implemented constructor by piece_index for enum PieceType

engine/src/piecetype.rs

@@ -16,3 +16,23 @@ pub enum PieceType {
     BlackKing,
 }
 
+impl PieceType {
+
+    pub(in super) fn from_index(idx: u8) -> Self {
+        return match idx {
+            0 => PieceType::WhitePawn,
+            1 => PieceType::WhiteKnight,
+            2 => PieceType::WhiteBishop,
+            3 => PieceType::WhiteRook,
+            4 => PieceType::WhiteQueen,
+            5 => PieceType::WhiteKing,
+            6 => PieceType::BlackPawn,
+            7 => PieceType::BlackKnight,
+            8 => PieceType::BlackBishop,
+            9 => PieceType::BlackRook,
+            10 => PieceType::BlackQueen,
+            11 => PieceType::BlackKing,
+            _ => panic!("invalid piece index! should NEVER appear")
+        }
+    }
+}