#pragma once
#include <cstdint>
#include <new>
#include <utility>

template <typename T, int N>
struct callback_list {
    struct node {
        alignas(T) uint8_t storage[sizeof(T)];
        node* prev = nullptr;
        node* next = nullptr;
        bool active = false;

        T& value() { return *reinterpret_cast<T*>(storage); }
        const T& value() const { return *reinterpret_cast<const T*>(storage); }
    };

    node nodes[N];
    node* free_head = &nodes[0];
    node sentinel;

    callback_list() {
        for (int i = 0; i < N - 1; i++) nodes[i].next = &nodes[i + 1];
        nodes[N - 1].next = nullptr;
        sentinel.prev = &sentinel;
        sentinel.next = &sentinel;
    }

    bool empty() const { return sentinel.next == &sentinel; }

    node* insert(T value) {
        if (!free_head) return nullptr;
        node* n = alloc(std::move(value));
        link_before(&sentinel, n);
        return n;
    }

    template <typename Less>
    node* insert_sorted(T value, Less&& less) {
        if (!free_head) return nullptr;
        node* n = alloc(std::move(value));
        node* pos = sentinel.next;
        while (pos != &sentinel && !less(n->value(), pos->value()))
            pos = pos->next;
        link_before(pos, n);
        return n;
    }

    void remove(node* n) {
        if (!n || !n->active) return;
        n->prev->next = n->next;
        n->next->prev = n->prev;
        n->active = false;
        n->value().~T();
        n->next = free_head;
        n->prev = nullptr;
        free_head = n;
    }

    node* front() { return sentinel.next != &sentinel ? sentinel.next : nullptr; }

    template <typename Fn>
    void for_each(Fn&& fn) {
        node* cur = sentinel.next;
        while (cur != &sentinel) {
            node* next = cur->next;
            fn(cur);
            cur = next;
        }
    }

private:
    node* alloc(T value) {
        node* n = free_head;
        free_head = n->next;
        new (n->storage) T(std::move(value));
        n->active = true;
        return n;
    }

    void link_before(node* pos, node* n) {
        n->prev = pos->prev;
        n->next = pos;
        pos->prev->next = n;
        pos->prev = n;
    }
};