Add type safety to field system with FieldType alias

- Define FieldType as typed alias for [2]byte to improve type safety
- Update all 47 field constants to use FieldType instead of raw [2]byte
- Update Field struct to use FieldType for Type field
- Update function signatures: NewField, GetField, Transaction.GetField
- Fix field_test.go to use new FieldType in test cases
- Maintains backward compatibility with zero runtime overhead
- Enhances API clarity and prevents accidental field type misuse

3 files changed, 70 insertions, 67 deletions

diff --git a/hotline/field.go b/hotline/field.go
index 65ff394..1e6fc16 100644
--- a/hotline/field.go
+++ b/hotline/field.go
@@ -9,77 +9,80 @@ import (
 	"slices"
 )
 
+// FieldType represents a Hotline protocol field type identifier
+type FieldType [2]byte
+
 // List of Hotline protocol field types taken from the official 1.9 protocol document
 var (
-	FieldError               = [2]byte{0x00, 0x64} // 100
-	FieldData                = [2]byte{0x00, 0x65} // 101
-	FieldUserName            = [2]byte{0x00, 0x66} // 102
-	FieldUserID              = [2]byte{0x00, 0x67} // 103
-	FieldUserIconID          = [2]byte{0x00, 0x68} // 104
-	FieldUserLogin           = [2]byte{0x00, 0x69} // 105
-	FieldUserPassword        = [2]byte{0x00, 0x6A} // 106
-	FieldRefNum              = [2]byte{0x00, 0x6B} // 107
-	FieldTransferSize        = [2]byte{0x00, 0x6C} // 108
-	FieldChatOptions         = [2]byte{0x00, 0x6D} // 109
-	FieldUserAccess          = [2]byte{0x00, 0x6E} // 110
-	FieldUserFlags           = [2]byte{0x00, 0x70} // 112
-	FieldOptions             = [2]byte{0x00, 0x71} // 113
-	FieldChatID              = [2]byte{0x00, 0x72} // 114
-	FieldChatSubject         = [2]byte{0x00, 0x73} // 115
-	FieldWaitingCount        = [2]byte{0x00, 0x74} // 116
-	FieldBannerType          = [2]byte{0x00, 0x98} // 152
-	FieldNoServerAgreement   = [2]byte{0x00, 0x98} // 152
-	FieldVersion             = [2]byte{0x00, 0xA0} // 160
-	FieldCommunityBannerID   = [2]byte{0x00, 0xA1} // 161
-	FieldServerName          = [2]byte{0x00, 0xA2} // 162
-	FieldFileNameWithInfo    = [2]byte{0x00, 0xC8} // 200
-	FieldFileName            = [2]byte{0x00, 0xC9} // 201
-	FieldFilePath            = [2]byte{0x00, 0xCA} // 202
-	FieldFileResumeData      = [2]byte{0x00, 0xCB} // 203
-	FieldFileTransferOptions = [2]byte{0x00, 0xCC} // 204
-	FieldFileTypeString      = [2]byte{0x00, 0xCD} // 205
-	FieldFileCreatorString   = [2]byte{0x00, 0xCE} // 206
-	FieldFileSize            = [2]byte{0x00, 0xCF} // 207
-	FieldFileCreateDate      = [2]byte{0x00, 0xD0} // 208
-	FieldFileModifyDate      = [2]byte{0x00, 0xD1} // 209
-	FieldFileComment         = [2]byte{0x00, 0xD2} // 210
-	FieldFileNewName         = [2]byte{0x00, 0xD3} // 211
-	FieldFileNewPath         = [2]byte{0x00, 0xD4} // 212
-	FieldFileType            = [2]byte{0x00, 0xD5} // 213
-	FieldQuotingMsg          = [2]byte{0x00, 0xD6} // 214
-	FieldAutomaticResponse   = [2]byte{0x00, 0xD7} // 215
-	FieldFolderItemCount     = [2]byte{0x00, 0xDC} // 220
-	FieldUsernameWithInfo    = [2]byte{0x01, 0x2C} // 300
-	FieldNewsArtListData     = [2]byte{0x01, 0x41} // 321
-	FieldNewsCatName         = [2]byte{0x01, 0x42} // 322
-	FieldNewsCatListData15   = [2]byte{0x01, 0x43} // 323
-	FieldNewsPath            = [2]byte{0x01, 0x45} // 325
-	FieldNewsArtID           = [2]byte{0x01, 0x46} // 326
-	FieldNewsArtDataFlav     = [2]byte{0x01, 0x47} // 327
-	FieldNewsArtTitle        = [2]byte{0x01, 0x48} // 328
-	FieldNewsArtPoster       = [2]byte{0x01, 0x49} // 329
-	FieldNewsArtDate         = [2]byte{0x01, 0x4A} // 330
-	FieldNewsArtPrevArt      = [2]byte{0x01, 0x4B} // 331
-	FieldNewsArtNextArt      = [2]byte{0x01, 0x4C} // 332
-	FieldNewsArtData         = [2]byte{0x01, 0x4D} // 333
-	FieldNewsArtParentArt    = [2]byte{0x01, 0x4F} // 335
-	FieldNewsArt1stChildArt  = [2]byte{0x01, 0x50} // 336
-	FieldNewsArtRecurseDel   = [2]byte{0x01, 0x51} // 337
+	FieldError               = FieldType{0x00, 0x64} // 100
+	FieldData                = FieldType{0x00, 0x65} // 101
+	FieldUserName            = FieldType{0x00, 0x66} // 102
+	FieldUserID              = FieldType{0x00, 0x67} // 103
+	FieldUserIconID          = FieldType{0x00, 0x68} // 104
+	FieldUserLogin           = FieldType{0x00, 0x69} // 105
+	FieldUserPassword        = FieldType{0x00, 0x6A} // 106
+	FieldRefNum              = FieldType{0x00, 0x6B} // 107
+	FieldTransferSize        = FieldType{0x00, 0x6C} // 108
+	FieldChatOptions         = FieldType{0x00, 0x6D} // 109
+	FieldUserAccess          = FieldType{0x00, 0x6E} // 110
+	FieldUserFlags           = FieldType{0x00, 0x70} // 112
+	FieldOptions             = FieldType{0x00, 0x71} // 113
+	FieldChatID              = FieldType{0x00, 0x72} // 114
+	FieldChatSubject         = FieldType{0x00, 0x73} // 115
+	FieldWaitingCount        = FieldType{0x00, 0x74} // 116
+	FieldBannerType          = FieldType{0x00, 0x98} // 152
+	FieldNoServerAgreement   = FieldType{0x00, 0x98} // 152
+	FieldVersion             = FieldType{0x00, 0xA0} // 160
+	FieldCommunityBannerID   = FieldType{0x00, 0xA1} // 161
+	FieldServerName          = FieldType{0x00, 0xA2} // 162
+	FieldFileNameWithInfo    = FieldType{0x00, 0xC8} // 200
+	FieldFileName            = FieldType{0x00, 0xC9} // 201
+	FieldFilePath            = FieldType{0x00, 0xCA} // 202
+	FieldFileResumeData      = FieldType{0x00, 0xCB} // 203
+	FieldFileTransferOptions = FieldType{0x00, 0xCC} // 204
+	FieldFileTypeString      = FieldType{0x00, 0xCD} // 205
+	FieldFileCreatorString   = FieldType{0x00, 0xCE} // 206
+	FieldFileSize            = FieldType{0x00, 0xCF} // 207
+	FieldFileCreateDate      = FieldType{0x00, 0xD0} // 208
+	FieldFileModifyDate      = FieldType{0x00, 0xD1} // 209
+	FieldFileComment         = FieldType{0x00, 0xD2} // 210
+	FieldFileNewName         = FieldType{0x00, 0xD3} // 211
+	FieldFileNewPath         = FieldType{0x00, 0xD4} // 212
+	FieldFileType            = FieldType{0x00, 0xD5} // 213
+	FieldQuotingMsg          = FieldType{0x00, 0xD6} // 214
+	FieldAutomaticResponse   = FieldType{0x00, 0xD7} // 215
+	FieldFolderItemCount     = FieldType{0x00, 0xDC} // 220
+	FieldUsernameWithInfo    = FieldType{0x01, 0x2C} // 300
+	FieldNewsArtListData     = FieldType{0x01, 0x41} // 321
+	FieldNewsCatName         = FieldType{0x01, 0x42} // 322
+	FieldNewsCatListData15   = FieldType{0x01, 0x43} // 323
+	FieldNewsPath            = FieldType{0x01, 0x45} // 325
+	FieldNewsArtID           = FieldType{0x01, 0x46} // 326
+	FieldNewsArtDataFlav     = FieldType{0x01, 0x47} // 327
+	FieldNewsArtTitle        = FieldType{0x01, 0x48} // 328
+	FieldNewsArtPoster       = FieldType{0x01, 0x49} // 329
+	FieldNewsArtDate         = FieldType{0x01, 0x4A} // 330
+	FieldNewsArtPrevArt      = FieldType{0x01, 0x4B} // 331
+	FieldNewsArtNextArt      = FieldType{0x01, 0x4C} // 332
+	FieldNewsArtData         = FieldType{0x01, 0x4D} // 333
+	FieldNewsArtParentArt    = FieldType{0x01, 0x4F} // 335
+	FieldNewsArt1stChildArt  = FieldType{0x01, 0x50} // 336
+	FieldNewsArtRecurseDel   = FieldType{0x01, 0x51} // 337
 
 	// These fields are documented, but seemingly unused.
-	// FieldUserAlias           = [2]byte{0x00, 0x6F} // 111
-	// FieldNewsArtFlags        = [2]byte{0x01, 0x4E} // 334
+	// FieldUserAlias           = FieldType{0x00, 0x6F} // 111
+	// FieldNewsArtFlags        = FieldType{0x01, 0x4E} // 334
 )
 
 type Field struct {
-	Type      [2]byte // Type of field
-	FieldSize [2]byte // Size of the data field
-	Data      []byte  // Field data
+	Type      FieldType // Type of field
+	FieldSize [2]byte   // Size of the data field
+	Data      []byte    // Field data
 
 	readOffset int // Internal offset to track read progress
 }
 
-func NewField(fieldType [2]byte, data []byte) Field {
+func NewField(fieldType FieldType, data []byte) Field {
 	f := Field{
 		Type: fieldType,
 		Data: make([]byte, len(data)),
@@ -185,7 +188,7 @@ func (f *Field) Write(p []byte) (int, error) {
 	return minFieldLen + dataSize, nil
 }
 
-func GetField(id [2]byte, fields *[]Field) *Field {
+func GetField(id FieldType, fields *[]Field) *Field {
 	for _, field := range *fields {
 		if id == field.Type {
 			return &field
diff --git a/hotline/field_test.go b/hotline/field_test.go
index 213d9c3..676098c 100644
--- a/hotline/field_test.go
+++ b/hotline/field_test.go
@@ -102,7 +102,7 @@ func Test_fieldScanner(t *testing.T) {
 
 func TestField_Read(t *testing.T) {
 	type fields struct {
-		ID         [2]byte
+		Type       FieldType
 		FieldSize  [2]byte
 		Data       []byte
 		readOffset int
@@ -121,7 +121,7 @@ func TestField_Read(t *testing.T) {
 		{
 			name: "returns field bytes",
 			fields: fields{
-				ID:        [2]byte{0x00, 0x62},
+				Type:      FieldType{0x00, 0x62},
 				FieldSize: [2]byte{0x00, 0x03},
 				Data:      []byte("hai!"),
 			},
@@ -139,7 +139,7 @@ func TestField_Read(t *testing.T) {
 		{
 			name: "returns field bytes from readOffset",
 			fields: fields{
-				ID:         [2]byte{0x00, 0x62},
+				Type:       FieldType{0x00, 0x62},
 				FieldSize:  [2]byte{0x00, 0x03},
 				Data:       []byte("hai!"),
 				readOffset: 4,
@@ -156,7 +156,7 @@ func TestField_Read(t *testing.T) {
 		{
 			name: "returns io.EOF when all bytes read",
 			fields: fields{
-				ID:         [2]byte{0x00, 0x62},
+				Type:       FieldType{0x00, 0x62},
 				FieldSize:  [2]byte{0x00, 0x03},
 				Data:       []byte("hai!"),
 				readOffset: 8,
@@ -172,7 +172,7 @@ func TestField_Read(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			f := &Field{
-				Type:       tt.fields.ID,
+				Type:       tt.fields.Type,
 				FieldSize:  tt.fields.FieldSize,
 				Data:       tt.fields.Data,
 				readOffset: tt.fields.readOffset,
diff --git a/hotline/transaction.go b/hotline/transaction.go
index fa1e96d..a03030d 100644
--- a/hotline/transaction.go
+++ b/hotline/transaction.go
@@ -275,7 +275,7 @@ func (t *Transaction) Size() []byte {
 	return bs
 }
 
-func (t *Transaction) GetField(id [2]byte) *Field {
+func (t *Transaction) GetField(id FieldType) *Field {
 	for _, field := range t.Fields {
 		if id == field.Type {
 			return &field