From d581347ed06695b1d752805d8e20181063fa345c Mon Sep 17 00:00:00 2001
From: requilence <requilence@gmail.com>
Date: Fri, 15 Jun 2018 13:52:26 +0200
Subject: [PATCH] fix #84

Allow to overwrite struct field with empty value in case corresponding source map's key exists and indicated to zero-value
---
 map.go   |  1 +
 merge.go | 17 ++++++++++-------
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/map.go b/map.go
index 6ea38e6..3f5afa8 100644
--- a/map.go
+++ b/map.go
@@ -72,6 +72,7 @@ func deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, conf
 	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)
diff --git a/merge.go b/merge.go
index 706b220..c84da07 100644
--- a/merge.go
+++ b/merge.go
@@ -25,9 +25,10 @@ func hasExportedField(dst reflect.Value) (exported bool) {
 }
 
 type Config struct {
-	Overwrite    bool
-	AppendSlice  bool
-	Transformers Transformers
+	Overwrite               bool
+	AppendSlice             bool
+	Transformers            Transformers
+	overwriteWithEmptyValue bool
 }
 
 type Transformers interface {
@@ -39,6 +40,8 @@ type Transformers interface {
 // short circuiting on recursive types.
 func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, config *Config) (err error) {
 	overwrite := config.Overwrite
+	overwriteWithEmptySrc := config.overwriteWithEmptyValue
+	config.overwriteWithEmptyValue = false
 
 	if !src.IsValid() {
 		return
@@ -73,7 +76,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 				}
 			}
 		} else {
-			if dst.CanSet() && !isEmptyValue(src) && (overwrite || isEmptyValue(dst)) {
+			if dst.CanSet() && (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) {
 				dst.Set(src)
 			}
 		}
@@ -124,7 +127,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 						dstSlice = reflect.ValueOf(dstElement.Interface())
 					}
 
-					if !isEmptyValue(src) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
+					if (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
 						dstSlice = srcSlice
 					} else if config.AppendSlice {
 						dstSlice = reflect.AppendSlice(dstSlice, srcSlice)
@@ -147,7 +150,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		if !dst.CanSet() {
 			break
 		}
-		if !isEmptyValue(src) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
+		if (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) && !config.AppendSlice {
 			dst.Set(src)
 		} else if config.AppendSlice {
 			dst.Set(reflect.AppendSlice(dst, src))
@@ -184,7 +187,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 			return
 		}
 	default:
-		if dst.CanSet() && !isEmptyValue(src) && (overwrite || isEmptyValue(dst)) {
+		if dst.CanSet() && (!isEmptyValue(src) || overwriteWithEmptySrc) && (overwrite || isEmptyValue(dst)) {
 			dst.Set(src)
 		}
 	}