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96 lines
4.0 KiB
Python
96 lines
4.0 KiB
Python
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# Copyright (c) 2002-2005, Jeremiah Fincher
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# All rights reserved.
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#
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# Redistribution and use in source and binary forms, with or without
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# modification, are permitted provided that the following conditions are met:
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#
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# * Redistributions of source code must retain the above copyright notice,
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# this list of conditions, and the following disclaimer.
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# * Redistributions in binary form must reproduce the above copyright notice,
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# this list of conditions, and the following disclaimer in the
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# documentation and/or other materials provided with the distribution.
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# * Neither the name of the author of this software nor the name of
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# contributors to this software may be used to endorse or promote products
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# derived from this software without specific prior written consent.
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#
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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
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# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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# POSSIBILITY OF SUCH DAMAGE.
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###
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def window(L, size):
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"""list * size -> window iterable
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Returns a sliding 'window' through the list L of size size."""
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assert not isinstance(L, int), 'Argument order swapped: window(L, size)'
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if size < 1:
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raise ValueError('size <= 0 disallowed.')
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for i in xrange(len(L) - (size-1)):
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yield L[i:i+size]
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def mapinto(f, L):
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for (i, x) in enumerate(L):
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L[i] = f(x)
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def renumerate(L):
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for i in xrange(len(L)-1, -1, -1):
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yield (i, L[i])
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def dameraulevenshtein(seq1, seq2):
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"""Calculate the Damerau-Levenshtein distance between sequences.
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This distance is the number of additions, deletions, substitutions,
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and transpositions needed to transform the first sequence into the
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second. Although generally used with strings, any sequences of
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comparable objects will work.
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Transpositions are exchanges of *consecutive* characters; all other
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operations are self-explanatory.
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This implementation is O(N*M) time and O(M) space, for N and M the
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lengths of the two sequences.
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>>> dameraulevenshtein('ba', 'abc')
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2
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>>> dameraulevenshtein('fee', 'deed')
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2
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It works with arbitrary sequences too:
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>>> dameraulevenshtein('abcd', ['b', 'a', 'c', 'd', 'e'])
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2
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"""
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# codesnippet:D0DE4716-B6E6-4161-9219-2903BF8F547F
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# Conceptually, this is based on a len(seq1) + 1 * len(seq2) + 1 matrix.
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# However, only the current and two previous rows are needed at once,
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# so we only store those.
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# Sourced from http://mwh.geek.nz/2009/04/26/python-damerau-levenshtein-distance/
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oneago = None
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thisrow = range(1, len(seq2) + 1) + [0]
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for x in xrange(len(seq1)):
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# Python lists wrap around for negative indices, so put the
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# leftmost column at the *end* of the list. This matches with
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# the zero-indexed strings and saves extra calculation.
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twoago, oneago, thisrow = oneago, thisrow, [0] * len(seq2) + [x + 1]
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for y in xrange(len(seq2)):
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delcost = oneago[y] + 1
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addcost = thisrow[y - 1] + 1
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subcost = oneago[y - 1] + (seq1[x] != seq2[y])
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thisrow[y] = min(delcost, addcost, subcost)
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# This block deals with transpositions
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if (x > 0 and y > 0 and seq1[x] == seq2[y - 1]
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and seq1[x-1] == seq2[y] and seq1[x] != seq2[y]):
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thisrow[y] = min(thisrow[y], twoago[y - 2] + 1)
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return thisrow[len(seq2) - 1]
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# vim:set shiftwidth=4 softtabstop=4 expandtab textwidth=79:
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