CLK/Storage/Disk/DiskImage/Formats/DiskCopy42.cpp

//
//  DiskCopy42.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 02/06/2019.
//  Copyright © 2019 Thomas Harte. All rights reserved.
//

#include "DiskCopy42.hpp"

/*
	File format specifications as referenced below are largely
	sourced from the documentation at
	https://wiki.68kmla.org/DiskCopy_4.2_format_specification
*/

using namespace Storage::Disk;

DiskCopy42::DiskCopy42(const std::string &file_name) :
	file_(file_name) {

	// File format starts with 64 bytes dedicated to the disk name;
	// this is a Pascal-style string though there is apparently a
	// bug in one version of Disk Copy that can cause the length to
	// be one too high.
	//
	// Validate the length, then skip the rest of the string.
	const auto name_length = file_.get8();
	if(name_length > 64)
		throw Error::InvalidFormat;

	// Get the length of the data and tag blocks.
	file_.seek(64, SEEK_SET);
	const auto data_block_length = file_.get32be();
	const auto tag_block_length = file_.get32be();
	const auto data_checksum = file_.get32be();
	const auto tag_checksum = file_.get32be();

	// Check that this is a comprehensible disk encoding.
	const auto encoding = file_.get8();
	switch(encoding) {
		default: throw Error::InvalidFormat;

		case 0:	encoding_ = Encoding::GCR400;	break;
		case 1:	encoding_ = Encoding::GCR800;	break;
		case 2:	encoding_ = Encoding::MFM720;	break;
		case 3:	encoding_ = Encoding::MFM1440;	break;
	}
	format_ = file_.get8();

	// Check the magic number.
	const auto magic_number = file_.get16be();
	if(magic_number != 0x0100)
		throw Error::InvalidFormat;

	// Read the data and tags, and verify that enough data
	// was present.
	data_ = file_.read(data_block_length);
	tags_ = file_.read(tag_block_length);

	if(data_.size() != data_block_length || tags_.size() != tag_block_length)
		throw Error::InvalidFormat;

	// Verify the two checksums.
	const auto computed_data_checksum = checksum(data_);
	const auto computed_tag_checksum = checksum(tags_, 12);

	if(computed_tag_checksum != tag_checksum || computed_data_checksum != data_checksum)
		throw Error::InvalidFormat;
}

uint32_t DiskCopy42::checksum(const std::vector<uint8_t> &data, size_t bytes_to_skip) {
	uint32_t result = 0;

	// Checksum algorith is: take each two bytes as a big-endian word; add that to a
	// 32-bit accumulator and then rotate the accumulator right one position.
	for(size_t c = bytes_to_skip; c < data.size(); c += 2) {
		const uint16_t next_word = uint16_t((data[c] << 8) | data[c+1]);
		result += next_word;
		result = (result >> 1) | (result << 31);
	}

	return result;
}

HeadPosition DiskCopy42::get_maximum_head_position() {
	return HeadPosition(80);
}

int DiskCopy42::get_head_count() {
	// Bit 5 in the format field indicates whether this disk is double
	// sided, regardless of whether it is GCR or MFM.
	return 1 + ((format_ & 0x20) >> 5);
}

bool DiskCopy42::get_is_read_only() {
	return true;
}

std::shared_ptr<::Storage::Disk::Track> DiskCopy42::get_track_at_position(::Storage::Disk::Track::Address address) {
	/*
		The format_ byte has the following meanings:

		GCR:
			This byte appears on disk as the GCR format nibble in every sector tag.
			The low five bits are an interleave factor, either:

				'2' for 0 8 1 9 2 10 3 11 4 12 5 13 6 14 7 15; or
				'4' for 0 4 8 12 1 5 9 13 2 6 10 14 3 7 11 15.

			Bit 5 indicates double sided or not.

		MFM:
			The low five bits provide sector size as a multiple of 256 bytes.
			Bit 5 indicates double sided or not.
	*/

	// TODO.
	return nullptr;
}