Initial

MareSynchronos/Utils/PngHdr.cs

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+namespace MareSynchronos.Utils;
+
+public class PngHdr
+{
+	private static readonly byte[] _magicSignature = [137, 80, 78, 71, 13, 10, 26, 10];
+	private static readonly byte[] _IHDR = [(byte)'I', (byte)'H', (byte)'D', (byte)'R'];
+	public static readonly (int Width, int Height) InvalidSize = (0, 0);
+
+	public static (int Width, int Height) TryExtractDimensions(Stream stream)
+	{
+		Span<byte> buffer = stackalloc byte[8];
+
+		try
+		{
+			stream.ReadExactly(buffer[..8]);
+
+			// All PNG files start with the same 8 bytes
+			if (!buffer.SequenceEqual(_magicSignature))
+				return InvalidSize;
+
+			stream.ReadExactly(buffer[..8]);
+
+			uint ihdrLength = BitConverter.ToUInt32(buffer);
+
+			// The next four bytes will be the length of the IHDR section (it should be 13 bytes but we only need 8)
+			if (ihdrLength < 8)
+				return InvalidSize;
+
+			// followed by ASCII "IHDR"
+			if (!buffer[4..].SequenceEqual(_IHDR))
+				return InvalidSize;
+
+			stream.ReadExactly(buffer[..8]);
+
+			uint width = BitConverter.ToUInt32(buffer);
+			uint height = BitConverter.ToUInt32(buffer[4..]);
+
+			// Validate the width/height are non-negative and... that's all we care about!
+			if (width > int.MaxValue || height > int.MaxValue)
+				return InvalidSize;
+
+			return ((int)width, (int)height);
+		}
+		catch (EndOfStreamException)
+		{
+			return InvalidSize;
+		}
+	}
+}