Edit_distance now computes the actual Damerau-Levenshtein edit-distance

nltk/metrics/distance.py

@@ -34,7 +34,13 @@ def _edit_dist_init(len1, len2):
     return lev
 
 
-def _edit_dist_step(lev, i, j, s1, s2, substitution_cost=1, transpositions=False):
+def _last_left_t_init(sigma):
+    return {c: 0 for c in sigma}
+
+
+def _edit_dist_step(
+    lev, i, j, s1, s2, last_left, last_right, substitution_cost=1, transpositions=False
+):
     c1 = s1[i - 1]
     c2 = s2[j - 1]
 
@@ -47,9 +53,8 @@ def _edit_dist_step(lev, i, j, s1, s2, substitution_cost=1, transpositions=False
 
     # transposition
     d = c + 1  # never picked by default
-    if transpositions and i > 1 and j > 1:
-        if s1[i - 2] == c2 and s2[j - 2] == c1:
-            d = lev[i - 2][j - 2] + 1
+    if transpositions and last_left > 0 and last_right > 0:
+        d = lev[last_left - 1][last_right - 1] + i - last_left + j - last_right - 1
 
     # pick the cheapest
     lev[i][j] = min(a, b, c, d)
@@ -85,18 +90,33 @@ def edit_distance(s1, s2, substitution_cost=1, transpositions=False):
     len2 = len(s2)
     lev = _edit_dist_init(len1 + 1, len2 + 1)
 
+    # retrieve alphabet
+    sigma = set()
+    sigma.update(s1)
+    sigma.update(s2)
+
+    # set up table to remember positions of last seen occurrence in s1
+    last_left_t = _last_left_t_init(sigma)
+
     # iterate over the array
     for i in range(len1):
+        last_right = 0
         for j in range(len2):
+            last_left = last_left_t[s2[j]]
             _edit_dist_step(
                 lev,
                 i + 1,
                 j + 1,
                 s1,
                 s2,
+                last_left,
+                last_right,
                 substitution_cost=substitution_cost,
                 transpositions=transpositions,
             )
+            if s1[i] == s2[j]:
+                last_right = j + 1
+            last_left_t[s1[i]] = i + 1
     return lev[len1][len2]
 
 
@@ -162,6 +182,8 @@ def edit_distance_align(s1, s2, substitution_cost=1):
                 j + 1,
                 s1,
                 s2,
+                0,
+                0,
                 substitution_cost=substitution_cost,
                 transpositions=False,
             )

nltk/test/unit/test_distance.py

@@ -0,0 +1,123 @@
+from typing import Tuple
+
+import pytest
+
+from nltk.metrics.distance import edit_distance
+
+
+class TestEditDistance:
+    @pytest.mark.parametrize(
+        "left,right,substitution_cost,expecteds",
+        [
+            # Allowing transpositions reduces the number of edits required.
+            # with transpositions:
+            # e.g. "abc" -T-> "cba" -D-> "ca": 2 steps
+            #
+            # without transpositions:
+            # e.g. "abc" -D-> "ab" -D-> "a" -I-> "ca": 3 steps
+            ("abc", "ca", 1, (2, 3)),
+            ("abc", "ca", 5, (2, 3)),  # Doesn't *require* substitutions
+            # Note, a substition_cost of higher than 2 doesn't make much
+            # sense, as a deletion + insertion is identical, and always
+            # costs 2.
+            #
+            #
+            # Transpositions don't always reduce the number of edits required:
+            # with or without transpositions:
+            # e.g. "wants" -D-> "wats" -D-> "was" -I-> "wasp": 3 steps
+            ("wants", "wasp", 1, (3, 3)),
+            ("wants", "wasp", 5, (3, 3)),  # Doesn't *require* substitutions
+            #
+            #
+            # Ought to have the same results with and without transpositions
+            # with or without transpositions:
+            # e.g. "rain" -S-> "sain" -S-> "shin" -I-> "shine": 3 steps
+            # (but cost 5 if substitution_cost=2)
+            ("rain", "shine", 1, (3, 3)),
+            ("rain", "shine", 2, (5, 5)),  # Does *require* substitutions
+            #
+            #
+            # Several potentially interesting typos
+            # with transpositions:
+            # e.g. "acbdef" -T-> "abcdef": 1 step
+            #
+            # without transpositions:
+            # e.g. "acbdef" -D-> "abdef" -I-> "abcdef": 2 steps
+            ("acbdef", "abcdef", 1, (1, 2)),
+            ("acbdef", "abcdef", 2, (1, 2)),  # Doesn't *require* substitutions
+            #
+            #
+            # with transpositions:
+            # e.g. "lnaguaeg" -T-> "languaeg" -T-> "language": 2 steps
+            #
+            # without transpositions:
+            # e.g. "lnaguaeg" -D-> "laguaeg" -I-> "languaeg" -D-> "languag" -I-> "language": 4 steps
+            ("lnaguaeg", "language", 1, (2, 4)),
+            ("lnaguaeg", "language", 2, (2, 4)),  # Doesn't *require* substitutions
+            #
+            #
+            # with transpositions:
+            # e.g. "lnaugage" -T-> "lanugage" -T-> "language": 2 steps
+            #
+            # without transpositions:
+            # e.g. "lnaugage" -S-> "lnangage" -D-> "langage" -I-> "language": 3 steps
+            # (but one substitution, so a cost of 4 if substition_cost = 2)
+            ("lnaugage", "language", 1, (2, 3)),
+            ("lnaugage", "language", 2, (2, 4)),
+            # Does *require* substitutions if no transpositions
+            #
+            #
+            # with transpositions:
+            # e.g. "lngauage" -T-> "lnaguage" -T-> "language": 2 steps
+            # without transpositions:
+            # e.g. "lngauage" -I-> "lanaguage" -D-> "language": 2 steps
+            ("lngauage", "language", 1, (2, 2)),
+            ("lngauage", "language", 2, (2, 2)),  # Doesn't *require* substitutions
+            #
+            #
+            # with or without transpositions:
+            # e.g. "wants" -S-> "sants" -S-> "swnts" -S-> "swits" -S-> "swims" -D-> "swim": 5 steps
+            #
+            # with substitution_cost=2 and transpositions:
+            # e.g. "wants" -T-> "santw" -D-> "sntw" -D-> "stw" -D-> "sw"
+            # -I-> "swi" -I-> "swim": 6 steps
+            #
+            # with substitution_cost=2 and no transpositions:
+            # e.g. "wants" -I-> "swants" -D-> "swant" -D-> "swan" -D-> "swa" -D-> "sw"
+            # -I-> "swi" -I-> "swim": 7 steps
+            ("wants", "swim", 1, (5, 5)),
+            ("wants", "swim", 2, (6, 7)),
+            #
+            #
+            # with or without transpositions:
+            # e.g. "kitten" -S-> "sitten" -s-> "sittin" -I-> "sitting": 3 steps
+            # (but cost 5 if substitution_cost=2)
+            ("kitten", "sitting", 1, (3, 3)),
+            ("kitten", "sitting", 2, (5, 5)),
+        ],
+    )
+    def test_with_transpositions(
+        self, left: str, right: str, substitution_cost: int, expecteds: Tuple[int, int]
+    ):
+        """Test `edit_distance` between two strings, given some `substitution_cost`,
+        and whether transpositions are allowed.
+
+        Args:
+            left (str): First input string to `edit_distance`.
+            right (str): Second input string to `edit_distance`.
+            substitution_cost (int): The cost of a substitution action in `edit_distance`.
+            expecteds (Tuple[int, int]): A tuple of expected outputs, such that `expecteds[0]` is
+                the expected output with `transpositions=True`, and `expecteds[1]` is
+                the expected output with `transpositions=False`.
+        """
+        # Test the input strings in both orderings
+        for s1, s2 in ((left, right), (right, left)):
+            # zip with [True, False] to get the transpositions value
+            for expected, transpositions in zip(expecteds, [True, False]):
+                predicted = edit_distance(
+                    s1,
+                    s2,
+                    substitution_cost=substitution_cost,
+                    transpositions=transpositions,
+                )
+                assert predicted == expected