Allow empty maps for required map fields (issue #256)

As reported in issue #256, currently, an empty map is unmarshalled into nil by easyjson
If a field is marked "required" or "!omitempty", then the empty map is a valid input and
we should unmarshal it into an empty map.

Similar logic for marshalling.

This change is to fix the behavior

.gitignore

@@ -3,3 +3,4 @@
 *.iml
 .idea
 *.swp
+bin/*

gen/decoder.go

@@ -228,16 +228,21 @@ func (g *Generator) genTypeDecoderNoCheck(t reflect.Type, out string, tags field
 		} // else assume the caller knows what they are doing and that the custom unmarshaler performs the translation from string or integer keys to the key type
 		elem := t.Elem()
 		tmpVar := g.uniqueVarName()
+		keepEmpty := tags.required || tags.noOmitEmpty || (!g.omitEmpty && !tags.omitEmpty)
 
 		fmt.Fprintln(g.out, ws+"if in.IsNull() {")
 		fmt.Fprintln(g.out, ws+"  in.Skip()")
 		fmt.Fprintln(g.out, ws+"} else {")
 		fmt.Fprintln(g.out, ws+"  in.Delim('{')")
-		fmt.Fprintln(g.out, ws+"  if !in.IsDelim('}') {")
+		if !keepEmpty {
+			fmt.Fprintln(g.out, ws+"  if !in.IsDelim('}') {")
+		}
 		fmt.Fprintln(g.out, ws+"  "+out+" = make("+g.getType(t)+")")
-		fmt.Fprintln(g.out, ws+"  } else {")
-		fmt.Fprintln(g.out, ws+"  "+out+" = nil")
-		fmt.Fprintln(g.out, ws+"  }")
+		if !keepEmpty {
+			fmt.Fprintln(g.out, ws+"  } else {")
+			fmt.Fprintln(g.out, ws+"  "+out+" = nil")
+			fmt.Fprintln(g.out, ws+"  }")
+		}
 
 		fmt.Fprintln(g.out, ws+"  for !in.IsDelim('}') {")
 		// NOTE: extra check for TextUnmarshaler. It overrides default methods.

tests/data.go

@@ -678,6 +678,12 @@ type RequiredOptionalStruct struct {
 	Lastname  string `json:"last_name"`
 }
 
+type RequiredOptionalMap struct {
+	ReqMap         map[int]string `json:"req_map,required"`
+	OmitEmptyMap   map[int]string `json:"oe_map,omitempty"`
+	NoOmitEmptyMap map[int]string `json:"noe_map,!omitempty"`
+}
+
 //easyjson:json
 type EncodingFlagsTestMap struct {
 	F map[string]string

tests/required_test.go

@@ -2,6 +2,7 @@ package tests
 
 import (
 	"fmt"
+	"reflect"
 	"testing"
 )
 
@@ -26,3 +27,25 @@ func TestRequiredField(t *testing.T) {
 		}
 	}
 }
+
+func TestRequiredOptionalMap(t *testing.T) {
+	baseJson := `{"req_map":{}, "oe_map":{}, "noe_map":{}, "oe_slice":[]}`
+	wantDecoding := RequiredOptionalMap{MapIntString{}, nil, MapIntString{}}
+
+	var v RequiredOptionalMap
+	if err := v.UnmarshalJSON([]byte(baseJson)); err != nil {
+		t.Errorf("%s. UnmarshalJSON didn't expect error: %v", baseJson, err)
+	}
+	if !reflect.DeepEqual(v, wantDecoding) {
+		t.Errorf("%s. UnmarshalJSON expected to gen: %v. got: %v", baseJson, wantDecoding, v)
+	}
+
+	baseStruct := RequiredOptionalMap{MapIntString{}, MapIntString{}, MapIntString{}}
+	wantJson := `{"req_map":{},"noe_map":{}}`
+	data, err := baseStruct.MarshalJSON()
+	if err != nil {
+		t.Errorf("MarshalJSON didn't expect error: %v on %v", err, data)
+	} else if string(data) != wantJson {
+		t.Errorf("%v. MarshalJSON wanted: %s got %s", baseStruct, wantJson, string(data))
+	}
+}