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170 lines
5.2 KiB
C++
170 lines
5.2 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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0x00, 0x01, 0xff, 0xff,
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0x02, 0x03, 0xff, 0x04, 0x05, 0x06, 0xff, 0xff,
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0xff, 0xff, 0xff, 0xff, 0x07, 0x08, 0xff, 0xff,
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0xff, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0xff, 0xff,
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0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0xff, 0x14,
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0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0xff, 0xff,
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0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
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0xff, 0x1b, 0xff, 0x1c, 0x1d, 0x1e, 0xff, 0xff,
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0xff, 0x1f, 0xff, 0xff, 0x20, 0x21, 0xff, 0x22,
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0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0xff, 0xff,
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0xff, 0xff, 0xff, 0x29, 0x2a, 0x2b, 0xff, 0x2c,
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0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0xff, 0xff,
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0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0xff, 0x39,
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0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, 0xff, 0xff
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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 1 and 1/8th times; that's long enough to make sure that any sector which straddles the
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// end of the track is caught. Since they're put into a map, it doesn't matter if they're caught twice.
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unsigned int extended_length = segment.number_of_bits + (segment.number_of_bits >> 3);
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for(unsigned int bit = 0; bit < extended_length; ++bit) {
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shift_register = static_cast<uint_fast8_t>((shift_register << 1) | segment.bit(bit % segment.number_of_bits));
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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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// 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.reset(new Sector);
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new_sector->data.reserve(412);
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} else {
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sector_location = static_cast<std::size_t>(bit % segment.number_of_bits);
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}
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}
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} else {
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if(new_sector) {
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new_sector->data.push_back(value);
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// If this is potentially a complete sector, check it out.
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if(new_sector->data.size() == 343) {
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// TODO: allow for 13-sector form.
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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 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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if(c < 0x96 || six_and_two_unmapping[c - 0x96] == 0xff) {
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out_of_bounds = true;
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break;
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}
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c = six_and_two_unmapping[c - 0x96];
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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.
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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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result.insert(std::make_pair(sector_location, std::move(*sector)));
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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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