Reduce memory allocation (mini-software#427)

* Reduce memory allocation when using MemoryStream

* Add System.Memory pacakge

* Reduce memory allocation in GetImageFormat()

Author: cupsos

src/MiniExcel/MiniExcelLibs.csproj

@@ -42,4 +42,7 @@ Todo : https://github.com/shps951023/MiniExcel/projects/1?fullscreen=true
   <ItemGroup>
     <None Include="icon.png" Pack="true" PackagePath="\" />
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'net45' Or '$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="System.Memory" Version="4.5.5" />
+  </ItemGroup>
 </Project>

src/MiniExcel/OpenXml/ExcelOpenXmlSheetReader.cs

@@ -678,12 +678,12 @@ private void ConvertCellValue(string rawValue, string aT, int xfIndex, out objec
                         if (v != null && v.StartsWith("@@@fileid@@@,",StringComparison.Ordinal))
                         {
                             var path = v.Substring(13);
-                            var stream = _archive.GetEntry(path).Open();
-                            byte[] bytes;
-                            using (var ms = new MemoryStream())
+                            var entry = _archive.GetEntry(path);
+                            byte[] bytes = new byte[entry.Length];
+                            using (var stream = entry.Open())
+                            using (var ms = new MemoryStream(bytes))
                             {
                                 stream.CopyTo(ms);
-                                bytes = ms.ToArray();
                             }
                             value = bytes;
                         }

src/MiniExcel/Utils/ImageHelper.cs

@@ -1,5 +1,6 @@
 namespace MiniExcelLibs.Utils
 {
+    using System;
     using System.Linq;
     using System.Text;
 
@@ -15,36 +16,35 @@ public enum ImageFormat
             unknown
         }
 
-        public static ImageFormat GetImageFormat(byte[] bytes)
+        public static ImageFormat GetImageFormat(ReadOnlySpan<byte> bytes)
         {
-            // see http://www.mikekunz.com/image_file_header.html  
-            var bmp = Encoding.ASCII.GetBytes("BM");     // BMP
-            var gif = Encoding.ASCII.GetBytes("GIF");    // GIF
-            var png = new byte[] { 137, 80, 78, 71 };    // PNG
-            var tiff = new byte[] { 73, 73, 42 };         // TIFF
-            var tiff2 = new byte[] { 77, 77, 42 };         // TIFF
-            var jpeg = new byte[] { 255, 216, 255, 224 }; // jpeg
-            var jpeg2 = new byte[] { 255, 216, 255, 225 }; // jpeg canon
-
-            if (bmp.SequenceEqual(bytes.Take(bmp.Length)))
+            ReadOnlySpan<byte> bmp = stackalloc byte[] { (byte)'B', (byte)'M' };            // BMP
+            ReadOnlySpan<byte> gif = stackalloc byte[] { (byte)'G', (byte)'I', (byte)'F' }; // GIF
+            ReadOnlySpan<byte> png = stackalloc byte[] { 137, 80, 78, 71 };                 // PNG
+            ReadOnlySpan<byte> tiff = stackalloc byte[] { 73, 73, 42 };                     // TIFF
+            ReadOnlySpan<byte> tiff2 = stackalloc byte[] { 77, 77, 42 };                    // TIFF
+            ReadOnlySpan<byte> jpeg = stackalloc byte[] { 255, 216, 255, 224 };             // jpeg
+            ReadOnlySpan<byte> jpeg2 = stackalloc byte[] { 255, 216, 255, 225 };            // jpeg canon
+
+            if (bytes.StartsWith(bmp))
                 return ImageFormat.bmp;
 
-            if (gif.SequenceEqual(bytes.Take(gif.Length)))
+            if (bytes.StartsWith(gif))
                 return ImageFormat.gif;
 
-            if (png.SequenceEqual(bytes.Take(png.Length)))
+            if (bytes.StartsWith(png))
                 return ImageFormat.png;
 
-            if (tiff.SequenceEqual(bytes.Take(tiff.Length)))
+            if (bytes.StartsWith(tiff))
                 return ImageFormat.tiff;
 
-            if (tiff2.SequenceEqual(bytes.Take(tiff2.Length)))
+            if (bytes.StartsWith(tiff2))
                 return ImageFormat.tiff;
 
-            if (jpeg.SequenceEqual(bytes.Take(jpeg.Length)))
+            if (bytes.StartsWith(jpeg))
                 return ImageFormat.jpg;
 
-            if (jpeg2.SequenceEqual(bytes.Take(jpeg2.Length)))
+            if (bytes.StartsWith(jpeg2))
                 return ImageFormat.jpg;
 
             return ImageFormat.unknown;