Revert "Get rid of unnecessary CRC32Checksum.cs"

This reverts commit 558adfa325.

src/BizHawk.Common/checksums/CRC32Checksum.cs

@@ -0,0 +1,33 @@
+using System;
+
+using BizHawk.Common.BufferExtensions;
+
+namespace BizHawk.Common
+{
+	/// <summary>uses <see cref="CRC32">custom implementation</see> of CRC-32 (i.e. POSIX cksum)</summary>
+	/// <seealso cref="MD5Checksum"/>
+	/// <seealso cref="SHA1Checksum"/>
+	/// <seealso cref="SHA256Checksum"/>
+	public static class CRC32Checksum
+	{
+		/// <remarks>in bits</remarks>
+		internal const int EXPECTED_LENGTH = 32;
+
+		internal const string PREFIX = "CRC32";
+
+		public static byte[] BytesAsDigest(uint digest)
+		{
+			var a = BitConverter.GetBytes(digest);
+			return new[] { a[3], a[2], a[1], a[0] };
+		}
+
+		public static byte[] Compute(ReadOnlySpan<byte> data)
+			=> BytesAsDigest(CRC32.Calculate(data));
+
+		public static string ComputeDigestHex(ReadOnlySpan<byte> data)
+			=> Compute(data).BytesToHexString();
+
+		public static string ComputePrefixedHex(ReadOnlySpan<byte> data)
+			=> $"{PREFIX}:{ComputeDigestHex(data)}";
+	}
+}

src/BizHawk.Common/checksums/MD5Checksum.cs

@@ -7,6 +7,7 @@ using BizHawk.Common.BufferExtensions;
 namespace BizHawk.Common
 {
 	/// <summary>uses <see cref="MD5"/> implementation from BCL</summary>
+	/// <seealso cref="CRC32Checksum"/>
 	/// <seealso cref="SHA1Checksum"/>
 	/// <seealso cref="SHA256Checksum"/>
 	public static class MD5Checksum

src/BizHawk.Common/checksums/SHA1Checksum.cs

@@ -7,6 +7,7 @@ using BizHawk.Common.BufferExtensions;
 namespace BizHawk.Common
 {
 	/// <summary>uses <see cref="SHA1"/> implementation from BCL</summary>
+	/// <seealso cref="CRC32Checksum"/>
 	/// <seealso cref="MD5Checksum"/>
 	/// <seealso cref="SHA256Checksum"/>
 	public static class SHA1Checksum

src/BizHawk.Common/checksums/SHA256Checksum.cs

@@ -7,6 +7,7 @@ using BizHawk.Common.BufferExtensions;
 namespace BizHawk.Common
 {
 	/// <summary>uses <see cref="SHA256"/> implementation from BCL</summary>
+	/// <seealso cref="CRC32Checksum"/>
 	/// <seealso cref="MD5Checksum"/>
 	/// <seealso cref="SHA1Checksum"/>
 	public static class SHA256Checksum

src/BizHawk.Emulation.Common/Database/Database.cs

@@ -220,7 +220,7 @@ namespace BizHawk.Emulation.Common
 		{
 			acquire.WaitOne();
 
-			var hashCRC32 = CRC32.Calculate(romData).ToString("X8");
+			var hashCRC32 = CRC32Checksum.ComputeDigestHex(romData);
 			if (DB.TryGetValue(hashCRC32, out var cgi))
 			{
 				return new GameInfo(cgi);

src/BizHawk.Emulation.DiscSystem/DiscHasher.cs

@@ -54,7 +54,7 @@ namespace BizHawk.Emulation.DiscSystem
 				crc.Add(buffer2352);
 			}
 
-			return crc.Result.ToString("X8");
+			return CRC32Checksum.BytesAsDigest(crc.Result).BytesToHexString();
 		}
 
 		/// <summary>
@@ -102,4 +102,4 @@ namespace BizHawk.Emulation.DiscSystem
 			return "no data track found";
 		}
 	}
-}
+}