Package ais :: Module aisstring

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Module aisstring

source code

Handle encoding and decoding AIS strings.

Bugs:

need some more interesting doctests!
needs to throw an exception if the character is not in the LUT
what to do about string with trailing @@@ or " " (white space)

Author: Kurt Schwehr

Version: 2068

Copyright: 2006

Date: 2006-05-02

Functions

[hide private]

str

decode(bits, dropAfterFirstAt=False)
Decode bits as a string.

source code

BitVector

encode(string, bitSize=None)
Returns: enocded bits for the string

source code

str

unpad(string, removeBlanks=True)
Remove AIS string padding

source code

str

pad(string, length)
pad a string out to the proper length with the @ character as required by the ais spec

source code

Variables

[hide private]

list characterLUT = ['@', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', '...
lookup table for decode to fetch characters faster

characterDict = {' ': 32, '!': 33, '"': 34, '#': 35, '$': 36, ...
Fast lookup for the AIS int code for a character

dict characterBits = {' ': <ais.BitVector.BitVector object at 0x148...
lookup table for going from a single character to a 6 bit BitVector

Function Details

[hide private]

decode(bits, dropAfterFirstAt=False)

source code

Decode bits as a string. Does not remove the end space or @@@@. Must be an multiple of 6 bits.

Parameters:

bits (BitVector) - n*6 bits that represent a string.

Returns: str

string with pad spaces or @@@@

encode(string, bitSize=None)

source code

Parameters:

string (str) - python ascii string to encode.
bitSize (int) - how many bits should this take. must be a multiple of 6

Returns: BitVector

enocded bits for the string

Bugs:

force to upper case
building this in reverse may be faster
check that bitSize is a multple of 6
pad with "@" to reach requested bitSize

unpad(string, removeBlanks=True)

source code

Remove AIS string padding

>>> unpad('@')
''
>>> unpad('A@')
'A'
>>> unpad('ABCDEF1234@@@@@')
'ABCDEF1234'

FIX: is this the correct response?

>>> unpad('A@B')
'A@B'

This is non standard behavior, but some AIS systems space pad the right

>>> unpad(' ')
''
>>> unpad('MY SHIP NAME    ')
'MY SHIP NAME'

The standard implies this behavior with is less fun

>>> unpad('MY SHIP NAME    ',removeBlanks=False)
'MY SHIP NAME    '

Parameters:

string (str) - string to cleanup
removeBlanks (bool) - set to true to strip spaces on the right

Returns: str

cleaned up string

Bug: use a faster algorithm for truncating the string

pad(string, length)

source code

pad a string out to the proper length with the @ character as required by the ais spec

>>> pad('',0)
''
>>> pad('',1)
'@'
>>> pad('A',1)
'A'
>>> pad('A',2)
'A@'
>>> pad('MY SHIP NAME',20)
'MY SHIP NAME@@@@@@@@'

Parameters:

string (str) - string to pad out
length (int) - number of characters that the string must be

Returns: str

str of len length

Bug: Use a list and join to make the string building faster

Variables Details

[hide private]

characterLUT

lookup table for decode to fetch characters faster

Type:: list

Value:

['@',
 'A',
 'B',
 'C',
 'D',
 'E',
 'F',
 'G',
...

characterDict

Fast lookup for the AIS int code for a character

Value:

{' ': 32,
 '!': 33,
 '"': 34,
 '#': 35,
 '$': 36,
 '%': 37,
 '&': 38,
 '(': 40,
...

characterBits

lookup table for going from a single character to a 6 bit BitVector

Type:: dict

Value:

{}