lib/encoder: fix decoding a trailing character after an escaped quote

lib/encoder/encoder.go

@@ -682,6 +682,21 @@ func (mask MultiEncoder) Encode(in string) string {
 	return out.String()
 }
 
+// quoteLen is the length in bytes of QuoteRune
+var quoteLen = utf8.RuneLen(QuoteRune)
+
+// endsWithQuote returns true if in ends with a QuoteRune which quotes
+// the following character, rather than one which is part of an
+// escaped QuoteRune (QuoteRune QuoteRune).
+func endsWithQuote(in string) bool {
+	n := 0
+	for strings.HasSuffix(in, string(QuoteRune)) {
+		in = in[:len(in)-quoteLen]
+		n++
+	}
+	return n%2 == 1
+}
+
 // Decode takes a name and undoes any substitutions made by Encode
 func (mask MultiEncoder) Decode(in string) string {
 	if mask == EncodeRaw {
@@ -727,22 +742,22 @@ func (mask MultiEncoder) Decode(in string) string {
 	suffix := ""
 	if r, l := utf8.DecodeLastRuneInString(in); mask.Has(EncodeRightSpace) && r == '␠' { // SYMBOL FOR SPACE
 		in = in[:len(in)-l]
-		if q, l2 := utf8.DecodeLastRuneInString(in); q == QuoteRune {
-			suffix, in = "␠", in[:len(in)-l2]
+		if endsWithQuote(in) {
+			suffix, in = "␠", in[:len(in)-quoteLen]
 		} else {
 			suffix = " "
 		}
 	} else if mask.Has(EncodeRightPeriod) && r == '．' { // FULLWIDTH FULL STOP
 		in = in[:len(in)-l]
-		if q, l2 := utf8.DecodeLastRuneInString(in); q == QuoteRune {
-			suffix, in = "．", in[:len(in)-l2]
+		if endsWithQuote(in) {
+			suffix, in = "．", in[:len(in)-quoteLen]
 		} else {
 			suffix = "."
 		}
 	} else if mask.Has(EncodeRightCrLfHtVt) && (r == '␀'+'\t' || r == '␀'+'\n' || r == '␀'+'\v' || r == '␀'+'\r') {
 		in = in[:len(in)-l]
-		if q, l2 := utf8.DecodeLastRuneInString(in); q == QuoteRune {
-			suffix, in = string(r), in[:len(in)-l2]
+		if endsWithQuote(in) {
+			suffix, in = string(r), in[:len(in)-quoteLen]
 		} else {
 			suffix = string(r - '␀')
 		}

lib/encoder/encoder_test.go

@@ -255,6 +255,44 @@ func TestDecodeHalf(t *testing.T) {
 	}
 }
 
+func TestEncodeQuoteBeforeRightEdge(t *testing.T) {
+	for i, tc := range []testCase{
+		{
+			mask: EncodeRightSpace,
+			in:   "a‛ ",
+			out:  "a‛‛␠",
+		}, {
+			mask: EncodeRightSpace,
+			in:   "‛‛ ",
+			out:  "‛‛‛‛␠",
+		}, {
+			mask: EncodeRightSpace,
+			in:   "a‛␠",
+			out:  "a‛‛‛␠",
+		}, {
+			mask: EncodeRightPeriod,
+			in:   "a‛.",
+			out:  "a‛‛．",
+		}, {
+			mask: EncodeRightCrLfHtVt,
+			in:   "a‛\t",
+			out:  "a‛‛␉",
+		},
+	} {
+		e := tc.mask
+		t.Run(strconv.FormatInt(int64(i), 10), func(t *testing.T) {
+			got := e.Encode(tc.in)
+			if got != tc.out {
+				t.Errorf("Encode(%q) want %q got %q", tc.in, tc.out, got)
+			}
+			got2 := e.Decode(got)
+			if got2 != tc.in {
+				t.Errorf("Decode(%q) want %q got %q", got, tc.in, got2)
+			}
+		})
+	}
+}
+
 const oneDrive = (Standard |
 	EncodeWin |
 	EncodeBackSlash |