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294 lines
9.9 KiB
C++
294 lines
9.9 KiB
C++
//
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// SegmentParser.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 04/05/2018.
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// Copyright 2018 Thomas Harte. All rights reserved.
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//
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#include "SegmentParser.hpp"
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#include "Encoder.hpp"
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#include <array>
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namespace {
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const uint8_t six_and_two_unmapping[] = {
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/* 0x96 */ 0x00, 0x01,
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/* 0x98 */ 0xff, 0xff, 0x02, 0x03, 0xff, 0x04, 0x05, 0x06,
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/* 0xa0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x07, 0x08,
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/* 0xa8 */ 0xff, 0xff, 0xff, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
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/* 0xb0 */ 0xff, 0xff, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13,
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/* 0xb8 */ 0xff, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a,
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/* 0xc0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
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/* 0xc8 */ 0xff, 0xff, 0xff, 0x1b, 0xff, 0x1c, 0x1d, 0x1e,
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/* 0xd0 */ 0xff, 0xff, 0xff, 0x1f, 0xff, 0xff, 0x20, 0x21,
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/* 0xd8 */ 0xff, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28,
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/* 0xe0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x29, 0x2a, 0x2b,
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/* 0xe8 */ 0xff, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32,
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/* 0xf0 */ 0xff, 0xff, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38,
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/* 0xf8 */ 0xff, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
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};
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uint8_t unmap_six_and_two(uint8_t source) {
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if(source < 0x96) return 0xff;
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return six_and_two_unmapping[source - 0x96];
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}
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}
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using namespace Storage::Encodings::AppleGCR;
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std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment(const Disk::PCMSegment &segment) {
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std::map<std::size_t, Sector> result;
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uint_fast8_t shift_register = 0;
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const std::size_t scanning_sentinel = std::numeric_limits<std::size_t>::max();
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std::unique_ptr<Sector> new_sector;
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std::size_t sector_location = 0;
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std::size_t pointer = scanning_sentinel;
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std::array<uint_fast8_t, 8> header{{0, 0, 0, 0, 0, 0, 0, 0}};
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std::array<uint_fast8_t, 3> scanner{{0, 0, 0}};
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// Scan the track while either all bits haven't been seen yet, or a potential
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// sector is still being parsed.
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size_t bit = 0;
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int header_delay = 0;
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while(bit < segment.data.size() || pointer != scanning_sentinel || header_delay) {
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shift_register = static_cast<uint_fast8_t>((shift_register << 1) | (segment.data[bit % segment.data.size()] ? 1 : 0));
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++bit;
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// Apple GCR parsing: bytes always have the top bit set.
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if(!(shift_register&0x80)) continue;
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if(header_delay) --header_delay;
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// Grab the byte.
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const uint_fast8_t value = shift_register;
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shift_register = 0;
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scanner[0] = scanner[1];
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scanner[1] = scanner[2];
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scanner[2] = value;
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if(pointer == scanning_sentinel) {
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if(
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scanner[0] == header_prologue[0] &&
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scanner[1] == header_prologue[1] &&
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(
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scanner[2] == header_prologue[2] ||
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scanner[2] == data_prologue[2]
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)) {
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pointer = 0;
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// If this is the start of a data section, and at least
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// one header has been witnessed, start a sector.
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if(scanner[2] == data_prologue[2]) {
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new_sector = std::make_unique<Sector>();
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new_sector->data.reserve(710);
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} else {
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sector_location = static_cast<std::size_t>(bit % segment.data.size());
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header_delay = 200; // Allow up to 200 bytes to find the body, if the
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// track split comes in between.
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}
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}
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} else {
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if(new_sector) {
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// Check whether the value just read is a legal GCR byte, in six-and-two
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// encoding (which is a strict superset of five-and-three).
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if(unmap_six_and_two(value) == 0xff) {
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// The second byte of the standard epilogue is illegal, so this still may
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// be a valid sector. If the final byte was the first byte of an epilogue,
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// chop it off and see whether the sector is otherwise intelligible.
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if(new_sector->data.empty() || new_sector->data.back() != epilogue[0]) {
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// No sector found; reset scanning procedure.
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new_sector.reset();
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pointer = scanning_sentinel;
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continue;
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}
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// Chop off the last byte.
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new_sector->data.resize(new_sector->data.size() - 1);
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// Move the sector elsewhere for processing; there's definitely no way to proceed with
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// the prospective sector if it doesn't parse.
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std::unique_ptr<Sector> sector = std::move(new_sector);
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new_sector.reset();
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pointer = scanning_sentinel;
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// Check for valid decoding options.
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switch(sector->data.size()) {
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default: // This is not a decodeable sector.
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break;
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// TODO: check for five-and-three / 13-sector form sectors.
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case 343: { // Potentially this is an Apple II six-and-two sector.
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// Check for apparent four and four encoding.
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uint_fast8_t header_mask = 0xff;
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for(auto c : header) header_mask &= c;
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header_mask &= 0xaa;
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if(header_mask != 0xaa) continue;
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sector->address.volume = ((header[0] << 1) | 1) & header[1];
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sector->address.track = ((header[2] << 1) | 1) & header[3];
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sector->address.sector = ((header[4] << 1) | 1) & header[5];
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// Check the header checksum.
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uint_fast8_t checksum = ((header[6] << 1) | 1) & header[7];
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if(checksum != (sector->address.volume^sector->address.track^sector->address.sector)) continue;
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// Unmap the sector contents as 6 and 2 data.
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bool out_of_bounds = false;
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for(auto &c : sector->data) {
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c = unmap_six_and_two(c);
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if(c == 0xff) {
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out_of_bounds = true;
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break;
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}
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}
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if(out_of_bounds) continue;
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// Undo the XOR step on sector contents and check that checksum.
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for(std::size_t c = 1; c < sector->data.size(); ++c) {
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sector->data[c] ^= sector->data[c-1];
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}
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if(sector->data.back()) continue;
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// Having checked the checksum, remove it.
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sector->data.resize(sector->data.size() - 1);
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// Undo the 6 and 2 mapping.
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const uint8_t bit_reverse[] = {0, 2, 1, 3};
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#define unmap(byte, nibble, shift) \
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sector->data[86 + byte] = static_cast<uint8_t>(\
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(sector->data[86 + byte] << 2) | bit_reverse[(sector->data[nibble] >> shift)&3]);
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for(std::size_t c = 0; c < 84; ++c) {
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unmap(c, c, 0);
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unmap(c+86, c, 2);
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unmap(c+172, c, 4);
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}
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unmap(84, 84, 0);
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unmap(170, 84, 2);
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unmap(85, 85, 0);
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unmap(171, 85, 2);
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#undef unmap
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// Throw away the collection of two-bit chunks from the start of the sector.
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sector->data.erase(sector->data.begin(), sector->data.end() - 256);
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// Add this sector to the map.
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sector->encoding = Sector::Encoding::SixAndTwo;
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result.insert(std::make_pair(sector_location, std::move(*sector)));
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} break;
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case 704: { // Potentially this is a Macintosh sector.
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// Attempt a six-and-two unmapping of the header.
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std::array<uint_fast8_t, 5> decoded_header;
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bool out_of_bounds = false;
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for(size_t c = 0; c < decoded_header.size(); ++c) {
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decoded_header[c] = unmap_six_and_two(header[c]);
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if(decoded_header[c] == 0xff) {
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out_of_bounds = true;
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break;
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}
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}
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if(out_of_bounds) {
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continue;
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}
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// Test the checksum.
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if(decoded_header[4] != (decoded_header[0] ^ decoded_header[1] ^ decoded_header[2] ^ decoded_header[3]))
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sector->has_header_checksum_error = true;
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// Decode the header.
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sector->address.track = uint8_t(decoded_header[0] | ((decoded_header[2]&0x1f) << 6));
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sector->address.sector = decoded_header[1];
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sector->address.format = decoded_header[3];
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sector->address.is_side_two = decoded_header[2] & 0x20;
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// Reverse the GCR encoding of the sector contents to get back to 6-bit data.
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for(auto &c: sector->data) {
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c = unmap_six_and_two(c);
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if(c == 0xff) {
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out_of_bounds = true;
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break;
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}
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}
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if(out_of_bounds) {
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continue;
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}
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// The first byte in the sector is a repeat of the sector number; test it
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// for correctness.
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if(sector->data[0] != sector->address.sector) {
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continue;
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}
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// Cf. the corresponding section of Encoder.cpp for logic below.
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int checksum[3] = {0, 0, 0};
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for(size_t c = 0; c < 175; ++c) {
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// Calculate the rolling checcksum in order to decode the bytes.
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checksum[0] = (checksum[0] << 1) | (checksum[0] >> 7);
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// All offsets are +1 below, to skip the initial sector number duplicate.
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const uint8_t top_bits = sector->data[1 + c*4];
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// Decode first byte.
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sector->data[0 + c * 3] = uint8_t((sector->data[2 + c*4] + ((top_bits & 0x30) << 2)) ^ checksum[0]);
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checksum[2] += sector->data[0 + c * 3] + (checksum[0] >> 8);
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// Decode second byte;
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sector->data[1 + c * 3] = uint8_t((sector->data[3 + c*4] + ((top_bits & 0x0c) << 4)) ^ checksum[2]);
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checksum[1] += sector->data[1 + c * 3] + (checksum[2] >> 8);
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// Decode third byte, if there is one.
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if(c != 174) {
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sector->data[2 + c * 3] = uint8_t((sector->data[4 + c*4] + ((top_bits & 0x03) << 6)) ^ checksum[1]);
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checksum[0] += sector->data[2 + c * 3] + (checksum[1] >> 8);
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}
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// Reset carries.
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checksum[0] &= 0xff;
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checksum[1] &= 0xff;
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checksum[2] &= 0xff;
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}
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// Test the checksum.
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if(
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checksum[0] != uint8_t(sector->data[703] + ((sector->data[700] & 0x03) << 6)) ||
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checksum[1] != uint8_t(sector->data[702] + ((sector->data[700] & 0x0c) << 4)) ||
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checksum[2] != uint8_t(sector->data[701] + ((sector->data[700] & 0x30) << 2))
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) sector->has_data_checksum_error = true;
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// Chop to size, and that's that.
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sector->data.resize(524);
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// Add this sector to the map.
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sector->encoding = Sector::Encoding::Macintosh;
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result.insert(std::make_pair(sector_location, std::move(*sector)));
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} break;
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}
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} else {
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new_sector->data.push_back(value);
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}
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} else {
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// Just capture the header in place; it'll be decoded
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// once a whole sector has been read.
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header[pointer] = value;
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++pointer;
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if(pointer == 8) {
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pointer = scanning_sentinel;
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}
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}
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}
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}
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return result;
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}
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