add Leaderboard class

App/Data/Leaderboard.cs

@@ -0,0 +1,33 @@
+namespace Game.Data {
+    internal class Leaderboard {
+        public static void WriteScore(string playerName, int score) {
+            var oldScores = ReadScores();
+            for (int i = 0; i < oldScores.Count; ++i) {
+                if (oldScores[i].Item1 == playerName) {
+                    oldScores.RemoveAt(i);
+                    break;
+                }
+            }
+            oldScores.Add((playerName, score));
+            oldScores.Sort((a, b) => a.Item2.CompareTo(b.Item2));
+            File.WriteAllLines("leaders.txt", oldScores.Select(score => score.Item1 + "\t" + score.Item2.ToString()));
+        }
+
+        public static List<(string, int)> ReadScores() {
+            using StreamReader rd = new StreamReader("leaders.txt");
+            List<(string, int)> scores = new List<(string, int)>();
+            string? line = null;
+            while ((line = rd.ReadLine()) != null) {
+                if (line.Length > 0) {
+                    string[] fields = line.Split('\t');
+                    if (fields.Length < 2) continue;
+                    int score;
+                    if (!int.TryParse(fields[1], out score)) continue;
+                    scores.Add((fields[0], score));
+                }
+            }
+            scores.Sort((a, b) => a.Item2.CompareTo(b.Item2));
+            return scores;
+        }
+    }
+}