Fast and memory efficient C++ implementations of the following string distances: LCS (Longest common subsequence), Levenshtein, Damerau-Levenshtein

StringDistances.h

// Copyright 2024 Johan Rade (johan.rade@gmail.com)
// Distributed under the MIT license (https://opensource.org/licenses/MIT)

inline size_t lcsDistance(const std::string& s, const std::string& t)
{
    size_t m = s.size() + 1;
    size_t n = t.size() + 1;
    const char* p = s.data();
    const char* q = t.data();

    if (m > n) {
        std::swap(m, n);
        std::swap(p, q);
    }

    static thread_local std::vector<size_t> buf;
    buf.resize(std::max(2 * m, buf.size()));
    size_t* prevDist = buf.data();
    size_t* dist = buf.data() + m;

    for (size_t i = 0; i != m; ++i)
        dist[i] = i;

    for (size_t j = 1; j != n; ++j) {
        std::swap(prevDist, dist);
        dist[0] = j;
        for (size_t i = 1; i != m; ++i) {
            const size_t cost = (p[i - 1] == q[j - 1]) ? 0 : 2;
            dist[i] = std::min(prevDist[i - 1] + cost, std::min(prevDist[i], dist[i - 1]) + 1);
        }
    }

    return dist[m - 1];
}

inline size_t levenshteinDistance(const std::string& s, const std::string& t)
{
    size_t m = s.size() + 1;
    size_t n = t.size() + 1;
    const char* p = s.data();
    const char* q = t.data();

    if (m > n) {
        std::swap(m, n);
        std::swap(p, q);
    }

    static thread_local std::vector<size_t> buf;
    buf.resize(std::max(2 * m, buf.size()));
    size_t* prevDist = buf.data();
    size_t* dist = buf.data() + m;

    for (size_t i = 0; i != m; ++i)
        dist[i] = i;

    for (size_t j = 1; j != n; ++j) {
        std::swap(prevDist, dist);
        dist[0] = j;
        for (size_t i = 1; i != m; ++i) {
            const size_t cost = (p[i - 1] == q[j - 1]) ? 0 : 1;
            dist[i] = std::min(prevDist[i - 1] + cost, std::min(prevDist[i], dist[i - 1]) + 1);
        }
    }

    return dist[m - 1];
}

inline size_t damerauLevenshteinDistance(const std::string& s, const std::string& t)
{
    size_t m = s.size() + 1;
    size_t n = t.size() + 1;
    const char* p = s.data();
    const char* q = t.data();

    if (m > n) {
        std::swap(m, n);
        std::swap(p, q);
    }
    if (m == 1)
        return n - 1;

    static thread_local std::vector<size_t> buf;
    buf.resize(std::max(3 * m, buf.size()));
    size_t* prevPrevDist = buf.data();
    size_t* prevDist = buf.data() + m;
    size_t* dist = buf.data() + 2 * m;

    // j = 0
    for (size_t i = 0; i != m; ++i)
        dist[i] = i;

    // j = 1
    std::swap(dist, prevDist);
    dist[0] = 1;
    for (size_t i = 1; i != m; ++i) {
        const size_t substCost = (p[i - 1] == q[0]) ? 0 : 1;
        dist[i] = std::min(prevDist[i - 1] + substCost, std::min(prevDist[i], dist[i - 1]) + 1);
    }

    for (size_t j = 2; j != n; ++j) {
        std::swap(prevDist, prevPrevDist);
        std::swap(dist, prevDist);
        dist[0] = j;
        {
            const size_t substCost = (p[0] == q[j - 1]) ? 0 : 1;
            dist[1] = std::min(prevDist[0] + substCost, std::min(prevDist[1], dist[0]) + 1);
        }
        for (size_t i = 2; i != m; ++i) {
            const size_t substCost = (p[i - 1] == q[j - 1]) ? 0 : 1;
            const size_t transCost = (p[i - 1] == q[j - 2] && p[i - 2] == q[j - 1]) ? 1 : 2;
            dist[i] = std::min(
                std::min(prevDist[i - 1] + substCost, prevPrevDist[i - 2] + transCost),
                std::min(prevDist[i], dist[i - 1]) + 1);
        }
    }

    return dist[m - 1];
}