diff --git a/app/chess_sim/test.py b/app/chess_sim/test.py
index c901463..4a7b9d7 100644
--- a/app/chess_sim/test.py
+++ b/app/chess_sim/test.py
@@ -3,6 +3,9 @@ from dataclasses import dataclass
 from typing import Iterator, Optional, List, Tuple
 import random
 
+FILES = "abcdefgh"
+RANKS = "12345678"
+
 class PieceType(Enum):
     PAWN = auto()
     KNIGHT = auto()
@@ -513,16 +516,24 @@ class ChessBoard:
     # https://en.wikipedia.org/wiki/Algebraic_notation_(chess)
     @staticmethod
     def pos_to_algebraic(pos: BoardPos) -> str:
-        file = chr(ord('a') + pos.y)
-        rank = str(8 - pos.x)
-        return f"{file}{rank}"
+        file = FILES[pos.y]
+        rank = RANKS[7 - pos.x]
+        return file + rank
 
     @staticmethod
     def algebraic_to_pos(s: str) -> BoardPos:
         if len(s) != 2:
             raise ValueError("invalid algebraic square")
-        col = ord(s[0].lower()) - ord('a')
-        row = 8 - int(s[1])
+
+        file_char = s[0].lower()
+        rank_char = s[1]
+
+        if file_char not in FILES or rank_char not in RANKS:
+            raise ValueError("invalid algebraic square")
+
+        col = FILES.index(file_char)
+        row = 7 - RANKS.index(rank_char)
+
         return BoardPos((row, col))
 
     # https://www.chess.com/de/terms/forsyth-edwards-notation-fen