Re: Set parity of a string

[Peter Hansen]

Peter Hansen wrote:
snacktime wrote:
Is there a module that sets the parity of a string?  
As to the specific question: a module is not really required.
But here's one for you anyway.  It raises an exception if any
input character is non-ASCII, otherwise when you call set_parity()
with a string and a parity parameter of 'even', 'mark', 'none',
'odd', or 'space', it returns a string of the same length with
the high (parity) bits set appropriately.
'''parity module for python'''
# Copyright 2005 Engenuity Corporation
# Released under the "MIT license"
# at http://www.opensource.org/licenses/mit-license.php
import string
import operator
def _makeTable(parity):
    bitMap = {'even': [0,128], 'odd': [128,0], 'mark': [128,128]}
    table = []
    for c in range(128):
        even_odd = (sum(bool(c & 1<<b) for b in range(8))) & 1
        cp = chr(c | bitMap.get(parity, [0,0])[even_odd])
        table.append(cp)
    table.extend(chr(c) for c in range(128, 256))
    table = ''.join(table)
    return table, table[128:]
_map = {}
for parity in 'even odd mark space none'.split():
    _map[parity] = _makeTable(parity)
def set_parity(data, parity='none'):
    '''return string with parity bits, accepting only ASCII characters 
(0..127) as input'''
    out = string.translate(data, *_map[parity.lower()])
    if len(out) != len(data):
        raise ValueError('non-ASCII characters in input')
    else:
        return out

if __name__ == '__main__':
    print 'Running self-tests for parity.py'
    # check for invalid input handling
    for input in [ '\xff', '\x80', 'test\xA0ing' ]:
        try:
            set_parity(input)
        except ValueError:
            pass
        else:
            assert False, 'no exception for non-ASCII input'
    # check for various valid inputs
    for i, (parity, input, expected) in enumerate([
        ('space', 'test', 'test'),
        ('SPACE', '\x00\x7f', '\x00\x7f'),
        ('mark', 'test', '\xf4\xe5\xf3\xf4'),
        ('Odd', '\x00test\x7f', '\x80\xf4\xe5s\xf4\x7f'),
        ('even', '\x00\x01\x02\x03test\x7d\x7e\x7f', 
'\x00\x81\x82\x03te\xf3t\x7d\x7e\xff'),
        ]):
        actual = set_parity(input, parity)
        assert expected == actual, 'case %d: %r != %r' % (i, expected, actual)

    print 'All tests passed.'
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