|
| 1 | +from fields import NBITS_AUTO, Uint |
| 2 | + |
| 3 | + |
| 4 | +class BitsCoder: |
| 5 | + """ |
| 6 | + Tool to encode/decode binary data. |
| 7 | +
|
| 8 | + BitsCoder specifies format of data stored in binary format |
| 9 | + using fields.Fields. When the format is specified you can either |
| 10 | + decode from binary or encode data to binary format. |
| 11 | +
|
| 12 | + Parameters |
| 13 | + ---------- |
| 14 | + fields: list |
| 15 | + List of fields which inherit from `fields.Field` |
| 16 | + byteorder: unicode |
| 17 | + Byte order, choices are: 'big' or 'little' |
| 18 | +
|
| 19 | + """ |
| 20 | + |
| 21 | + def __init__(self, fields, byteorder='big'): |
| 22 | + assert byteorder in ['big', 'little'], \ |
| 23 | + "byteorder must be either 'little' or 'big'" |
| 24 | + self.byteorder = byteorder |
| 25 | + self._fields = fields |
| 26 | + self._add_field_for_remaining_bits() |
| 27 | + self._map = {} |
| 28 | + self._create_fields_map() |
| 29 | + |
| 30 | + def _add_field_for_remaining_bits(self): |
| 31 | + bits_count = sum( |
| 32 | + field.nbits |
| 33 | + for field in self._fields |
| 34 | + if field.nbits != NBITS_AUTO |
| 35 | + ) |
| 36 | + remaining_bits = bits_count % 8 |
| 37 | + if remaining_bits: |
| 38 | + self._fields.append(Uint(nbits=remaining_bits, value=0)) |
| 39 | + |
| 40 | + def _create_fields_map(self): |
| 41 | + underscore_counter = 1 |
| 42 | + for field in self._fields: |
| 43 | + if not field.name: |
| 44 | + field.name = '___{}'.format(underscore_counter) |
| 45 | + underscore_counter += 1 |
| 46 | + self._map[field.name] = field.value |
| 47 | + |
| 48 | + def validate_fields_for_encoding(self): |
| 49 | + if not all(field.enc_value is not None for field in self._fields): |
| 50 | + raise ValueError('All fields need to have a value for encoding') |
| 51 | + |
| 52 | + @property |
| 53 | + def map(self): |
| 54 | + """Filed name to field value map.""" |
| 55 | + self._create_fields_map() |
| 56 | + return self._map |
| 57 | + |
| 58 | + @property |
| 59 | + def list(self): |
| 60 | + """List of field's values.""" |
| 61 | + if not hasattr(self, '_list'): |
| 62 | + self._list = [field.value for field in self._fields] |
| 63 | + return self._list |
| 64 | + |
| 65 | + def encode(self): |
| 66 | + """Encode all fields to bytearray.""" |
| 67 | + self.validate_fields_for_encoding() |
| 68 | + byte_array = bytearray() |
| 69 | + last_byte = 0 |
| 70 | + free_bits_left = 8 |
| 71 | + |
| 72 | + for field in self._fields: |
| 73 | + shift = free_bits_left - field.nbits |
| 74 | + if shift >= 0: |
| 75 | + last_byte |= (field.enc_value << shift) |
| 76 | + free_bits_left = shift |
| 77 | + if shift == 0: |
| 78 | + byte_array.append(last_byte) |
| 79 | + last_byte = 0 |
| 80 | + else: |
| 81 | + shift = abs(shift) |
| 82 | + last_byte |= (field.enc_value >> shift) |
| 83 | + byte_array.append(last_byte) |
| 84 | + full_bytes, remaining_bits = divmod(shift, 8) |
| 85 | + if full_bytes: |
| 86 | + remaining_full_bytes_value = ( |
| 87 | + (field.enc_value >> remaining_bits) & |
| 88 | + (2 ** (8 * full_bytes) - 1) |
| 89 | + ) |
| 90 | + byte_array.extend( |
| 91 | + remaining_full_bytes_value.to_bytes( |
| 92 | + full_bytes, byteorder='big') |
| 93 | + ) |
| 94 | + if remaining_bits: |
| 95 | + free_bits_left = 8 - remaining_bits |
| 96 | + last_byte = ( |
| 97 | + (field.enc_value & (2 ** remaining_bits - 1)) << |
| 98 | + (8 - remaining_bits) |
| 99 | + ) |
| 100 | + else: |
| 101 | + free_bits_left = 8 |
| 102 | + last_byte = 0 |
| 103 | + if 0 < free_bits_left < 8: |
| 104 | + byte_array.append(last_byte) |
| 105 | + if self.byteorder == 'little': |
| 106 | + byte_array = byte_array[::-1] |
| 107 | + |
| 108 | + return byte_array |
| 109 | + |
| 110 | + def decode(self, pld): |
| 111 | + """Decode values from bytearray |
| 112 | +
|
| 113 | + Parameters |
| 114 | + ---------- |
| 115 | + pld: bytearray, str(hex) |
| 116 | + Payload to be decoded. |
| 117 | +
|
| 118 | + """ |
| 119 | + if isinstance(pld, str): |
| 120 | + pld = bytearray.fromhex(pld) |
| 121 | + byte_number = 0 |
| 122 | + bits_on_byte_left = 8 |
| 123 | + if self.byteorder == 'little': |
| 124 | + pld = pld[::-1] |
| 125 | + for field in self._fields: |
| 126 | + bytes_to_decode = bytearray([pld[byte_number]]) |
| 127 | + if field.nbits > bits_on_byte_left: |
| 128 | + add_bytes, last_bits_used = divmod( |
| 129 | + field.nbits - bits_on_byte_left, 8) |
| 130 | + end_byte_n = byte_number + add_bytes + (last_bits_used != 0) |
| 131 | + bytes_to_decode = pld[byte_number:end_byte_n + 1] |
| 132 | + bits_on_byte_left = 8 - last_bits_used |
| 133 | + byte_number += add_bytes + 1 |
| 134 | + else: |
| 135 | + bits_on_byte_left -= field.nbits |
| 136 | + if not bits_on_byte_left: |
| 137 | + byte_number += 1 |
| 138 | + value = ( |
| 139 | + ( |
| 140 | + int.from_bytes(bytes_to_decode, byteorder='big') >> |
| 141 | + (bits_on_byte_left % 8) |
| 142 | + ) & |
| 143 | + (2 ** field.nbits - 1) |
| 144 | + ) |
| 145 | + field.decode(value) |
0 commit comments