Introduces enough DMK support to progress to failure to parse a track.

Storage/Disk/DiskImage/Formats/DMK.cpp

@@ -0,0 +1,75 @@
+//
+//  DMK.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 08/01/2018.
+//  Copyright © 2018 Thomas Harte. All rights reserved.
+//
+
+#include "DMK.hpp"
+
+using namespace Storage::Disk;
+
+DMK::DMK(const char *file_name) :
+	file_(file_name) {
+	// Determine whether this DMK represents a read-only disk (whether intentionally,
+	// or by virtue of placement).
+	uint8_t read_only_byte = file_.get8();
+	if(read_only_byte != 0x00 && read_only_byte != 0xff) throw ErrorNotDMK;
+	is_read_only_ = (read_only_byte == 0xff) || file_.get_is_known_read_only();
+
+	// Read track count and size.
+	head_position_count_ = static_cast<int>(file_.get8());
+	track_length_ = static_cast<long>(file_.get16le());
+
+	// Track length must be at least 0x80, as that's the size of the IDAM
+	// table before track contents.
+	if(track_length_ < 0x80) throw ErrorNotDMK;
+
+	// Read the file flags and apply them.
+	uint8_t flags = file_.get8();
+	head_count_ = 2 - ((flags & 0x10) >> 4);
+	head_position_count_ /= head_count_;
+	is_purely_single_density_ = !!(flags & 0x40);
+
+	// Skip to the end of the header and check that this is
+	// "in the emulator's native format".
+	file_.seek(0xc, SEEK_SET);
+	uint32_t format = file_.get32le();
+	if(format) throw ErrorNotDMK;
+}
+
+int DMK::get_head_position_count() {
+	return head_position_count_;
+}
+
+int DMK::get_head_count() {
+	return head_count_;
+}
+
+bool DMK::get_is_read_only() {
+	return is_read_only_;
+}
+
+long DMK::get_file_offset_for_position(Track::Address address) {
+	return (address.head*head_count_ + address.position) * track_length_ + 16;
+}
+
+std::shared_ptr<::Storage::Disk::Track> DMK::get_track_at_position(::Storage::Disk::Track::Address address) {
+	file_.seek(get_file_offset_for_position(address), SEEK_SET);
+
+	uint16_t idam_locations[64];
+	std::size_t index = 0;
+	for(std::size_t c = 0; c < sizeof(idam_locations); ++c) {
+		idam_locations[index] = file_.get16le();
+		if((idam_locations[index] & 0x7fff) >= 128) {
+			index++;
+		}
+	}
+
+	// `index` is now the final (sensical) entry in the IDAM location table.
+	// TODO: parse track contents.
+	printf("number of IDAMs: %d", index);
+
+	return nullptr;
+}