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Add new DamerauLevenshtein... classes #84

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merged 6 commits into from
Sep 18, 2022
Merged

Add new DamerauLevenshtein... classes #84

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742edf5
Add new DamerauLevenshteinRestricted and DamerauLevenshteinUnrestrict…
Sep 4, 2022
de1bb8e
Merge branch 'master' into split-damerau-levenshtein
orsinium Sep 9, 2022
789893b
DamerauLevenshtein: make `restricted` a flag
orsinium Sep 9, 2022
983e9c0
cleanup docstring
orsinium Sep 9, 2022
47703b2
add OSA from rapidfuzz for DL
orsinium Sep 18, 2022
efd915c
clean-up DL a bit
orsinium Sep 18, 2022
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30 changes: 24 additions & 6 deletions tests/test_edit/test_damerau_levenshtein.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4,11 +4,9 @@
# project
import textdistance


ALG = textdistance.DamerauLevenshtein


@pytest.mark.parametrize('left, right, expected', [
COMMON = [
('test', 'text', 1),
('test', 'tset', 1),
('test', 'qwy', 4),
Expand All @@ -24,15 +22,35 @@
('ab', 'ba', 1),
('ab', 'cde', 3),
('ab', 'ac', 1),
('ab', 'ba', 1),
('ab', 'bc', 2),
]


@pytest.mark.parametrize('left, right, expected', COMMON + [
('ab', 'bca', 3),
('abcd', 'bdac', 4),
])
def test_distance(left, right, expected):
def test_distance_restricted(left, right, expected):
actual = ALG(external=False)(left, right)
assert actual == expected

actual = ALG(external=True)(left, right)
assert actual == expected

actual = ALG()._pure_python(left, right)
actual = ALG()._pure_python_restricted(left, right)
assert actual == expected


@pytest.mark.parametrize('left, right, expected', COMMON + [
('ab', 'bca', 2),
('abcd', 'bdac', 3),
])
def test_distance_unrestricted(left, right, expected):
actual = ALG(external=False, restricted=False)(left, right)
assert actual == expected

actual = ALG(external=True, restricted=False)(left, right)
assert actual == expected

actual = ALG()._pure_python_unrestricted(left, right)
assert actual == expected
57 changes: 54 additions & 3 deletions textdistance/algorithms/edit_based.py
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Original file line number Diff line number Diff line change
Expand Up @@ -148,6 +148,10 @@ class DamerauLevenshtein(_Base):
* substitution: ABC -> ABE, ADC, FBC..
* transposition: ABC -> ACB, BAC

If `restricted=False`, it will calculate unrestricted distance,
where the same character can be touched more than once.
So the distance between BA and ACB is 2: BA -> AB -> ACB.

https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance
"""

Expand All @@ -156,10 +160,12 @@ def __init__(
qval: int = 1,
test_func: TestFunc | None = None,
external: bool = True,
restricted: bool = True,
) -> None:
self.qval = qval
self.test_func = test_func or self._ident
self.external = external
self.restricted = restricted

def _numpy(self, s1: Sequence[T], s2: Sequence[T]) -> int:
# TODO: doesn't pass tests, need improve
Expand Down Expand Up @@ -194,11 +200,54 @@ def _numpy(self, s1: Sequence[T], s2: Sequence[T]) -> int:

return d[len(s1) - 1][len(s2) - 1]

def _pure_python(self, s1: Sequence[T], s2: Sequence[T]) -> int:
def _pure_python_unrestricted(self, s1: Sequence[T], s2: Sequence[T]) -> int:
"""https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance
"""
d: dict[tuple[int, int], int] = {}
da: defaultdict[T, int] = defaultdict(int)

len1 = len(s1)
len2 = len(s2)

maxdist = len1 + len2
d[-1, -1] = maxdist

# matrix
for i in range(len(s1) + 1):
d[i, -1] = maxdist
d[i, 0] = i
for j in range(len(s2) + 1):
d[-1, j] = maxdist
d[0, j] = j

for i, cs1 in enumerate(s1):
i += 1
db = 0
for j, cs2 in enumerate(s2):
j += 1
i1 = da[cs2]
j1 = db
if self.test_func(cs1, cs2):
cost = 0
db = j
else:
cost = 1

d[i, j] = min(
d[i - 1, j - 1] + cost, # substitution
d[i, j - 1] + 1, # insertion
d[i - 1, j] + 1, # deletion
d[i1 - 1, j1 - 1] + (i - i1) - 1 + (j - j1), # transposition
)
da[cs1] = i

return d[len1, len2]
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is this really correct? we write into d[i, j] which should be at most d[len1-1, len2-1] from my understanding.

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@orsinium orsinium Sep 18, 2022 •
edited
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I don't remember much about the algorithms, I can only rely on tests.

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@orsinium orsinium Sep 18, 2022 •
edited
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I checked the code. What a mess it is, oh my. We first set these values by doing iteration over range(len(s1) + 1) when initializing the matrix, And then when we do enumeration for i, cs1 in enumerate(s1):, we on the next line shift the index: i += 1 🧠 I'll try to clean it up a tiny bit.

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This is deliberate! The boundary values are needed, but are never modified later, as the algorithm looks at the preceding values in the array when calculating the new values. I tried to come up with a way of avoiding the i+=1 business, but I kept messing things up, so I decided to be somewhat non-Pythonic and stick to the algorithm as presented on Wikipedia!

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I tried to come up with a way of avoiding the i+=1 business, but I kept messing things up

Tip of the day: enumerate accepts an argument start, where you can specify the initial value. I've changed the code to use it. It became a bit slower, for some reason, but IMHO a bit more readable.

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Yes I completely missed the += 1 before

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I tried to come up with a way of avoiding the i+=1 business, but I kept messing things up

Tip of the day: enumerate accepts an argument start, where you can specify the initial value. I've changed the code to use it. It became a bit slower, for some reason, but IMHO a bit more readable.

Oh, how neat! Thanks for that tip. I agree, more readable is better.


def _pure_python_restricted(self, s1: Sequence[T], s2: Sequence[T]) -> int:
"""
https://www.guyrutenberg.com/2008/12/15/damerau-levenshtein-distance-in-python/
"""
d = {}
d: dict[tuple[int, int], int] = {}

# matrix
for i in range(-1, len(s1) + 1):
Expand Down Expand Up @@ -241,7 +290,9 @@ def __call__(self, s1: Sequence[T], s2: Sequence[T]) -> int:
# if numpy:
# return self._numpy(s1, s2)
# else:
return self._pure_python(s1, s2)
if self.restricted:
return self._pure_python_restricted(s1, s2)
return self._pure_python_unrestricted(s1, s2)


class JaroWinkler(_BaseSimilarity):
Expand Down
11 changes: 7 additions & 4 deletions textdistance/libraries.py
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Original file line number Diff line number Diff line change
Expand Up @@ -166,10 +166,13 @@ class SameLengthTextLibrary(SameLengthLibrary, TextLibrary):
reg = prototype.register

alg = 'DamerauLevenshtein'
reg(alg, LibraryBase('abydos.distance', 'DamerauLevenshtein', presets={}, attr='dist_abs'))
reg(alg, LibraryBase('pyxdameraulevenshtein', 'damerau_levenshtein_distance'))
reg(alg, TextLibrary('jellyfish', 'damerau_levenshtein_distance'))
reg(alg, LibraryBase('rapidfuzz.distance.DamerauLevenshtein', 'distance'))
reg(alg, LibraryBase(
'abydos.distance', 'DamerauLevenshtein', presets={}, attr='dist_abs',
conditions=dict(restricted=False),
))
reg(alg, LibraryBase('pyxdameraulevenshtein', 'damerau_levenshtein_distance', conditions=dict(restricted=True)))
reg(alg, TextLibrary('jellyfish', 'damerau_levenshtein_distance', conditions=dict(restricted=False)))
reg(alg, LibraryBase('rapidfuzz.distance.DamerauLevenshtein', 'distance', conditions=dict(restricted=False)))
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alg = 'Hamming'
reg(alg, LibraryBase('abydos.distance', 'Hamming', presets={}, attr='dist_abs'))
Expand Down