First Light

#pragma once
#include <cstddef>
#include <memory_resource>
#include <string>

namespace prism {

// An instrumented interface, not an owner of upstream. Declare upstream first,
// this resource second, and its borrowing containers last (reverse destruction).
// This deliberately single-threaded teaching resource needs external locking
// if shared. Counters describe live requested bytes, not arena physical capacity.
class CountingResource final : public std::pmr::memory_resource {
public:
    explicit CountingResource(std::pmr::memory_resource& upstream) noexcept : upstream_{upstream} {}
    ~CountingResource() override = default;
    CountingResource(const CountingResource&) = delete;
    CountingResource& operator=(const CountingResource&) = delete;
    // Borrowers retain this resource's address and equality is identity-based.
    CountingResource(CountingResource&&) = delete;
    CountingResource& operator=(CountingResource&&) = delete;
    [[nodiscard]] std::size_t allocations() const noexcept { return allocations_; }
    [[nodiscard]] std::size_t live_bytes() const noexcept { return live_; }
    [[nodiscard]] std::size_t peak_bytes() const noexcept { return peak_; }
private:
    void* do_allocate(std::size_t bytes, std::size_t alignment) override;
    void do_deallocate(void* pointer, std::size_t bytes, std::size_t alignment) override;
    bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {
        return this == &other;
    }
    std::pmr::memory_resource& upstream_;
    std::size_t allocations_{}, live_{}, peak_{};
};

struct MemoryReport {
    std::string promoted_title; // Independent ordinary owner survives arena release.
    std::size_t allocations{}, peak_bytes{}, live_after_scope{};
    bool bounded_failure{};
};
[[nodiscard]] MemoryReport run_memory_lab();
} // namespace prism