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Move decompressed data length field into common compressed header

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Previously it was handled separately (but identically) by both the
system and application decompression functions.
This commit is contained in:
dgelessus 2019-08-21 23:30:20 +02:00
parent 5bc2c0cc81
commit 3d444bda10
1 changed files with 17 additions and 19 deletions
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36
rsrcfork/compress.py
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@ -18,15 +18,15 @@ COMPRESSED_TYPE_SYSTEM = 0x0901
# 4 bytes: Signature (see above).
# 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")
# 4 bytes: Length of the data after decompression.
STRUCT_COMPRESSED_HEADER = struct.Struct(">4sHHI")
# Header continuation part for an "application" compressed resource.
# 4 bytes: Length of the data after decompression.
# 1 byte: "Working buffer fractional size" - the ratio of the compressed data size to the uncompressed data size, times 256.
# 1 byte: "Expansion buffer size" - the maximum number of bytes that the data might grow during decompression.
# 2 bytes: The ID of the 'dcmp' resource that can decompress this resource. Currently only ID 0 is supported.
# 2 bytes: Reserved (always zero).
STRUCT_COMPRESSED_APPLICATION_HEADER = struct.Struct(">IBBhH")
STRUCT_COMPRESSED_APPLICATION_HEADER = struct.Struct(">BBhH")
# Lookup table for codes in range(0x4b, 0xfe) in "application" compressed resources.
# This table was obtained by decompressing a manually created compressed resource that refers to every possible table entry. Detailed steps:
@ -69,12 +69,11 @@ COMPRESSED_APPLICATION_TABLE = [COMPRESSED_APPLICATION_TABLE_DATA[i:i + 2] for i
assert len(COMPRESSED_APPLICATION_TABLE) == len(range(0x4b, 0xfe))
# 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 CompressedSystemFlags enum below for details.
STRUCT_COMPRESSED_SYSTEM_HEADER = struct.Struct(">IhHBB")
STRUCT_COMPRESSED_SYSTEM_HEADER = struct.Struct(">hHBB")
# 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.
@ -164,11 +163,10 @@ def _read_variable_length_integer(data: bytes, position: int) -> typing.Tuple[in
return int.from_bytes(data[position:position+1], "big", signed=True), 1
def _decompress_application(data: bytes, *, debug: bool=False) -> bytes:
decompressed_length, working_buffer_fractional_size, expansion_buffer_size, dcmp_id, reserved = STRUCT_COMPRESSED_APPLICATION_HEADER.unpack_from(data)
def _decompress_application(data: bytes, decompressed_length: int, *, debug: bool=False) -> bytes:
working_buffer_fractional_size, expansion_buffer_size, dcmp_id, reserved = STRUCT_COMPRESSED_APPLICATION_HEADER.unpack_from(data)
if debug:
print(f"Decompressed length: {decompressed_length}")
print(f"Working buffer fractional size: {working_buffer_fractional_size} (=> {len(data) * 256 / working_buffer_fractional_size})")
print(f"Expansion buffer size: {expansion_buffer_size}")
@ -431,9 +429,6 @@ def _decompress_application(data: bytes, *, debug: bool=False) -> bytes:
# This is necessary because nearly all codes generate data in groups of 2 or 4 bytes, so it is basically impossible to represent data with an odd length using this compression format.
decompressed = decompressed[:-1]
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
@ -494,8 +489,8 @@ def _decompress_system_tagged(data: bytes, decompressed_length: int, table: typi
return b"".join(parts)
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)
def _decompress_system(data: bytes, decompressed_length: int, *, debug: bool=False) -> bytes:
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:
@ -534,17 +529,14 @@ def _decompress_system(data: bytes, *, debug: bool=False) -> bytes:
else:
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
return decompress_func(data[data_start:], decompressed_length, table, debug=debug)
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)
signature, header_length, compression_type, decompressed_length = STRUCT_COMPRESSED_HEADER.unpack_from(data)
except struct.error:
raise DecompressError(f"Invalid header")
if signature != COMPRESSED_SIGNATURE:
@ -559,4 +551,10 @@ def decompress(data: bytes, *, debug: bool=False) -> bytes:
else:
raise DecompressError(f"Unsupported compression type: 0x{compression_type:>04x}")
return decompress_func(data[STRUCT_COMPRESSED_HEADER.size:], debug=debug)
if debug:
print(f"Decompressed length: {decompressed_length}")
decompressed = decompress_func(data[STRUCT_COMPRESSED_HEADER.size:], decompressed_length, 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