From da839988af1edcd153ea74a907e5b68bf5ba03fd Mon Sep 17 00:00:00 2001
From: Kurt <kaphotics@gmail.com>
Date: Sat, 21 Jul 2018 19:24:09 -0700
Subject: [PATCH] Simplify RBY checksum calc

utilize unchecked byte overflow
remove unnecessary zeroing -- t's immediately after the checksummed
region. even if it was included, chk += 0 is still 0.
---
 PKHeX.Core/Saves/SAV1.cs | 41 ++++++++++++++--------------------------
 1 file changed, 14 insertions(+), 27 deletions(-)

diff --git a/PKHeX.Core/Saves/SAV1.cs b/PKHeX.Core/Saves/SAV1.cs
index 11e6fffab..5c82a11d2 100644
--- a/PKHeX.Core/Saves/SAV1.cs
+++ b/PKHeX.Core/Saves/SAV1.cs
@@ -205,35 +205,22 @@ protected override byte[] Write(bool DSV)
         public override bool IsPKMPresent(int Offset) => PKX.IsPKMPresentGB(Data, Offset);
 
         // Checksums
-        protected override void SetChecksums()
-        {
-            int CHECKSUM_OFS = Japanese ? 0x3594 : 0x3523;
-            Data[CHECKSUM_OFS] = 0;
-            uint chksum = 0;
-            for (int i = 0x2598; i < CHECKSUM_OFS; i++)
-            {
-                chksum += Data[i];
-            }
-
-            chksum = ~chksum;
-            chksum &= 0xFF;
-
-            Data[CHECKSUM_OFS] = (byte)chksum;
-        }
-        public override bool ChecksumsValid
-        {
-            get
-            {
-                int CHECKSUM_OFS = Japanese ? 0x3594 : 0x3523;
-                byte temp = Data[CHECKSUM_OFS]; // cache current chk
-                SetChecksums(); // chksum is recalculated (after being set to 0 to perform check)
-                byte chk = Data[CHECKSUM_OFS]; // correct checksum
-                Data[CHECKSUM_OFS] = temp; // restore old chk
-                return temp == chk;
-            }
-        }
+        protected override void SetChecksums() => Data[CHECKSUM_OFS] = GetRBYChecksum(CHECKSUM_OFS);
+        public override bool ChecksumsValid => Data[CHECKSUM_OFS] == GetRBYChecksum(CHECKSUM_OFS);
         public override string ChecksumInfo => ChecksumsValid ? "Checksum valid." : "Checksum invalid";
 
+        private const int CHECKSUM_START = 0x2598;
+        private int CHECKSUM_OFS => Japanese ? 0x3594 : 0x3523;
+
+        private byte GetRBYChecksum(int end)
+        {
+            byte chksum = 0;
+            for (int i = CHECKSUM_START; i < end; i++)
+                chksum += Data[i];
+            chksum ^= 0xFF;
+            return chksum;
+        }
+
         // Trainer Info
         public override GameVersion Version { get; protected set; }