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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
// Levenshtein distance algorithm is based off Java implementation from apache commons-text library licensed under the Apache 2.0 license.
#include "levenshtein_distance.h"
#include <limits>
#include <vector>
std::optional<uint32_t>
vespalib::LevenshteinDistance::calculate(std::span<const uint32_t> left, std::span<const uint32_t> right, uint32_t threshold)
{
uint32_t n = left.size();
uint32_t m = right.size();
if (n > m) {
return calculate(right, left, threshold);
}
// if one string is empty, the edit distance is necessarily the length
// of the other
if (n == 0) {
return m <= threshold ? std::optional(m) : std::nullopt;
}
if (m == 0) {
return n <= threshold ? std::optional(n) : std::nullopt;
}
// the edit distance cannot be less than the length difference
if (m - n > threshold) {
return std::nullopt;
}
std::vector<uint32_t> p(n+1); // 'previous' cost array, horizontally
std::vector<uint32_t> d(n+1); // cost array, horizontally
const uint32_t boundary = std::min(n, threshold) + 1;
for (uint32_t i = 0; i < boundary; ++i) {
p[i] = i;
}
// these fills ensure that the value above the rightmost entry of our
// stripe will be ignored in following loop iterations
for (uint32_t i = boundary; i < p.size(); ++i) {
p[i] = std::numeric_limits<uint32_t>::max();
}
for (uint32_t i = 0; i < d.size(); ++i) {
d[i] = std::numeric_limits<uint32_t>::max();
}
// iterates through t
for (uint32_t j = 1; j <= m; ++j) {
uint32_t rightJ = right[j - 1]; // jth character of right
d[0] = j;
int32_t min = std::max(1, static_cast<int32_t>(j) - static_cast<int32_t>(threshold));
uint32_t max = j > std::numeric_limits<uint32_t>::max() - threshold ?
n : std::min(n, j + threshold);
// ignore entry left of leftmost
if (min > 1) {
d[min - 1] = std::numeric_limits<uint32_t>::max();
}
uint32_t lowerBound = std::numeric_limits<uint32_t>::max();
for (uint32_t i = min; i <= max; ++i) {
if (left[i - 1] == rightJ) {
// diagonally left and up
d[i] = p[i - 1];
} else {
// 1 + minimum of cell to the left, to the top, diagonally
// left and up
d[i] = 1 + std::min(std::min(d[i - 1], p[i]), p[i - 1]);
}
lowerBound = std::min(lowerBound, d[i]);
}
if (lowerBound > threshold) {
return std::nullopt;
}
std::swap(p, d);
}
if (p[n] <= threshold) {
return {p[n]};
}
return std::nullopt;
}
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