Do not calculate Adler checksum if it is not necessary.

src/ICSharpCode.SharpZipLib/Zip/Compression/Deflater.cs

@@ -187,7 +187,7 @@ public Deflater(int level, bool noZlibHeaderOrFooter)
 			}
 
 			pending = new DeflaterPending();
-			engine = new DeflaterEngine(pending);
+			engine = new DeflaterEngine(pending, noZlibHeaderOrFooter);
 			this.noZlibHeaderOrFooter = noZlibHeaderOrFooter;
 			SetStrategy(DeflateStrategy.Default);
 			SetLevel(level);

src/ICSharpCode.SharpZipLib/Zip/Compression/DeflaterEngine.cs

@@ -56,15 +56,33 @@ public class DeflaterEngine
 
 		/// <summary>
 		/// Construct instance with pending buffer
+		/// Adler calculation will be peformed
 		/// </summary>
 		/// <param name="pending">
 		/// Pending buffer to use
-		/// </param>>
+		/// </param>
 		public DeflaterEngine(DeflaterPending pending)
+			: this (pending, false)
+		{
+		}
+
+
+
+		/// <summary>
+		/// Construct instance with pending buffer
+		/// </summary>
+		/// <param name="pending">
+		/// Pending buffer to use
+		/// </param>
+		/// <param name="noAdlerCalculation">
+		/// If no adler calculation should be performed
+		/// </param>
+		public DeflaterEngine(DeflaterPending pending, bool noAdlerCalculation)
 		{
 			this.pending = pending;
 			huffman = new DeflaterHuffman(pending);
-			adler = new Adler32();
+			if (!noAdlerCalculation)
+				adler = new Adler32();
 
 			window = new byte[2 * DeflaterConstants.WSIZE];
 			head = new short[DeflaterConstants.HASH_SIZE];
@@ -185,7 +203,7 @@ public void SetDictionary(byte[] buffer, int offset, int length)
 				throw new InvalidOperationException("strstart not 1");
 			}
 #endif
-			adler.Update(new ArraySegment<byte>(buffer, offset, length));
+			adler?.Update(new ArraySegment<byte>(buffer, offset, length));
 			if (length < DeflaterConstants.MIN_MATCH)
 			{
 				return;
@@ -216,7 +234,7 @@ public void SetDictionary(byte[] buffer, int offset, int length)
 		public void Reset()
 		{
 			huffman.Reset();
-			adler.Reset();
+			adler?.Reset();
 			blockStart = strstart = 1;
 			lookahead = 0;
 			totalIn = 0;
@@ -239,7 +257,7 @@ public void Reset()
 		/// </summary>
 		public void ResetAdler()
 		{
-			adler.Reset();
+			adler?.Reset();
 		}
 
 		/// <summary>
@@ -249,7 +267,7 @@ public int Adler
 		{
 			get
 			{
-				return unchecked((int)adler.Value);
+				return (adler != null) ? unchecked((int)adler.Value) : 0;
 			}
 		}
 
@@ -368,7 +386,7 @@ public void FillWindow()
 				}
 
 				System.Array.Copy(inputBuf, inputOff, window, strstart + lookahead, more);
-				adler.Update(new ArraySegment<byte>(inputBuf, inputOff, more));
+				adler?.Update(new ArraySegment<byte>(inputBuf, inputOff, more));
 
 				inputOff += more;
 				totalIn += more;

src/ICSharpCode.SharpZipLib/Zip/Compression/Inflater.cs

@@ -172,7 +172,8 @@ public Inflater() : this(false)
 		public Inflater(bool noHeader)
 		{
 			this.noHeader = noHeader;
-			this.adler = new Adler32();
+			if (!noHeader)
+				this.adler = new Adler32();
 			input = new StreamManipulator();
 			outputWindow = new OutputWindow();
 			mode = noHeader ? DECODE_BLOCKS : DECODE_HEADER;
@@ -195,7 +196,7 @@ public void Reset()
 			litlenTree = null;
 			distTree = null;
 			isLastBlock = false;
-			adler.Reset();
+			adler?.Reset();
 		}
 
 		/// <summary>
@@ -407,9 +408,9 @@ private bool DecodeChksum()
 				neededBits -= 8;
 			}
 
-			if ((int)adler.Value != readAdler)
+			if ((int)adler?.Value != readAdler)
 			{
-				throw new SharpZipBaseException("Adler chksum doesn't match: " + (int)adler.Value + " vs. " + readAdler);
+				throw new SharpZipBaseException("Adler chksum doesn't match: " + (int)adler?.Value + " vs. " + readAdler);
 			}
 
 			mode = FINISHED;
@@ -607,13 +608,13 @@ public void SetDictionary(byte[] buffer, int index, int count)
 				throw new InvalidOperationException("Dictionary is not needed");
 			}
 
-			adler.Update(new ArraySegment<byte>(buffer, index, count));
+			adler?.Update(new ArraySegment<byte>(buffer, index, count));
 
-			if ((int)adler.Value != readAdler)
+			if (adler != null && (int)adler.Value != readAdler)
 			{
 				throw new SharpZipBaseException("Wrong adler checksum");
 			}
-			adler.Reset();
+			adler?.Reset();
 			outputWindow.CopyDict(buffer, index, count);
 			mode = DECODE_BLOCKS;
 		}
@@ -759,7 +760,7 @@ public int Inflate(byte[] buffer, int offset, int count)
 					int more = outputWindow.CopyOutput(buffer, offset, count);
 					if (more > 0)
 					{
-						adler.Update(new ArraySegment<byte>(buffer, offset, more));
+						adler?.Update(new ArraySegment<byte>(buffer, offset, more));
 						offset += more;
 						bytesCopied += more;
 						totalOut += (long)more;
@@ -823,7 +824,18 @@ public int Adler
 		{
 			get
 			{
-				return IsNeedingDictionary ? readAdler : (int)adler.Value;
+				if (IsNeedingDictionary)
+				{
+					return readAdler;
+				}
+				else if (adler != null)
+				{
+					return (int)adler.Value;
+				}
+				else
+				{
+					return 0;
+				}
 			}
 		}