asm/lexer: Fix handling of EOF and add test case to verify the implem…

…entation. Updates #8.

asm/lexer/lexer.go

@@ -61,7 +61,11 @@ func (l *lexer) lex() {
 // errorf appends an error token at the current position.
 func (l *lexer) errorf(format string, args ...interface{}) {
 	err := fmt.Sprintf(format, args...)
-	_, width := utf8.DecodeLastRuneInString(l.input[:l.cur])
+	// Backup one rune if not at EOF.
+	width := 0
+	if !(l.cur == len(l.input) && l.width == 0) {
+		_, width = utf8.DecodeLastRuneInString(l.input[:l.cur])
+	}
 	tok := token.Token{
 		Kind: token.Error,
 		Val:  err,

asm/lexer/lexer_test.go

@@ -182,6 +182,15 @@ func TestParse(t *testing.T) {
 				{Kind: token.EOF, Pos: 12},
 			},
 		},
+		// i=16
+		{
+			input: `"fooλbar" "baz`,
+			want: []token.Token{
+				{Kind: token.String, Val: "fooλbar", Pos: 0},
+				{Kind: token.Error, Val: "unexpected eof in quoted string", Pos: 15},
+				{Kind: token.EOF, Pos: 15},
+			},
+		},
 	}
 	for i, g := range golden {
 		got := Parse(g.input)

asm/lexer/state.go

@@ -169,6 +169,7 @@ func lexAt(l *lexer) stateFn {
 	case l.accept(`"`):
 		s, ok := readString(l)
 		if !ok {
+			l.emitEOF()
 			// Terminate the lexer with a nil state function.
 			return nil
 		}
@@ -207,6 +208,7 @@ func lexPercent(l *lexer) stateFn {
 	case l.accept(`"`):
 		s, ok := readString(l)
 		if !ok {
+			l.emitEOF()
 			// Terminate the lexer with a nil state function.
 			return nil
 		}
@@ -269,6 +271,7 @@ func lexDollar(l *lexer) stateFn {
 	case l.accept(`"`):
 		s, ok := readString(l)
 		if !ok {
+			l.emitEOF()
 			// Terminate the lexer with a nil state function.
 			return nil
 		}
@@ -362,6 +365,7 @@ func lexQuote(l *lexer) stateFn {
 	// Consume a string constant ("foo", "fo\6F").
 	s, ok := readString(l)
 	if !ok {
+		l.emitEOF()
 		// Terminate the lexer with a nil state function.
 		return nil
 	}