vendor/github.com/imdario/mergo/merge.go

// Copyright 2013 Dario Castañé. All rights reserved.
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

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

package mergo

import (
	"reflect"
)

// 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(dst, src reflect.Value, visited map[uintptr]*visit, depth int, overwrite bool) (err error) {
	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
			}
		}
		// Remember, remember...
		visited[h] = &visit{addr, typ, seen}
	}
	switch dst.Kind() {
	case reflect.Struct:
		for i, n := 0, dst.NumField(); i < n; i++ {
			if err = deepMerge(dst.Field(i), src.Field(i), visited, depth+1, overwrite); err != nil {
				return
			}
		}
	case reflect.Map:
		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.Ptr, reflect.Interface, reflect.Slice:
				if srcElement.IsNil() {
					continue
				}
				fallthrough
			default:
				switch reflect.TypeOf(srcElement.Interface()).Kind() {
				case reflect.Struct:
					fallthrough
				case reflect.Ptr:
					fallthrough
				case reflect.Map:
					if err = deepMerge(dstElement, srcElement, visited, depth+1, overwrite); err != nil {
						return
					}
				}
			}
			if !isEmptyValue(srcElement) && (overwrite || (!dstElement.IsValid() || isEmptyValue(dst))) {
				if dst.IsNil() {
					dst.Set(reflect.MakeMap(dst.Type()))
				}
				dst.SetMapIndex(key, srcElement)
			}
		}
	case reflect.Ptr:
		fallthrough
	case reflect.Interface:
		if src.IsNil() {
			break
		} else if dst.IsNil() {
			if dst.CanSet() && (overwrite || isEmptyValue(dst)) {
				dst.Set(src)
			}
		} else if err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, overwrite); err != nil {
			return
		}
	default:
		if dst.CanSet() && !isEmptyValue(src) && (overwrite || isEmptyValue(dst)) {
			dst.Set(src)
		}
	}
	return
}

// Merge sets fields' values in dst from src if they have a zero
// value of their type.
// 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{}) error {
	return merge(dst, src, false)
}

func MergeWithOverwrite(dst, src interface{}) error {
	return merge(dst, src, true)
}

func merge(dst, src interface{}, overwrite bool) error {
	var (
		vDst, vSrc reflect.Value
		err        error
	)
	if vDst, vSrc, err = resolveValues(dst, src); err != nil {
		return err
	}
	if vDst.Type() != vSrc.Type() {
		return ErrDifferentArgumentsTypes
	}
	return deepMerge(vDst, vSrc, make(map[uintptr]*visit), 0, overwrite)
}