tpl_persistent_vector.H

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/*
                          Aleph_w
 
  Data structures & Algorithms
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
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  of this software and associated documentation files (the "Software"), to deal
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*/
 
#ifndef TPL_PERSISTENT_VECTOR_H
#define TPL_PERSISTENT_VECTOR_H
 
#include <array>
#include <concepts>
#include <cstddef>
#include <limits>
#include <memory>
#include <type_traits>
#include <utility>
 
#include <ah-errors.H>
#include <tpl_array.H>
 
namespace Aleph
{
 
template <typename T>
class PersistentVector
{
  static constexpr size_t branch_bits = 5;
  static constexpr size_t branch_factor = size_t{1} << branch_bits;
  static constexpr size_t branch_mask = branch_factor - 1;
 
  using ValuePtr = std::shared_ptr<const T>;
 
  struct Node
  {
    explicit Node(const bool leaf = true) noexcept : is_leaf(leaf) {}
 
    bool is_leaf = true;
    std::array<std::shared_ptr<const Node>, branch_factor> children{};
    std::array<ValuePtr, branch_factor> values{};
  };
 
  using NodePtr = std::shared_ptr<const Node>;
 
  NodePtr root_;
  size_t size_ = 0;
  size_t shift_ = 0;  // 0 means root is a leaf; otherwise high child shift.
 
  [[nodiscard]] static size_t child_index(const size_t index, const size_t shift) noexcept
  {
    return (index >> shift) & branch_mask;
  }
 
  [[nodiscard]] static size_t capacity_for_shift(const size_t shift) noexcept
  {
    constexpr size_t digits = std::numeric_limits<size_t>::digits;
    if (shift >= digits - branch_bits)
      return std::numeric_limits<size_t>::max();
    return size_t{1} << (shift + branch_bits);
  }
 
  [[nodiscard]] static NodePtr set_rec(const NodePtr &node,
                                       const size_t shift,
                                       const size_t index,
                                       ValuePtr value)
  {
    auto copy = node == nullptr ? std::make_shared<Node>(shift == 0)
                                : std::make_shared<Node>(*node);
    copy->is_leaf = shift == 0;
 
    if (shift == 0)
      {
        copy->values[child_index(index, 0)] = std::move(value);
        return copy;
      }
 
    const size_t slot = child_index(index, shift);
    copy->children[slot] = set_rec(copy->children[slot], shift - branch_bits,
                                   index, std::move(value));
    return copy;
  }
 
  [[nodiscard]] static bool leaf_has_values(const std::shared_ptr<Node> &node) noexcept
  {
    for (const auto &value : node->values)
      if (value != nullptr)
        return true;
    return false;
  }
 
  [[nodiscard]] static bool internal_has_children(const std::shared_ptr<Node> &node) noexcept
  {
    for (const auto &child : node->children)
      if (child != nullptr)
        return true;
    return false;
  }
 
  [[nodiscard]] static NodePtr clear_rec(const NodePtr &node,
                                         const size_t shift,
                                         const size_t index)
  {
    if (node == nullptr)
      return nullptr;
 
    auto copy = std::make_shared<Node>(*node);
    copy->is_leaf = shift == 0;
 
    if (shift == 0)
      {
        copy->values[child_index(index, 0)] = nullptr;
        return leaf_has_values(copy) ? NodePtr(copy) : nullptr;
      }
 
    const size_t slot = child_index(index, shift);
    copy->children[slot] = clear_rec(copy->children[slot], shift - branch_bits, index);
    return internal_has_children(copy) ? NodePtr(copy) : nullptr;
  }
 
  [[nodiscard]] static const T *get_ptr(const NodePtr &root,
                                        size_t shift,
                                        const size_t index) noexcept
  {
    const Node *curr = root.get();
    while (curr != nullptr and shift > 0)
      {
        curr = curr->children[child_index(index, shift)].get();
        shift -= branch_bits;
      }
    if (curr == nullptr)
      return nullptr;
    const ValuePtr &value = curr->values[child_index(index, 0)];
    return value == nullptr ? nullptr : value.get();
  }
 
  [[nodiscard]] static size_t count_values_rec(const NodePtr &node,
                                               const size_t shift,
                                               bool &ok) noexcept
  {
    if (node == nullptr)
      return 0;
    if (node->is_leaf != (shift == 0))
      {
        ok = false;
        return 0;
      }
 
    size_t count = 0;
    if (shift == 0)
      {
        for (const auto &child : node->children)
          if (child != nullptr)
            ok = false;
        for (const auto &value : node->values)
          if (value != nullptr)
            ++count;
        return count;
      }
 
    for (const auto &value : node->values)
      if (value != nullptr)
        ok = false;
    for (const auto &child : node->children)
      count += count_values_rec(child, shift - branch_bits, ok);
    return count;
  }
 
  explicit PersistentVector(NodePtr root, const size_t n, const size_t shift) noexcept
    : root_(std::move(root)), size_(n), shift_(shift)
  {
    // Empty.
  }
 
public:
  PersistentVector() noexcept = default;
 
  [[nodiscard]] static constexpr size_t branching_factor() noexcept
  {
    return branch_factor;
  }
 
  [[nodiscard]] bool is_empty() const noexcept { return size_ == 0; }
 
  [[nodiscard]] size_t size() const noexcept { return size_; }
 
  [[nodiscard]] const T &get(const size_t index) const
  {
    ah_out_of_range_error_if(index >= size_)
      << "PersistentVector::get(): index out of range";
    const T *value = get_ptr(root_, shift_, index);
    ah_logic_error_if(value == nullptr)
      << "PersistentVector::get(): missing value inside vector trie";
    return *value;
  }
 
  [[nodiscard]] const T &operator [] (const size_t index) const
  {
    return get(index);
  }
 
  [[nodiscard]] PersistentVector push_back(const T &value) const
  {
    return push_back(ValuePtr(std::make_shared<const T>(value)));
  }
 
  [[nodiscard]] PersistentVector push_back(T &&value) const
  {
    return push_back(ValuePtr(std::make_shared<const T>(std::move(value))));
  }
 
  [[nodiscard]] PersistentVector set(const size_t index, const T &value) const
  {
    ah_out_of_range_error_if(index >= size_)
      << "PersistentVector::set(): index out of range";
    return set(index, ValuePtr(std::make_shared<const T>(value)));
  }
 
  [[nodiscard]] PersistentVector set(const size_t index, T &&value) const
  {
    ah_out_of_range_error_if(index >= size_)
      << "PersistentVector::set(): index out of range";
    return set(index, ValuePtr(std::make_shared<const T>(std::move(value))));
  }
 
  [[nodiscard]] PersistentVector pop_back() const
  {
    ah_underflow_error_if(is_empty())
      << "PersistentVector::pop_back(): empty vector";
 
    if (size_ == 1)
      return PersistentVector();
 
    const size_t new_size = size_ - 1;
    NodePtr root = clear_rec(root_, shift_, new_size);
    size_t shift = shift_;
 
    while (shift > 0 and new_size <= capacity_for_shift(shift - branch_bits))
      {
        root = root == nullptr ? nullptr : root->children[0];
        shift -= branch_bits;
      }
 
    return PersistentVector(std::move(root), new_size, shift);
  }
 
  [[nodiscard]] Array<T> to_array() const
    requires (std::is_copy_constructible_v<T> and std::is_copy_assignable_v<T> and
              std::is_move_constructible_v<T> and std::is_move_assignable_v<T>)
  {
    Array<T> out;
    out.reserve(size_);
    for (size_t i = 0; i < size_; ++i)
      {
        const T &value = get(i);
        out.append(value);
      }
    return out;
  }
 
  [[nodiscard]] bool verify() const noexcept
  {
    if (size_ == 0)
      return root_ == nullptr and shift_ == 0;
    if (root_ == nullptr)
      return false;
    if (size_ > capacity_for_shift(shift_))
      return false;
 
    bool ok = true;
    if (const size_t count = count_values_rec(root_, shift_, ok); not ok or count != size_)
      return false;
    for (size_t i = 0; i < size_; ++i)
      if (get_ptr(root_, shift_, i) == nullptr)
        return false;
    return true;
  }
 
private:
  [[nodiscard]] PersistentVector push_back(ValuePtr value) const
  {
    ah_length_error_if(size_ == std::numeric_limits<size_t>::max())
      << "PersistentVector::push_back(): size_t capacity exhausted";
 
    if (root_ == nullptr)
      return PersistentVector(set_rec(nullptr, 0, 0, std::move(value)), 1, 0);
 
    NodePtr root = root_;
    size_t shift = shift_;
    if (size_ == capacity_for_shift(shift_))
      {
        ah_length_error_if(shift_ >= std::numeric_limits<size_t>::digits - branch_bits)
          << "PersistentVector::push_back(): trie depth exhausted";
        auto grown = std::make_shared<Node>(false);
        grown->children[0] = root_;
        root = grown;
        shift += branch_bits;
      }
 
    root = set_rec(root, shift, size_, std::move(value));
    return PersistentVector(std::move(root), size_ + 1, shift);
  }
 
  [[nodiscard]] PersistentVector set(const size_t index, ValuePtr value) const
  {
    ah_out_of_range_error_if(index >= size_)
      << "PersistentVector::set(): index out of range";
    return PersistentVector(set_rec(root_, shift_, index, std::move(value)),
                            size_, shift_);
  }
};
 
} // namespace Aleph
 
#endif // TPL_PERSISTENT_VECTOR_H