code/breadcrumb/server/decoding.py

   1 # -*- coding: utf8 -*-
   2 """Functions for decoding crumbs."""
   3
   4 # Copyright (C) 2008 Laurens Van Houtven <lvh at laurensvh.be>
   5 # This program is free software: you can redistribute it and/or modify
   6 # it under the terms of the GNU General Public License as published by
   7 # the Free Software Foundation, either version 3 of the License, or
   8 # (at your option) any later version.
   9 #
  10 # This program is distributed in the hope that it will be useful,
  11 # but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13 # GNU General Public License for more details.
  14 #
  15 # You should have received a copy of the GNU General Public License
  16 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
  17
  18 import os
  19 import sys
  20 import struct
  21 import logging
  22
  23 sys.path.append(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
  24 from common.checksum import generate_byte_checksum as generate_checksum
  25 from common.consts import CRUMB_FIELDS
  26
  27 def decode_crumb(crumb):
  28     """Decodes a crumb into a dictionary."""
  29     # Unpack the crumb
  30     header, content, checksum = crumb[0], crumb[1:-1], crumb[-1]
  31
  32     logging.debug("Got crumb: %s" % ':'.join('%02X' % ord(c) for c in crumb))
  33
  34     # Checksum
  35     if generate_checksum(header + content) != checksum:
  36         logging.error("Got invalid checksum, ignoring this crumb...")
  37         return
  38
  39     keys = []
  40     formats = ['<'] # forced little-endianness prevents useless padding
  41
  42     fields = zip(unpack_bools(header), CRUMB_FIELDS)
  43     for present, field_data in fields:
  44         if present: # the field is contained in the crumb
  45             attr_name, attr_keys, attr_format = field_data
  46             logging.debug("Decoding attribute %s from crumb..." % attr_name)
  47             logging.debug("Keys: %s, format: %s" % (attr_keys, attr_format))
  48
  49             keys.extend(attr_keys)
  50             formats.append(attr_format)
  51
  52     logging.debug("Done decoding attributes from the header byte.")
  53
  54     format = ''.join(formats)
  55     values = struct.unpack(format, content)
  56
  57     point = dict(zip(keys, values))
  58     logging.debug("Point data: %s" % repr(point))
  59
  60     return point
  61
  62 def unpack_bools(byte):
  63     """Unpacks a byte into a list of eight booleans.
  64
  65     The byte is expected to be little-endian.
  66
  67     >>> unpackbools('\\x00')
  68     [False, False, False, False, False, False, False, False]
  69
  70     >>> unpackbools('\\x01')
  71     [True, False, False, False, False, False, False, False]
  72
  73     >>> unpackbools('\\xff')
  74     [True, True, True, True, True, True, True, True]
  75     """
  76     return [bool((ord(byte) >> i) & 1) for i in xrange(8)]