3DS ECD archive decompression

decompress_ecd.py

def bit_is_set(byteval,idx):
    return ((byteval&(1<<idx))!=0)

def read_int_bbo(f, s, signed=False):
	return int.from_bytes(f.read(s), byteorder='big', signed=signed)

def decompressECD(f, out_fname): #file handle at ECD start position, out file name
	magic = f.read(3)
	if not magic == b'ECD':
		return
	version = f.read(1)
	skip = read_int_bbo(f, 4)
	compSize = read_int_bbo(f, 4)
	uncompSize = read_int_bbo(f, 4)
	
	output = bytearray()
	buffer = bytearray(bytes(1024))
	output.extend(f.read(skip))
	while True:
		flag = f.read(1)
		if len(flag) == 0:
			break
		flagi = int.from_bytes(flag, 'big')
		for i in range(0, 8):
			if bit_is_set(flagi, i):
				c = read_int_bbo(f, 1)
				buffer[len(output) % 1024] = c
				output.append(c)
			else:
				b1 = read_int_bbo(f, 1)
				b2 = read_int_bbo(f, 1)
				l = b2 % 64 + 3
				off = ((b2 >> 6) << 8) + b1 + 74
				for i in range(off, off+l):
					offset=i % 1024
					c = buffer[offset]
					buffer[len(output) % 1024] = c
					output.append(c)
			if len(output) >= uncompSize:
				break
		else:
			continue
		break
	with open(out_fname, 'wb') as x:
		x.write(output)

with open('test_ecd.bin', 'rb') as r:
	decompressECD(r, 'test_ecd_decompressed.bin')