Backtracking.H

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/*
                          Aleph_w
 
  Data structures & Algorithms
  version 2.0.0b
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
 
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
 
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
*/
 
# ifndef BACKTRACKING_H
# define BACKTRACKING_H
 
#include <utility>
 
#include <ah-errors.H>
#include <state_search_common.H>
 
namespace Aleph {
 
namespace state_search_detail {
 
template <SearchState State, SearchMove Move>
struct PreferShallowerSolution
{
  bool operator()(const SearchSolution<State, Move> &lhs,
                  const SearchSolution<State, Move> &rhs) const noexcept
  {
    return lhs.depth < rhs.depth;
  }
};
 
}  // end namespace state_search_detail
 
template <BacktrackingDomain Domain>
class Depth_First_Backtracking
{
public:
  using Domain_Type = Domain;
  using State = typename Domain::State;
  using Move = typename Domain::Move;
  using Solution = SearchSolution<State, Move>;
  using Solution_Compare = state_search_detail::PreferShallowerSolution<State, Move>;
  using Result = SearchResult<Solution, Solution_Compare>;
 
  static constexpr bool supports_best_first = false;
 
  explicit Depth_First_Backtracking(Domain domain,
                                    ExplorationPolicy policy = {},
                                    const SearchLimits &limits = {},
                                    Solution_Compare compare = {})
    : domain_(std::move(domain)), policy_(policy), limits_(limits), compare_(std::move(compare))
  {
    // empty
  }
 
  [[nodiscard]] const Domain &domain() const noexcept
  {
    return domain_;
  }
 
  [[nodiscard]] Domain &domain() noexcept
  {
    return domain_;
  }
 
  [[nodiscard]] const ExplorationPolicy &policy() const noexcept
  {
    return policy_;
  }
 
  [[nodiscard]] const SearchLimits &limits() const noexcept
  {
    return limits_;
  }
 
  void set_policy(const ExplorationPolicy &policy) noexcept
  {
    policy_ = policy;
  }
 
  void set_limits(const SearchLimits &limits) noexcept
  {
    limits_ = limits;
  }
 
  [[nodiscard]] Result search(State initial_state)
  {
    ContinueSearch on_solution;
    return search(std::move(initial_state), on_solution);
  }
 
  template <typename OnSolution>
    requires SearchSolutionVisitor<OnSolution, Solution>
  [[nodiscard]] Result search(State initial_state, OnSolution &on_solution)
  {
    ah_invalid_argument_if(policy_.strategy != ExplorationPolicy::Strategy::Depth_First)
      << "Depth_First_Backtracking only supports depth-first strategy";
    ah_invalid_argument_if(limits_.max_solutions == 0)
      << "SearchLimits::max_solutions must be positive or Search_Unlimited";
 
    Result result(compare_);
    result.policy = policy_;
    result.limits = limits_;
    const auto start_time = SearchClock::now();
 
    SearchPath<Move> path;
    reserve_search_path(path, limits_);
    (void) dfs(initial_state, path, 0, result, on_solution);
 
    if (result.status == SearchStatus::NotStarted)
      result.status = SearchStatus::Exhausted;
 
    result.stats.elapsed_ms = search_elapsed_ms(SearchClock::now() - start_time);
 
    return result;
  }
 
  template <typename OnSolution>
    requires SearchSolutionVisitor<OnSolution, Solution>
  [[nodiscard]] Result search(State initial_state, OnSolution &&on_solution)
  {
    auto handler = std::forward<OnSolution>(on_solution);
    return search(std::move(initial_state), handler);
  }
 
  template <typename VisitedSet>
    requires SearchStateKeyProvider<Domain>
         and VisitedBoundMap<VisitedSet, typename Domain::State_Key, size_t>
  [[nodiscard]] Result search(State initial_state, VisitedSet &visited)
  {
    ContinueSearch on_solution;
    return search(std::move(initial_state), visited, on_solution);
  }
 
  template <typename VisitedSet, typename OnSolution>
    requires SearchStateKeyProvider<Domain>
         and VisitedBoundMap<VisitedSet, typename Domain::State_Key, size_t>
         and SearchSolutionVisitor<OnSolution, Solution>
  [[nodiscard]] Result search(State initial_state, VisitedSet &visited, OnSolution &on_solution)
  {
    ah_invalid_argument_if(policy_.strategy != ExplorationPolicy::Strategy::Depth_First)
      << "Depth_First_Backtracking only supports depth-first strategy";
    ah_invalid_argument_if(limits_.max_solutions == 0)
      << "SearchLimits::max_solutions must be positive or Search_Unlimited";
 
    Result result(compare_);
    result.policy = policy_;
    result.limits = limits_;
    const auto start_time = SearchClock::now();
 
    SearchPath<Move> path;
    reserve_search_path(path, limits_);
    (void) dfs_visited(initial_state, path, 0, result, on_solution, visited);
 
    if (result.status == SearchStatus::NotStarted)
      result.status = SearchStatus::Exhausted;
 
    result.stats.elapsed_ms = search_elapsed_ms(SearchClock::now() - start_time);
 
    return result;
  }
 
private:
  Domain domain_;
  ExplorationPolicy policy_;
  SearchLimits limits_;
  Solution_Compare compare_;
 
  [[nodiscard]] bool stop_after_solution(Result &result) const
  {
    return search_engine_detail::stop_after_solution(result, policy_, limits_);
  }
 
  [[nodiscard]] bool expansion_limit_reached(Result &result) const
  {
    return search_engine_detail::expansion_limit_reached(result, limits_);
  }
 
  template <typename OnSolution>
    requires SearchSolutionVisitor<OnSolution, Solution>
  [[nodiscard]] bool dfs(State &state,
                         SearchPath<Move> &path,
                         const size_t depth,
                         Result &result,
                         OnSolution &on_solution)
  {
    ++result.stats.visited_states;
    if (depth > result.stats.max_depth_reached)
      result.stats.max_depth_reached = depth;
 
    if (domain_.is_goal(state))
      {
        ++result.stats.solutions_found;
        Solution solution{state, path, depth};
        result.best_solution.consider(solution);
 
        if (not on_solution(solution))
          {
            result.status = SearchStatus::StoppedOnSolution;
            return true;
          }
 
        return stop_after_solution(result);
      }
 
    if (search_engine_detail::is_terminal_state(domain_, state))
      {
        ++result.stats.terminal_states;
        return false;
      }
 
    if (depth >= limits_.max_depth)
      {
        ++result.stats.pruned_by_depth;
        return false;
      }
 
    if (search_engine_detail::should_prune_state(domain_, state, depth))
      {
        ++result.stats.pruned_by_domain;
        return false;
      }
 
    if (expansion_limit_reached(result))
      return true;
 
    ++result.stats.expanded_states;
 
    bool stop = false;
    (void) domain_.for_each_successor(state,
                                      [&](const Move &move) -> bool
    {
      ++result.stats.generated_successors;
      path.append(move);
      domain_.apply(state, move);
      try
        {
          stop = dfs(state, path, depth + 1, result, on_solution);
        }
      catch (...)
        {
          domain_.undo(state, move);
          (void) path.remove_last();
          throw;
        }
      domain_.undo(state, move);
      (void) path.remove_last();
      return not stop;
    });
 
    return stop;
  }
 
  template <typename OnSolution, typename VisitedSet>
    requires SearchSolutionVisitor<OnSolution, Solution>
         and VisitedBoundMap<VisitedSet, typename Domain::State_Key, size_t>
  [[nodiscard]] bool dfs_visited(State &state,
                                 SearchPath<Move> &path,
                                 const size_t depth,
                                 Result &result,
                                 OnSolution &on_solution,
                                 VisitedSet &visited)
  {
    ++result.stats.visited_states;
    if (depth > result.stats.max_depth_reached)
      result.stats.max_depth_reached = depth;
 
    const auto key = domain_.state_key(state);
    if (auto *entry = visited.search(key);
        entry != nullptr and entry->second <= depth)
      {
        ++result.stats.pruned_by_visited;
        return false;
      }
 
    if (domain_.is_goal(state))
      {
        ++result.stats.solutions_found;
        Solution solution{state, path, depth};
        result.best_solution.consider(solution);
 
        if (not on_solution(solution))
          {
            result.status = SearchStatus::StoppedOnSolution;
            return true;
          }
 
        return stop_after_solution(result);
      }
 
    if (search_engine_detail::is_terminal_state(domain_, state))
      {
        ++result.stats.terminal_states;
        return false;
      }
 
    if (depth >= limits_.max_depth)
      {
        ++result.stats.pruned_by_depth;
        return false;
      }
 
    if (search_engine_detail::should_prune_state(domain_, state, depth))
      {
        ++result.stats.pruned_by_domain;
        return false;
      }
 
    if (expansion_limit_reached(result))
      return true;
 
    if (auto *entry = visited.search(key); entry != nullptr)
      entry->second = depth;
    else
      visited.insert(key, depth);
 
    ++result.stats.expanded_states;
 
    bool stop = false;
    (void) domain_.for_each_successor(state,
                                      [&](const Move &move) -> bool
    {
      ++result.stats.generated_successors;
      path.append(move);
      domain_.apply(state, move);
      try
        {
          stop = dfs_visited(state, path, depth + 1, result, on_solution, visited);
        }
      catch (...)
        {
          domain_.undo(state, move);
          (void) path.remove_last();
          throw;
        }
      domain_.undo(state, move);
      (void) path.remove_last();
      return not stop;
    });
 
    return stop;
  }
};
 
template <BacktrackingDomain Domain>
[[nodiscard]] auto backtracking_search(Domain domain,
                                       typename Domain::State initial_state,
                                       ExplorationPolicy policy = {},
                                       SearchLimits limits = {})
{
  Depth_First_Backtracking<Domain> engine(std::move(domain), policy, limits);
  return engine.search(std::move(initial_state));
}
 
template <BacktrackingDomain Domain, typename OnSolution>
  requires SearchSolutionVisitor<OnSolution, SearchSolution<typename Domain::State, typename Domain::Move>>
[[nodiscard]] auto backtracking_search(Domain domain,
                                       typename Domain::State initial_state,
                                       OnSolution &on_solution,
                                       ExplorationPolicy policy = {},
                                       SearchLimits limits = {})
{
  Depth_First_Backtracking<Domain> engine(std::move(domain), policy, limits);
  return engine.search(std::move(initial_state), on_solution);
}
 
}  // end namespace Aleph
 
# endif  // BACKTRACKING_H