Transcribes interesting 2MG fields, albeit without acting on them.

Storage/Disk/DiskImage/Formats/2MG.cpp

@@ -11,7 +11,65 @@
 using namespace Storage::Disk;
 
 DiskImageHolderBase *Disk2MG::open(const std::string &file_name) {
-	(void)file_name;
+	FileHolder file(file_name);
+
+	// Check the signature.
+	if(!file.check_signature("2IMG")) throw Error::InvalidFormat;
+
+	// Skip the creator.
+	file.seek(4, SEEK_CUR);
+
+	// Grab the header size, version number and image format.
+	const uint16_t header_size = file.get16le();
+	const uint16_t version = file.get16le();
+	const uint32_t format = file.get32le();
+	const uint32_t flags = file.get32le();
+
+	// Skip the number of ProDOS blocks; this is surely implicit from the data size?
+	file.seek(4, SEEK_CUR);
+
+	// Get the offset and size of the disk image data.
+	const uint32_t data_start = file.get32le();
+	const uint32_t data_size = file.get32le();
+
+	// Skipped:
+	//
+	//	four bytes, offset to comment
+	//	four bytes, length of comment
+	//	four bytes, offset to creator-specific data
+	//	four bytes, length of creator-specific data
+	//
+	// (all of which relate to optional appendages).
+
+	// Validate.
+	if(header_size < 0x40 || header_size >= file.stats().st_size) {
+		throw Error::InvalidFormat;
+	}
+	if(version > 1) {
+		throw Error::InvalidFormat;
+	}
+
+	// TODO: based on format, instantiate a suitable disk image.
+	switch(format) {
+		default: throw Error::InvalidFormat;
+		case 0:
+			// TODO: DOS 3.3 sector order.
+		break;
+		case 1:
+			// TODO: ProDOS sector order.
+		break;
+		case 2:
+			// TODO: NIB data (yuck!).
+		break;
+	}
+
+	// So: maybe extend FileHolder to handle hiding the header before instantiating
+	// a proper holder? Probably more valid than having each actual disk class do
+	// its own range logic?
+	(void)flags;
+	(void)data_start;
+	(void)data_size;
+
 	throw Error::InvalidFormat;
 	return nullptr;
 }