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Avoid off-by-one error in sector decoding.

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Specific issue: retaining the 256 bytes up to _and including_ the checksum, rather than excluding.
This commit is contained in:
Thomas Harte 2022-09-16 15:47:38 -04:00
parent 9cba56237d
commit d17fadbe0b
1 changed files with 9 additions and 6 deletions
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15
Storage/Disk/Encodings/AppleGCR/SegmentParser.cpp
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@ -175,11 +175,12 @@ std::unique_ptr<Sector> decode_appleii_sector(const std::array<uint_fast8_t, 8>
} }
} }
// Undo the XOR step on sector contents and check that checksum. // Undo the XOR step on sector contents, then check and discard the checksum.
for(std::size_t c = 1; c < sector->data.size(); ++c) { for(std::size_t c = 1; c < sector->data.size(); ++c) {
sector->data[c] ^= sector->data[c-1]; sector->data[c] ^= sector->data[c-1];
} }
if(sector->data.back()) return nullptr; if(sector->data.back()) return nullptr;
sector->data.resize(sector->data.size() - 1);
if(is_five_and_three) { if(is_five_and_three) {
// TODO: the below is almost certainly incorrect; Beneath Apple DOS partly documents // TODO: the below is almost certainly incorrect; Beneath Apple DOS partly documents
@ -275,7 +276,8 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
scanner[2] == five_and_three_header_prologue[2] || scanner[2] == five_and_three_header_prologue[2] ||
scanner[2] == header_prologue[2] || scanner[2] == header_prologue[2] ||
scanner[2] == data_prologue[2] scanner[2] == data_prologue[2]
)) { )
) {
pointer = 0; pointer = 0;
if(scanner[2] != data_prologue[2]) { if(scanner[2] != data_prologue[2]) {
@ -309,20 +311,21 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
new_sector.reset(); new_sector.reset();
pointer = scanning_sentinel; pointer = scanning_sentinel;
const bool had_header = has_header;
has_header = false;
// Potentially this is a Macintosh sector. // Potentially this is a Macintosh sector.
auto macintosh_sector = decode_macintosh_sector(has_header ? &header : nullptr, sector); auto macintosh_sector = decode_macintosh_sector(had_header ? &header : nullptr, sector);
if(macintosh_sector) { if(macintosh_sector) {
result.insert(std::make_pair(sector_location, std::move(*macintosh_sector))); result.insert(std::make_pair(sector_location, std::move(*macintosh_sector)));
continue; continue;
} }
// Apple II then? // Apple II then?
auto appleii_sector = decode_appleii_sector(has_header ? &header : nullptr, sector, is_five_and_three); auto appleii_sector = decode_appleii_sector(had_header ? &header : nullptr, sector, is_five_and_three);
if(appleii_sector) { if(appleii_sector) {
result.insert(std::make_pair(sector_location, std::move(*appleii_sector))); result.insert(std::make_pair(sector_location, std::move(*appleii_sector)));
} }
has_header = false;
} else { } else {
new_sector->data.push_back(value); new_sector->data.push_back(value);
} }