feat(notation): add pgn reading and writing

Pgn.scala

@@ -0,0 +1,109 @@
+package gambit
+
+final case class PgnGame(
+    tags: List[(String, String)],
+    startFen: String,
+    moves: List[Int],
+    result: String
+)
+
+object Pgn:
+  private val resultTokens = Set("1-0", "0-1", "1/2-1/2", "*")
+
+  def write(
+      startFen: String,
+      moves: List[Int],
+      result: String,
+      tags: List[(String, String)] = Nil
+  ): String =
+    val header = tagSection(startFen, result, tags)
+    val movetext = wrap(renderMoves(startFen, moves) :+ result)
+    s"$header\n$movetext\n"
+
+  private def tagSection(startFen: String, result: String, extra: List[(String, String)]): String =
+    val provided = extra.map(_._1).toSet
+    val roster = List(
+      "Event" -> "gambit self-play",
+      "Site" -> "local",
+      "Date" -> "????.??.??",
+      "Round" -> "-",
+      "White" -> "gambit",
+      "Black" -> "gambit",
+      "Result" -> result
+    ).filterNot(pair => provided.contains(pair._1))
+    val setup =
+      if startFen == Fen.startPosition then Nil
+      else List("SetUp" -> "1", "FEN" -> startFen)
+    val all = extra ++ roster ++ setup
+    all.map((k, v) => s"""[$k "$v"]""").mkString("\n")
+
+  private def renderMoves(startFen: String, moves: List[Int]): List[String] =
+    val pos = Fen.parse(startFen)
+    val out = scala.collection.mutable.ListBuffer[String]()
+    var first = true
+    var index = 0
+    while index < moves.length do
+      val m = moves(index)
+      if pos.sideToMove == Color.White then out += s"${pos.fullmove}."
+      else if first then out += s"${pos.fullmove}..."
+      out += San.toSan(pos, m)
+      pos.make(m)
+      first = false
+      index += 1
+    out.toList
+
+  private def wrap(tokens: List[String], width: Int = 80): String =
+    val sb = new StringBuilder
+    var lineLen = 0
+    for token <- tokens do
+      val add = if lineLen == 0 then token.length else token.length + 1
+      if lineLen > 0 && lineLen + add > width then
+        sb.append('\n')
+        sb.append(token)
+        lineLen = token.length
+      else
+        if lineLen > 0 then sb.append(' ')
+        sb.append(token)
+        lineLen += add
+    sb.toString
+
+  def read(text: String): PgnGame =
+    val tags = parseTags(text)
+    val startFen = tags.find(_._1 == "FEN").map(_._2).getOrElse(Fen.startPosition)
+    val body = text.linesIterator.filterNot(_.trim.startsWith("[")).mkString(" ")
+    val cleaned = stripDecorations(body)
+    val tokens = cleaned.split("\\s+").filter(_.nonEmpty).toList
+    val pos = Fen.parse(startFen)
+    val moves = scala.collection.mutable.ListBuffer[Int]()
+    var result = "*"
+    for raw <- tokens do
+      val token = raw.replaceAll("^\\d+\\.+", "")
+      if token.isEmpty then ()
+      else if resultTokens.contains(token) then result = token
+      else
+        val m = San.parse(pos, token)
+        if m != 0 then
+          moves += m
+          pos.make(m)
+    PgnGame(tags, startFen, moves.toList, result)
+
+  private def parseTags(text: String): List[(String, String)] =
+    val pattern = """\[(\w+)\s+"([^"]*)"\]""".r
+    pattern.findAllMatchIn(text).map(m => (m.group(1), m.group(2))).toList
+
+  private def stripDecorations(body: String): String =
+    val noComments = removePaired(body, '{', '}')
+    val noVariations = removePaired(noComments, '(', ')')
+    noVariations.replaceAll("\\$\\d+", " ")
+
+  private def removePaired(s: String, open: Char, close: Char): String =
+    val sb = new StringBuilder
+    var depth = 0
+    var i = 0
+    while i < s.length do
+      val c = s.charAt(i)
+      if c == open then depth += 1
+      else if c == close then (if depth > 0 then depth -= 1)
+      else if depth == 0 then sb.append(c)
+      i += 1
+    sb.toString