CLK/Storage/Disk/Encodings/AppleGCR/Encoder.cpp

//
//  AppleGCR.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 21/04/2018.
//  Copyright © 2018 Thomas Harte. All rights reserved.
//

#include "Encoder.hpp"

namespace {

const uint8_t five_and_three_mapping[] = {
	0xab, 0xad, 0xae, 0xaf, 0xb5, 0xb6, 0xb7, 0xba,
	0xbb, 0xbd, 0xbe, 0xbf, 0xd6, 0xd7, 0xda, 0xdb,
	0xdd, 0xde, 0xdf, 0xea, 0xeb, 0xed, 0xee, 0xef,
	0xf5, 0xf6, 0xf7, 0xfa, 0xfb, 0xfd, 0xfe, 0xff
};

const uint8_t six_and_two_mapping[] = {
	0x96, 0x97, 0x9a, 0x9b, 0x9d, 0x9e, 0x9f, 0xa6,
	0xa7, 0xab, 0xac, 0xad, 0xae, 0xaf, 0xb2, 0xb3,
	0xb4, 0xb5, 0xb6, 0xb7, 0xb9, 0xba, 0xbb, 0xbc,
	0xbd, 0xbe, 0xbf, 0xcb, 0xcd, 0xce, 0xcf, 0xd3,
	0xd6, 0xd7, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde,
	0xdf, 0xe5, 0xe6, 0xe7, 0xe9, 0xea, 0xeb, 0xec,
	0xed, 0xee, 0xef, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6,
	0xf7, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff
};

/*!
	Produces a PCM segment containing @c length sync bytes, each aligned to the beginning of
	a @c bit_size -sized window.
*/
Storage::Disk::PCMSegment sync(int length, int bit_size) {
	Storage::Disk::PCMSegment segment;

	// Allocate sufficient storage.
	segment.data.resize(static_cast<size_t>(((length * bit_size) + 7) >> 3), 0);

	while(length--) {
		segment.data[segment.number_of_bits >> 3] |= 0xff >> (segment.number_of_bits & 7);
		if(segment.number_of_bits & 7) {
			segment.data[1 + (segment.number_of_bits >> 3)] |= 0xff << (8 - (segment.number_of_bits & 7));
		}
		segment.number_of_bits += static_cast<unsigned int>(bit_size);
	}

	return segment;
}

}

using namespace Storage::Encodings;

Storage::Disk::PCMSegment AppleGCR::six_and_two_sync(int length) {
	return sync(length, 10);
}

Storage::Disk::PCMSegment AppleGCR::five_and_three_sync(int length) {
	return sync(length, 9);
}

Storage::Disk::PCMSegment AppleGCR::header(uint8_t volume, uint8_t track, uint8_t sector) {
	const uint8_t checksum = volume ^ track ^ sector;

	// Apple headers are encoded using an FM-esque scheme rather than 6 and 2, or 5 and 3.
	Storage::Disk::PCMSegment segment;
	segment.data.resize(14);
	segment.number_of_bits = 14*8;

	segment.data[0] = header_prologue[0];
	segment.data[1] = header_prologue[1];
	segment.data[2] = header_prologue[2];

#define WriteFM(index, value)	\
	segment.data[index+0] = static_cast<uint8_t>(((value) >> 1) | 0xaa);	\
	segment.data[index+1] = static_cast<uint8_t>((value) | 0xaa);	\

	WriteFM(3, volume);
	WriteFM(5, track);
	WriteFM(7, sector);
	WriteFM(9, checksum);

#undef WriteFM

	segment.data[11] = epilogue[0];
	segment.data[12] = epilogue[1];
	segment.data[13] = epilogue[2];

	return segment;
}

Storage::Disk::PCMSegment AppleGCR::five_and_three_data(const uint8_t *source) {
	Storage::Disk::PCMSegment segment;

	segment.data.resize(410 + 7);
	segment.data[0] = data_prologue[0];
	segment.data[1] = data_prologue[1];
	segment.data[2] = data_prologue[2];

	segment.data[414] = epilogue[0];
	segment.data[411] = epilogue[1];
	segment.data[416] = epilogue[2];

//	std::size_t source_pointer = 0;
//	std::size_t destination_pointer = 3;
//	while(source_pointer < 255) {
//		encode_five_and_three_block(&segment.data[destination_pointer], &source[source_pointer]);
//
//		source_pointer += 5;
//		destination_pointer += 8;
//	}

	// Map five-bit values up to full bytes.
	for(std::size_t c = 0; c < 410; ++c) {
		segment.data[3 + c] = five_and_three_mapping[segment.data[3 + c]];
	}

	return segment;
}

Storage::Disk::PCMSegment AppleGCR::six_and_two_data(const uint8_t *source) {
	Storage::Disk::PCMSegment segment;

	segment.data.resize(349);
	segment.number_of_bits = static_cast<unsigned int>(segment.data.size() * 8);

	// Add the prologue and epilogue.
	segment.data[0] = data_prologue[0];
	segment.data[1] = data_prologue[1];
	segment.data[2] = data_prologue[2];

	segment.data[346] = epilogue[0];
	segment.data[347] = epilogue[1];
	segment.data[348] = epilogue[2];

	// Fill in byte values: the first 86 bytes contain shuffled
	// and combined copies of the bottom two bits of the sector
	// contents; the 256 bytes afterwards are the remaining
	// six bits.
	const uint8_t bit_shuffle[] = {0, 2, 1, 3};
	for(std::size_t c = 0; c < 84; ++c) {
		segment.data[3 + c] =
			static_cast<uint8_t>(
				bit_shuffle[source[c]&3] |
				(bit_shuffle[source[c + 86]&3] << 2) |
				(bit_shuffle[source[c + 172]&3] << 4)
			);
	}
	segment.data[87] =
			static_cast<uint8_t>(
				(bit_shuffle[source[84]&3] << 0) |
				(bit_shuffle[source[170]&3] << 2)
			);
	segment.data[88] =
			static_cast<uint8_t>(
				(bit_shuffle[source[85]&3] << 0) |
				(bit_shuffle[source[171]&3] << 2)
			);

	for(std::size_t c = 0; c < 256; ++c) {
		segment.data[3 + 86 + c] = source[c] >> 2;
	}

	// Exclusive OR each byte with the one before it.
	segment.data[345] = segment.data[344];
	std::size_t location = 344;
	while(location > 3) {
		segment.data[location] ^= segment.data[location-1];
		--location;
	}

	// Map six-bit values up to full bytes.
	for(std::size_t c = 0; c < 343; ++c) {
		segment.data[3 + c] = six_and_two_mapping[segment.data[3 + c]];
	}

	return segment;
}