fs_helpers.py

import struct
from io import BytesIO

PADDING_BYTES = b"This is padding data to alignme"

class InvalidOffsetError(Exception):
  pass

def data_len(data):
  data_length = data.seek(0, 2)
  return data_length

def make_copy_data(data):
  copy_data = read_all_bytes(data)
  return BytesIO(copy_data)


def read_all_bytes(data):
  data.seek(0)
  return data.read()

def read_bytes(data, offset, length):
  data.seek(offset)
  return data.read(length)

def write_bytes(data, offset, raw_bytes):
  data.seek(offset)
  data.write(raw_bytes)

def read_and_unpack_bytes(data, offset, length, format_string):
  data.seek(offset)
  requested_data = data.read(length)
  unpacked_data = struct.unpack(format_string, requested_data)
  return unpacked_data

def write_and_pack_bytes(data, offset, new_values, format_string):
  packed_data = struct.pack(format_string, *new_values)
  data.seek(offset)
  data.write(packed_data)


def read_str(data, offset, length):
  data_length = data.seek(0, 2)
  if offset+length > data_length:
    raise InvalidOffsetError("Offset %X, length %X is past the end of the data (length %X)." % (offset, length, data_length))
  data.seek(offset)
  string = data.read(length).decode("shift_jis")
  string = string.rstrip("\0") # Remove trailing null bytes
  return string

def try_read_str(data, offset, length):
  try:
    return read_str(data, offset, length)
  except UnicodeDecodeError:
    return None
  except InvalidOffsetError:
    return None

def read_str_until_null_character(data, offset):
  data_length = data.seek(0, 2)
  if offset > data_length:
    raise InvalidOffsetError("Offset %X is past the end of the data (length %X)." % (offset, data_length))
  
  temp_offset = offset
  str_length = 0
  while temp_offset < data_length:
    data.seek(temp_offset)
    char = data.read(1)
    if char == b"\0":
      break
    else:
      str_length += 1
    temp_offset += 1
  
  data.seek(offset)
  str = data.read(str_length).decode("shift_jis")
  
  return str

def write_str(data, offset, new_string, max_length):
  # Writes a fixed-length string.
  # Although it is fixed-length, it still must have a null character terminating it, so the real max length is one less than the passed max_length argument.
  
  str_len = len(new_string)
  if str_len >= max_length:
    raise Exception("String \"%s\" is too long (max length including null byte: %X)" % (new_string, max_length))
  
  padding_length = max_length - str_len
  null_padding = b"\x00"*padding_length
  new_value = new_string.encode("shift_jis") + null_padding
  
  data.seek(offset)
  data.write(new_value)

def write_magic_str(data, offset, new_string, max_length):
  # Writes a fixed-length string that does not have to end with a null byte.
  # This is for magic file format identifiers.
  
  str_len = len(new_string)
  if str_len > max_length:
    raise Exception("String %s is too long (max length %X)" % (new_string, max_length))
  
  padding_length = max_length - str_len
  null_padding = b"\x00"*padding_length
  new_value = new_string.encode("shift_jis") + null_padding
  
  data.seek(offset)
  data.write(new_value)

def write_str_with_null_byte(data, offset, new_string):
  # Writes a non-fixed length string.
  
  str_len = len(new_string)
  write_str(data, offset, new_string, str_len+1)


def read_u8(data, offset):
  data.seek(offset)
  return struct.unpack(">B", data.read(1))[0]

def read_u16(data, offset):
  data.seek(offset)
  return struct.unpack(">H", data.read(2))[0]

def read_u32(data, offset):
  data.seek(offset)
  return struct.unpack(">I", data.read(4))[0]

def read_float(data, offset):
  data.seek(offset)
  return struct.unpack(">f", data.read(4))[0]


def read_s8(data, offset):
  data.seek(offset)
  return struct.unpack(">b", data.read(1))[0]

def read_s16(data, offset):
  data.seek(offset)
  return struct.unpack(">h", data.read(2))[0]

def read_s32(data, offset):
  data.seek(offset)
  return struct.unpack(">i", data.read(4))[0]


def write_u8(data, offset, new_value):
  new_value = struct.pack(">B", new_value)
  data.seek(offset)
  data.write(new_value)

def write_u16(data, offset, new_value):
  new_value = struct.pack(">H", new_value)
  data.seek(offset)
  data.write(new_value)

def write_u32(data, offset, new_value):
  new_value = struct.pack(">I", new_value)
  data.seek(offset)
  data.write(new_value)

def write_float(data, offset, new_value):
  new_value = struct.pack(">f", new_value)
  data.seek(offset)
  data.write(new_value)


def write_s8(data, offset, new_value):
  new_value = struct.pack(">b", new_value)
  data.seek(offset)
  data.write(new_value)

def write_s16(data, offset, new_value):
  new_value = struct.pack(">h", new_value)
  data.seek(offset)
  data.write(new_value)

def write_s32(data, offset, new_value):
  new_value = struct.pack(">i", new_value)
  data.seek(offset)
  data.write(new_value)


def align_data_to_nearest(data, size, padding_bytes=PADDING_BYTES):
  current_end = data_len(data)
  next_offset = current_end + (size - current_end % size) % size
  padding_needed = next_offset - current_end
  data.seek(current_end)
  padding = padding_bytes*(padding_needed // len(padding_bytes))
  padding += padding_bytes[:padding_needed % len(padding_bytes)]
  data.write(padding)

def pad_offset_to_nearest(offset, size):
  next_offset = offset + (size - offset % size) % size
  return next_offset