94 lines
3.5 KiB
C++
94 lines
3.5 KiB
C++
// Copyright 2022 The Abseil Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "absl/strings/internal/damerau_levenshtein_distance.h"
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#include <algorithm>
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#include <array>
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#include <numeric>
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#include "absl/strings/string_view.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace strings_internal {
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// Calculate DamerauLevenshtein (adjacent transpositions) distance
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// between two strings,
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// https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance. The
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// algorithm follows the condition that no substring is edited more than once.
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// While this can reduce is larger distance, it's a) a much simpler algorithm
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// and b) more realistic for the case that typographic mistakes should be
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// detected.
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// When the distance is larger than cutoff, or one of the strings has more
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// than MAX_SIZE=100 characters, the code returns min(MAX_SIZE, cutoff) + 1.
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uint8_t CappedDamerauLevenshteinDistance(absl::string_view s1,
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absl::string_view s2, uint8_t cutoff) {
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const uint8_t MAX_SIZE = 100;
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const uint8_t _cutoff = std::min(MAX_SIZE, cutoff);
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const uint8_t cutoff_plus_1 = static_cast<uint8_t>(_cutoff + 1);
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if (s1.size() > s2.size()) std::swap(s1, s2);
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if (s1.size() + _cutoff < s2.size() || s2.size() > MAX_SIZE)
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return cutoff_plus_1;
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if (s1.empty())
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return static_cast<uint8_t>(s2.size());
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// Lower diagonal bound: y = x - lower_diag
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const uint8_t lower_diag =
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_cutoff - static_cast<uint8_t>(s2.size() - s1.size());
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// Upper diagonal bound: y = x + upper_diag
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const uint8_t upper_diag = _cutoff;
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// d[i][j] is the number of edits required to convert s1[0, i] to s2[0, j]
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std::array<std::array<uint8_t, MAX_SIZE + 2>, MAX_SIZE + 2> d;
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std::iota(d[0].begin(), d[0].begin() + upper_diag + 1, 0);
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d[0][cutoff_plus_1] = cutoff_plus_1;
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for (size_t i = 1; i <= s1.size(); ++i) {
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// Deduce begin of relevant window.
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size_t j_begin = 1;
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if (i > lower_diag) {
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j_begin = i - lower_diag;
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d[i][j_begin - 1] = cutoff_plus_1;
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} else {
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d[i][0] = static_cast<uint8_t>(i);
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}
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// Deduce end of relevant window.
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size_t j_end = i + upper_diag;
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if (j_end > s2.size()) {
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j_end = s2.size();
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} else {
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d[i][j_end + 1] = cutoff_plus_1;
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}
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for (size_t j = j_begin; j <= j_end; ++j) {
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const uint8_t deletion_distance = d[i - 1][j] + 1;
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const uint8_t insertion_distance = d[i][j - 1] + 1;
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const uint8_t mismatched_tail_cost = s1[i - 1] == s2[j - 1] ? 0 : 1;
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const uint8_t mismatch_distance = d[i - 1][j - 1] + mismatched_tail_cost;
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uint8_t transposition_distance = _cutoff + 1;
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if (i > 1 && j > 1 && s1[i - 1] == s2[j - 2] && s1[i - 2] == s2[j - 1])
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transposition_distance = d[i - 2][j - 2] + 1;
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d[i][j] = std::min({cutoff_plus_1, deletion_distance, insertion_distance,
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mismatch_distance, transposition_distance});
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}
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}
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return d[s1.size()][s2.size()];
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}
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} // namespace strings_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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