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hid_parser: parse unaligned fields correctly #21

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47 changes: 18 additions & 29 deletions hid_parser/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -85,37 +85,21 @@ def _data_bit_shift(data: Sequence[int], offset: int, length: int) -> Sequence[i
if not length > 0:
raise ValueError(f'Invalid specified length: {length}')

left_extra = offset % 8
right_extra = 8 - (offset + length) % 8
start_offset = offset // 8
end_offset = (offset + length - 1) // 8
byte_length = (length - 1) // 8 + 1
byte_offset = offset // 8
byte_len = (length + 7) // 8
in_end = (length + offset + 7) // 8

if not end_offset < len(data):
raise ValueError(f'Invalid data length: {len(data)} (expecting {end_offset + 1})')
if in_end > len(data):
raise ValueError(f'Invalid data length: {len(data)} (expecting {in_end})')

shifted = [0] * byte_length

if right_extra == 8:
right_extra = 0

i = end_offset
shifted_offset = byte_length - 1
while shifted_offset >= 0:
shifted[shifted_offset] = data[i] >> right_extra

if i - start_offset >= 0:
shifted[shifted_offset] |= (data[i - 1] & (0xff >> (8 - right_extra))) << (8 - right_extra)

shifted_offset -= 1
i -= 1

shifted[0] &= 0xff >> ((left_extra + right_extra) % 8)

if not len(shifted) == byte_length:
raise ValueError('Invalid data')

return shifted
in_slice = data[byte_offset:in_end]
# Take advantage of long ints in Python >= 3
val = int.from_bytes(in_slice, byteorder='little')
# Shift leading LSBs
val >>= offset % 8
# Mask trailing MSBs
val &= (1 << length) - 1
return val.to_bytes(byte_len, byteorder='little')


class BitNumber(int):
Expand Down Expand Up @@ -333,6 +317,7 @@ def __init__(
self._logical_max = logical_max
self._physical_min = physical_min
self._physical_max = physical_max
self._signed = (logical_min < 0) or (logical_max < 0)
# TODO: unit

@property
Expand Down Expand Up @@ -428,6 +413,10 @@ def parse(self, data: Sequence[int]) -> UsageValue:
)
): # int
value = int.from_bytes(data, byteorder='little')
# Sign extend
if self._signed and (value >> (self.size - 1)) != 0:
# Subtracting twice the sign bit saves explicit clearing
value -= (1 << self.size)
elif (
hid_parser.data.UsageTypes.ON_OFF_CONTROL in self.usage.usage_types
and not self.preferred_state
Expand Down