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

//
//  STX.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 13/11/2019.
//  Copyright © 2019 Thomas Harte. All rights reserved.
//

#include "STX.hpp"

#include "../../Encodings/MFM/Constants.hpp"
#include "../../Encodings/MFM/Shifter.hpp"
#include "../../Encodings/MFM/Encoder.hpp"
#include "../../Track/PCMTrack.hpp"

#include "Utility/ImplicitSectors.hpp"

#include <array>
#include <cstdlib>
#include <cstring>

using namespace Storage::Disk;

namespace {

class TrackConstructor {
public:
	constexpr static uint16_t NoFirstOffset = std::numeric_limits<uint16_t>::max();

	struct Sector {
		// Records explicitly present in the sector table.
		uint32_t data_offset = 0;
		size_t bit_position = 0;
		uint16_t data_duration = 0;
		std::array<uint8_t, 6> address = {0, 0, 0, 0, 0, 0};
		uint8_t status = 0;

		// Other facts that will either be supplied by the STX or which
		// will be empty.
		std::vector<uint8_t> fuzzy_mask;
		std::vector<uint8_t> contents;
		std::vector<uint16_t> timing;

		// Accessors.

		/// @returns The byte size of this sector, according to its address mark.
		uint32_t data_size() const {
			return uint32_t(128 << (address[3]&3));
		}

		struct Fragment {
			int prior_syncs = 1;
			std::vector<uint8_t> contents;
		};

		/// @returns The byte stream this sector address would produce if a WD read track command were to observe it.
		std::vector<Fragment> get_track_address_fragments() const {
			return track_fragments(address.begin(), address.begin() + 4, {0xa1, 0xa1, 0xfe});
		}

		/// @returns The byte stream this sector data would produce if a WD read track command were to observe it.
		std::vector<Fragment> get_track_data_fragments() const {
			return track_fragments(contents.begin(), contents.end(), {0xa1, 0xa1, 0xfb});
		}

		/*!
			Acts like std::search except that it tries to find a start location from which all of the members of @c fragments
			can be found in successive order with no more than a 'permissible' amount of gap between them.

			Where 'permissible' is derived empirically from trial and error; in practice it's a measure of the number of bytes
			a WD may produce when it has encountered a false sync, and I don't have documentation on that. So it's
			derived from in-practice testing of STXs (which, hopefully, contain an accurate copy of what a WD would do,
			so are themselves possibly a way to research that).
		*/
		template <typename Iterator> static Iterator find_fragments(Iterator begin, Iterator end, const std::vector<Fragment> &fragments) {
			while(true) {
				// To match the fragments, they must all be found, in order, with at most two bytes of gap.
				auto this_begin = begin;
				std::vector<uint8_t>::const_iterator first_location = end;
				bool is_found = true;
				bool is_first = true;
				for(auto fragment: fragments) {
					auto location = std::search(this_begin, end, fragment.contents.begin(), fragment.contents.end());

					// If fragment wasn't found at all, it's never going to be found. So game over.
					if(location == end) {
						return location;
					}

					// Otherwise, either mark
					if(is_first) {
						first_location = location;
					} else if(location > this_begin + 5*fragment.prior_syncs) {
						is_found = false;
						break;
					}

					is_first = false;
					this_begin = location + ssize_t(fragment.contents.size());
				}

				if(is_found) {
					return first_location;
				}

				// TODO: can I assume more than this?
				++begin;
			}
			return end;
		}

	private:
		/// @returns The effect of encoding @c prefix followed by the bytes from @c begin to @c end as MFM data and then decoding them as if
		/// observed by a WD read track command, split into fragments separated by any instances of false sync — since it's still unclear to me exactly what
		/// a WD should put out in those instances.
		template <typename T> static std::vector<Fragment> track_fragments(T begin, T end, std::initializer_list<uint8_t> prefix) {
			std::vector<Fragment> result;
			result.reserve(size_t(end - begin) + prefix.size());

			PCMSegment segment;
			std::unique_ptr<Storage::Encodings::MFM::Encoder> encoder = Storage::Encodings::MFM::GetMFMEncoder(segment.data);

			// Encode prefix.
			for(auto c: prefix) {
				encoder->add_byte(c);
			}

			// Encode body.
			while(begin != end) {
				encoder->add_byte(*begin);
				++begin;
			}

			// Decode, starting a new segment upon any false sync since I don't have good documentation
			// presently on exactly how a WD should react to those.
			using Shifter = Storage::Encodings::MFM::Shifter;
			Shifter shifter;
			shifter.set_should_obey_syncs(true);
			shifter.set_is_mfm(true);

			result.emplace_back();

			// Add whatever comes from the track.
			int ignore_count = 0;
			for(auto bit: segment.data) {
				shifter.add_input_bit(int(bit));

				const auto token = shifter.get_token();
				if(token != Shifter::None) {
					if(ignore_count) {
						--ignore_count;
						continue;
					}

					// If anything other than a byte is encountered,
					// skip it and the next thing to be reported,
					// beginning a new fragment.
					if(token != Shifter::Token::Byte) {
						ignore_count = 1;

						if(!result.back().contents.empty()) {
							result.emplace_back();
						} else {
							++result.back().prior_syncs;
						}

						continue;
					}

					// This was an ordinary byte, retain it.
					result.back().contents.push_back(shifter.get_byte());
				}
			}

			return result;
		}
	};


	TrackConstructor(const std::vector<uint8_t> &track_data, const std::vector<Sector> &sectors, size_t track_size, uint16_t first_sync) :
		track_data_(track_data), sectors_(sectors), track_size_(track_size), first_sync_(first_sync) {
		(void)first_sync_;
	}

	std::shared_ptr<PCMTrack> get_track() {
		// If no contents are supplied, return an unformatted track.
		if(sectors_.empty() && track_data_.empty()) {
			return nullptr;
		}

		// If no sectors are on this track, just encode the track data. STX allows speed
		// changes and fuzzy bits in sectors only.
		if(sectors_.empty()) {
			PCMSegment segment;
			std::unique_ptr<Storage::Encodings::MFM::Encoder> encoder = Storage::Encodings::MFM::GetMFMEncoder(segment.data);
			for(auto c: track_data_) {
				encoder->add_byte(c);
			}
			return std::make_shared<PCMTrack>(segment);
		}

		// Otherwise, seek to encode the sectors, using the track data to
		// fill in the gaps (if provided).
		std::unique_ptr<Storage::Encodings::MFM::Encoder> encoder;
		std::vector<PCMSegment> segments;

		// To reconcile the list of sectors with the WD get track-style track image,
		// use sector bodies as definitive and refer to the track image for in-fill.
		auto track_position = track_data_.begin();
		const auto sync_mark = {0xa1, 0xa1};
		struct Location {
			enum Type {
				Address, Data
			} type;
			std::vector<uint8_t>::const_iterator position;
			const Sector &sector;

			Location(Type type, std::vector<uint8_t>::const_iterator position, const Sector &sector) : type(type), position(position), sector(sector) {}
		};
		std::vector<Location> locations;
		for(const auto &sector: sectors_) {
			{
				// Find out what the address would look like, if found in a read track.
				const auto address_fragments = sector.get_track_address_fragments();

				// Try to locate the header within the track image; if it can't be found then settle for
				// the next thing that looks like a header of any sort.
				auto address_position = TrackConstructor::Sector::find_fragments(track_position, track_data_.end(), address_fragments);
				if(address_position == track_data_.end()) {
					address_position = std::search(track_position, track_data_.end(), sync_mark.begin(), sync_mark.end());
				}

				// Place this address only if somewhere to put it was found.
				if(address_position != track_data_.end()) {
					locations.emplace_back(Location::Address, address_position, sector);

					// Advance the track position.
					track_position = address_position + 6;
				}
			}

			// Do much the same thing for the data, if it exists.
			if(!(sector.status & 0x10)) {
				const auto data_fragments = sector.get_track_data_fragments();

				auto data_position = TrackConstructor::Sector::find_fragments(track_position, track_data_.end(), data_fragments);
				if(data_position == track_data_.end()) {
					data_position = std::search(track_position, track_data_.end(), sync_mark.begin(), sync_mark.end());
				}
				if(data_position == track_data_.end()) {
					// Desperation: guess from the given offset.
					data_position = track_data_.begin() + (sector.bit_position / 16);
				}

				locations.emplace_back(Location::Data, data_position, sector);
				track_position = data_position + sector.data_size();
			}
		}

		const auto encoder_at_rate = [&encoder, &segments](unsigned int rate) -> Storage::Encodings::MFM::Encoder* {
			if(!encoder) {
				segments.emplace_back();
				segments.back().length_of_a_bit = Storage::Time(int(rate + 1), 1);
				encoder = Storage::Encodings::MFM::GetMFMEncoder(segments.back().data, &segments.back().fuzzy_mask);
			} else if(segments.back().length_of_a_bit.length != rate) {
				segments.emplace_back();
				segments.back().length_of_a_bit = Storage::Time(int(rate + 1), 1);
				encoder->reset_target(segments.back().data, &segments.back().fuzzy_mask);
			}
			return encoder.get();
		};

		// Write out, being wary of potential overlapping sectors, and copying from track_data_ to fill in gaps.
		auto location = locations.begin();
		track_position = track_data_.begin();
		while(location != locations.end()) {
//			assert(location->position >= track_position && location->position < track_data_.end());

			// Advance to location.position.
			auto default_rate_encoder = encoder_at_rate(127);
			while(track_position < location->position) {
				default_rate_encoder->add_byte(*track_position);
				++track_position;
			}

			// Write the relevant mark and fill in a default number of bytes to write.
			size_t bytes_to_write;
			switch(location->type) {
				default:
				case Location::Address:
					default_rate_encoder->add_ID_address_mark();
					bytes_to_write = 6;
				break;
				case Location::Data:
					if(location->sector.status & 0x20)
						default_rate_encoder->add_deleted_data_address_mark();
					else
						default_rate_encoder->add_data_address_mark();
					bytes_to_write = location->sector.data_size() + 2;
				break;
			}
			track_position += 3;

			// Decide how much data to write for real; this [partially] allows for overlapping sectors.
			auto next_location = location + 1;
			if(next_location != locations.end()) {
				bytes_to_write = std::min(bytes_to_write, size_t(next_location->position - track_position));
			}

			// Skip that many bytes from the underlying track image.
			track_position += ssize_t(bytes_to_write);

			// Write bytes.
			switch(location->type) {
				default:
				case Location::Address:
					for(size_t c = 0; c < bytes_to_write; ++c)
						default_rate_encoder->add_byte(location->sector.address[c]);
				break;
				case Location::Data: {
					const auto body_bytes = std::min(bytes_to_write, size_t(location->sector.data_size()));

					// If timing information is attached to this sector, write each byte at the proper speed.
					// (TODO: is there any benefit to optiming number of calls to encoder_at_rate?)
					if(!location->sector.timing.empty()) {
						for(size_t c = 0; c < body_bytes; ++c) {
							encoder_at_rate(location->sector.timing[c >> 4])->add_byte(
								location->sector.contents[c],
								location->sector.fuzzy_mask.empty() ? 0x00 : location->sector.fuzzy_mask[c]
							);
						}
					} else {
						for(size_t c = 0; c < body_bytes; ++c) {
							default_rate_encoder->add_byte(
								location->sector.contents[c],
								location->sector.fuzzy_mask.empty() ? 0x00 : location->sector.fuzzy_mask[c]
							);
						}
					}

					// Add a CRC only if it fits (TODO: crop if necessary?).
					if(bytes_to_write & 127) {
						default_rate_encoder = encoder_at_rate(127);
						default_rate_encoder->add_crc((location->sector.status & 0x18) == 0x10);
					}
				} break;
			}

			// Advance location.
			++location;
		}

		// Write anything remaining from the track image.
		while(track_position < track_data_.end()) {
			encoder->add_byte(*track_position);
			++track_position;
		}

		// Count total size of track.
		size_t track_size = 0;
		for(auto &segment: segments) {
			track_size += segment.data.size();
		}

		// Write generic padding up until the specified track size.
		while(track_size < track_size_ * 16) {
			encoder->add_byte(0x4e);
			track_size += 16;
		}

		// Pad out to the minimum size a WD can actually make sense of.
		// I've no idea why it's valid for tracks to be shorter than this,
		// so likely I'm suffering a comprehansion deficiency.
		// TODO: determine why this isn't correct (or, possibly, is).
		while(track_size < 5750 * 16) {
			encoder->add_byte(0x4e);
			track_size += 16;
		}

		return std::make_shared<PCMTrack>(segments);
	}

private:
	const std::vector<uint8_t> &track_data_;
	const std::vector<Sector> &sectors_;
	const size_t track_size_;
	const uint16_t first_sync_;
};

}

STX::STX(const std::string &file_name) : file_(file_name) {
	// Require that this be a version 3 Pasti.
	if(!file_.check_signature("RSY", 4)) throw Error::InvalidFormat;
	if(file_.get16le() != 3) throw Error::InvalidFormat;

	// Skip: tool used, 2 reserved bytes.
	file_.seek(4, SEEK_CUR);

	// Skip the track count, test for a new-style encoding, skip a reserved area.
	file_.seek(1, SEEK_CUR);
	is_new_format_ = file_.get8() == 2;
	file_.seek(4, SEEK_CUR);

	// Set all tracks absent.
	memset(offset_by_track_, 0, sizeof(offset_by_track_));

	// Parse the tracks table to fill in offset_by_track_. The only available documentation
	// for STX is unofficial and makes no promise about track order. Hence the bucket sort,
	// effectively putting them into track order.
	//
	//	Track descriptor layout:
	//
	//	0	4	Record size.
	//	4	4	Number of bytes in fuzzy mask record.
	//	8	2	Number of sectors on track.
	//	10	2	Track flags.
	//	12	2	Total number of bytes on track.
	//	14	1	Track number (b7 = side, b0-b6 = track).
	//	15	1	Track type.
	track_count_ = 0;
	head_count_ = 1;
	while(true) {
		const long offset = file_.tell();
		const uint32_t size = file_.get32le();
		if(file_.eof()) break;

		// Skip fields other than track position, then fill in table position and advance.
		file_.seek(10, SEEK_CUR);

		const uint8_t track_position = file_.get8();
		offset_by_track_[track_position] = offset;

		// Update the maximum surface dimensions.
		track_count_ = std::max(track_count_, track_position & 0x7f);
		head_count_ = std::max(head_count_, ((track_position & 0x80) >> 6));

		// Seek next track start.
		file_.seek(offset + size, SEEK_SET);
	}
}

HeadPosition STX::get_maximum_head_position() {
	return HeadPosition(track_count_ + 1);	// Same issue as MSA; must fix!
}

int STX::get_head_count() {
	return head_count_;
}

std::shared_ptr<::Storage::Disk::Track> STX::get_track_at_position(::Storage::Disk::Track::Address address) {
	// These images have two sides, at most.
	if(address.head > 1) return nullptr;

	// If no track was found, there's nothing to do here.
	const int track_index = (address.head * 0x80) + address.position.as_int();
	if(!offset_by_track_[track_index]) return nullptr;

	// Seek to the track (skipping the record size field).
	file_.seek(offset_by_track_[track_index] + 4, SEEK_SET);

	// Grab the track description.
	const uint32_t fuzzy_size = file_.get32le();
	const uint16_t sector_count = file_.get16le();
	const uint16_t flags = file_.get16le();
	const size_t track_length = file_.get16le();
	file_.seek(2, SEEK_CUR);		// Skip track type; despite being named, it's apparently unused.

	// If this is a trivial .ST-style sector dump, life is easy.
	if(!(flags & 1)) {
		const auto sector_contents = file_.read(sector_count * 512);
		return track_for_sectors(sector_contents.data(), sector_count, uint8_t(address.position.as_int()), uint8_t(address.head), 1, 2, Storage::Encodings::MFM::Density::Double);
	}

	// Grab sector records, if provided.
	std::vector<TrackConstructor::Sector> sectors;
	std::vector<uint8_t> track_data;
	uint16_t first_sync = TrackConstructor::NoFirstOffset;

	// Sector records come first.
	for(uint16_t c = 0; c < sector_count; ++c) {
		sectors.emplace_back();
		sectors.back().data_offset = file_.get32le();
		sectors.back().bit_position = file_.get16le();
		sectors.back().data_duration = file_.get16le();
		file_.read(sectors.back().address);
		sectors.back().status = file_.get8();
		file_.seek(1, SEEK_CUR);
	}

	// If fuzzy masks are specified, attach them to their corresponding sectors.
	if(fuzzy_size) {
		uint32_t fuzzy_bytes_read = 0;
		for(auto &sector: sectors) {
			// Check for the fuzzy bit mask; if it's not set then
			// there's nothing for this sector.
			if(!(sector.status & 0x80)) continue;

			// Make sure there are enough bytes left.
			const uint32_t expected_bytes = sector.data_size();
			if(fuzzy_bytes_read + expected_bytes > fuzzy_size) break;

			// Okay, there are, so read them.
			sector.fuzzy_mask = file_.read(expected_bytes);
			fuzzy_bytes_read += expected_bytes;
		}

		// It should be true that the number of fuzzy masks caused
		// exactly the correct number of fuzzy bytes to be read.
		// But, just in case, check and possibly skip some.
		file_.seek(long(fuzzy_size) - fuzzy_bytes_read, SEEK_CUR);
	}

	// There may or may not be a track image. Grab it if so.

	// Grab the read-track-esque track contents, if available.
	long sector_start = file_.tell();
	if(flags & 0x40) {
		// Bit 6 => there is a track to read;
		// bit
		if(flags & 0x80) {
			first_sync = file_.get16le();
			const uint16_t image_size = file_.get16le();
			track_data = file_.read(image_size);
		} else {
			const uint16_t image_size = file_.get16le();
			track_data = file_.read(image_size);
		}
	}

	// Grab sector contents.
	long end_of_data = file_.tell();
	for(auto &sector: sectors) {
		// If the FDC record-not-found flag is set, there's no sector body to find.
		// Otherwise there's a sector body in the file somewhere.
		if(!(sector.status & 0x10)) {
			file_.seek(sector.data_offset + sector_start, SEEK_SET);
			sector.contents = file_.read(sector.data_size());
			end_of_data = std::max(end_of_data, file_.tell());
		}
	}
	file_.seek(end_of_data, SEEK_SET);

	// Grab timing info if available.
	file_.seek(4, SEEK_CUR);	// Skip the timing descriptor, as it includes no new information.
	for(auto &sector: sectors) {
		// Skip any sector with no intra-sector bit width variation.
		if(!(sector.status&1)) continue;

		const auto timing_record_size = sector.data_size() >> 4;	// Use one entry per 16 bytes.
		sector.timing.resize(timing_record_size);

		if(!is_new_format_) {
			// Generate timing records for Macrodos/Speedlock.
			// Timing is specified in quarters. Which might or might not be
			// quantities of 128 bytes, who knows?
			for(size_t c = 0; c < timing_record_size; ++c) {
				if(c < (timing_record_size >> 2)) {
					sector.timing[c] = 127;
				} else if(c < ((timing_record_size*2) >> 2)) {
					sector.timing[c] = 133;
				} else if(c < ((timing_record_size*3) >> 2)) {
					sector.timing[c] = 121;
				} else {
					sector.timing[c] = 127;
				}
			}

			continue;
		}

		// This is going to be a new-format record.
		for(size_t c = 0; c < timing_record_size; ++c) {
			sector.timing[c] = file_.get16be();		// These values are big endian, unlike the rest of the file.
		}
	}

	// Sort the sectors by starting position. It's perfectly possible that they're always
	// sorted in STX but, again, the reverse-engineered documentation doesn't make the
	// promise, so that's that.
	std::sort(sectors.begin(), sectors.end(),
		[] (TrackConstructor::Sector &lhs, TrackConstructor::Sector &rhs) {
			return lhs.bit_position < rhs.bit_position;
	});


	/*
		Having reached here, the actual stuff of parsing the file structure should be done.
		So hand off to the TrackConstructor.

	*/

	TrackConstructor constructor(track_data, sectors, track_length, first_sync);
	return constructor.get_track();
}