Implement function to calculate Damerau-Levenshtein distance between two strings.

PiperOrigin-RevId: 481568970
Change-Id: Icb132348f62fed4c0168aac4963b3313a060890b

3 files changed, 227 insertions, 0 deletions

diff --git a/absl/strings/internal/damerau_levenshtein_distance.cc b/absl/strings/internal/damerau_levenshtein_distance.cc
new file mode 100644
index 00000000..7cc23acd
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+++ b/absl/strings/internal/damerau_levenshtein_distance.cc
@@ -0,0 +1,93 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/damerau_levenshtein_distance.h"
+
+#include <algorithm>
+#include <array>
+#include <numeric>
+
+#include "absl/strings/string_view.h"
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+// Calculate DamerauLevenshtein (adjacent transpositions) distance
+// between two strings,
+// https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance. The
+// algorithm follows the condition that no substring is edited more than once.
+// While this can reduce is larger distance, it's a) a much simpler algorithm
+// and b) more realistic for the case that typographic mistakes should be
+// detected.
+// When the distance is larger than cutoff, or one of the strings has more
+// than MAX_SIZE=100 characters, the code returns min(MAX_SIZE, cutoff) + 1.
+size_t CappedDamerauLevenshteinDistance(absl::string_view s1,
+                                        absl::string_view s2, uint8_t cutoff) {
+  const uint8_t MAX_SIZE = 100;
+  const uint8_t _cutoff = std::min(MAX_SIZE, cutoff);
+  const uint8_t cutoff_plus_1 = static_cast<uint8_t>(_cutoff + 1);
+
+  if (s1.size() > s2.size()) std::swap(s1, s2);
+  if (s1.size() + _cutoff < s2.size() || s2.size() > MAX_SIZE)
+    return cutoff_plus_1;
+
+  if (s1.empty())
+    return std::min(static_cast<size_t>(cutoff_plus_1), s2.size());
+
+  // Lower diagonal bound: y = x - lower_diag
+  const uint8_t lower_diag =
+      _cutoff - static_cast<uint8_t>(s2.size() - s1.size());
+  // Upper diagonal bound: y = x + upper_diag
+  const uint8_t upper_diag = _cutoff;
+
+  // d[i][j] is the number of edits required to convert s1[0, i] to s2[0, j]
+  std::array<std::array<uint8_t, MAX_SIZE + 2>, MAX_SIZE + 2> d;
+  std::iota(d[0].begin(), d[0].begin() + upper_diag + 1, 0);
+  d[0][cutoff_plus_1] = cutoff_plus_1;
+  for (size_t i = 1; i <= s1.size(); ++i) {
+    // Deduce begin of relevant window.
+    size_t j_begin = 1;
+    if (i > lower_diag) {
+      j_begin = i - lower_diag;
+      d[i][j_begin - 1] = cutoff_plus_1;
+    } else {
+      d[i][0] = static_cast<uint8_t>(i);
+    }
+
+    // Deduce end of relevant window.
+    size_t j_end = i + upper_diag;
+    if (j_end > s2.size()) {
+      j_end = s2.size();
+    } else {
+      d[i][j_end + 1] = cutoff_plus_1;
+    }
+
+    for (size_t j = j_begin; j <= j_end; ++j) {
+      const uint8_t deletion_distance = d[i - 1][j] + 1;
+      const uint8_t insertion_distance = d[i][j - 1] + 1;
+      const uint8_t mismatched_tail_cost = s1[i - 1] == s2[j - 1] ? 0 : 1;
+      const uint8_t mismatch_distance = d[i - 1][j - 1] + mismatched_tail_cost;
+      uint8_t transposition_distance = _cutoff + 1;
+      if (i > 1 && j > 1 && s1[i - 1] == s2[j - 2] && s1[i - 2] == s2[j - 1])
+        transposition_distance = d[i - 2][j - 2] + 1;
+      d[i][j] = std::min({cutoff_plus_1, deletion_distance, insertion_distance,
+                          mismatch_distance, transposition_distance});
+    }
+  }
+  return d[s1.size()][s2.size()];
+}
+
+}  // namespace strings_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/damerau_levenshtein_distance.h b/absl/strings/internal/damerau_levenshtein_distance.h
new file mode 100644
index 00000000..b9bb6fe1
--- /dev/null
+++ b/absl/strings/internal/damerau_levenshtein_distance.h
@@ -0,0 +1,35 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_DAMERAU_LEVENSHTEIN_DISTANCE_H_
+#define ABSL_STRINGS_INTERNAL_DAMERAU_LEVENSHTEIN_DISTANCE_H_
+
+#include <numeric>
+#include <vector>
+
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+// Calculate DamerauLevenshtein distance between two strings.
+// When the distance is larger than cutoff, the code just returns cutoff + 1.
+size_t CappedDamerauLevenshteinDistance(absl::string_view s1,
+                                        absl::string_view s2, uint8_t cutoff);
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_DAMERAU_LEVENSHTEIN_DISTANCE_H_
diff --git a/absl/strings/internal/damerau_levenshtein_distance_test.cc b/absl/strings/internal/damerau_levenshtein_distance_test.cc
new file mode 100644
index 00000000..85871941
--- /dev/null
+++ b/absl/strings/internal/damerau_levenshtein_distance_test.cc
@@ -0,0 +1,99 @@
+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/damerau_levenshtein_distance.h"
+
+#include <cstdint>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+using absl::strings_internal::CappedDamerauLevenshteinDistance;
+
+TEST(Distance, TestDistances) {
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("ab", "ab", 6), 0);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("a", "b", 6), 1);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("ca", "abc", 6), 3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "ad", 6), 2);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "cadb", 6), 4);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "bdac", 6), 4);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("ab", "ab", 0), 0);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("", "", 0), 0);
+  // combinations for 3-character strings:
+  // 1, 2, 3 removals, insertions or replacements and transpositions
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", "abc", 6), 0);
+  for (auto res :
+       {"", "ca", "efg", "ea", "ce", "ceb", "eca", "cae", "cea", "bea"}) {
+    EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", res, 6), 3);
+    EXPECT_THAT(CappedDamerauLevenshteinDistance(res, "abc", 6), 3);
+  }
+  for (auto res :
+       {"a",   "b",   "c",   "ba",  "cb",  "bca", "cab", "cba", "ace",
+        "efc", "ebf", "aef", "ae",  "be",  "eb",  "ec",  "ecb", "bec",
+        "bce", "cbe", "ace", "eac", "aeb", "bae", "eab", "eba"}) {
+    EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", res, 6), 2);
+    EXPECT_THAT(CappedDamerauLevenshteinDistance(res, "abc", 6), 2);
+  }
+  for (auto res : {"ab", "ac", "bc", "acb", "bac", "ebc", "aec", "abe"}) {
+    EXPECT_THAT(CappedDamerauLevenshteinDistance("abc", res, 6), 1);
+    EXPECT_THAT(CappedDamerauLevenshteinDistance(res, "abc", 6), 1);
+  }
+}
+
+TEST(Distance, TestCutoff) {
+  // Returing cutoff + 1 if the value is larger than cutoff or string longer
+  // than MAX_SIZE.
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "a", 3), 3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "a", 2), 3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcd", "a", 1), 2);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("abcdefg", "a", 2), 3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance("a", "abcde", 2), 3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(102, 'a'),
+                                               std::string(102, 'a'), 105),
+              101);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(100, 'a'), 100),
+              0);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(100, 'b'), 100),
+              100);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(99, 'a'), 2),
+              1);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(101, 'a'), 2),
+              3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(100, 'a'),
+                                               std::string(101, 'a'), 2),
+              3);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(UINT8_MAX + 1, 'a'),
+                                               std::string(UINT8_MAX + 1, 'b'),
+                                               UINT8_MAX),
+              101);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(UINT8_MAX - 1, 'a'),
+                                               std::string(UINT8_MAX - 1, 'b'),
+                                               UINT8_MAX),
+              101);
+  EXPECT_THAT(
+      CappedDamerauLevenshteinDistance(std::string(UINT8_MAX, 'a'),
+                                       std::string(UINT8_MAX, 'b'), UINT8_MAX),
+      101);
+  EXPECT_THAT(CappedDamerauLevenshteinDistance(std::string(UINT8_MAX - 1, 'a'),
+                                               std::string(UINT8_MAX - 1, 'a'),
+                                               UINT8_MAX),
+              101);
+}
+}  // namespace