feat: support automatic camel case field key

Add camel case encoding option
Add camel case key conversion
Add camel case test

encode_test.go

@@ -2388,3 +2388,29 @@ func TestIssue324(t *testing.T) {
 		t.Fatalf("failed to encode. expected %q but got %q", expected, got)
 	}
 }
+
+func TestIssue271(t *testing.T) {
+	type T struct {
+		FieldA bool
+	}
+
+	type T2 struct {
+		FieldA bool `json:"fieldA"`
+	}
+
+	v := T{FieldA: true}
+	got, err := json.MarshalWithOption(v, json.EnableCamelCase())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	v2 := T2{FieldA: true}
+	expected, err := json.Marshal(v2)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !bytes.Equal(expected, got) {
+		t.Fatalf("failed to encode. expected %q but got %q", expected, got)
+	}
+}

internal/encoder/option.go

@@ -5,7 +5,7 @@ import (
 	"io"
 )
 
-type OptionFlag uint8
+type OptionFlag uint16
 
 const (
 	HTMLEscapeOption OptionFlag = 1 << iota
@@ -16,6 +16,7 @@ const (
 	ContextOption
 	NormalizeUTF8Option
 	FieldQueryOption
+	CamelCaseOption
 )
 
 type Option struct {

internal/encoder/vm/util.go

@@ -3,6 +3,7 @@ package vm
 import (
 	"encoding/json"
 	"fmt"
+	"unicode"
 	"unsafe"
 
 	"github.com/goccy/go-json/internal/encoder"
@@ -184,7 +185,17 @@ func appendStructHead(_ *encoder.RuntimeContext, b []byte) []byte {
 	return append(b, '{')
 }
 
-func appendStructKey(_ *encoder.RuntimeContext, code *encoder.Opcode, b []byte) []byte {
+func appendStructKey(e *encoder.RuntimeContext, code *encoder.Opcode, b []byte) []byte {
+	if e.Option.Flag&encoder.CamelCaseOption > 0 {
+		key := []rune(code.Key)
+		for i := range key {
+			if unicode.IsLetter(key[i]) {
+				key[i] = unicode.ToLower(key[i])
+				break
+			}
+		}
+		return append(b, string(key)...)
+	}
 	return append(b, code.Key...)
 }
 

option.go

@@ -24,6 +24,13 @@ func DisableHTMLEscape() EncodeOptionFunc {
 	}
 }
 
+// EnableCamelCase convert the keys to camel case when encoding a struct.
+func EnableCamelCase() EncodeOptionFunc {
+	return func(opt *EncodeOption) {
+		opt.Flag |= encoder.CamelCaseOption
+	}
+}
+
 // DisableNormalizeUTF8
 // By default, when encoding string, UTF8 characters in the range of 0x80 - 0xFF are processed by applying \ufffd for invalid code and escaping for \u2028 and \u2029.
 // This option disables this behaviour. You can expect faster speeds by applying this option, but be careful.