Add Parse Method and Check Stream

Signed-off-by: 舰队的偶像-岛风酱! <frg2089@outlook.com>

src/ImageSharp/Formats/Icon/IconDecoderCore.cs

@@ -139,12 +139,18 @@ internal abstract class IconDecoderCore : IImageDecoderInternals
 
     protected void ReadHeader(Stream stream)
     {
-        // TODO: Check length and throw if the header cannot be read.
-        _ = Read(stream, out this.fileHeader, IconDir.Size);
+        Span<byte> buffer = stackalloc byte[IconDirEntry.Size];
+
+        // ICONDIR
+        _ = IconAssert.EndOfStream(stream.Read(buffer[..IconDir.Size]), IconDir.Size);
+        this.fileHeader = IconDir.Parse(buffer);
+
+        // ICONDIRENTRY
         this.Entries = new IconDirEntry[this.FileHeader.Count];
         for (int i = 0; i < this.Entries.Length; i++)
         {
-            _ = Read(stream, out this.Entries[i], IconDirEntry.Size);
+            _ = IconAssert.EndOfStream(stream.Read(buffer[..IconDirEntry.Size]), IconDirEntry.Size);
+            this.Entries[i] = IconDirEntry.Parse(buffer);
         }
 
         int width = 0;
@@ -175,18 +181,6 @@ internal abstract class IconDecoderCore : IImageDecoderInternals
         this.Dimensions = new(width, height);
     }
 
-    private static int Read<T>(Stream stream, out T data, int size)
-        where T : unmanaged
-    {
-        // TODO: Use explicit parsing methods for each T type.
-        // See PngHeader.Parse() for example.
-        Span<byte> buffer = stackalloc byte[size];
-
-        _ = IconAssert.EndOfStream(stream.Read(buffer), size);
-        data = MemoryMarshal.Cast<byte, T>(buffer)[0];
-        return size;
-    }
-
     private IImageDecoderInternals GetDecoder(bool isPng)
     {
         if (isPng)

src/ImageSharp/Formats/Icon/IconDir.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
 using System.Runtime.InteropServices;
@@ -15,12 +15,21 @@ internal struct IconDir
 
     public IconDir(IconFileType type)
         : this(type, 0)
-        => this.Type = type;
+    {
+    }
 
     public IconDir(IconFileType type, ushort count)
+        : this(0, type, count)
+    {
+    }
+
+    public IconDir(ushort reserved, IconFileType type, ushort count)
     {
-        this.Reserved = 0;
+        this.Reserved = reserved;
         this.Type = type;
         this.Count = count;
     }
+
+    public static IconDir Parse(in ReadOnlySpan<byte> data)
+        => MemoryMarshal.Cast<byte, IconDir>(data)[0];
 }

src/ImageSharp/Formats/Icon/IconDirEntry.cs

@@ -25,4 +25,7 @@ internal struct IconDirEntry
     public uint BytesInRes;
 
     public uint ImageOffset;
+
+    public static IconDirEntry Parse(in ReadOnlySpan<byte> data)
+        => MemoryMarshal.Cast<byte, IconDirEntry>(data)[0];
 }