Add debug utility to byteswap N64 roms

src/BizHawk.Client.EmuHawk/MainForm.Debug.cs

@@ -56,6 +56,7 @@ namespace BizHawk.Client.EmuHawk
 					{
 						DropDownItems =
 						{
+							new DebugVSystemChildItem(N64RomByteswapToolForm.TOOL_NAME, OpenTool<N64RomByteswapToolForm>) { RequiresLoadedRom = false },
 							new DebugVSystemChildItem(N64VideoSettingsFuzzToolForm.TOOL_NAME, OpenTool<N64VideoSettingsFuzzToolForm>),
 						},
 					},

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

@@ -0,0 +1,96 @@
+#nullable enable
+
+using System;
+using System.IO;
+using System.Windows.Forms;
+
+using BizHawk.Client.Common;
+using BizHawk.Common;
+using BizHawk.Emulation.Common;
+using BizHawk.WinForms.Controls;
+
+namespace BizHawk.Client.EmuHawk.ForDebugging
+{
+	internal sealed class N64RomByteswapToolForm : ToolFormBase
+	{
+		public const string TOOL_NAME = "Manual N64 ROM Byteswapping Tool";
+
+		protected override string WindowTitleStatic
+			=> TOOL_NAME;
+
+		public N64RomByteswapToolForm()
+		{
+			SzTextBoxEx txtBaseFile = new() { Size = new(224, 23) };
+			void ChooseBaseFile()
+			{
+				using OpenFileDialog ofd = new() { InitialDirectory = Config!.PathEntries.RomAbsolutePath() };
+				this.ShowDialogAsChild(ofd);
+				txtBaseFile.Text = ofd.FileName;
+			}
+			ComboBox comboFormats = new()
+			{
+				Items = { "rare little-endian (.n64)", "byte-swapped (.v64)", "native (.z64)" },
+				SelectedIndex = 2,
+				Size = new(160, 23),
+			};
+			SzTextBoxEx txtTargetFile = new() { Size = new(224, 23) };
+			void ChooseTargetFile()
+			{
+				using SaveFileDialog sfd = new()
+				{
+					DefaultExt = comboFormats.SelectedIndex switch
+					{
+						0 => "n64",
+						1 => "v64",
+						2 => "z64",
+						_ => string.Empty
+					},
+					InitialDirectory = Config!.PathEntries.RomAbsolutePath(),
+				};
+				this.ShowDialogAsChild(sfd);
+				txtTargetFile.Text = sfd.FileName;
+			}
+			void DoConvert()
+			{
+				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
+					{
+						case 0:
+							N64RomByteswapper.ToN64LittleEndian(rom);
+							break;
+						case 1:
+							N64RomByteswapper.ToV64ByteSwapped(rom);
+							break;
+						case 2:
+						default:
+							N64RomByteswapper.ToZ64Native(rom);
+							break;
+					}
+					File.WriteAllBytes(txtTargetFile.Text, rom);
+					this.ModalMessageBox($"wrote {txtTargetFile.Text}\n{SHA1Checksum.ComputePrefixedHex(rom)}");
+				}
+				catch (Exception e)
+				{
+					this.ModalMessageBox($"caught {e.GetType().Name}:\n{e}");
+				}
+			}
+			ClientSize = new(320, 200);
+			SuspendLayout();
+			Controls.Add(new SingleColumnFLP
+			{
+				Controls =
+				{
+					new LabelEx { Text = "convert" },
+					new SingleRowFLP { Controls = { txtBaseFile, GenControl.Button("(browse)", width: 75, ChooseBaseFile) } },
+					new LabelEx { Text = "from <autodetected> (assumes .z64 on fail)" },
+					new SingleRowFLP { Controls = { new LabelEx { Text = "to" }, comboFormats } },
+					new SingleRowFLP { Controls = { txtTargetFile, GenControl.Button("(browse)", width: 75, ChooseTargetFile) } },
+					GenControl.Button("--> convert", width: 75, DoConvert),
+				}
+			});
+			ResumeLayout();
+		}
+	}
+}

src/BizHawk.Common/EndiannessUtils.cs

@@ -45,5 +45,13 @@ namespace BizHawk.Common
 			for (var i = 0; i < ints.Length; i++) ints[i] = BinaryPrimitives.ReverseEndianness(ints[i]);
 #endif
 		}
+
+		/// <summary>swaps pairs of 16-bit words in-place: <c>0xAABBIIJJPPQQYYZZ</c> &lt;=> <c>0xIIJJAABBYYZZPPQQ</c></summary>
+		public static void MutatingShortSwap32(Span<byte> a)
+		{
+			// no need to optimise this further, it's only used in the manual byteswap tool and only between the two less common formats
+			MutatingByteSwap32(a); // calls can be in either order, though this order ensures the length-mod-4 assert is hit immediately
+			MutatingByteSwap16(a);
+		}
 	}
 }

src/BizHawk.Emulation.Common/N64RomByteswapper.cs

@@ -11,6 +11,22 @@ namespace BizHawk.Emulation.Common
 		/// <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 };
 
+		/// <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)
+		{
+			if (rom[0] == MAGIC_BYTES[0]) EndiannessUtils.MutatingByteSwap32(rom); // native (.z64)
+			else if (rom[0] == MAGIC_BYTES[1]) EndiannessUtils.MutatingShortSwap32(rom); // byte-swapped (.v64)
+			// else already rare little-endian .n64
+		}
+
+		/// <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)
+		{
+			if (rom[0] == MAGIC_BYTES[0]) EndiannessUtils.MutatingByteSwap16(rom); // native (.z64)
+			else if (rom[0] == MAGIC_BYTES[3]) EndiannessUtils.MutatingShortSwap32(rom); // rare little-endian .n64
+			// else already byte-swapped (.v64)
+		}
+
 		/// <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)
 		{

src/BizHawk.Tests/Common/EndiannessUtils/EndiannessUtilsTests.cs

@@ -32,5 +32,16 @@ namespace BizHawk.Tests.Common
 			EndiannessUtils.MutatingByteSwap32(a);
 			Assert.IsTrue(a.SequenceEqual(b));
 		}
+
+		[TestMethod]
+		public void TestShortSwap32()
+		{
+			var b = new byte[] { 0x45, 0x67, 0x01, 0x23, 0xCD, 0xEF, 0x89, 0xAB }.AsSpan();
+			var a = b.ToArray().AsSpan();
+			EndiannessUtils.MutatingShortSwap32(a);
+			Assert.IsTrue(a.SequenceEqual(expected));
+			EndiannessUtils.MutatingShortSwap32(a);
+			Assert.IsTrue(a.SequenceEqual(b));
+		}
 	}
 }