# -*- coding: utf-8 -*-
# Copyright 2014-2018 by Christopher C. Little.
# This file is part of Abydos.
#
# Abydos is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# Abydos is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with Abydos. If not, see <http://www.gnu.org/licenses/>.
"""abydos.compression._bwt.
Burrows-Wheeler Transform encoder/decoder
"""
from __future__ import (
absolute_import,
division,
print_function,
unicode_literals,
)
from six.moves import range
__all__ = ['BWT', 'bwt_decode', 'bwt_encode']
[docs]class BWT(object):
"""Burrows-Wheeler Transform.
The Burrows-Wheeler transform is an attempt at placing similar characters
together to improve compression.
Cf. :cite:`Burrows:1994`.
"""
[docs] def encode(self, word, terminator='\0'):
r"""Return the Burrows-Wheeler transformed form of a word.
Parameters
----------
word : str
The word to transform using BWT
terminator : str
A character added to signal the end of the string
Returns
-------
str
Word encoded by BWT
Raises
------
ValueError
Specified terminator absent from code.
Examples
--------
>>> bwt = BWT()
>>> bwt.encode('align')
'n\x00ilag'
>>> bwt.encode('banana')
'annb\x00aa'
>>> bwt.encode('banana', '@')
'annb@aa'
"""
if word:
if terminator in word:
raise ValueError(
'Specified terminator, {}, already in word.'.format(
terminator if terminator != '\0' else '\\0'
)
)
else:
word += terminator
wordlist = sorted(
word[i:] + word[:i] for i in range(len(word))
)
return ''.join([w[-1] for w in wordlist])
else:
return terminator
[docs] def decode(self, code, terminator='\0'):
r"""Return a word decoded from BWT form.
Parameters
----------
code : str
The word to transform from BWT form
terminator : str
A character added to signal the end of the string
Returns
-------
str
Word decoded by BWT
Raises
------
ValueError
Specified terminator absent from code.
Examples
--------
>>> bwt = BWT()
>>> bwt.decode('n\x00ilag')
'align'
>>> bwt.decode('annb\x00aa')
'banana'
>>> bwt.decode('annb@aa', '@')
'banana'
"""
if code:
if terminator not in code:
raise ValueError(
'Specified terminator, {}, absent from code.'.format(
terminator if terminator != '\0' else '\\0'
)
)
else:
wordlist = [''] * len(code)
for i in range(len(code)):
wordlist = sorted(
code[i] + wordlist[i] for i in range(len(code))
)
rows = [w for w in wordlist if w[-1] == terminator][0]
return rows.rstrip(terminator)
else:
return ''
[docs]def bwt_encode(word, terminator='\0'):
r"""Return the Burrows-Wheeler transformed form of a word.
This is a wrapper for :py:meth:`BWT.encode`.
Parameters
----------
word : str
The word to transform using BWT
terminator : str
A character added to signal the end of the string
Returns
-------
str
Word encoded by BWT
Examples
--------
>>> bwt_encode('align')
'n\x00ilag'
>>> bwt_encode('banana')
'annb\x00aa'
>>> bwt_encode('banana', '@')
'annb@aa'
"""
return BWT().encode(word, terminator)
[docs]def bwt_decode(code, terminator='\0'):
r"""Return a word decoded from BWT form.
This is a wrapper for :py:meth:`BWT.decode`.
Parameters
----------
code : str
The word to transform from BWT form
terminator : str
A character added to signal the end of the string
Returns
-------
str
Word decoded by BWT
Examples
--------
>>> bwt_decode('n\x00ilag')
'align'
>>> bwt_decode('annb\x00aa')
'banana'
>>> bwt_decode('annb@aa', '@')
'banana'
"""
return BWT().decode(code, terminator)
if __name__ == '__main__':
import doctest
doctest.testmod()