ฟังก์ชันแฮชแบบกำหนดเอง

#include <iostream>
#include <unordered_set>
#include <string>

int main() {
    std::unordered_set<std::string> s{"hi"};
    std::cout << s.count("hi") << '\n';
    return 0;
}

#include <iostream>
#include <functional>
#include <string>

int main() {
    std::hash<std::string> h;
    std::cout << "hash exists and returns a size_t\n";
    std::size_t v = h("hello");
    std::cout << (v != 0 ? "non-zero hash" : "zero") << '\n';
    return 0;
}

#include <iostream>

struct Point {
    int x, y;
    bool operator==(const Point& o) const { return x == o.x && y == o.y; }
};

int main() {
    Point a{1, 2}, b{1, 2};
    std::cout << std::boolalpha << (a == b) << '\n';
    return 0;
}

#include <iostream>
#include <functional>

struct Point { int x, y; };

struct PointHash {
    std::size_t operator()(const Point& p) const {
        return std::hash<int>()(p.x) ^ (std::hash<int>()(p.y) << 1);
    }
};

int main() {
    PointHash h;
    std::cout << "hashed: " << (h({3, 4}) != 0 ? "ok" : "zero") << '\n';
    return 0;
}

#include <iostream>
#include <unordered_set>
#include <functional>

struct Point {
    int x, y;
    bool operator==(const Point& o) const { return x == o.x && y == o.y; }
};

struct PointHash {
    std::size_t operator()(const Point& p) const {
        return std::hash<int>()(p.x) ^ (std::hash<int>()(p.y) << 1);
    }
};

int main() {
    std::unordered_set<Point, PointHash> pts;
    pts.insert({1, 2});
    pts.insert({1, 2});
    std::cout << pts.size() << '\n';
    return 0;
}

#include <iostream>
#include <unordered_set>

struct Point {
    int x, y;
    bool operator==(const Point& o) const { return x == o.x && y == o.y; }
};

struct PointHash {
    std::size_t operator()(const Point& p) const {
        return std::hash<int>()(p.x * 31 + p.y);
    }
};

int main() {
    std::unordered_set<Point, PointHash> s{{1, 1}, {2, 2}};
    std::cout << s.count({1, 1}) << '\n';
    return 0;
}

#include <iostream>
#include <unordered_map>
#include <functional>

struct Point {
    int x, y;
    bool operator==(const Point& o) const { return x == o.x && y == o.y; }
};

struct PointHash {
    std::size_t operator()(const Point& p) const {
        return std::hash<int>()(p.x) ^ (std::hash<int>()(p.y) << 1);
    }
};

int main() {
    std::unordered_map<Point, std::string, PointHash> m;
    m[{0, 0}] = "origin";
    std::cout << m[{0, 0}] << '\n';
    return 0;
}

#include <iostream>
#include <functional>

std::size_t combine(std::size_t seed, std::size_t v) {
    return seed ^ (v + 0x9e3779b9 + (seed << 6) + (seed >> 2));
}

int main() {
    std::size_t h = 0;
    h = combine(h, std::hash<int>()(10));
    h = combine(h, std::hash<int>()(20));
    std::cout << (h != 0 ? "combined ok" : "zero") << '\n';
    return 0;
}

#include <iostream>
#include <functional>

struct Bad { std::size_t operator()(int) const { return 0; } };
struct Good { std::size_t operator()(int x) const { return std::hash<int>()(x); } };

int main() {
    std::cout << Bad()(5) << ' ' << (Good()(5) != 0 ? "varies" : "0") << '\n';
    return 0;
}

#include <iostream>
#include <unordered_set>

struct Point {
    int x, y;
    bool operator==(const Point& o) const { return x == o.x && y == o.y; }
};

namespace std {
    template <> struct hash<Point> {
        std::size_t operator()(const Point& p) const {
            return hash<int>()(p.x) ^ (hash<int>()(p.y) << 1);
        }
    };
}

int main() {
    std::unordered_set<Point> s{{1, 2}};
    std::cout << s.count({1, 2}) << '\n';
    return 0;
}

#include <iostream>
#include <unordered_set>

int main() {
    auto h = [](int x) { return std::hash<int>()(x * 2654435761u); };
    std::unordered_set<int, decltype(h)> s(8, h);
    s.insert(42);
    std::cout << s.count(42) << '\n';
    return 0;
}