C++ levenshtein edit distance

README.md

When working on fuzzywuzzy I required a C++ implementation of the Levenshtein edit distance. I found different ways to implement this, so I took a bit of everything, and tried to simplify it as much as I could.

Majority of credit goes to Martin Ettl, for his submission on Rosetta Code.

Licese

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levelshtein_edit_distance.cpp

#include <vector>                    // std::vector
#include <utility>                   // std::pair

#include "levenshtein_edit_distance.hpp"
#include "utils.hpp"

using std::experimental::string_view;
using std::vector;

namespace algorithm {

size_t algorithm::levenshtein_distance(const string_view &s1, const string_view &s2)
{
    size_t len1 = s1.size();
    size_t len2 = s2.size();

    size_t i, j;

    /* Base case: a string is empty. */
    if (len1 == 0) return len2;
    if (len2 == 0) return len1;

    /*
     * Initialize rows[i].first (the previous row of distances).
     * This first row represents the edit distance againts an
     * empty string, so the distance is just the numbers of
     * characters to delete from the non-empty string.
     */
    vector<std::pair<size_t, size_t>> rows(lenb + 1);
    for (i = 0; i < rows.size(); i++)
        rows[i].first = i;

    for (i = 0; i < lena; i++) {
        /* Edit distance is delete (i+1) chars from non-empty to empty. */
        rows[0].second = i + 1;

        /* Fill in the rest of the row. */
        for (j = 0; j < lenb; j++) {
            auto cost = (s1[i] == s2[j]) ? 0 : 1;

            rows[j + 1].second = utils::min(rows[j].second + 1,
                                            rows[j + 1].first + 1,
                                            rows[j].first + cost);
        }

        /*
         * Copy the current row to the previous
         * one in preparation for next iteration.
         */
        for (auto &row : rows)
            row.first = row.second;
    }

    return rows[lenb].second;
}
    
}

levenshtein_edit_distance.hpp

#pragma once

#include <experimental/string_view>  // std::experimental::string_view

namespace algorithm {

size_t levenshtein_dist(const string_view &str1, const string_view &str2);
  
}

utils.hpp

#pragma once

#include <algorithm>  // std::min_element()
#include <vector>     // std::vector

/*
 * An "extension" of std::min() so that more than two arguments
 * can be passed. The first argument decides What everything else
 * is casted too.
 *
 * Hopefully the compiler will complain if we pass this something stupid.
 */
template <typename T, typename... Args>
constexpr auto min(T first, Args... args)
{
    static_assert(sizeof...(Args) > 1, "use std::min() instead, since it's only two arguments.");

    std::vector<T> vec = {first, static_cast<T>(args)...};
    auto min = std::min_element(vec.begin(), vec.end());

    return *min;
}