feat(movegen): add pseudo-legal and legal move generation with perft

Perft.scala

@@ -0,0 +1,33 @@
+package gambit
+
+object Perft:
+  def perft(pos: Position, depth: Int): Long =
+    if depth == 0 then 1L
+    else
+      val moves = new MoveList()
+      MoveGen.generateLegal(pos, moves)
+      if depth == 1 then moves.size.toLong
+      else
+        var nodes = 0L
+        var i = 0
+        while i < moves.size do
+          val m = moves(i)
+          val undo = pos.make(m)
+          nodes += perft(pos, depth - 1)
+          pos.unmake(m, undo)
+          i += 1
+        nodes
+
+  def divide(pos: Position, depth: Int): List[(String, Long)] =
+    val moves = new MoveList()
+    MoveGen.generateLegal(pos, moves)
+    val out = List.newBuilder[(String, Long)]
+    var i = 0
+    while i < moves.size do
+      val m = moves(i)
+      val undo = pos.make(m)
+      val count = if depth <= 1 then 1L else perft(pos, depth - 1)
+      pos.unmake(m, undo)
+      out += (Move.uci(m) -> count)
+      i += 1
+    out.result()