Refactor IconImageFormatDetector.

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

src/ImageSharp/Formats/Icon/IconImageFormatDetector.cs

@@ -2,7 +2,6 @@
 // Licensed under the Six Labors Split License.
 
 using System.Diagnostics.CodeAnalysis;
-using System.Runtime.InteropServices;
 
 namespace SixLabors.ImageSharp.Formats.Icon;
 
@@ -12,22 +11,58 @@ namespace SixLabors.ImageSharp.Formats.Icon;
 public class IconImageFormatDetector : IImageFormatDetector
 {
     /// <inheritdoc/>
-    public int HeaderSize { get; } = 4;
+    public int HeaderSize { get; } = IconDir.Size + IconDirEntry.Size;
 
     /// <inheritdoc/>
     public bool TryDetectFormat(ReadOnlySpan<byte> header, [NotNullWhen(true)] out IImageFormat? format)
     {
-        switch (MemoryMarshal.Cast<byte, uint>(header)[0])
+        format = this.IsSupportedFileFormat(header) switch
         {
-            case Ico.IcoConstants.FileHeader:
-                format = Ico.IcoFormat.Instance;
-                return true;
-            case Cur.CurConstants.FileHeader:
-                format = Cur.CurFormat.Instance;
-                return true;
-            default:
-                format = default;
-                return false;
+            true => Ico.IcoFormat.Instance,
+            false => Cur.CurFormat.Instance,
+            null => default
+        };
+
+        return format is not null;
+    }
+
+    private bool? IsSupportedFileFormat(ReadOnlySpan<byte> header)
+    {
+        // There are no magic bytes in the first few bytes of a tga file,
+        // so we try to figure out if its a valid tga by checking for valid tga header bytes.
+        if (header.Length < this.HeaderSize)
+        {
+            return null;
+        }
+
+        IconDir dir = IconDir.Parse(header);
+        if (dir is not { Reserved: 0 } // Should be 0.
+            or not { Type: IconFileType.ICO or IconFileType.CUR } // Unknown Type.
+            or { Count: 0 }) // Should not be 0.
+        {
+            return null;
+        }
+
+        IconDirEntry entry = IconDirEntry.Parse(header[IconDir.Size..]);
+        if (entry is not { Reserved: 0 } // Should be 0.
+            or { BytesInRes: 0 } // Should not be 0.
+            || entry.ImageOffset < IconDir.Size + (dir.Count * IconDirEntry.Size))
+        {
+            return null;
+        }
+
+        if (dir.Type is IconFileType.ICO)
+        {
+            if (entry is not { BitCount: 1 or 4 or 8 or 16 or 24 or 32 } or not { Planes: 0 or 1 })
+            {
+                return null;
+            }
+
+            return true;
+        }
+        else
+        {
+            return false;
         }
     }
 }