LinearProbingHashST.hpp

/*
 * Copyright (C) 2024-2026  landerrosette <57791410+landerrosette@users.noreply.github.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#ifndef ALGS4_LINEARPROBINGHASHST_HPP
#define ALGS4_LINEARPROBINGHASHST_HPP

#include <concepts>
#include <cstddef>
#include <functional>
#include <optional>
#include <utility>
#include <vector>

#include "Queue.hpp"
#include "ST.hpp"

namespace algs4 {
template <std::copyable Key, std::movable Value>
    requires std::equality_comparable<Key>
class LinearProbingHashST : public ST<Key, Value> {
private:
    std::ptrdiff_t N_ = 0;   // number of key-value pairs in the table
    std::ptrdiff_t M_ = 16;  // size of linear-probing table
    std::vector<std::optional<Key>> keys_;
    std::vector<std::optional<Value>> vals_;

    explicit constexpr LinearProbingHashST(std::ptrdiff_t M) : M_(M), keys_(M), vals_(M) {}

    constexpr auto hash(const Key& key) const { return std::hash<Key>()(key) % M_; }

    constexpr void resize(std::ptrdiff_t cap) {
        LinearProbingHashST t(cap);
        for (decltype(M_) i = 0; i < M_; ++i)
            if (keys_[i]) t.put(std::move(*keys_[i]), std::move(*vals_[i]));
        keys_ = std::move(t.keys_);
        vals_ = std::move(t.vals_);
        M_ = t.M_;
    }

public:
    constexpr LinearProbingHashST() : keys_(M_), vals_(M_) {}

    constexpr void put(Key key, Value val) override {
        if (N_ >= M_ / 2) resize(2 * M_);

        auto i = hash(key);
        for (; keys_[i]; i = (i + 1) % M_)
            if (keys_[i] == key) {
                vals_[i] = std::move(val);
                return;
            }
        keys_[i] = std::move(key);
        vals_[i] = std::move(val);
        ++N_;
    }

    using ST<Key, Value>::get;
    constexpr const Value* get(const Key& key) const override {
        for (auto i = hash(key); keys_[i]; i = (i + 1) % M_)
            if (keys_[i] == key) return &*vals_[i];
        return nullptr;
    }

    constexpr void remove(const Key& key) override {
        if (!this->contains(key)) return;
        auto i = hash(key);
        while (keys_[i] != key) i = (i + 1) % M_;
        keys_[i] = std::nullopt;
        vals_[i] = std::nullopt;
        i = (i + 1) % M_;
        while (keys_[i]) {
            Key keyToRedo = std::move(*keys_[i]);
            Value valToRedo = std::move(*vals_[i]);
            keys_[i] = std::nullopt;
            vals_[i] = std::nullopt;
            --N_;
            put(std::move(keyToRedo), std::move(valToRedo));
            i = (i + 1) % M_;
        }
        --N_;
        if (N_ > 0 && N_ == M_ / 8) resize(M_ / 2);
    }

    constexpr std::ptrdiff_t size() const override { return N_; }

    Queue<Key> keys() const override {
        Queue<Key> queue;
        for (decltype(M_) i = 0; i < M_; ++i)
            if (keys_[i]) queue.enqueue(*keys_[i]);
        return queue;
    }
};
}  // namespace algs4

#endif  // ALGS4_LINEARPROBINGHASHST_HPP