Be less lazy about N64 header detection in byteswapper

fixes 82c3b471a, 9660c16a0

src/BizHawk.Client.Common/RomGame.cs

@@ -96,7 +96,7 @@ namespace BizHawk.Client.Common
 				RomData = DeInterleaveSMD(RomData);
 			}
 
-			if (file.Extension is ".n64" or ".v64" or ".z64") N64RomByteswapper.ToZ64Native(RomData); //TODO don't use file extension for N64 rom detection (yes that means detecting all formats before converting to Z64)
+			if (file.Extension is ".n64" or ".v64" or ".z64") _ = N64RomByteswapper.ToZ64Native(RomData); //TODO don't use file extension for N64 rom detection (yes that means detecting all formats before converting to Z64)
 
 			// note: this will be taking several hashes, of a potentially large amount of data.. yikes!
 			GameInfo = Database.GetGameInfo(RomData, file.Name);

src/BizHawk.Client.EmuHawk/debug/N64RomByteswapToolForm.cs

@@ -51,19 +51,12 @@ namespace BizHawk.Client.EmuHawk.ForDebugging
 				try
 				{
 					var rom = File.ReadAllBytes(txtBaseFile.Text);
-					switch (comboFormats.SelectedIndex) // can't have Action<Span<byte>> (System.Buffers.SpanAction isn't suitable) or I'd be able to have a tiny switch expr >:( --yoshi
+					_ = comboFormats.SelectedIndex switch
 					{
-						case 0:
+						0 => N64RomByteswapper.ToN64LittleEndian(rom),
-							N64RomByteswapper.ToN64LittleEndian(rom);
+						1 => N64RomByteswapper.ToV64ByteSwapped(rom),
-							break;
+						_ => N64RomByteswapper.ToZ64Native(rom)
-						case 1:
+					};
-							N64RomByteswapper.ToV64ByteSwapped(rom);
-							break;
-						case 2:
-						default:
-							N64RomByteswapper.ToZ64Native(rom);
-							break;
-					}
 					File.WriteAllBytes(txtTargetFile.Text, rom);
 					this.ModalMessageBox($"wrote {txtTargetFile.Text}\n{SHA1Checksum.ComputePrefixedHex(rom)}");
 				}

src/BizHawk.Emulation.Common/N64RomByteswapper.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Runtime.InteropServices;
 
 using BizHawk.Common;
 
@@ -8,31 +9,67 @@ namespace BizHawk.Emulation.Common
 	/// <remarks>http://n64dev.org/romformats.html</remarks>
 	public static class N64RomByteswapper
 	{
+		private static readonly byte[] MAGIC_BYTES_LE = { 0x37, 0x80, 0x40, 0x12 };
+
 		/// <remarks>not actually magic, just always the same in commercial carts? https://n64brew.dev/wiki/ROM_Header works all the same</remarks>
-		private static readonly byte[] MAGIC_BYTES = { 0x80, 0x37, 0x12, 0x40 };
+		private static readonly byte[] MAGIC_BYTES_NATIVE = { 0x80, 0x37, 0x12, 0x40 };
+
+		private static readonly byte[] MAGIC_BYTES_SWAPPED = { 0x40, 0x12, 0x37, 0x80 };
 
 		/// <summary>ensures <paramref name="rom"/> is in the rare little-endian (<c>.n64</c>) format, mutating it in-place if necessary</summary>
-		public static void ToN64LittleEndian(Span<byte> rom)
+		/// <returns><see langword="true"/> iff <paramref name="rom"/> was one of the 3 valid formats</returns>
+		public static bool ToN64LittleEndian(Span<byte> rom)
 		{
-			if (rom[0] == MAGIC_BYTES[0]) EndiannessUtils.MutatingByteSwap32(rom); // native (.z64)
+			var romMagicBytes = rom.Slice(start: 0, length: 4);
-			else if (rom[0] == MAGIC_BYTES[1]) EndiannessUtils.MutatingShortSwap32(rom); // byte-swapped (.v64)
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_NATIVE))
-			// else already rare little-endian .n64
+			{
+				EndiannessUtils.MutatingByteSwap32(rom);
+				return true;
+			}
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_SWAPPED))
+			{
+				EndiannessUtils.MutatingShortSwap32(rom);
+				return true;
+			}
+			return romMagicBytes.SequenceEqual(MAGIC_BYTES_LE);
 		}
 
 		/// <summary>ensures <paramref name="rom"/> is in the byte-swapped (<c>.v64</c>) format, mutating it in-place if necessary</summary>
-		public static void ToV64ByteSwapped(Span<byte> rom)
+		/// <returns><see langword="true"/> iff <paramref name="rom"/> was one of the 3 valid formats</returns>
+		public static bool ToV64ByteSwapped(Span<byte> rom)
 		{
-			if (rom[0] == MAGIC_BYTES[0]) EndiannessUtils.MutatingByteSwap16(rom); // native (.z64)
+			var romMagicBytes = rom.Slice(start: 0, length: 4);
-			else if (rom[0] == MAGIC_BYTES[3]) EndiannessUtils.MutatingShortSwap32(rom); // rare little-endian .n64
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_NATIVE))
-			// else already byte-swapped (.v64)
+			{
+				EndiannessUtils.MutatingByteSwap16(rom);
+				return true;
+			}
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_SWAPPED)) return true;
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_LE))
+			{
+				EndiannessUtils.MutatingShortSwap32(rom);
+				return true;
+			}
+			return false;
 		}
 
 		/// <summary>ensures <paramref name="rom"/> is in the native (<c>.z64</c>) format, mutating it in-place if necessary</summary>
-		public static void ToZ64Native(Span<byte> rom)
+		/// <returns><see langword="true"/> iff <paramref name="rom"/> was one of the 3 valid formats</returns>
+		public static bool ToZ64Native(Span<byte> rom)
 		{
-			if (rom[0] == MAGIC_BYTES[1]) EndiannessUtils.MutatingByteSwap16(rom); // byte-swapped (.v64)
+			var romMagicBytes = rom.Slice(start: 0, length: 4);
-			else if (rom[0] == MAGIC_BYTES[3]) EndiannessUtils.MutatingByteSwap32(rom); // rare little-endian .n64
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_NATIVE)) return true;
-			// else already native (.z64)
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_SWAPPED))
+			{
+				EndiannessUtils.MutatingByteSwap16(rom);
+				return true;
+			}
+			if (romMagicBytes.SequenceEqual(MAGIC_BYTES_LE))
+			{
+				EndiannessUtils.MutatingByteSwap32(rom);
+				return true;
+			}
+			return false;
 		}
 	}
 }

src/BizHawk.Emulation.Cores/Consoles/Nintendo/Ares64/Ares64.cs

@@ -69,17 +69,7 @@ namespace BizHawk.Emulation.Cores.Consoles.Nintendo.Ares64
 			// TODO: this is normally handled frontend side
 			// except XML files don't go through RomGame
 			// (probably should, but needs refactoring)
-			foreach (var d in lp.Roms.Select(static r => r.RomData))
+			foreach (var r in lp.Roms) _ = N64RomByteswapper.ToZ64Native(r.RomData); // no-op if N64 magic bytes not present
-			{
-				// magic N64 rom bytes are 0x80, 0x37, 0x12, 0x40
-				// this should hopefully only ever detect N64 roms and not other kinds of files sent through here...
-				if ((d[1] is 0x80 && d[0] is 0x37 && d[3] is 0x12 && d[2] is 0x40) // .v64 byteswapped
-					|| (d[3] is 0x80 && d[2] is 0x37 && d[1] is 0x12 && d[0] is 0x40) // .n64 little-endian
-					|| (d[0] is 0x80 && d[1] is 0x37 && d[2] is 0x12 && d[3] is 0x40)) // .z64 native
-				{
-					N64RomByteswapper.ToZ64Native(d);
-				}
-			}
 
 			var gbRoms = lp.Roms.FindAll(r => IsGBRom(r.FileData)).Select(r => r.FileData).ToArray();
 			var rom = lp.Roms.Find(r => !gbRoms.Contains(r.FileData) && (char)r.RomData[0x3B] is 'N' or 'C')?.RomData;