129 lines
5.4 KiB
C++
129 lines
5.4 KiB
C++
#include "./levenshtein.h"
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#include "./math.h"
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#include "./multiarray.h"
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#include <iostream>
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#include <limits>
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#include <memory>
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using namespace std;
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namespace CppUtilities {
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/// \cond
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/// \brief The DistanceArray is a 2D array which is allocated either on the stack or the heap.
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using DistanceArray = MultiArray<size_t, NoneOwningMultiArray, size_t, size_t>;
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/// \brief Initializes the distance array.
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///
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/// For size1=5 and size2=4 it would look like this:
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/// ```
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/// 9 9 9 9 9 9
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/// 9 0 1 2 3 4
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/// 9 1 0 0 0 0
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/// 9 2 0 0 0 0
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/// 9 3 0 0 0 0
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/// 9 4 0 0 0 0
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/// 9 5 0 0 0 0
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/// ```
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void initDistanceArray(DistanceArray &distanceArray, const size_t size1, const size_t size2)
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{
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const auto maxDistance(size1 + size2);
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distanceArray.at(0, 0) = maxDistance;
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for (size_t i = 0; i <= size1; ++i) {
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distanceArray.at(i + 1, 1) = i;
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distanceArray.at(i + 1, 0) = maxDistance;
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}
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for (size_t i = 0; i <= size2; ++i) {
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distanceArray.at(1, i + 1) = i;
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distanceArray.at(0, i + 1) = maxDistance;
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}
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}
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/// \brief Performs the actual Damerau–Levenshtein algorithm.
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/// \sa computeDamerauLevenshteinDistance() for more details on the algorithm.
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size_t performDamerauLevenshteinAlgorithm(
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DistanceArray &distanceArray, const char *const str1, const size_t size1, const char *const str2, const size_t size2)
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{
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size_t dist1[std::numeric_limits<unsigned char>::max() + 1] = { 0 };
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for (size_t index1 = 1; index1 <= size1; ++index1) {
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size_t dist2 = 0;
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for (size_t index2 = 1; index2 <= size2; ++index2) {
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const size_t substitution((str1[index1 - 1] == str2[index2 - 1]) ? 0 : 1);
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const size_t transposition1(dist1[static_cast<unsigned char>(str2[index2 - 1])]);
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const size_t transposition2(dist2);
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if (!substitution) {
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dist2 = index2;
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}
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// clang-format off
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distanceArray.at(index1 + 1, index2 + 1) = CppUtilities::min(
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distanceArray.at(index1, index2) + substitution, // substitution
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distanceArray.at(index1 + 1, index2) + 1, // insertion
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distanceArray.at(index1, index2 + 1) + 1, // deletion
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distanceArray.at(transposition1, transposition2) + (index1 - transposition1 - 1) + 1 + (index2 - transposition2 - 1) // transposition
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);
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// clang-format on
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}
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dist1[static_cast<int>(str1[index1 - 1])] = index1;
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}
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return distanceArray.at(size1 + 1, size2 + 1);
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}
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/// \brief Allocates the distance array on the heap and performs the Damerau–Levenshtein algorithm.
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template <typename DistanceArray>
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size_t performDamerauLevenshteinAlgorithmAllocatingOnHeap(
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DistanceArray &distanceArray, const char *const str1, const size_t size1, const char *const str2, const size_t size2)
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{
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std::vector<size_t> buffer(distanceArray.totalSize());
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distanceArray.buffer() = buffer.data();
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initDistanceArray(distanceArray, size1, size2);
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return performDamerauLevenshteinAlgorithm(distanceArray, str1, size1, str2, size2);
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}
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/// \brief Allocates the distance array on the stack and performs the Damerau–Levenshtein algorithm.
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/// \remarks The totalSize() of \a distanceArray mustn't exceed 128 byte.
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template <typename DistanceArray>
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size_t performDamerauLevenshteinAlgorithmAllocatingOnStack(
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DistanceArray &distanceArray, const char *const str1, const size_t size1, const char *const str2, const size_t size2)
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{
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size_t buffer[128] = { 0 };
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distanceArray.buffer() = buffer;
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initDistanceArray(distanceArray, size1, size2);
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return performDamerauLevenshteinAlgorithm(distanceArray, str1, size1, str2, size2);
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}
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/// \endcond
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/*!
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* \brief Computes Damerau–Levenshtein distance with adjacent transpositions.
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*
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* \returns Returns the number of editing steps required to turn \a str1 into \a str2.
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* The following operations are considered as editing steps:
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* - substitution: replace one character with another character
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* - insertion: insert one character at any position
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* - deletion: delete one character at any position
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* - transposition: swap any pair of adjacent characters
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*
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* \remarks
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* - Computing "Optimal string alignment distance" is a *different* thing.
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* - The algorithm operates on byte-level. So characters requiring more than one byte in
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* the used character encoding (e.g. UTF-8 encoded German umlauts) are counted as multiple
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* characters (e.g. substitution of those umlauts with non-umlauts requires 2 editing steps).
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* - The memory consumption of this algorithm is considerably. The required memory increases
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* with the product of \a size1 and \a size2. Pass only short words to this function!
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*/
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std::size_t computeDamerauLevenshteinDistance(const char *const str1, const size_t size1, const char *const str2, const size_t size2)
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{
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// allocate distance array
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auto distanceArray(makeNoneOwningMultiArray<std::size_t>(size1 + 2, size2 + 2));
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if (distanceArray.totalSize() <= 128) {
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return performDamerauLevenshteinAlgorithmAllocatingOnStack(distanceArray, str1, size1, str2, size2);
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} else {
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return performDamerauLevenshteinAlgorithmAllocatingOnHeap(distanceArray, str1, size1, str2, size2);
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}
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}
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} // namespace CppUtilities
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