weaseldb/src/reference.hpp

#pragma once

#include <algorithm>
#include <atomic>
#include <cstdint>
#include <cstdio>
#include <cstdlib>

/**
 * @brief Thread-safe reference counting abstraction with shared/weak pointer
 * semantics
 *
 * TODO: Implement custom reference counting system with:
 * - Thread-safe reference counting using atomic operations
 * - Weak reference support to break circular dependencies
 * - Move semantics for efficient transfers
 * - Custom deleter support
 * - Zero-overhead when not using weak references
 */

namespace detail {
struct ControlBlock {
  std::atomic<uint32_t> strong_count;
  std::atomic<uint32_t> weak_count;

  ControlBlock()
      : strong_count(1), weak_count(1) {
  } // Start with 1 strong, 1 weak (biased)

  /**
   * @brief Increment strong reference count
   * @return Previous strong count
   */
  uint32_t increment_strong() noexcept {
    return strong_count.fetch_add(1, std::memory_order_relaxed);
  }

  /**
   * @brief Decrement strong reference count
   * @return Previous strong count
   */
  uint32_t decrement_strong() noexcept {
    return strong_count.fetch_sub(1, std::memory_order_acq_rel);
  }

  /**
   * @brief Increment weak reference count
   * @return Previous weak count
   */
  uint32_t increment_weak() noexcept {
    return weak_count.fetch_add(1, std::memory_order_relaxed);
  }

  /**
   * @brief Decrement weak reference count
   * @return Previous weak count
   */
  uint32_t decrement_weak() noexcept {
    return weak_count.fetch_sub(1, std::memory_order_acq_rel);
  }
};
} // namespace detail

template <typename T> struct Ref {
  /**
   * @brief Get raw pointer to managed object
   */
  T *get() const noexcept { return ptr; }

  /**
   * @brief Dereference operator
   */
  T &operator*() const { return *ptr; }

  /**
   * @brief Arrow operator
   */
  T *operator->() const { return ptr; }

  /**
   * @brief Check if Ref is valid (not empty)
   */
  explicit operator bool() const noexcept { return ptr != nullptr; }

  /**
   * @brief Destructor - decrements strong reference count
   */
  ~Ref() { release(); }

  /**
   * @brief Copy constructor - increments strong reference count
   */
  Ref(const Ref &other) noexcept
      : ptr(other.ptr), control_block(other.control_block) {
    if (control_block) {
      control_block->increment_strong();
    }
  }

  /**
   * @brief Copy assignment operator
   */
  Ref &operator=(const Ref &other) noexcept {
    if (this != &other) {
      release();
      ptr = other.ptr;
      control_block = other.control_block;
      if (control_block) {
        control_block->increment_strong();
      }
    }
    return *this;
  }

  /**
   * @brief Move constructor - transfers ownership
   */
  Ref(Ref &&other) noexcept
      : ptr(other.ptr), control_block(other.control_block) {
    other.ptr = nullptr;
    other.control_block = nullptr;
  }

  /**
   * @brief Move assignment operator
   */
  Ref &operator=(Ref &&other) noexcept {
    if (this != &other) {
      release();
      ptr = other.ptr;
      control_block = other.control_block;
      other.ptr = nullptr;
      other.control_block = nullptr;
    }
    return *this;
  }

  /**
   * @brief Reset to empty state
   */
  void reset() noexcept {
    release();
    ptr = nullptr;
    control_block = nullptr;
  }

  /**
   * @brief Equality comparison
   */
  bool operator==(const Ref &other) const noexcept {
    return control_block == other.control_block;
  }

  /**
   * @brief Inequality comparison
   */
  bool operator!=(const Ref &other) const noexcept { return !(*this == other); }

  /**
   * @brief Default constructor - creates empty Ref
   */
  Ref() : ptr(nullptr), control_block(nullptr) {}

private:
  explicit Ref(T *object_ptr, detail::ControlBlock *cb)
      : ptr(object_ptr), control_block(cb) {}

  T *ptr;
  detail::ControlBlock *control_block;

  /**
   * @brief Release current reference and handle cleanup
   */
  void release() noexcept {
    if (control_block) {
      uint32_t prev_strong = control_block->decrement_strong();

      // If this was the last strong reference, destroy the object
      if (prev_strong == 1) {
        ptr->~T();

        // Release the bias - decrement weak count for strong references
        uint32_t prev_weak = control_block->decrement_weak();

        // If weak count hits 0, destroy and free control block
        if (prev_weak == 1) {
          control_block->~ControlBlock();
          std::free(control_block);
        }
      }
    }
  }

  template <typename U, typename... Args>
  friend Ref<U> make_ref(Args &&...args);

  template <typename U> friend struct WeakRef;
};

template <typename T> struct WeakRef {
  /**
   * @brief Attempt to promote WeakRef to Ref
   * @return Valid Ref if object still alive, empty Ref otherwise
   */
  Ref<T> lock() {
    if (!control_block) {
      return Ref<T>();
    }

    // Try to increment strong count if it's not zero
    uint32_t expected_strong =
        control_block->strong_count.load(std::memory_order_relaxed);
    while (expected_strong > 0) {
      // Try to increment the strong count
      if (control_block->strong_count.compare_exchange_weak(
              expected_strong, expected_strong + 1, std::memory_order_acquire,
              std::memory_order_relaxed)) {
        // Success - we incremented the strong count
        return Ref<T>(get_object_ptr(), control_block);
      }
      // CAS failed, expected_strong now contains the current value, retry
    }

    // Strong count was 0, object is being destroyed
    return Ref<T>();
  }

  /**
   * @brief Destructor - decrements weak reference count
   */
  ~WeakRef() { release(); }

  /**
   * @brief Copy constructor from WeakRef
   */
  WeakRef(const WeakRef &other) noexcept : control_block(other.control_block) {
    if (control_block) {
      control_block->increment_weak();
    }
  }

  /**
   * @brief Copy constructor from Ref
   */
  WeakRef(const Ref<T> &ref) noexcept : control_block(ref.control_block) {
    if (control_block) {
      control_block->increment_weak();
    }
  }

  /**
   * @brief Copy assignment from WeakRef
   */
  WeakRef &operator=(const WeakRef &other) noexcept {
    if (this != &other) {
      release();
      control_block = other.control_block;
      if (control_block) {
        control_block->increment_weak();
      }
    }
    return *this;
  }

  /**
   * @brief Copy assignment from Ref
   */
  WeakRef &operator=(const Ref<T> &ref) noexcept {
    release();
    control_block = ref.control_block;
    if (control_block) {
      control_block->increment_weak();
    }
    return *this;
  }

  /**
   * @brief Move constructor
   */
  WeakRef(WeakRef &&other) noexcept : control_block(other.control_block) {
    other.control_block = nullptr;
  }

  /**
   * @brief Move assignment
   */
  WeakRef &operator=(WeakRef &&other) noexcept {
    if (this != &other) {
      release();
      control_block = other.control_block;
      other.control_block = nullptr;
    }
    return *this;
  }

  /**
   * @brief Reset to empty state
   */
  void reset() noexcept {
    release();
    control_block = nullptr;
  }

  /**
   * @brief Default constructor - creates empty WeakRef
   */
  WeakRef() : control_block(nullptr) {}

private:
  explicit WeakRef(detail::ControlBlock *cb) : control_block(cb) {}

  detail::ControlBlock *control_block;

  // Helper to calculate object pointer from control block
  T *get_object_ptr() const {
    if (!control_block)
      return nullptr;
    constexpr size_t cb_size = sizeof(detail::ControlBlock);
    constexpr size_t alignment = alignof(T);
    constexpr size_t padded_cb_size =
        (cb_size + alignment - 1) & ~(alignment - 1);
    return reinterpret_cast<T *>(reinterpret_cast<char *>(control_block) +
                                 padded_cb_size);
  }

  /**
   * @brief Release current weak reference and handle cleanup
   */
  void release() noexcept {
    if (control_block) {
      uint32_t prev_weak = control_block->decrement_weak();

      // If weak count hits 0, destroy and free control block
      if (prev_weak == 1) {
        control_block->~ControlBlock();
        std::free(control_block);
      }
    }
  }

  template <typename U> friend struct Ref;
};

/**
 * @brief Create a new Ref with object constructed in-place after control block
 */
template <typename T, typename... Args> Ref<T> make_ref(Args &&...args) {
  constexpr size_t cb_size = sizeof(detail::ControlBlock);
  constexpr size_t alignment = alignof(T);
  constexpr size_t padded_cb_size =
      (cb_size + alignment - 1) & ~(alignment - 1);

  constexpr size_t total_alignment =
      std::max(alignof(detail::ControlBlock), alignment);
  constexpr size_t total_size = padded_cb_size + sizeof(T);
  constexpr size_t aligned_total_size =
      (total_size + total_alignment - 1) & ~(total_alignment - 1);

  char *buf = reinterpret_cast<char *>(
      std::aligned_alloc(total_alignment, aligned_total_size));
  if (!buf) {
    std::fprintf(stderr, "Out of memory\n");
    std::abort();
  }

  auto *cb = new (buf) detail::ControlBlock();
  T *obj = new (buf + padded_cb_size) T{std::forward<Args>(args)...};
  return Ref<T>(obj, cb);
}