Data commit

Task/Generate-Chess960-starting-position/Swift/generate-chess960-starting-position.swift

@@ -0,0 +1,91 @@
+func isValid960Position(_ firstRank: String) -> Bool {
+  var rooksPlaced = 0
+  var bishopColor = -1
+
+  for (i, piece) in firstRank.enumerated() {
+    switch piece {
+    case "♚" where rooksPlaced != 1:
+      return false
+    case "♜":
+      rooksPlaced += 1
+    case "♝" where bishopColor == -1:
+      bishopColor = i & 1
+    case "♝" where bishopColor == i & 1:
+      return false
+    case _:
+      continue
+    }
+  }
+
+  return true
+}
+
+struct Chess960Counts {
+  var king = 0, queen = 0, rook = 0, bishop = 0, knight = 0
+
+  subscript(_ piece: String) -> Int {
+    get {
+      switch piece {
+      case "♚": return king
+      case "♛": return queen
+      case "♜": return rook
+      case "♝": return bishop
+      case "♞": return knight
+      case _:   fatalError()
+      }
+    }
+
+    set {
+      switch piece {
+      case "♚": king = newValue
+      case "♛": queen = newValue
+      case "♜": rook = newValue
+      case "♝": bishop = newValue
+      case "♞": knight = newValue
+      case _:   fatalError()
+      }
+    }
+  }
+}
+
+func get960Position() -> String {
+  var counts = Chess960Counts()
+  var bishopColor = -1 // 0 - white 1 - black
+  var output = ""
+
+  for i in 1...8 {
+    let validPieces = [
+      counts["♜"] == 1 && counts["♚"] == 0 ? "♚" : nil, // king
+      i == 1 || (counts["♛"] == 0) ? "♛" : nil, // queen
+      i == 1 || (counts["♜"] == 0 || counts["♜"] < 2 && counts["♚"] == 1) ? "♜" : nil, // rook
+      i == 1 || (counts["♝"] < 2 && bishopColor == -1 || bishopColor != i & 1) ? "♝" : nil, // bishop
+      i == 1 || (counts["♞"] < 2) ? "♞" : nil // knight
+    ].lazy.compactMap({ $0 })
+
+    guard let chosenPiece = validPieces.randomElement() else {
+      // Need to swap last piece with a bishop
+      output.insert("♝", at: output.index(before: output.endIndex))
+
+      break
+    }
+
+    counts[chosenPiece] += 1
+    output += chosenPiece
+
+    if bishopColor == -1 && chosenPiece == "♝" {
+      bishopColor = i & 1
+    }
+  }
+
+  assert(isValid960Position(output), "invalid 960 position \(output)")
+
+  return output
+}
+
+var positions = Set<String>()
+
+while positions.count != 960 {
+  positions.insert(get960Position())
+}
+
+print(positions.count, positions.randomElement()!)