fix: Supports bufio use also for strings larger than 4096 bytes.

fix lint error

main.go

@@ -32,15 +32,25 @@ func (t *replacer) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err e
 	if len(_src) == 0 && atEOF {
 		return
 	}
-	if !utf8.Valid(_src) {
-		// If not a string, do not process
-		err = ErrInvalidUTF8
-		return
-	}
 
 	for len(_src) > 0 {
-		_, n := utf8.DecodeRune(_src)
-		buf := _src[:n]
+		r, size := utf8.DecodeRune(_src)
+		if r < utf8.RuneSelf {
+			size = 1
+		} else if size == 1 {
+			// All valid runes of size 1 (those below utf8.RuneSelf) were
+			// handled above. We have invalid UTF-8, or we haven't seen the
+			// full character yet.
+			if !atEOF && !utf8.FullRune(_src) {
+				err = transform.ErrShortSrc
+				break
+			}
+			//　If the last string cannot be converted to rune, it is not replaced.
+			if atEOF && !utf8.FullRune(_src) {
+				break
+			}
+		}
+		buf := _src[:size]
 		if _, encErr := t.enc.Bytes(buf); encErr != nil {
 			// Replace strings that cannot be converted
 			buf = []byte(string(t.replaceRune))
@@ -54,9 +64,9 @@ func (t *replacer) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err e
 		if dstN <= 0 {
 			break
 		}
-		nSrc += n
+		nSrc += size
 		nDst += dstN
-		_src = _src[n:]
+		_src = _src[size:]
 	}
 	return
 }

main_test.go

@@ -1,6 +1,7 @@
 package garbledreplacer_test
 
 import (
+	"bufio"
 	"bytes"
 	"errors"
 	"github.com/tomtwinkle/garbledreplacer"
@@ -46,10 +47,10 @@ func TestNewTransformer(t *testing.T) {
 			want:     "?",
 		},
 		"UTF-8->ShiftJIS:Invalid UTF-8 character": {
-			encoding:  japanese.ShiftJIS,
-			in:        "\xe4",
-			replace:   '?',
-			wantError: garbledreplacer.ErrInvalidUTF8,
+			encoding: japanese.ShiftJIS,
+			in:       "\xe4",
+			replace:  '?',
+			want:     "",
 		},
 		"UTF-8->EUCJP:with garbled text": {
 			encoding: japanese.EUCJP,
@@ -63,40 +64,77 @@ func TestNewTransformer(t *testing.T) {
 			replace:  '?',
 			want:     strings.Repeat("咖呸咕咀呻?呷咄咒咆呼咐?呱呶和咚呢", 3000),
 		},
+		"UTF-8->ShiftJIS:with garbled text:larger than 4096bytes": {
+			encoding: japanese.ShiftJIS,
+			in:       strings.Repeat("一二三四🍣五六七八九🍺十拾壱", 4000),
+			replace:  '?',
+			want:     strings.Repeat("一二三四?五六七八九?十拾壱", 4000),
+		},
+		"UTF-8->ShiftJIS:all garbled text:larger than 4096bytes": {
+			encoding: japanese.ShiftJIS,
+			in:       strings.Repeat("🍣🍣🍣🍺🍣🍣🍣", 4000),
+			replace:  '?',
+			want:     strings.Repeat("???????", 4000),
+		},
+	}
+
+	assertFunc := func(t *testing.T, want string, actual bytes.Buffer, decoder *encoding.Decoder) {
+		var assertBuf bytes.Buffer
+		aw := transform.NewWriter(&assertBuf, decoder)
+		if _, err := aw.Write(actual.Bytes()); err != nil {
+			t.Error(err)
+		}
+		if err := aw.Close(); err != nil {
+			t.Error(err)
+		}
+
+		if len([]rune(want)) != len([]rune(assertBuf.String())) {
+			t.Errorf("string length does not match %d=%d", len([]rune(want)), len([]rune(assertBuf.String())))
+		}
+		if want != assertBuf.String() {
+			t.Errorf("string does not match\n%s", assertBuf.String())
+		}
 	}
 
 	for n, v := range tests {
 		name := n
 		tt := v
 
-		t.Run(name, func(t *testing.T) {
+		t.Run("[transform.NewWriter]"+name, func(t *testing.T) {
 			var buf bytes.Buffer
 			w := transform.NewWriter(&buf, garbledreplacer.NewTransformer(tt.encoding, tt.replace))
-			if _, err := w.Write([]byte(tt.in)); err != nil {
-				if tt.wantError != nil && errors.Is(err, tt.wantError) {
+			_, err := w.Write([]byte(tt.in))
+			if tt.wantError != nil {
+				if err == nil {
+					t.Errorf("want error %v, got nil", tt.wantError)
+				}
+				if errors.Is(err, tt.wantError) {
 					return
 				}
 				t.Error(err)
 			}
 			if err := w.Close(); err != nil {
 				t.Error(err)
 			}
-
-			var actual bytes.Buffer
-			aw := transform.NewWriter(&actual, tt.encoding.NewDecoder())
-			if _, err := aw.Write(buf.Bytes()); err != nil {
+			assertFunc(t, tt.want, buf, tt.encoding.NewDecoder())
+		})
+		t.Run("[transform.NewWriter with bufio.NewWriter]"+name, func(t *testing.T) {
+			var buf bytes.Buffer
+			w := bufio.NewWriter(transform.NewWriter(&buf, garbledreplacer.NewTransformer(tt.encoding, tt.replace)))
+			_, err := w.WriteString(tt.in)
+			if tt.wantError != nil {
+				if err == nil {
+					t.Errorf("want error %v, got nil", tt.wantError)
+				}
+				if errors.Is(err, tt.wantError) {
+					return
+				}
 				t.Error(err)
 			}
-			if err := aw.Close(); err != nil {
+			if err := w.Flush(); err != nil {
 				t.Error(err)
 			}
-
-			if len([]rune(tt.want)) != len([]rune(actual.String())) {
-				t.Errorf("string length does not match %d=%d", len([]rune(tt.want)), len([]rune(actual.String())))
-			}
-			if tt.want != actual.String() {
-				t.Errorf("string does not match\n%s", actual.String())
-			}
+			assertFunc(t, tt.want, buf, tt.encoding.NewDecoder())
 		})
 	}
 }