Optimize Crc32 calculations

src/UglyToad.PdfPig/Images/Png/Crc32.cs

@@ -1,12 +1,11 @@
 namespace UglyToad.PdfPig.Images.Png
 {
     using System;
-    using System.Collections.Generic;
 
     /// <summary>
     /// 32-bit Cyclic Redundancy Code used by the PNG for checking the data is intact.
     /// </summary>
-    internal static class Crc32
+    internal class Crc32
     {
         private const uint Polynomial = 0xEDB88320;
 
@@ -49,21 +48,6 @@
             return crc32 ^ uint.MaxValue;
         }
 
-        /// <summary>
-        /// Calculate the CRC32 for data.
-        /// </summary>
-        public static uint Calculate(List<byte> data)
-        {
-            var crc32 = uint.MaxValue;
-            for (var i = 0; i < data.Count; i++)
-            {
-                var index = (crc32 ^ data[i]) & 0xFF;
-                crc32 = (crc32 >> 8) ^ Lookup[index];
-            }
-
-            return crc32 ^ uint.MaxValue;
-        }
-
         /// <summary>
         /// Calculate the combined CRC32 for data.
         /// </summary>
@@ -84,5 +68,30 @@
 
             return crc32 ^ uint.MaxValue;
         }
+
+
+        private uint state = uint.MaxValue;
+
+        /// <summary>
+        /// Calculate the CRC32 for data.
+        /// </summary>
+        public void Append(ReadOnlySpan<byte> data)
+        {
+            for (var i = 0; i < data.Length; i++)
+            {
+                var index = (state ^ data[i]) & 0xFF;
+                state = (state >> 8) ^ Lookup[index];
+            }
+        }
+
+        public uint GetCurrentHashAsUInt32()
+        {
+            return state ^ uint.MaxValue;
+        }
+
+        public void Reset()
+        {
+            state = uint.MaxValue;
+        }
     }
 }

src/UglyToad.PdfPig/Images/Png/PngStreamWriteHelper.cs

@@ -1,14 +1,13 @@
 namespace UglyToad.PdfPig.Images.Png
 {
     using System;
-    using System.Collections.Generic;
+    using System.Buffers.Binary;
     using System.IO;
-    using System.Linq;
 
-    internal class PngStreamWriteHelper : Stream
+    internal sealed class PngStreamWriteHelper : Stream
     {
         private readonly Stream inner;
-        private readonly List<byte> written = new List<byte>();
+        private readonly Crc32 crc = new();
 
         public override bool CanRead => inner.CanRead;
 
@@ -33,7 +32,7 @@
 
         public void WriteChunkHeader(ReadOnlySpan<byte> header)
         {
-            written.Clear();
+            crc.Reset();
             Write(header);
         }
 
@@ -50,28 +49,33 @@
 
         public override void Write(byte[] buffer, int offset, int count)
         {
-            written.AddRange(buffer.Skip(offset).Take(count));
+            crc.Append(buffer.AsSpan(offset, count));
             inner.Write(buffer, offset, count);
         }
 
 #if NET8_0_OR_GREATER
         public override void Write(ReadOnlySpan<byte> buffer)
         {
-            written.AddRange(buffer);
+            crc.Append(buffer);
             inner.Write(buffer);
         }
 #else
         public void Write(ReadOnlySpan<byte> buffer)
         {
-            written.AddRange(buffer.ToArray());
+            crc.Append(buffer);
             inner.Write(buffer);
         }
 #endif
 
         public void WriteCrc()
         {
-            var result = (int)Crc32.Calculate(written);
-            StreamHelper.WriteBigEndianInt32(inner, result);
+            Span<byte> buffer = stackalloc byte[4];
+
+            var result = crc.GetCurrentHashAsUInt32();
+
+            BinaryPrimitives.WriteUInt32BigEndian(buffer, result);
+
+            inner.Write(buffer);
         }
     }
 }