Fix sav4 active block detect

Closes #2697 ty @Ammako !

lazy slice check has flaws; just compare the major/minor counters in the save footer and check the uninitialized cases

nobody is gonna save the game 2^32 times, but check for the overflow case too!
first ff'd is preferred when the second is ff'd-1!

PKHeX.Core/Saves/SAV4.cs

@@ -149,30 +149,10 @@ namespace PKHeX.Core
 
         private static int GetActiveBlock(byte[] data, int begin, int length)
         {
-            // Check to see if the save is initialized completely
-            // if the block is not initialized, fall back to the other save.
-            var start = begin;
-            if (data.IsRangeAll((byte)0, start, 10) || data.IsRangeAll((byte)0xFF, start, 10))
-                return 1;
-            start += PartitionSize; // check other save
-            if (data.IsRangeAll((byte)0, start, 10) || data.IsRangeAll((byte)0xFF, start, 10))
-                return 0;
-
-            // Fall back to highest value save counter
-            return GetActiveBlockViaCounter(data, begin, length);
+            int offset = begin + length - 0x14;
+            return SAV4BlockDetection.CompareFooters(data, offset, offset + PartitionSize);
         }
 
-        private static int GetActiveBlockViaCounter(byte[] data, int begin, int length)
-        {
-            int ofs = GetBlockSaveCounterOffset(begin, length);
-            var block0 = BitConverter.ToUInt16(data, ofs);
-            var block1 = BitConverter.ToUInt16(data, ofs + PartitionSize);
-            bool first = block0 >= block1;
-            return first ? 0 : 1;
-        }
-
-        private static int GetBlockSaveCounterOffset(int start, int length) => start + length - 0x10;
-
         protected int WondercardFlags = int.MinValue;
         protected int AdventureInfo = int.MinValue;
         protected int Seal = int.MinValue;

PKHeX.Core/Saves/SAV4BlockDetection.cs

@@ -0,0 +1,53 @@
+using System;
+
+namespace PKHeX.Core
+{
+    /// <summary>
+    /// Finds the index of the most recent save block for <see cref="SAV4"/> blocks.
+    /// </summary>
+    public static class SAV4BlockDetection
+    {
+        private const int First = 0;
+        private const int Second = 1;
+        private const int Same = 2;
+
+        /// <summary>
+        /// Compares the footers of the two blocks to determine which is newest.
+        /// </summary>
+        /// <returns>0=Primary, 1=Secondary.</returns>
+        public static int CompareFooters(byte[] data, int offset1, int offset2)
+        {
+            // Major Counters
+            var major1 = BitConverter.ToUInt32(data, offset1);
+            var major2 = BitConverter.ToUInt32(data, offset2);
+            var result1 = CompareCounters(major1, major2);
+            if (result1 == First)
+                return First;
+            if (result1 == Second)
+                return Second;
+
+            // Minor Counters
+            var minor1 = BitConverter.ToUInt32(data, offset1 + 4);
+            var minor2 = BitConverter.ToUInt32(data, offset2 + 4);
+            var result2 = CompareCounters(minor1, minor2);
+            return result2 == Second ? Second : First; // Same -> First, shouldn't happen for valid saves.
+        }
+
+        private static int CompareCounters(uint counter1, uint counter2)
+        {
+            // Uninitialized
+            if (counter1 == uint.MaxValue && counter2 == 0)
+                return Second;
+            if (counter1 == 0 && counter2 == uint.MaxValue)
+                return First;
+
+            // Different
+            if (counter1 > counter2)
+                return First;
+            if (counter1 < counter2)
+                return Second;
+
+            return Same;
+        }
+    }
+}