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346 lines
12 KiB
C++
346 lines
12 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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using namespace Storage::Encodings::AppleGCR;
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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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const uint8_t five_and_three_unmapping[] = {
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/* 0xab */ 0x00, 0xff, 0x01, 0x02, 0x03,
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/* 0xb0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x04, 0x05, 0x06,
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/* 0xb8 */ 0xff, 0xff, 0x07, 0x08, 0xff, 0x09, 0x0a, 0x0b,
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/* 0xc0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
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/* 0xc8 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
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/* 0xd0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0c, 0x0d,
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/* 0xd8 */ 0xff, 0xff, 0x0e, 0x0f, 0xff, 0x10, 0x11, 0x12,
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/* 0xe0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
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/* 0xe8 */ 0xff, 0xff, 0x13, 0x14, 0xff, 0x15, 0x16, 0x17,
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/* 0xf0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x18, 0x19, 0x1a,
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/* 0xf8 */ 0xff, 0xff, 0x1b, 0x1c, 0xff, 0x1d, 0x1e, 0x1f,
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};
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uint8_t unmap_five_and_three(uint8_t source) {
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if(source < 0xab) return 0xff;
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return five_and_three_unmapping[source - 0xab];
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}
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std::unique_ptr<Sector> decode_macintosh_sector(const std::array<uint_fast8_t, 8> *header, const std::unique_ptr<Sector> &original) {
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// There must be a header and at least 704 bytes to decode from.
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if(!header || original->data.size() < 704) return nullptr;
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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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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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return nullptr;
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}
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}
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// Allocate a sector.
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auto sector = std::make_unique<Sector>();
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sector->data.resize(704);
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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(size_t index = 0; index < sector->data.size(); ++index) {
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sector->data[index] = unmap_six_and_two(original->data[index]);
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if(sector->data[index] == 0xff) {
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return nullptr;
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}
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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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return nullptr;
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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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// Report success.
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sector->data.resize(524);
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sector->encoding = Sector::Encoding::Macintosh;
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return sector;
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}
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std::unique_ptr<Sector> decode_appleii_sector(const std::array<uint_fast8_t, 8> *header, const std::unique_ptr<Sector> &original, bool is_five_and_three) {
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// There must be at least 411 bytes to decode a five-and-three sector from;
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// there must be only 343 if this is a six-and-two sector.
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const size_t data_size = is_five_and_three ? 411 : 343;
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if(original->data.size() < data_size) return nullptr;
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// Allocate a sector.
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auto sector = std::make_unique<Sector>();
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sector->data.resize(data_size);
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// If there is a header, check for apparent four and four encoding.
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if(header) {
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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) return nullptr;
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// Fill the header fields.
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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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const 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)) return nullptr;
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}
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// Unmap the sector contents.
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for(size_t index = 0; index < data_size; ++index) {
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sector->data[index] = is_five_and_three ? unmap_five_and_three(original->data[index]) : unmap_six_and_two(original->data[index]);
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if(sector->data[index] == 0xff) {
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return nullptr;
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}
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}
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// Undo the XOR step on sector contents, then check and discard the 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()) return nullptr;
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sector->data.resize(sector->data.size() - 1);
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if(is_five_and_three) {
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// TODO: the below is almost certainly incorrect; Beneath Apple DOS partly documents
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// the process, enough to give the basic outline below of how five source bytes are
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// mapped to eight five-bit quantities, but isn't clear on the order those bytes will
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// end up in on disk.
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std::vector<uint8_t> buffer(256);
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for(size_t c = 0; c < 0x33; ++c) {
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const uint8_t *const base = §or->data[0x032 - c];
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buffer[(c * 5) + 0] = uint8_t((base[0x000] << 3) | (base[0x100] >> 2));
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buffer[(c * 5) + 1] = uint8_t((base[0x033] << 3) | (base[0x133] >> 2));
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buffer[(c * 5) + 2] = uint8_t((base[0x066] << 3) | (base[0x166] >> 2));
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buffer[(c * 5) + 3] = uint8_t((base[0x099] << 3) | ((base[0x100] & 2) << 1) | (base[0x133] & 2) | ((base[0x166] & 2) >> 1));
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buffer[(c * 5) + 4] = uint8_t((base[0x0cc] << 3) | ((base[0x100] & 1) << 2) | ((base[0x133] & 1) << 1) | (base[0x166] & 1));
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}
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buffer[255] = uint8_t((sector->data[0x0ff] << 3) | (sector->data[0x199] >> 2));
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sector->data = std::move(buffer);
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sector->encoding = Sector::Encoding::FiveAndThree;
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} else {
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// Undo the 6 and 2 mapping.
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constexpr uint8_t bit_reverse[] = {0, 2, 1, 3};
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const auto unmap = [&](std::size_t byte, std::size_t nibble, int shift) {
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sector->data[86 + byte] = uint8_t(
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(sector->data[86 + byte] << 2) | bit_reverse[(sector->data[nibble] >> shift)&3]
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);
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};
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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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// 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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sector->encoding = Sector::Encoding::SixAndTwo;
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}
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// Return successfully.
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return sector;
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}
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}
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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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bool is_five_and_three = false;
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bool has_header = false;
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while(bit < segment.data.size() || pointer != scanning_sentinel || header_delay) {
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shift_register = 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] == five_and_three_header_prologue[2] ||
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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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) {
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pointer = 0;
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if(scanner[2] != data_prologue[2]) {
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is_five_and_three = scanner[2] == five_and_three_header_prologue[2];
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}
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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 { // i.e. the third symbol is from either of the header prologues.
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sector_location = 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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has_header = true;
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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, for this sector;
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// if not, or if
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const bool is_invalid = is_five_and_three ? (unmap_five_and_three(value) == 0xff) : (unmap_six_and_two(value) == 0xff);
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if(is_invalid || new_sector->data.size() >= 704) {
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// The second byte of the standard epilogue is 'illegal', as is the first byte of
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// all prologues. So either a whole sector has been captured up to now, or it hasn't.
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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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const bool had_header = has_header;
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has_header = false;
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// Potentially this is a Macintosh sector.
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auto macintosh_sector = decode_macintosh_sector(had_header ? &header : nullptr, sector);
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if(macintosh_sector) {
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result.insert({sector_location, std::move(*macintosh_sector)});
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continue;
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}
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// Apple II then?
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auto appleii_sector = decode_appleii_sector(had_header ? &header : nullptr, sector, is_five_and_three);
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if(appleii_sector) {
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result.insert({sector_location, std::move(*appleii_sector)});
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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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