Fix PGN parsing of games w/ nested variations (#125)

* Fix PGN parsing of games w/ nested variations

Existing moveListWithComments() contains a parsing bug when the pgn in
question contains a game with nested variations. In this case the
moveListTokenRe regex doesn't work as expected and results in attempts
to parse an illegal game. To demonstrate the problem this commit adds
a TestScannerWithNested() test case along with a 0013.pgn fixture
which is an export of a lichess study. This commit also fixes the
problem by adding a stripVariations() preprocess step in
moveListWithComments(). I have a subsequent commit coming which adds a
Scanner option to include variations in the game list when processing
a pgn.

* simplified tests

---------

Co-authored-by: Logan Spears <[email protected]>

fixtures/pgns/0013.pgn

@@ -0,0 +1,39 @@
+[Event "Test Nested Variations for https://github.com/notnil/chess: Smith-Morra Accepted; Paulsen Formation"]
+[Site "https://lichess.org/study/lOO87pCX/BZELlaSA"]
+[Result "*"]
+[UTCDate "2023.05.26"]
+[UTCTime "16:09:59"]
+[Variant "Standard"]
+[ECO "B21"]
+[Opening "Sicilian Defense: Smith-Morra Gambit Accepted, Paulsen Formation"]
+[Annotator "https://lichess.org/@/bughouse26"]
+
+1. e4 c5 2. d4 cxd4 3. c3 dxc3 4. Nxc3 Nc6 5. Nf3 e6 6. Bc4 a6 7. O-O b5 { move comment with with braces  ?  } 8. Bb3 *
+
+
+[Event "Test Nested Variations for https://github.com/notnil/chess: Smith-Morra Declined; Alapin Formation"]
+[Site "https://lichess.org/study/lOO87pCX/fC2Vw8Fz"]
+[Result "*"]
+[UTCDate "2023.05.26"]
+[UTCTime "16:13:20"]
+[Variant "Standard"]
+[ECO "B22"]
+[Opening "Sicilian Defense: Alapin Variation, Smith-Morra Declined"]
+[Annotator "https://lichess.org/@/bughouse26"]
+
+1. e4 c5 2. d4 cxd4 3. c3 Nf6 4. e5 Nd5 5. Nf3 Nc6 6. cxd4 d6 { chapter 2 main line move comment with parens ( test ) } 7. Bc4 Nb6 (7... dxe5 { chapter 2 variation move comment with unmatched paren ( } 8. dxe5) 8. Bb5 *
+
+
+[Event "Test Nested Variations for https://github.com/notnil/chess: Smith-Morra Declined; Push Variation"]
+[Site "https://lichess.org/study/lOO87pCX/XaAI645Y"]
+[Result "*"]
+[UTCDate "2023.05.26"]
+[UTCTime "16:16:43"]
+[Variant "Standard"]
+[ECO "B21"]
+[Opening "Sicilian Defense: Smith-Morra Gambit Declined, Push Variation"]
+[Annotator "https://lichess.org/@/bughouse26"]
+
+1. e4 c5 2. d4 cxd4 3. c3 d3 4. Bxd3 Nc6 { chapter 3 main line move comment } 5. Nf3 g6 6. O-O (6. c4 Bg7 { chapter 3 variation 1 comment } 7. O-O) 6... Bg7 7. Qe2 d6 (7... Nf6 8. Rd1 { chapter 3 variation 2 comment } 8... O-O (8... d6 9. e5 dxe5 { chapter 3 nested variation comment } 10. Bxg6 Qc7 { chapter 3 double nested variation comment with parens and moves ( 1. e4 e5 2. Nf3 ) } (10... Qxd1+ 11. Qxd1) 11. Bc2) 9. e5) 8. Rd1 Qc7 9. Na3 a6 10. Bf4 Nf6 11. e5 dxe5 (11... Nh5 12. exd6)  (11... Nd5 12. exd6) *
+
+

pgn.go

@@ -152,7 +152,11 @@ func (a multiDecoder) Decode(pos *Position, s string) (*Move, error) {
 
 func decodePGN(pgn string) (*Game, error) {
 	tagPairs := getTagPairs(pgn)
-	moveComments, outcome := moveListWithComments(pgn)
+	moveComments, outcome, err := moveListWithComments(pgn)
+	if err != nil {
+		return nil, err
+	}
+
 	gameFuncs := []func(*Game){}
 	for _, tp := range tagPairs {
 		if strings.ToLower(tp.Key) == "fen" {
@@ -179,6 +183,7 @@ func decodePGN(pgn string) (*Game, error) {
 		g.comments = append(g.comments, move.Comments)
 	}
 	g.outcome = outcome
+
 	return g, nil
 }
 
@@ -233,10 +238,15 @@ type moveWithComment struct {
 
 var moveListTokenRe = regexp.MustCompile(`(?:\d+\.)|(O-O(?:-O)?|\w*[abcdefgh][12345678]\w*(?:=[QRBN])?(?:\+|#)?)|(?:\{([^}]*)\})|(?:\([^)]*\))|(\*|0-1|1-0|1\/2-1\/2)`)
 
-func moveListWithComments(pgn string) ([]moveWithComment, Outcome) {
+func moveListWithComments(pgn string) ([]moveWithComment, Outcome, error) {
 	pgn = stripTagPairs(pgn)
 	var outcome Outcome
 	moves := []moveWithComment{}
+	// moveListTokenRe doesn't work w/ nested variations
+	pgn, err := stripVariations(pgn)
+	if err != nil {
+		return moves, outcome, err
+	}
 
 	for _, match := range moveListTokenRe.FindAllStringSubmatch(pgn, -1) {
 		move, commentText, outcomeText := match[1], match[2], match[3]
@@ -257,7 +267,7 @@ func moveListWithComments(pgn string) ([]moveWithComment, Outcome) {
 			moves = append(moves, moveWithComment{MoveStr: move})
 		}
 	}
-	return moves, outcome
+	return moves, outcome, nil
 }
 
 func stripTagPairs(pgn string) string {
@@ -271,3 +281,43 @@ func stripTagPairs(pgn string) string {
 	}
 	return strings.Join(cp, "\n")
 }
+
+func stripVariations(pgn string) (string, error) {
+	var ret strings.Builder
+
+	variationDepth := 0
+	inCommentSection := false
+
+	for _, c := range pgn {
+		if c == '{' {
+			if inCommentSection {
+				return "", fmt.Errorf("chess: pgn decode mismatched { in variation: %v", pgn)
+			}
+			inCommentSection = true
+		} else if c == '}' {
+			if !inCommentSection {
+				return "", fmt.Errorf("chess: pgn decode mismatched } in variation: %v", pgn)
+			}
+			inCommentSection = false
+		}
+		if !inCommentSection && c == '(' {
+			variationDepth++
+			continue
+		}
+		if !inCommentSection && c == ')' {
+			if variationDepth <= 0 {
+				return "", fmt.Errorf("chess: pgn decode mismatched parenthesis in variation: %v", pgn)
+			}
+			variationDepth--
+			continue
+		}
+		if variationDepth == 0 {
+			_, err := ret.WriteRune(c)
+			if err != nil {
+				return "", err
+			}
+		}
+	}
+
+	return ret.String(), nil
+}

pgn_test.go

@@ -136,7 +136,12 @@ func TestWriteComments(t *testing.T) {
 }
 
 func TestScanner(t *testing.T) {
-	for _, fname := range []string{"fixtures/pgns/0006.pgn", "fixtures/pgns/0007.pgn"} {
+	m := map[string]int{
+		"fixtures/pgns/0006.pgn": 5,
+		"fixtures/pgns/0007.pgn": 5,
+		"fixtures/pgns/0013.pgn": 3,
+	}
+	for fname, count := range m {
 		f, err := os.Open(fname)
 		if err != nil {
 			panic(err)
@@ -146,10 +151,13 @@ func TestScanner(t *testing.T) {
 		games := []*Game{}
 		for scanner.Scan() {
 			game := scanner.Next()
+			if len(game.Moves()) == 0 {
+				continue
+			}
 			games = append(games, game)
 		}
-		if len(games) != 5 {
-			t.Fatalf(fname+" expected 5 games but got %d", len(games))
+		if len(games) != count {
+			t.Fatalf(fname+" expected %d games but got %d", count, len(games))
 		}
 	}
 }