Revert Adler32 changes and add test

src/ICSharpCode.SharpZipLib/Checksum/Adler32.cs

@@ -133,10 +133,31 @@ public void Update(byte[] buffer)
 		/// </param>
 		public void Update(ArraySegment<byte> segment)
 		{
-			foreach (byte b in segment)
+			//(By Per Bothner)
+			uint s1 = checkValue & 0xFFFF;
+			uint s2 = checkValue >> 16;
+			var count = segment.Count;
+			var offset = segment.Offset;
+			while (count > 0)
 			{
-				Update(b);
+				// We can defer the modulo operation:
+				// s1 maximally grows from 65521 to 65521 + 255 * 3800
+				// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
+				int n = 3800;
+				if (n > count)
+				{
+					n = count;
+				}
+				count -= n;
+				while (--n >= 0)
+				{
+					s1 = s1 + (uint)(segment.Array[offset++] & 0xff);
+					s2 = s2 + s1;
+				}
+				s1 %= BASE;
+				s2 %= BASE;
 			}
+			checkValue = (s2 << 16) | s1;
 		}
 	}
 }

test/ICSharpCode.SharpZipLib.Tests/Checksum/ChecksumTests.cs

@@ -1,6 +1,7 @@
 using ICSharpCode.SharpZipLib.Checksum;
 using NUnit.Framework;
 using System;
+using System.Diagnostics;
 
 namespace ICSharpCode.SharpZipLib.Tests.Checksum
 {
@@ -27,6 +28,33 @@ public void Adler_32()
 			exceptionTesting(underTestAdler32);
 		}
 
+		const long BufferSize = 256 * 1024 * 1024;
+
+		[Test]
+		public void Adler_32_Performance()
+		{
+			var rand = new Random(1);
+
+			var buffer = new byte[BufferSize];
+			rand.NextBytes(buffer);
+
+			var adler = new Adler32();
+			Assert.AreEqual(0x00000001, adler.Value);
+
+			var sw = new Stopwatch();
+			sw.Start();
+
+			adler.Update(buffer);
+
+			sw.Stop();
+			Console.WriteLine($"Adler32 Hashing of 256 MiB: {sw.Elapsed.TotalSeconds:f4} second(s)");
+
+			adler.Update(check);
+			Assert.AreEqual(0xD4897DA3, adler.Value);
+
+			exceptionTesting(adler);
+		}
+
 		[Test]
 		public void CRC_32_BZip2()
 		{