misc fixes

utfbom.go

@@ -13,14 +13,7 @@ import (
 	"sync"
 )
 
-var (
-	_          io.Reader = (*Reader)(nil)
-	utf8BOM              = []byte{0xef, 0xbb, 0xbf}
-	utf16BEBOM           = []byte{0xfe, 0xff}
-	utf16LEBOM           = []byte{0xff, 0xfe}
-	utf32BEBOM           = []byte{0x00, 0x00, 0xfe, 0xff}
-	utf32LEBOM           = []byte{0xff, 0xfe, 0x00, 0x00}
-)
+var _ io.Reader = (*Reader)(nil)
 
 // ErrRead helps to trace error origin.
 var ErrRead = errors.New("utfbom: I/O error during BOM processing")
@@ -64,32 +57,32 @@ const (
 //   - UTF-16 Little Endian (BOM: 0xff 0xfe)
 //   - UTF-32 Big Endian (BOM: 0x00 0x00 0xfe 0xff)
 //   - UTF-32 Little Endian (BOM: 0xff 0xfe 0x00 0x00)
-func DetectEncoding[T string | []byte](input T) Encoding {
-	ibs := []byte(input)
+func DetectEncoding[T ~string | ~[]byte](input T) Encoding {
+	b := []byte(input)
 
-	if len(ibs) < 2 {
+	if len(b) < 2 {
 		return Unknown
 	}
 
-	if len(ibs) >= 3 && bytes.HasPrefix(ibs, utf8BOM) {
+	if len(b) >= 3 && bytes.HasPrefix(b, []byte{0xef, 0xbb, 0xbf}) {
 		return UTF8
 	}
 
-	if len(ibs) >= 4 {
-		if bytes.HasPrefix(ibs, utf32BEBOM) {
+	if len(b) >= 4 {
+		if bytes.HasPrefix(b, []byte{0x00, 0x00, 0xfe, 0xff}) {
 			return UTF32BigEndian
 		}
 
-		if bytes.HasPrefix(ibs, utf32LEBOM) {
+		if bytes.HasPrefix(b, []byte{0xff, 0xfe, 0x00, 0x00}) {
 			return UTF32LittleEndian
 		}
 	}
 
-	if bytes.HasPrefix(ibs, utf16BEBOM) {
+	if bytes.HasPrefix(b, []byte{0xfe, 0xff}) {
 		return UTF16BigEndian
 	}
 
-	if bytes.HasPrefix(ibs, utf16LEBOM) {
+	if bytes.HasPrefix(b, []byte{0xff, 0xfe}) {
 		return UTF16LittleEndian
 	}
 
@@ -108,7 +101,7 @@ func (e Encoding) AnyOf(es ...Encoding) bool {
 	return false
 }
 
-// Strings returns human-readable name of encoding.
+// String returns the human-readable name of the encoding.
 func (e Encoding) String() string {
 	switch e {
 	case UTF8:
@@ -146,21 +139,21 @@ func (e Encoding) Bytes() []byte {
 	default:
 		return nil
 	case UTF8:
-		return utf8BOM
+		return []byte{0xef, 0xbb, 0xbf}
 	case UTF16BigEndian:
-		return utf16BEBOM
+		return []byte{0xfe, 0xff}
 	case UTF16LittleEndian:
-		return utf16LEBOM
+		return []byte{0xff, 0xfe}
 	case UTF32BigEndian:
-		return utf32BEBOM
+		return []byte{0x00, 0x00, 0xfe, 0xff}
 	case UTF32LittleEndian:
-		return utf32LEBOM
+		return []byte{0xff, 0xfe, 0x00, 0x00}
 	}
 }
 
-// Trim removes the BOM prefix from the input `s` based on the encoding `enc`.
+// Trim removes the BOM prefix from the input.
 // Supports string or []byte inputs and returns the same type without the BOM.
-func Trim[T string | []byte](input T) (T, Encoding) {
+func Trim[T ~string | ~[]byte](input T) (T, Encoding) {
 	b := []byte(input)
 	enc := DetectEncoding(b)
 
@@ -174,7 +167,7 @@ func Trim[T string | []byte](input T) (T, Encoding) {
 // Prepend adds the corresponding Byte Order Mark (BOM) for a given encoding
 // to the beginning of a string or byte slice.
 // If the provided encoding is Unknown, the input is returned unmodified.
-func Prepend[T string | []byte](input T, enc Encoding) T {
+func Prepend[T ~string | ~[]byte](input T, enc Encoding) T {
 	if enc == Unknown {
 		return input
 	}
@@ -190,6 +183,8 @@ func Prepend[T string | []byte](input T, enc Encoding) T {
 
 // Reader implements automatic BOM (Unicode Byte Order Mark) checking and
 // removing as necessary for an io.Reader object.
+//
+// Reader is not safe for concurrent use.
 type Reader struct {
 	rd   *bufio.Reader
 	once sync.Once
@@ -198,6 +193,7 @@ type Reader struct {
 }
 
 // NewReader wraps an incoming reader.
+// Passing a nil reader will cause a panic on the first Read call.
 func NewReader(rd io.Reader) *Reader {
 	return &Reader{
 		rd:   bufio.NewReader(rd),
@@ -207,10 +203,8 @@ func NewReader(rd io.Reader) *Reader {
 }
 
 // Read implements the io.Reader interface.
-// On the first read call, it reads from the underlying Reader, detects and removes any Byte Order Mark (BOM).
-// Subsequent calls delegate directly to the underlying Reader without BOM handling.
-// Read is only safe for concurrent use during the first call due to sync.Once; after that, thread-safety
-// depends on the underlying Reader. It is best to assume unsafe concurrent use.
+// On the first call, it detects and removes any Byte Order Mark (BOM).
+// Subsequent calls delegate directly to the underlying Reader.
 func (r *Reader) Read(buf []byte) (int, error) {
 	const maxBOMLen = 4
 

utfbom_test.go

@@ -443,6 +443,34 @@ func TestEncoding_Bytes(t *testing.T) {
 	}
 }
 
+// TestBytes_NoAliasing checks that bytes returned by Bytes() are immutable.
+func TestBytes_NoAliasing(t *testing.T) {
+	t.Parallel()
+
+	encodings := []utfbom.Encoding{
+		utfbom.UTF8,
+		utfbom.UTF16BigEndian,
+		utfbom.UTF16LittleEndian,
+		utfbom.UTF32BigEndian,
+		utfbom.UTF32LittleEndian,
+	}
+
+	for _, enc := range encodings {
+		t.Run(enc.String(), func(t *testing.T) {
+			t.Parallel()
+
+			original := enc.Bytes()
+			originalCopy := make([]byte, len(original))
+			copy(originalCopy, original)
+
+			original[0] = 0x00
+
+			fresh := enc.Bytes()
+			be.Equal(t, fresh, originalCopy)
+		})
+	}
+}
+
 func TestPrepend(t *testing.T) {
 	t.Parallel()
 
@@ -487,3 +515,71 @@ func TestPrepend(t *testing.T) {
 		}
 	})
 }
+
+type CustomString string
+
+type CustomBytes []byte
+
+func TestDetectEncoding_TypeAliases(t *testing.T) {
+	t.Parallel()
+
+	t.Run("custom_string", func(t *testing.T) {
+		input := CustomString("\ufeffhello")
+		enc := utfbom.DetectEncoding(input)
+		be.Equal(t, enc, utfbom.UTF8)
+	})
+
+	t.Run("custom_bytes", func(t *testing.T) {
+		input := CustomBytes([]byte{0xfe, 0xff, 'h', 'i'})
+		enc := utfbom.DetectEncoding(input)
+		be.Equal(t, enc, utfbom.UTF16BigEndian)
+	})
+}
+
+func TestTrim_TypeAliases(t *testing.T) {
+	t.Parallel()
+
+	t.Run("custom_string", func(t *testing.T) {
+		input := CustomString("\ufeffhello")
+		out, enc := utfbom.Trim(input)
+		be.Equal(t, enc, utfbom.UTF8)
+		be.Equal(t, out, CustomString("hello"))
+	})
+
+	t.Run("custom_bytes", func(t *testing.T) {
+		input := CustomBytes([]byte{0xfe, 0xff, 'h', 'i'})
+		out, enc := utfbom.Trim(input)
+		be.Equal(t, enc, utfbom.UTF16BigEndian)
+		be.Equal(t, out, CustomBytes([]byte{'h', 'i'}))
+	})
+}
+
+func TestPrepend_TypeAliases(t *testing.T) {
+	t.Parallel()
+
+	t.Run("custom_string", func(t *testing.T) {
+		input := CustomString("hello")
+		out := utfbom.Prepend(input, utfbom.UTF8)
+		be.Equal(t, out, CustomString("\ufeffhello"))
+	})
+
+	t.Run("custom_bytes", func(t *testing.T) {
+		input := CustomBytes([]byte{'h', 'i'})
+		out := utfbom.Prepend(input, utfbom.UTF16BigEndian)
+		be.Equal(t, out, CustomBytes([]byte{0xfe, 0xff, 'h', 'i'}))
+	})
+}
+
+func TestNewReader_NilPanics(t *testing.T) {
+	t.Parallel()
+
+	rd := utfbom.NewReader(nil)
+
+	defer func() {
+		r := recover()
+		be.True(t, r != nil)
+	}()
+
+	buf := make([]byte, 10)
+	_, _ = rd.Read(buf)
+}