Add new WithSliceDeepMerge config option

merge.go

@@ -45,6 +45,7 @@ type Config struct {
 	overwriteWithEmptyValue      bool
 	overwriteSliceWithEmptyValue bool
 	sliceDeepCopy                bool
+	sliceDeepMerge               bool
 	debug                        bool
 }
 
@@ -61,6 +62,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 	overwriteWithEmptySrc := config.overwriteWithEmptyValue
 	overwriteSliceWithEmptySrc := config.overwriteSliceWithEmptyValue
 	sliceDeepCopy := config.sliceDeepCopy
+	sliceDeepMerge := config.sliceDeepMerge
 
 	if !src.IsValid() {
 		return
@@ -190,6 +192,39 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 							}
 						}
 
+					} else if sliceDeepMerge {
+						if srcSlice.Len() > dstSlice.Len() {
+							newSlice := reflect.MakeSlice(srcSlice.Type(), srcSlice.Len(), srcSlice.Len())
+
+							for i := 0; i < dstSlice.Len(); i++ {
+								newSlice.Index(i).Set(dstSlice.Index(i))
+							}
+
+							dstSlice = newSlice
+						}
+
+						for i := 0; i < srcSlice.Len(); i++ {
+							srcElem := srcSlice.Index(i)
+							dstElem := dstSlice.Index(i)
+
+							if srcElem.CanInterface() {
+								srcElem = reflect.ValueOf(srcElem.Interface())
+							}
+
+							if dstElem.CanInterface() {
+								dstElem = reflect.ValueOf(dstElem.Interface())
+							}
+
+							if dstSlice.Index(i).IsZero() {
+								dstSlice.Index(i).Set(srcElem)
+							} else {
+								if err = deepMerge(dstElem, srcElem, visited, depth+1, config); err != nil {
+									return
+								} else {
+									dstSlice.Index(i).Set(dstElem)
+								}
+							}
+						}
 					}
 					dst.SetMapIndex(key, dstSlice)
 				}
@@ -231,6 +266,39 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 					return
 				}
 			}
+		} else if sliceDeepMerge {
+			if src.Len() > dst.Len() {
+				newSlice := reflect.MakeSlice(dst.Type(), src.Len(), src.Len())
+
+				for i := 0; i < dst.Len(); i++ {
+					newSlice.Index(i).Set(dst.Index(i))
+				}
+
+				dst = newSlice
+			}
+
+			for i := 0; i < src.Len(); i++ {
+				srcElem := src.Index(i)
+				dstElem := dst.Index(i)
+
+				if srcElem.CanInterface() {
+					srcElem = reflect.ValueOf(srcElem.Interface())
+				}
+
+				if dstElem.CanInterface() {
+					dstElem = reflect.ValueOf(dstElem.Interface())
+				}
+
+				if dst.Index(i).IsZero() {
+					dst.Index(i).Set(srcElem)
+				} else {
+					if err = deepMerge(dstElem, srcElem, visited, depth+1, config); err != nil {
+						return
+					} else {
+						dst.Index(i).Set(dstElem)
+					}
+				}
+			}
 		}
 	case reflect.Ptr:
 		fallthrough
@@ -342,6 +410,11 @@ func WithSliceDeepCopy(config *Config) {
 	config.Overwrite = true
 }
 
+// WithSliceDeepMerge will make merge deep merge slice elements pairwise (resizing dst slice if needed)
+func WithSliceDeepMerge(config *Config) {
+	config.sliceDeepMerge = true
+}
+
 func merge(dst, src interface{}, opts ...func(*Config)) error {
 	if dst != nil && reflect.ValueOf(dst).Kind() != reflect.Ptr {
 		return ErrNonPointerAgument

pr180_test.go

@@ -0,0 +1,54 @@
+package mergo
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+)
+
+func pp(i interface{}) string {
+	b, _ := json.MarshalIndent(i, "", "    ")
+	return string(b)
+}
+
+func TestIssue121WithSliceDeepMerge(t *testing.T) {
+	dst := map[string]interface{}{
+		"a": "1",
+		"b": []map[string]interface{}{
+			map[string]interface{}{"c": "2"},
+		},
+	}
+
+	src := map[string]interface{}{
+		"b": []map[string]interface{}{
+			map[string]interface{}{"c": "3", "d": "1"},
+			map[string]interface{}{"e": "1", "f": "1", "g": []string{"1", "2"}},
+		},
+	}
+
+	if err := Merge(&dst, src, WithSliceDeepMerge); err != nil {
+		t.Fatalf("Error during the merge: %v", err)
+	}
+
+	fmt.Println(pp(dst))
+
+	if dst["a"].(string) != "1" {
+		t.Error("a should equal '1'")
+	}
+
+	if dst["b"].([]map[string]interface{})[0]["c"] != "2" {
+		t.Error("b.[0].c should equal '2'")
+	}
+
+	if dst["b"].([]map[string]interface{})[0]["d"] != "1" {
+		t.Error("b.[0].d should equal '2'")
+	}
+
+	if dst["b"].([]map[string]interface{})[1]["e"] != "1" {
+		t.Error("b.[1].e should equal '1'")
+	}
+
+	if dst["b"].([]map[string]interface{})[1]["g"].([]string)[0] != "1" {
+		t.Error("b.[1].g[0] should equal '1'")
+	}
+}