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CLK/Storage/Disk/Encodings/AppleGCR/SegmentParser.cpp
Thomas Harte ea26f4f7bf Eliminates test code, adds a caveat.
2020-03-25 21:22:30 -04:00

343 lines
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//
// SegmentParser.cpp
// Clock Signal
//
// Created by Thomas Harte on 04/05/2018.
// Copyright 2018 Thomas Harte. All rights reserved.
//
#include "SegmentParser.hpp"
#include "Encoder.hpp"
#include <array>
namespace {
const uint8_t six_and_two_unmapping[] = {
/* 0x96 */ 0x00, 0x01,
/* 0x98 */ 0xff, 0xff, 0x02, 0x03, 0xff, 0x04, 0x05, 0x06,
/* 0xa0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x07, 0x08,
/* 0xa8 */ 0xff, 0xff, 0xff, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
/* 0xb0 */ 0xff, 0xff, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13,
/* 0xb8 */ 0xff, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a,
/* 0xc0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xc8 */ 0xff, 0xff, 0xff, 0x1b, 0xff, 0x1c, 0x1d, 0x1e,
/* 0xd0 */ 0xff, 0xff, 0xff, 0x1f, 0xff, 0xff, 0x20, 0x21,
/* 0xd8 */ 0xff, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28,
/* 0xe0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x29, 0x2a, 0x2b,
/* 0xe8 */ 0xff, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32,
/* 0xf0 */ 0xff, 0xff, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38,
/* 0xf8 */ 0xff, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
};
uint8_t unmap_six_and_two(uint8_t source) {
if(source < 0x96) return 0xff;
return six_and_two_unmapping[source - 0x96];
}
const uint8_t five_and_three_unmapping[] = {
/* 0xab */ 0x00, 0xff, 0x01, 0x02, 0x03,
/* 0xb0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x04, 0x05, 0x06,
/* 0xb8 */ 0xff, 0xff, 0x07, 0x08, 0xff, 0x09, 0x0a, 0x0b,
/* 0xc0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xc8 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xd0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0c, 0x0d,
/* 0xd8 */ 0xff, 0xff, 0x0e, 0x0f, 0xff, 0x10, 0x11, 0x12,
/* 0xe0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xe8 */ 0xff, 0xff, 0x13, 0x14, 0xff, 0x15, 0x16, 0x17,
/* 0xf0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x18, 0x19, 0x1a,
/* 0xf8 */ 0xff, 0xff, 0x1b, 0x1c, 0xff, 0x1d, 0x1e, 0x1f,
};
uint8_t unmap_five_and_three(uint8_t source) {
if(source < 0xab) return 0xff;
return five_and_three_unmapping[source - 0xab];
}
}
using namespace Storage::Encodings::AppleGCR;
std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment(const Disk::PCMSegment &segment) {
std::map<std::size_t, Sector> result;
uint_fast8_t shift_register = 0;
const std::size_t scanning_sentinel = std::numeric_limits<std::size_t>::max();
std::unique_ptr<Sector> new_sector;
std::size_t sector_location = 0;
std::size_t pointer = scanning_sentinel;
std::array<uint_fast8_t, 8> header{{0, 0, 0, 0, 0, 0, 0, 0}};
std::array<uint_fast8_t, 3> scanner{{0, 0, 0}};
// Scan the track while either all bits haven't been seen yet, or a potential
// sector is still being parsed.
size_t bit = 0;
int header_delay = 0;
bool is_five_and_three = false;
while(bit < segment.data.size() || pointer != scanning_sentinel || header_delay) {
shift_register = static_cast<uint_fast8_t>((shift_register << 1) | (segment.data[bit % segment.data.size()] ? 1 : 0));
++bit;
// Apple GCR parsing: bytes always have the top bit set.
if(!(shift_register&0x80)) continue;
if(header_delay) --header_delay;
// Grab the byte.
const uint_fast8_t value = shift_register;
shift_register = 0;
scanner[0] = scanner[1];
scanner[1] = scanner[2];
scanner[2] = value;
if(pointer == scanning_sentinel) {
if(
scanner[0] == header_prologue[0] &&
scanner[1] == header_prologue[1] &&
(
scanner[2] == five_and_three_header_prologue[2] ||
scanner[2] == header_prologue[2] ||
scanner[2] == data_prologue[2]
)) {
pointer = 0;
if(scanner[2] != data_prologue[2]) {
is_five_and_three = scanner[2] == five_and_three_header_prologue[2];
}
// If this is the start of a data section, and at least
// one header has been witnessed, start a sector.
if(scanner[2] == data_prologue[2]) {
new_sector = std::make_unique<Sector>();
new_sector->data.reserve(710);
} else {
sector_location = static_cast<std::size_t>(bit % segment.data.size());
header_delay = 200; // Allow up to 200 bytes to find the body, if the
// track split comes in between.
}
}
} else {
if(new_sector) {
// Check whether the value just read is a legal GCR byte, in six-and-two
// encoding (which is a strict superset of five-and-three).
const bool is_invalid = is_five_and_three ? (unmap_five_and_three(value) == 0xff) : (unmap_six_and_two(value) == 0xff);
if(is_invalid) {
// The second byte of the standard epilogue is illegal, so this still may
// be a valid sector. If the final byte was the first byte of an epilogue,
// chop it off and see whether the sector is otherwise intelligible.
if(new_sector->data.empty() || new_sector->data.back() != epilogue[0]) {
// No sector found; reset scanning procedure.
new_sector.reset();
pointer = scanning_sentinel;
continue;
}
// Chop off the last byte.
new_sector->data.resize(new_sector->data.size() - 1);
// Move the sector elsewhere for processing; there's definitely no way to proceed with
// the prospective sector if it doesn't parse.
std::unique_ptr<Sector> sector = std::move(new_sector);
new_sector.reset();
pointer = scanning_sentinel;
// Check for valid decoding options.
switch(sector->data.size()) {
default: // This is not a decodeable sector.
break;
case 411: // Potentially this is an Apple II five-and-three sector.
case 343: { // Potentially this is an Apple II six-and-two sector.
// Check for apparent four and four encoding.
uint_fast8_t header_mask = 0xff;
for(auto c : header) header_mask &= c;
header_mask &= 0xaa;
if(header_mask != 0xaa) continue;
sector->address.volume = ((header[0] << 1) | 1) & header[1];
sector->address.track = ((header[2] << 1) | 1) & header[3];
sector->address.sector = ((header[4] << 1) | 1) & header[5];
// Check the header checksum.
// The 0x11 is reverse engineered from the game 'Alien Rain' and is present even on the boot sector,
// so probably isn't copy protection?
uint_fast8_t checksum = (((header[6] << 1) | 1) & header[7]) ^ (is_five_and_three ? 0x11 : 0x00);
if(checksum != (sector->address.volume^sector->address.track^sector->address.sector)) continue;
// Unmap the sector contents.
bool out_of_bounds = false;
for(auto &c : sector->data) {
c = is_five_and_three ? unmap_five_and_three(c) : unmap_six_and_two(c);
if(c == 0xff) {
out_of_bounds = true;
break;
}
}
if(out_of_bounds) continue;
// Undo the XOR step on sector contents and check that checksum.
for(std::size_t c = 1; c < sector->data.size(); ++c) {
sector->data[c] ^= sector->data[c-1];
}
if(sector->data.back()) continue;
// Having checked the checksum, remove it.
sector->data.resize(sector->data.size() - 1);
if(is_five_and_three) {
// TODO: the above is almost certainly incorrect; Beneath Apple DOS partly documents
// the process, enough to give the basic outline below of how five source bytes are
// mapped to eight five-bit quantities, but isn't clear on the order those bytes will
// end up in on disk.
std::vector<uint8_t> buffer(256);
for(size_t c = 0; c < 0x33; ++c) {
const uint8_t *const base = &sector->data[0x032 - c];
buffer[(c * 5) + 0] = uint8_t((base[0x000] << 3) | (base[0x100] >> 2));
buffer[(c * 5) + 1] = uint8_t((base[0x033] << 3) | (base[0x133] >> 2));
buffer[(c * 5) + 2] = uint8_t((base[0x066] << 3) | (base[0x166] >> 2));
buffer[(c * 5) + 3] = uint8_t((base[0x099] << 3) | ((base[0x100] & 2) << 1) | (base[0x133] & 2) | ((base[0x166] & 2) >> 1));
buffer[(c * 5) + 4] = uint8_t((base[0x0cc] << 3) | ((base[0x100] & 1) << 2) | ((base[0x133] & 1) << 1) | (base[0x166] & 1));
}
buffer[255] = uint8_t((sector->data[0x0ff] << 3) | (sector->data[0x199] >> 2));
sector->data = std::move(buffer);
sector->encoding = Sector::Encoding::FiveAndThree;
} else {
// Undo the 6 and 2 mapping.
const uint8_t bit_reverse[] = {0, 2, 1, 3};
#define unmap(byte, nibble, shift) \
sector->data[86 + byte] = static_cast<uint8_t>(\
(sector->data[86 + byte] << 2) | bit_reverse[(sector->data[nibble] >> shift)&3]);
for(std::size_t c = 0; c < 84; ++c) {
unmap(c, c, 0);
unmap(c+86, c, 2);
unmap(c+172, c, 4);
}
unmap(84, 84, 0);
unmap(170, 84, 2);
unmap(85, 85, 0);
unmap(171, 85, 2);
#undef unmap
// Throw away the collection of two-bit chunks from the start of the sector.
sector->data.erase(sector->data.begin(), sector->data.end() - 256);
sector->encoding = Sector::Encoding::SixAndTwo;
}
// Add this sector to the map.
result.insert(std::make_pair(sector_location, std::move(*sector)));
} break;
case 704: { // Potentially this is a Macintosh sector.
// Attempt a six-and-two unmapping of the header.
std::array<uint_fast8_t, 5> decoded_header;
bool out_of_bounds = false;
for(size_t c = 0; c < decoded_header.size(); ++c) {
decoded_header[c] = unmap_six_and_two(header[c]);
if(decoded_header[c] == 0xff) {
out_of_bounds = true;
break;
}
}
if(out_of_bounds) {
continue;
}
// Test the checksum.
if(decoded_header[4] != (decoded_header[0] ^ decoded_header[1] ^ decoded_header[2] ^ decoded_header[3]))
sector->has_header_checksum_error = true;
// Decode the header.
sector->address.track = uint8_t(decoded_header[0] | ((decoded_header[2]&0x1f) << 6));
sector->address.sector = decoded_header[1];
sector->address.format = decoded_header[3];
sector->address.is_side_two = decoded_header[2] & 0x20;
// Reverse the GCR encoding of the sector contents to get back to 6-bit data.
for(auto &c: sector->data) {
c = unmap_six_and_two(c);
if(c == 0xff) {
out_of_bounds = true;
break;
}
}
if(out_of_bounds) {
continue;
}
// The first byte in the sector is a repeat of the sector number; test it
// for correctness.
if(sector->data[0] != sector->address.sector) {
continue;
}
// Cf. the corresponding section of Encoder.cpp for logic below.
int checksum[3] = {0, 0, 0};
for(size_t c = 0; c < 175; ++c) {
// Calculate the rolling checcksum in order to decode the bytes.
checksum[0] = (checksum[0] << 1) | (checksum[0] >> 7);
// All offsets are +1 below, to skip the initial sector number duplicate.
const uint8_t top_bits = sector->data[1 + c*4];
// Decode first byte.
sector->data[0 + c * 3] = uint8_t((sector->data[2 + c*4] + ((top_bits & 0x30) << 2)) ^ checksum[0]);
checksum[2] += sector->data[0 + c * 3] + (checksum[0] >> 8);
// Decode second byte;
sector->data[1 + c * 3] = uint8_t((sector->data[3 + c*4] + ((top_bits & 0x0c) << 4)) ^ checksum[2]);
checksum[1] += sector->data[1 + c * 3] + (checksum[2] >> 8);
// Decode third byte, if there is one.
if(c != 174) {
sector->data[2 + c * 3] = uint8_t((sector->data[4 + c*4] + ((top_bits & 0x03) << 6)) ^ checksum[1]);
checksum[0] += sector->data[2 + c * 3] + (checksum[1] >> 8);
}
// Reset carries.
checksum[0] &= 0xff;
checksum[1] &= 0xff;
checksum[2] &= 0xff;
}
// Test the checksum.
if(
checksum[0] != uint8_t(sector->data[703] + ((sector->data[700] & 0x03) << 6)) ||
checksum[1] != uint8_t(sector->data[702] + ((sector->data[700] & 0x0c) << 4)) ||
checksum[2] != uint8_t(sector->data[701] + ((sector->data[700] & 0x30) << 2))
) sector->has_data_checksum_error = true;
// Chop to size, and that's that.
sector->data.resize(524);
// Add this sector to the map.
sector->encoding = Sector::Encoding::Macintosh;
result.insert(std::make_pair(sector_location, std::move(*sector)));
} break;
}
} else {
new_sector->data.push_back(value);
}
} else {
// Just capture the header in place; it'll be decoded
// once a whole sector has been read.
header[pointer] = value;
++pointer;
if(pointer == 8) {
pointer = scanning_sentinel;
}
}
}
}
return result;
}
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