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Refactor rsrcfork.compress to prepare for new compression type support

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dgelessus 2019-07-14 21:41:14 +02:00
parent 67a16d34a6
commit 360833f940
1 changed files with 49 additions and 30 deletions
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79
rsrcfork/compress.py
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@ -9,21 +9,26 @@ __all__ = [
# The signature of all compressed resource data, 0xa89f6572 in hex, or "®üer" in MacRoman.
COMPRESSED_SIGNATURE = b"\xa8\x9fer"
# The compression type commonly used for application resources.
COMPRESSED_TYPE_APPLICATION = 0x0801
# The compression type commonly used for System file resources.
COMPRESSED_TYPE_SYSTEM = 0x0901
# Header for a compressed resource.
# Common header for compressed resources of all types.
# 4 bytes: Signature (see above).
# 2 bytes: Length of the header. (This meaning is just a guess - the field's value is always 0x0012, so there's no way to know for certain what it means.)
# 2 bytes: Compression type. Known so far: 0x0901 is used in the System file's resources. 0x0801 is used in other files' resources. Currently only the first type is supported.
# 2 bytes: Length of the complete header (this common part and the type-specific part that follows it). (This meaning is just a guess - the field's value is always 0x0012, so there's no way to know for certain what it means.)
# 2 bytes: Compression type. Known so far: 0x0901 is used in the System file's resources. 0x0801 is used in other files' resources.
STRUCT_COMPRESSED_HEADER = struct.Struct(">4sHH")
# Header continuation part for a "system" compressed resource.
# 4 bytes: Length of the data after decompression.
# 2 bytes: The ID of the 'dcmp' resource that can decompress this resource. Currently only ID 2 is supported.
# 2 bytes: Unknown meaning, doesn't appear to have any effect on the decompression algorithm. Usually zero, sometimes set to a small integer (< 10). On 'lpch' resources, the value is always nonzero, and sometimes larger than usual.
# 1 byte: Number of entries in the custom lookup table minus one. Set to zero if the default lookup table is used.
# 1 byte: Flags. See the CompressedFlags enum below for details.
STRUCT_COMPRESSED_HEADER = struct.Struct(">4sHHIhHBB")
# 1 byte: Flags. See the CompressedSystemFlags enum below for details.
STRUCT_COMPRESSED_SYSTEM_HEADER = struct.Struct(">IhHBB")
# Default lookup table for compressed resources.
# Default lookup table for "system" compressed resources.
# If the custom table flag is set, a custom table (usually with fewer than 256 entries) is used instead of this one.
# This table was obtained by decompressing a manually created compressed resource that refers to every possible table entry. Detailed steps:
# 1. Create a file with a resource fork
@ -69,7 +74,7 @@ COMPRESSED_DEFAULT_TABLE_DATA = (
COMPRESSED_DEFAULT_TABLE = [COMPRESSED_DEFAULT_TABLE_DATA[i:i + 2] for i in range(0, len(COMPRESSED_DEFAULT_TABLE_DATA), 2)]
class CompressedFlags(enum.Flag):
class CompressedSystemFlags(enum.Flag):
TAGGED = 1 << 1 # The compressed data is tagged, meaning that it consists of "blocks" of a tag byte followed by 8 table references and/or literals. See comments in the decompress function for details.
CUSTOM_TABLE = 1 << 0 # A custom lookup table is included before the compressed data, which is used instead of the default table.
@ -94,7 +99,11 @@ def _split_bits(i: int) -> typing.Tuple[bool, bool, bool, bool, bool, bool, bool
)
def _decompress_untagged(data: bytes, decompressed_length: int, table: typing.Sequence[bytes], *, debug: bool=False) -> bytes:
def _decompress_application(data: bytes, *, debug: bool=False) -> bytes:
raise DecompressError('"Application" compression type not supported yet')
def _decompress_system_untagged(data: bytes, decompressed_length: int, table: typing.Sequence[bytes], *, debug: bool=False) -> bytes:
parts = []
i = 0
while i < len(data):
@ -113,7 +122,7 @@ def _decompress_untagged(data: bytes, decompressed_length: int, table: typing.Se
return b"".join(parts)
def _decompress_tagged(data: bytes, decompressed_length: int, table: typing.Sequence[bytes], *, debug: bool=False) -> bytes:
def _decompress_system_tagged(data: bytes, decompressed_length: int, table: typing.Sequence[bytes], *, debug: bool=False) -> bytes:
parts = []
i = 0
while i < len(data):
@ -151,20 +160,8 @@ def _decompress_tagged(data: bytes, decompressed_length: int, table: typing.Sequ
return b"".join(parts)
def decompress(data: bytes, *, debug: bool=False) -> bytes:
"""Decompress the given compressed resource data."""
try:
signature, header_length, compression_type, decompressed_length, dcmp_id, unknown, table_count_m1, flags_raw = STRUCT_COMPRESSED_HEADER.unpack_from(data, offset=0)
except struct.error:
raise DecompressError(f"Invalid header")
if signature != COMPRESSED_SIGNATURE:
raise DecompressError(f"Invalid signature: {signature!r}, expected {COMPRESSED_SIGNATURE}")
if header_length != STRUCT_COMPRESSED_HEADER.size:
raise DecompressError(f"Unsupported header length: 0x{header_length:>04x}, expected 0x{STRUCT_COMPRESSED_HEADER.size:>04x}")
if compression_type != COMPRESSED_TYPE_SYSTEM:
raise DecompressError(f"Unsupported compression type: 0x{compression_type:>04x}, expected 0x{COMPRESSED_TYPE_SYSTEM:>04x}")
def _decompress_system(data: bytes, *, debug: bool=False) -> bytes:
decompressed_length, dcmp_id, unknown, table_count_m1, flags_raw = STRUCT_COMPRESSED_SYSTEM_HEADER.unpack_from(data)
if dcmp_id != 2:
raise DecompressError(f"Unsupported 'dcmp' ID: {dcmp_id}, expected 2")
if debug:
@ -175,15 +172,15 @@ def decompress(data: bytes, *, debug: bool=False) -> bytes:
print(f"Table has {table_count} entries")
try:
flags = CompressedFlags(flags_raw)
flags = CompressedSystemFlags(flags_raw)
except ValueError:
raise DecompressError(f"Unsupported flags set: 0b{flags_raw:>08b}, currently only bits 0 and 1 are supported")
if debug:
print(f"Flags: {flags}")
if CompressedFlags.CUSTOM_TABLE in flags:
table_start = STRUCT_COMPRESSED_HEADER.size
if CompressedSystemFlags.CUSTOM_TABLE in flags:
table_start = STRUCT_COMPRESSED_SYSTEM_HEADER.size
data_start = table_start + table_count * 2
table = []
for i in range(table_start, data_start, 2):
@ -194,16 +191,38 @@ def decompress(data: bytes, *, debug: bool=False) -> bytes:
if table_count_m1 != 0:
raise DecompressError(f"table_count_m1 field is {table_count_m1}, but must be zero when the default table is used")
table = COMPRESSED_DEFAULT_TABLE
data_start = STRUCT_COMPRESSED_HEADER.size
data_start = STRUCT_COMPRESSED_SYSTEM_HEADER.size
if debug:
print("Using default table")
if CompressedFlags.TAGGED in flags:
decompress_func = _decompress_tagged
if CompressedSystemFlags.TAGGED in flags:
decompress_func = _decompress_system_tagged
else:
decompress_func = _decompress_untagged
decompress_func = _decompress_system_untagged
decompressed = decompress_func(data[data_start:], decompressed_length, table, debug=debug)
if len(decompressed) != decompressed_length:
raise DecompressError(f"Actual length of decompressed data ({len(decompressed)}) does not match length stored in resource ({decompressed_length})")
return decompressed
def decompress(data: bytes, *, debug: bool=False) -> bytes:
"""Decompress the given compressed resource data."""
try:
signature, header_length, compression_type = STRUCT_COMPRESSED_HEADER.unpack_from(data)
except struct.error:
raise DecompressError(f"Invalid header")
if signature != COMPRESSED_SIGNATURE:
raise DecompressError(f"Invalid signature: {signature!r}, expected {COMPRESSED_SIGNATURE}")
if header_length != 0x12:
raise DecompressError(f"Unsupported header length: 0x{header_length:>04x}, expected 0x12")
if compression_type == COMPRESSED_TYPE_APPLICATION:
decompress_func = _decompress_application
elif compression_type == COMPRESSED_TYPE_SYSTEM:
decompress_func = _decompress_system
else:
raise DecompressError(f"Unsupported compression type: 0x{compression_type:>04x}")
return decompress_func(data[STRUCT_COMPRESSED_HEADER.size:], debug=debug)