uid.h

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#pragma once
 
#include <atomic>
#include <concepts>
#include <cstddef>
#include <functional>
#include <iosfwd>
 
namespace osc
{
    // Unique ID
    //
    // an ID that is guaranteed to be unique upon non-copy/move construction
    class UID final {
    public:
        using element_type = intptr_t;
 
        static constexpr UID invalid()
        {
            return UID{-1};
        }
 
        static constexpr UID empty()
        {
            return UID{0};
        }
 
        template<std::integral T>
        requires (sizeof(T) <= sizeof(element_type))
        static constexpr UID from_int_unchecked(T i)
        {
            return UID{static_cast<element_type>(i)};
        }
 
        UID() : value_{allocate_next_id()}
        {}
        constexpr UID(const UID&) = default;
        constexpr UID(UID&&) noexcept = default;
        constexpr UID& operator=(const UID&) = default;
        constexpr UID& operator=(UID&&) noexcept = default;
        ~UID() noexcept = default;
 
        void reset()
        {
            value_ = allocate_next_id();
        }
 
        constexpr element_type get() const
        {
            return value_;
        }
 
        explicit constexpr operator bool() const
        {
            return value_ > 0;
        }
 
        friend auto operator<=>(const UID&, const UID&) = default;
 
    private:
        static constinit std::atomic<element_type> g_next_available_id;
 
        static element_type allocate_next_id()
        {
            return g_next_available_id.fetch_add(1, std::memory_order_relaxed);
        }
 
        constexpr UID(element_type value) : value_{value}
        {}
 
        element_type value_;
    };
 
    std::ostream& operator<<(std::ostream&, const UID&);
}
 
// hashing support for LogicalIDs
//
// lets them be used as associative lookup keys, etc.
template<>
struct std::hash<osc::UID> final {
    size_t operator()(const osc::UID& id) const noexcept
    {
        return static_cast<size_t>(id.get());
    }
};