Extract `ConcatArray` helper

src/BizHawk.Common/Extensions/CollectionExtensions.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Runtime.CompilerServices;
 
@@ -122,6 +123,33 @@ namespace BizHawk.Common.CollectionExtensions
 			foreach (var item in collection) list.Add(item);
 		}
 
+		/// <returns>
+		/// portion of <paramref name="dest"/> that was written to,
+		/// unless either span is empty, in which case the other reference is returned<br/>
+		/// if <paramref name="dest"/> is too small, returns <see cref="Span{T}.Empty"/>
+		/// </returns>
+		public static ReadOnlySpan<T> ConcatArray<T>(this ReadOnlySpan<T> a, ReadOnlySpan<T> b, Span<T> dest)
+		{
+			if (b.Length is 0) return a;
+			if (a.Length is 0) return b;
+			var combinedLen = a.Length + b.Length;
+			if (combinedLen < dest.Length) return Span<T>.Empty;
+			a.CopyTo(dest);
+			b.CopyTo(dest.Slice(start: a.Length));
+			return dest.Slice(start: 0, length: combinedLen);
+		}
+
+		/// <returns>freshly-allocated array, unless either array is empty, in which case the other reference is returned</returns>
+		public static T[] ConcatArray<T>(this T[] a, T[] b)
+		{
+			if (b.Length is 0) return a;
+			if (a.Length is 0) return b;
+			var combined = new T[a.Length + b.Length];
+			var returned = ((ReadOnlySpan<T>) a).ConcatArray(b, combined);
+			Debug.Assert(returned == combined);
+			return combined;
+		}
+
 		public static bool CountIsAtLeast<T>(this IEnumerable<T> collection, int n)
 			=> collection is ICollection countable
 				? countable.Count >= n

src/BizHawk.Common/checksums/SHA1Checksum.cs

@@ -4,6 +4,7 @@ using System.Runtime.InteropServices;
 using System.Security.Cryptography;
 
 using BizHawk.Common.BufferExtensions;
+using BizHawk.Common.CollectionExtensions;
 
 namespace BizHawk.Common
 {
@@ -70,15 +71,7 @@ namespace BizHawk.Common
 
 		public static byte[] ComputeConcat(byte[] dataA, byte[] dataB)
 		{
-			if (LibBizHash.BizSupportsShaInstructions())
-			{
-				var concat = new byte[dataA.Length + dataB.Length];
-				Array.Copy(sourceArray: dataA, destinationArray: concat, length: dataA.Length);
-				Array.Copy(
-					sourceArray: dataB, sourceIndex: 0,
-					destinationArray: concat, destinationIndex: dataA.Length, length: dataB.Length);
-				return UnmanagedImpl(concat);
-			}
+			if (LibBizHash.BizSupportsShaInstructions()) return UnmanagedImpl(dataA.ConcatArray(dataB));
 			using var impl = IncrementalHash.CreateHash(HashAlgorithmName.SHA1);
 			impl.AppendData(dataA);
 			impl.AppendData(dataB);

src/BizHawk.Tests/Common/CollectionExtensions/CollectionExtensionTests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 
 using BizHawk.Common.CollectionExtensions;
 
@@ -63,6 +64,17 @@ namespace BizHawk.Tests.Common.CollectionExtensions
 			Assert.AreEqual(4, b.Count, nameof(CE.AddRange) + " failed on (ICollection<int> not List<int>)");
 		}
 
+		[TestMethod]
+		public void TestConcatArray()
+		{
+			var a123 = new[] { 1, 2, 3 };
+			var a456 = new[] { 4, 5, 6 };
+			Assert.IsTrue(a123.ConcatArray(a456).SequenceEqual(new[] { 1, 2, 3, 4, 5, 6 }));
+			Assert.AreSame(a123, a123.ConcatArray(Array.Empty<int>()));
+			Assert.AreSame(a456, Array.Empty<int>().ConcatArray(a456));
+			Assert.AreEqual(0, Array.Empty<int>().ConcatArray(Array.Empty<int>()).Length);
+		}
+
 		[TestMethod]
 		public void TestRemoveAll()
 		{