Scanner: fix parsing of PGNs w/ starting position

PGNs can encode games starting from a specific position rather than
the normal starting position. Currently, Scanner will not parse these
correctly because it matches move lists only on lines beginning with
"1. ". This commit fixes this.

fixtures/pgns/0014.pgn

@@ -0,0 +1,55 @@
+[Event "Fantasy Caro-Kann: Starting Position"]
+[Result "*"]
+[UTCDate "2023.05.29"]
+[UTCTime "16:05:07"]
+[Variant "Standard"]
+[ECO "B12"]
+[Opening "Caro-Kann Defense: Maróczy Variation"]
+
+1. e4 c6 2. d4 d5 3. f3 *
+
+
+[Event "Fantasy Caro-Kann: 3. ... dxe4"]
+[Result "*"]
+[UTCDate "2023.05.29"]
+[UTCTime "16:05:48"]
+[Variant "Standard"]
+[FEN "rnbqkbnr/pp2pppp/2p5/3p4/3PP3/5P2/PPP3PP/RNBQKBNR b KQkq - 0 3"]
+[SetUp "1"]
+
+3... dxe4 4. fxe4 e5 5. Nf3 Bg4 (5... exd4 6. Bc4) 6. Bc4 Nd7 7. O-O Ngf6 8. c3 *
+
+
+[Event "Fantasy Caro-Kann: 3. ... e6"]
+[Result "*"]
+[UTCDate "2023.05.29"]
+[UTCTime "16:06:13"]
+[Variant "Standard"]
+[FEN "rnbqkbnr/pp2pppp/2p5/3p4/3PP3/5P2/PPP3PP/RNBQKBNR b KQkq - 0 3"]
+[SetUp "1"]
+
+3... e6 4. Nc3 Bb4 (4... Nf6 5. e5) 5. Bd2 Ne7 6. Bd3 *
+
+
+[Event "Fantasy Caro-Kann: 3. ... g6"]
+[Result "*"]
+[UTCDate "2023.05.29"]
+[UTCTime "16:06:42"]
+[Variant "Standard"]
+[FEN "rnbqkbnr/pp2pppp/2p5/3p4/3PP3/5P2/PPP3PP/RNBQKBNR b KQkq - 0 3"]
+[SetUp "1"]
+
+3... g6 4. Nc3 Bg7 5. Be3 *
+
+
+[Event "Fantasy Caro-Kann: 3. ... Qb6"]
+[Result "*"]
+[UTCDate "2023.05.29"]
+[UTCTime "16:07:08"]
+[Variant "Standard"]
+[FEN "rnbqkbnr/pp2pppp/2p5/3p4/3PP3/5P2/PPP3PP/RNBQKBNR b KQkq - 0 3"]
+[SetUp "1"]
+
+3... Qb6 4. Nc3 dxe4 5. fxe4 e5 6. Nf3 exd4 7. Nxd4 *
+
+

pgn.go

@@ -37,6 +37,8 @@ const (
 // a game or EOF was reached.  Running scan populates
 // data for Next() and Err().
 func (s *Scanner) Scan() bool {
+	moveListRe := regexp.MustCompile(`^\d+\.`)
+
 	if s.err == io.EOF {
 		return false
 	}
@@ -64,7 +66,7 @@ func (s *Scanner) Scan() bool {
 		}
 		line := strings.TrimSpace(s.scanr.Text())
 		isTagPair := strings.HasPrefix(line, "[")
-		isMoveSeq := strings.HasPrefix(line, "1. ")
+		isMoveSeq := moveListRe.MatchString(line)
 		switch state {
 		case notInPGN:
 			if !isTagPair {

pgn_test.go

@@ -154,6 +154,41 @@ func TestScanner(t *testing.T) {
 	}
 }
 
+func TestScannerWithFromPosFENs(t *testing.T) {
+	finalPositions := []string{
+		"rnbqkbnr/pp2pppp/2p5/3p4/3PP3/5P2/PPP3PP/RNBQKBNR b KQkq - 0 3",
+		"r2qkb1r/pp1n1ppp/2p2n2/4p3/2BPP1b1/2P2N2/PP4PP/RNBQ1RK1 b kq - 0 8",
+		"rnbqk2r/pp2nppp/2p1p3/3p4/1b1PP3/2NB1P2/PPPB2PP/R2QK1NR b KQkq - 5 6",
+		"rnbqk1nr/pp2ppbp/2p3p1/3p4/3PP3/2N1BP2/PPP3PP/R2QKBNR b KQkq - 3 5",
+		"rnb1kbnr/pp3ppp/1qp5/8/3NP3/2N5/PPP3PP/R1BQKB1R b KQkq - 0 7",
+	}
+	fname := "fixtures/pgns/0014.pgn"
+	f, err := os.Open(fname)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	scanner := NewScanner(f)
+	games := []*Game{}
+	for idx := 0; scanner.Scan(); {
+		game := scanner.Next()
+		if len(game.moves) == 0 {
+			continue
+		}
+		finalPos := game.Position().String()
+		if finalPos != finalPositions[idx] {
+			t.Fatalf(fname+" game %v expected final pos %v but got %v", idx,
+				finalPositions[idx], finalPos)
+		}
+		games = append(games, game)
+		idx++
+	}
+	if len(games) != len(finalPositions) {
+		t.Fatalf(fname+" expected %v games but got %v", len(finalPositions),
+			len(games))
+	}
+}
+
 func BenchmarkPGN(b *testing.B) {
 	pgn := mustParsePGN("fixtures/pgns/0001.pgn")
 	b.ResetTimer()