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Cleanup ASIF calculations #86

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bcfbe50
Cleanup ASIF calculations
Schamper Jun 16, 2026
2b3faae
Update asif.py
Schamper Jun 19, 2026
6e4462c
Expand unit test
Schamper Jun 19, 2026
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Update asif.py
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@Schamper
Schamper committed Jun 19, 2026
commit 2b3faaed0a78ea6073baf0c440069901f6ef00e8
44 changes: 25 additions & 19 deletions dissect/hypervisor/disk/asif.py
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Original file line number Diff line number Diff line change
Expand Up @@ -175,15 +175,19 @@ def entries(self) -> list[int]:
self.asif.fh.seek(self.offset + 8)
return c_asif.uint64[self.asif._num_tables](self.asif.fh)

def table(self, index: int) -> Table:
def table(self, index: int) -> Table | None:
"""Get a table from the directory.

Args:
index: Index of the table in the directory.
"""
if index >= self.asif._num_tables:
raise IndexError("Table index out of range")
return Table(self.asif, index, self.entries[index] * self.asif.chunk_size)

if (entry := self.entries[index]) == 0:
return None

return Table(self.asif, index, entry * self.asif.chunk_size)


class Table:
Expand Down Expand Up @@ -251,25 +255,27 @@ def __init__(self, asif: ASIF, reserved: bool = False):
def _read(self, offset: int, length: int) -> bytes:
result = []
while length:
table = self.directory.table(offset // self.asif._size_per_table)

# Calculate the relative chunk index within the table
relative_block_index = (offset // self.asif.block_size) - (table.virtual_offset // self.asif.block_size)
relative_chunk_index = relative_block_index // self.asif._blocks_per_chunk

# Calculate the chunk group
chunk_group = relative_chunk_index // self.asif._num_chunks_per_group
# Each chunk group has a bitmap entry, so we need to account for that in the entry index
entry_index = relative_chunk_index + chunk_group

chunk = table.entries[entry_index] & 0x7FFFFFFFFFFFFF

read_length = min(length, self.asif.chunk_size)
if chunk == 0:
if (table := self.directory.table(offset // self.asif._size_per_table)) is None:
read_length = min(length, self.asif._size_per_table)
result.append(b"\x00" * read_length)
else:
self.asif.fh.seek(chunk * self.asif.chunk_size)
result.append(self.asif.fh.read(read_length))
# Calculate the relative chunk index within the table
relative_block_index = (offset // self.asif.block_size) - (table.virtual_offset // self.asif.block_size)
relative_chunk_index = relative_block_index // self.asif._blocks_per_chunk

# Calculate the chunk group
chunk_group = relative_chunk_index // self.asif._num_chunks_per_group
# Each chunk group has a bitmap entry, so we need to account for that in the entry index
entry_index = relative_chunk_index + chunk_group

chunk = table.entries[entry_index] & 0x7FFFFFFFFFFFFF

read_length = min(length, self.asif.chunk_size)
if chunk == 0:
result.append(b"\x00" * read_length)
else:
self.asif.fh.seek(chunk * self.asif.chunk_size)
result.append(self.asif.fh.read(read_length))

offset += read_length
length -= read_length
Expand Down
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