
					srcPtr := srcElement.Addr()
					srcElement = reflect.ValueOf(srcPtr)
					srcKind = reflect.Ptr
				}
			}

			if !srcElement.IsValid() {
				continue
			}
			if srcKind == dstKind {
				if _, err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
					return
				}
			} else if dstKind == reflect.Interface && dstElement.Kind() == reflect.Interface {
				if _, err = deepMerge(dstElement, srcElement, visited, depth+1, config); err != nil {
					return
				}
			} else if srcKind == reflect.Map {
				if err = deepMap(dstElement, srcElement, visited, depth+1, config); err != nil {
					return
				}
			} else {
				return fmt.Errorf("type mismatch on %s field: found %v, expected %v", fieldName, srcKind, dstKind)
			}
		}
	}
	return
}

Map sets fields' values in dst from src. src can be a map with string keys or a struct. dst must be the opposite: if src is a map, dst must be a valid pointer to struct. If src is a struct, dst must be map[string]interface{}. It won't merge unexported (private) fields and will do recursively any exported field. If dst is a map, keys will be src fields' names in lower camel case. Missing key in src that doesn't match a field in dst will be skipped. This doesn't apply if dst is a map. This is separated method from Merge because it is cleaner and it keeps sane semantics: merging equal types, mapping different (restricted) types.

func Map(dst, src interface{}, opts ...func(*Config)) error {
	return _map(dst, src, opts...)
}

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

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

func _map(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

To be friction-less, we redirect equal-type arguments to deepMerge. Only because arguments can be anything.

	if vSrc.Kind() == vDst.Kind() {
		_, err := deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, config)
		return err
	}
	switch vSrc.Kind() {
	case reflect.Struct:
		if vDst.Kind() != reflect.Map {
			return ErrExpectedMapAsDestination
		}
	case reflect.Map:
		if vDst.Kind() != reflect.Struct {
			return ErrExpectedStructAsDestination
		}
	default:
		return ErrNotSupported
	}
	return deepMap(vDst, vSrc, make(map[uintptr]*visit), 0, config)