python-rsrcfork/rsrcfork/compress/dcmp2.py

import enum
import io
import struct
import typing

from . import common


# Parameters for a 'dcmp' (2)-compressed resource.
# 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 ParameterFlags enum below for details.
STRUCT_PARAMETERS = struct.Struct(">HBB")

# Default lookup table.
# 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 with the following contents:
# b'\xa8\x9fer\x00\x12\t\x01\x00\x00\x02\x00\x00\x02\x00\x00\x00\x00' + bytes(range(256))
DEFAULT_TABLE_DATA = (
	b"\x00\x00\x00\x08N\xba nNu\x00\x0c\x00\x04p\x00"
	b"\x00\x10\x00\x02Hn\xff\xfc`\x00\x00\x01H\xe7/."
	b"NV\x00\x06N^/\x00a\x00\xff\xf8/\x0b\xff\xff"
	b"\x00\x14\x00\n\x00\x18 _\x00\x0e P?<\xff\xf4"
	b"L\xee0.g\x00L\xdf&n\x00\x12\x00\x1cBg"
	b"\xff\xf00</\x0c\x00\x03N\xd0\x00 p\x01\x00\x16"
	b"-@H\xc0 xr\x00X\x8ff\x00O\xefB\xa7"
	b"g\x06\xff\xfaU\x8f(n?\x00\xff\xfe/<g\x04"
	b"Y\x8f k\x00$ \x1fA\xfa\x81\xe1f\x04g\x08"
	b"\x00\x1aN\xb9P\x8f .\x00\x07N\xb0\xff\xf2=@"
	b"\x00\x1e hf\x06\xff\xf6N\xf9\x08\x00\x0c@=|"
	b"\xff\xec\x00\x05 <\xff\xe8\xde\xfcJ.\x000\x00("
	b"/\x08 \x0b`\x02Bn-H S @\x18\x00"
	b"`\x04A\xee/(/\x01g\nH@ \x07f\x08"
	b"\x01\x18/\x070(?.0+\"n/+\x00,"
	b"g\x0c\"_`\x06\x00\xff0\x07\xff\xeeS@\x00@"
	b"\xff\xe4J@f\n\x00\x0fN\xadp\xff\"\xd8Hk"
	b"\x00\" Kg\x0eJ\xaeN\x90\xff\xe0\xff\xc0\x00*"
	b"'@g\x02Q\xc8\x02\xb6Hz\"x\xb0n\xff\xe6"
	b"\x00\t2.>\x00HA\xff\xeaC\xeeNqt\x00"
	b"/, l\x00<\x00&\x00P\x18\x800\x1f\"\x00"
	b"f\x0c\xff\xda\x008f\x020, \x0c-nB@"
	b"\xff\xe2\xa9\xf0\xff\x007|\xe5\x80\xff\xdcHhYO"
	b"\x004>\x1f`\x08/\x06\xff\xde`\np\x02\x002"
	b"\xff\xcc\x00\x80\"Q\x10\x1f1|\xa0)\xff\xd8R@"
	b"\x01\x00g\x10\xa0#\xff\xce\xff\xd4 \x06Hx\x00."
	b"POC\xfag\x12v\x00A\xe8Jn \xd9\x00Z"
	b"\x7f\xffQ\xca\x00\\.\x00\x02@H\xc7g\x14\x0c\x80"
	b".\x9f\xff\xd6\x80\x00\x10\x00HBJk\xff\xd2\x00H"
	b"JGN\xd1 o\x00A`\x0c*xB.2\x00"
	b"etg\x16\x00DHm \x08Hl\x0b|&@"
	b"\x04\x00\x00h m\x00\r*@\x00\x0b\x00>\x02 "
)
DEFAULT_TABLE = [DEFAULT_TABLE_DATA[i:i + 2] for i in range(0, len(DEFAULT_TABLE_DATA), 2)]


class ParameterFlags(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.


def _split_bits(i: int) -> typing.Tuple[bool, bool, bool, bool, bool, bool, bool, bool]:
	"""Split a byte (an int) into its 8 bits (a tuple of 8 bools)."""
	
	assert i in range(256)
	return (
		bool(i & (1 << 7)),
		bool(i & (1 << 6)),
		bool(i & (1 << 5)),
		bool(i & (1 << 4)),
		bool(i & (1 << 3)),
		bool(i & (1 << 2)),
		bool(i & (1 << 1)),
		bool(i & (1 << 0)),
	)


def _decompress_system_untagged(stream: "common.PeekableIO", decompressed_length: int, table: typing.Sequence[bytes], *, debug: bool=False) -> typing.Iterator[bytes]:
	while True: # Loop is terminated when EOF is reached.
		table_index_data = stream.read(1)
		if not table_index_data:
			# End of compressed data.
			break
		elif not stream.peek(1) and decompressed_length % 2 != 0:
			# Special case: if we are at the last byte of the compressed data, and the decompressed data has an odd length, the last byte is a single literal byte, and not a table reference.
			if debug:
				print(f"Last byte: {table_index_data}")
			yield table_index_data
			break
		
		# Compressed data is untagged, every byte is a table reference.
		(table_index,) = table_index_data
		if debug:
			print(f"Reference: {table_index} -> {table[table_index]}")
		yield table[table_index]

def _decompress_system_tagged(stream: "common.PeekableIO", decompressed_length: int, table: typing.Sequence[bytes], *, debug: bool=False) -> typing.Iterator[bytes]:
	while True: # Loop is terminated when EOF is reached.
		tag_data = stream.read(1)
		if not tag_data:
			# End of compressed data.
			break
		elif not stream.peek(1) and decompressed_length % 2 != 0:
			# Special case: if we are at the last byte of the compressed data, and the decompressed data has an odd length, the last byte is a single literal byte, and not a tag or a table reference.
			if debug:
				print(f"Last byte: {tag_data}")
			yield tag_data
			break
		
		# Compressed data is tagged, each tag byte is followed by 8 table references and/or literals.
		(tag,) = tag_data
		if debug:
			print(f"Tag: 0b{tag:>08b}")
		for is_ref in _split_bits(tag):
			if is_ref:
				# This is a table reference (a single byte that is an index into the table).
				table_index_data = stream.read(1)
				if not table_index_data:
					# End of compressed data.
					break
				(table_index,) = table_index_data
				if debug:
					print(f"Reference: {table_index} -> {table[table_index]}")
				yield table[table_index]
			else:
				# This is a literal (two uncompressed bytes that are literally copied into the output).
				literal = stream.read(2)
				if not literal:
					# End of compressed data.
					break
				# Note: the literal may be only a single byte long if it is located exactly at EOF. This is intended and expected - the 1-byte literal is yielded normally, and on the next iteration, decompression is terminated as EOF is detected.
				if debug:
					print(f"Literal: {literal}")
				yield literal


def decompress_stream(header_info: common.CompressedHeaderInfo, stream: typing.BinaryIO, *, debug: bool=False) -> typing.Iterator[bytes]:
	"""Decompress compressed data in the format used by 'dcmp' (2)."""
	
	if not isinstance(header_info, common.CompressedSystemHeaderInfo):
		raise common.DecompressError(f"Incorrect header type: {type(header_info).__qualname__}")
	
	unknown, table_count_m1, flags_raw = STRUCT_PARAMETERS.unpack(header_info.parameters)
	
	if debug:
		print(f"Value of unknown parameter field: 0x{unknown:>04x}")
	
	table_count = table_count_m1 + 1
	if debug:
		print(f"Table has {table_count} entries")
	
	try:
		flags = ParameterFlags(flags_raw)
	except ValueError:
		raise common.DecompressError(f"Unsupported flags set: 0b{flags_raw:>08b}, currently only bits 0 and 1 are supported")
	
	if debug:
		print(f"Flags: {flags}")
	
	if ParameterFlags.CUSTOM_TABLE in flags:
		table = []
		for _ in range(table_count):
			table.append(common.read_exact(stream, 2))
		if debug:
			print(f"Using custom table: {table}")
	else:
		if table_count_m1 != 0:
			raise common.DecompressError(f"table_count_m1 field is {table_count_m1}, but must be zero when the default table is used")
		table = DEFAULT_TABLE
		if debug:
			print("Using default table")
	
	if ParameterFlags.TAGGED in flags:
		decompress_func = _decompress_system_tagged
	else:
		decompress_func = _decompress_system_untagged
	
	yield from decompress_func(common.make_peekable(stream), header_info.decompressed_length, table, debug=debug)

def decompress(header_info: common.CompressedHeaderInfo, data: bytes, *, debug: bool=False) -> bytes:
	return b"".join(decompress_stream(header_info, io.BytesIO(data), debug=debug))