Linear + Diagonal moves

Sources/Engine/Square.swift

@@ -1,16 +1,35 @@
 public struct Square: Equatable {
 
 	public struct Position: Equatable {
-		public let file: Int8
 		public let rank: Int8
+		public let file: Int8
 
-		public var index: Int {
+		public var index: Int? {
+			guard (rank > 0 && rank < 9) && (file > 0 && file < 9) else {
+				return nil
+			}
 			return Int(8*(8-rank)+file-1)
 		}
 
 		public static func == (lhs: Position, rhs: Position) -> Bool {
 			return lhs.index == rhs.index
 		}
+
+		public static func + (lhs: Position, rhs: Position) -> Position {
+			.init(rank: lhs.rank + rhs.rank, file: lhs.file + rhs.file)
+		}
+
+		public static func + (lhs: Position, rhs: (Int8, Int8)) -> Position {
+			.init(rank: lhs.rank + rhs.0, file: lhs.file + rhs.1)
+		}
+
+		public static func += (lhs: inout Position, rhs: Position) {
+			lhs = lhs + rhs
+		}
+
+		public static func += (lhs: inout Position, rhs: (Int8, Int8)) {
+			lhs = lhs + rhs
+		}
 	}
 
 	public let position: Position