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Add a backup strategy for non-standard formats.

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This commit is contained in:
Thomas Harte 2023-12-07 21:52:51 -05:00
parent ac47b93542
commit d8d33ac419
1 changed files with 42 additions and 2 deletions
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44
Storage/Disk/DiskImage/Formats/NIB.cpp
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@ -103,6 +103,28 @@ std::shared_ptr<::Storage::Disk::Track> NIB::get_track_at_position(::Storage::Di
}
}
// If searching for sector prologues didn't work, look for runs of FF FF FF FF FF.
if(sync_starts.empty()) {
for(size_t index = 0; index < track_data.size(); ++index) {
if(track_data[index] == 0xff) {
size_t length = 0;
size_t end = index;
while(track_data[end] == 0xff && length < 5) {
end = (end + 1) % track_data.size();
++length;
}
if(length == 5) {
sync_starts.insert(index);
while(track_data[index] == 0xff && index < track_data.size()) {
++index;
}
}
}
}
}
PCMSegment segment;
// If the track started in a sync block, write sync first.
@ -110,6 +132,15 @@ std::shared_ptr<::Storage::Disk::Track> NIB::get_track_at_position(::Storage::Di
segment += Encodings::AppleGCR::six_and_two_sync(int(start_index));
}
// Cap slip bits per location to avoid packing too many bits onto the track
// and thereby making it over-dense.
//
// The magic constant 51,024 comes from the quantity that most DSKs are encoded to;
// the minimum of 5 is the minimum number of FFs that must have slip bits in order to
// guarantee synchronisation.
const int max_slip_bytes_per_location =
std::max(5, int((51024 - (track_data.size() * 8)) / sync_starts.size()));
std::size_t index = start_index;
for(const auto location: sync_starts) {
// Write data from index to sync_start.
@ -129,8 +160,17 @@ std::shared_ptr<::Storage::Disk::Track> NIB::get_track_at_position(::Storage::Di
while(index < track_length && track_data[index] == 0xff)
++index;
if(index - location)
segment += Encodings::AppleGCR::six_and_two_sync(int(index - location));
int leadin_length = int(index - location);
if(leadin_length) {
// If this is more bytes than are permitted slip bits, encode the first bunch as non-slipping FFs.
if(leadin_length > max_slip_bytes_per_location) {
std::vector<uint8_t> ffs(size_t(leadin_length - max_slip_bytes_per_location), 0xff);
segment += PCMSegment(ffs);
leadin_length = max_slip_bytes_per_location;
}
segment += Encodings::AppleGCR::six_and_two_sync(leadin_length);
}
}
// If there's still data remaining on the track, write it out. If a sync ran over