Clean up and enable SNES Game Genie cheatcode converter

Author: YoshiRulz

src/BizHawk.Client.Common/cheats/GameSharkDecoder.cs

@@ -125,12 +125,7 @@ private static IDecodeResult Sms(string code)
 
 		private static IDecodeResult Snes(string code)
 		{
-			if (code.Contains("-") && code.Length == 9)
-			{
-				return new InvalidCheatCode("Game genie codes are not currently supported for SNES");
-				////return SnesGameGenieDecoder.Decode(code);
-			}
-
+			if (SnesGameGenieDecoder.Decode(code) is DecodeResult validSNES) return validSNES; // decoder checks format, will not return invalid for any well-formed input
 			if (code.Length == 8)
 			{
 				return GbGameSharkDecoder.Decode(code);

src/BizHawk.Client.Common/cheats/SnesGameGenieDecoder.cs

@@ -1,32 +1,39 @@
 using System;
 using System.Collections.Generic;
 
+using BizHawk.Common.StringExtensions;
+
 namespace BizHawk.Client.Common.cheats
 {
 	public static class SnesGameGenieDecoder
 	{
-		// including transposition
-		// Code: D F 4 7 0 9 1 5 6 B C 8 A 2 3 E
-		// Hex:  0 1 2 3 4 5 6 7 8 9 A B C D E F
-		// This only applies to the SNES
-		private static readonly Dictionary<char, int> SNESGameGenieTable = new Dictionary<char, int>
+		private const string ERR_MSG_MALFORMED = "Game genie codes must be 9 characters with a format of xxyy-yyyy";
+
+		private static void RotLeft16(ref uint value, int offset)
+			=> value = ((value << offset) & 0xFFFF) | (value >> (16 - offset));
+
+		/// <remarks>
+		/// encr: D F 4 7 0 9 1 5 6 B C 8 A 2 3 E
+		/// decr: 0 1 2 3 4 5 6 7 8 9 A B C D E F
+		/// </remarks>
+		private static readonly Dictionary<char, byte> NybbleDecodeLookup = new()
 		{
-			['D'] = 0,  // 0000
-			['F'] = 1,  // 0001
-			['4'] = 2,  // 0010
-			['7'] = 3,  // 0011
-			['0'] = 4,  // 0100
-			['9'] = 5,  // 0101
-			['1'] = 6,  // 0110
-			['5'] = 7,  // 0111
-			['6'] = 8,  // 1000
-			['B'] = 9,  // 1001
-			['C'] = 10, // 1010
-			['8'] = 11, // 1011
-			['A'] = 12, // 1100
-			['2'] = 13, // 1101
-			['3'] = 14, // 1110
-			['E'] = 15  // 1111
+			['0'] = 0x4,
+			['1'] = 0x6,
+			['2'] = 0xD,
+			['3'] = 0xE,
+			['4'] = 0x2,
+			['5'] = 0x7,
+			['6'] = 0x8,
+			['7'] = 0x3,
+			['8'] = 0xB,
+			['9'] = 0x5,
+			['A'] = 0xC,
+			['B'] = 0x9,
+			['C'] = 0xA,
+			['D'] = 0x0,
+			['E'] = 0xF,
+			['F'] = 0x1,
 		};
 
 		public static IDecodeResult Decode(string code)
@@ -35,11 +42,9 @@ public static IDecodeResult Decode(string code)
 			{
 				throw new ArgumentNullException(nameof(code));
 			}
-
-			if (!code.Contains("-") && code.Length != 9)
-			{
-				return new InvalidCheatCode("Game genie codes must be 9 characters with a format of xxyy-yyyy");
-			}
+			if (code.Length is not 9 || code[4] is not '-') return new InvalidCheatCode(ERR_MSG_MALFORMED);
+			code = code.OnlyHex();
+			if (code.Length is not 8) return new InvalidCheatCode(ERR_MSG_MALFORMED);
 
 			// Code: D F 4 7 0 9 1 5 6 B C 8 A 2 3 E
 			// Hex:  0 1 2 3 4 5 6 7 8 9 A B C D E F
@@ -48,65 +53,25 @@ public static IDecodeResult Decode(string code)
 			// Bit  # |3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|
 			// maps to|     Value     |i|j|k|l|q|r|s|t|o|p|a|b|c|d|u|v|w|x|e|f|g|h|m|n|
 			// order  |     Value     |a|b|c|d|e|f|g|h|i|j|k|l|m|n|o|p|q|r|s|t|u|v|w|x|
-			var result = new DecodeResult { Size = WatchSize.Byte };
-
-			int x;
-
-			// Value
-			if (code.Length > 0)
+			var addrBitsIJKL = (uint) NybbleDecodeLookup[code[2]];
+			var addrBitsQRST = (uint) NybbleDecodeLookup[code[3]];
+			var bunchaBits = unchecked((uint) ((NybbleDecodeLookup[code[4]] << 12)
+				| (NybbleDecodeLookup[code[5]] << 8)
+				| (NybbleDecodeLookup[code[6]] << 4)
+				| NybbleDecodeLookup[code[7]]));
+			RotLeft16(ref bunchaBits, 2);
+			var addr = ((bunchaBits & 0xF000U) << 8) // offset 12 to 20
+				| ((bunchaBits & 0x00F0U) << 12) // offset 4 to 16
+				| (addrBitsIJKL << 12)
+				| ((bunchaBits & 0x000FU) << 8) // offset 0 to 8
+				| (addrBitsQRST << 4)
+				| ((bunchaBits & 0x0F00U) >> 8); // offset 8 to 0
+			return new DecodeResult
 			{
-				SNESGameGenieTable.TryGetValue(code[0], out x);
-				result.Value = x << 4;
-			}
-
-			if (code.Length > 1)
-			{
-				SNESGameGenieTable.TryGetValue(code[1], out x);
-				result.Value |= x;
-			}
-
-			// Address
-			if (code.Length > 2)
-			{
-				SNESGameGenieTable.TryGetValue(code[2], out x);
-				result.Address = x << 12;
-			}
-
-			if (code.Length > 3)
-			{
-				SNESGameGenieTable.TryGetValue(code[3], out x);
-				result.Address |= x << 4;
-			}
-
-			if (code.Length > 4)
-			{
-				SNESGameGenieTable.TryGetValue(code[4], out x);
-				result.Address |= (x & 0xC) << 6;
-				result.Address |= (x & 0x3) << 22;
-			}
-
-			if (code.Length > 5)
-			{
-				SNESGameGenieTable.TryGetValue(code[5], out x);
-				result.Address |= (x & 0xC) << 18;
-				result.Address |= (x & 0x3) << 2;
-			}
-
-			if (code.Length > 6)
-			{
-				SNESGameGenieTable.TryGetValue(code[6], out x);
-				result.Address |= (x & 0xC) >> 2;
-				result.Address |= (x & 0x3) << 18;
-			}
-
-			if (code.Length > 7)
-			{
-				SNESGameGenieTable.TryGetValue(code[7], out x);
-				result.Address |= (x & 0xC) << 14;
-				result.Address |= (x & 0x3) << 10;
-			}
-
-			return result;
+				Address = unchecked((int) addr),
+				Size = WatchSize.Byte,
+				Value = (NybbleDecodeLookup[code[0]] << 4) | NybbleDecodeLookup[code[1]],
+			};
 		}
 	}
 }