From 6c48af34346dd494d26feabeb011c04a121359d2 Mon Sep 17 00:00:00 2001
From: "Admiral H. Curtiss" <pikachu025@gmail.com>
Date: Sat, 7 Jun 2014 23:43:20 +0200
Subject: [PATCH] Fix checksums for devices that care about that.

---
 Crc16.cs          | 51 +++++++++++++++++++++++++++++++++++++++++++++++
 Program.cs        | 10 ++++++++++
 WfcPatcher.csproj |  1 +
 3 files changed, 62 insertions(+)
 create mode 100644 Crc16.cs

diff --git a/Crc16.cs b/Crc16.cs
new file mode 100644
index 0000000..8c35edd
--- /dev/null
+++ b/Crc16.cs
@@ -0,0 +1,51 @@
+// modified from http://www.sanity-free.org/134/standard_crc_16_in_csharp.html
+
+using System;
+using System.IO;
+
+namespace WfcPatcher {
+	public class Crc16 {
+		private ushort[] table = new ushort[256];
+
+		public ushort ComputeChecksum( byte[] bytes ) {
+			ushort crc = 0;
+			for ( int i = 0; i < bytes.Length; ++i ) {
+				byte index = (byte)( crc ^ bytes[i] );
+				crc = (ushort)( ( crc >> 8 ) ^ table[index] );
+			}
+			return crc;
+		}
+
+		public ushort ComputeChecksum( Stream stream, int length, ushort init = 0 ) {
+			ushort crc = init;
+			for ( int i = 0; i < length; ++i ) {
+				byte index = (byte)( ( crc ^ stream.ReadByte() ) & 0xFF );
+				crc = (ushort)( ( crc >> 8 ) ^ table[index] );
+			}
+			return crc;
+		}
+
+		public byte[] ComputeChecksumBytes( byte[] bytes ) {
+			ushort crc = ComputeChecksum( bytes );
+			return BitConverter.GetBytes( crc );
+		}
+
+		public Crc16( ushort polynomial = 0xA001 ) {
+			ushort value;
+			ushort temp;
+			for ( ushort i = 0; i < table.Length; ++i ) {
+				value = 0;
+				temp = i;
+				for ( byte j = 0; j < 8; ++j ) {
+					if ( ( ( value ^ temp ) & 0x0001 ) != 0 ) {
+						value = (ushort)( ( value >> 1 ) ^ polynomial );
+					} else {
+						value >>= 1;
+					}
+					temp >>= 1;
+				}
+				table[i] = value;
+			}
+		}
+	}
+}
diff --git a/Program.cs b/Program.cs
index fc24a5e..13c4dcf 100644
--- a/Program.cs
+++ b/Program.cs
@@ -177,6 +177,16 @@ static void PatchArm9( System.IO.FileStream nds, uint pos, uint len ) {
 				byte[] newSizeBytes = BitConverter.GetBytes( newSize );
 				nds.Position = 0x2C;
 				nds.Write( newSizeBytes, 0, 4 );
+
+				// recalculate checksums
+				nds.Position = pos;
+				ushort secureChecksum = new Crc16().ComputeChecksum( nds, 0x4000, 0xFFFF );
+				nds.Position = 0x6C;
+				nds.Write( BitConverter.GetBytes( secureChecksum ), 0, 2 );
+
+				nds.Position = 0;
+				ushort headerChecksum = new Crc16().ComputeChecksum( nds, 0x15E, 0xFFFF );
+				nds.Write( BitConverter.GetBytes( headerChecksum ), 0, 2 );
 			}
 		}
 
diff --git a/WfcPatcher.csproj b/WfcPatcher.csproj
index f2217aa..ab4cfa2 100644
--- a/WfcPatcher.csproj
+++ b/WfcPatcher.csproj
@@ -43,6 +43,7 @@
   <ItemGroup>
     <Compile Include="blz.cs" />
     <Compile Include="ByteArrayRocks.cs" />
+    <Compile Include="Crc16.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Util.cs" />