State_Search_IDA_Star.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
 
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  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
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  copies of the Software, and to permit persons to whom the Software is
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*/
 
# ifndef STATE_SEARCH_IDA_STAR_H
# define STATE_SEARCH_IDA_STAR_H
 
#include <limits>
#include <type_traits>
#include <utility>
 
#include <ah-errors.H>
#include <state_search_common.H>
#include <Backtracking.H>
 
namespace Aleph {
 
template <typename Domain>
concept IDAStarDomain
  = BacktrackingDomain<Domain> and HeuristicEvaluator<Domain> and ActionCostProvider<Domain>;
 
template <typename Distance>
struct IDAStarIteration
{
  Distance threshold = Distance{};       
  size_t visited_states = 0;             
  bool found_solution = false;           
  Distance next_threshold = Distance{};  
};
 
template <typename Solution, typename Distance>
struct IDAStarResult : SearchResult<Solution>
{
  using Base = SearchResult<Solution>;
  using Distance_Type = Distance;
 
  Distance total_cost = Distance{};  
  Array<IDAStarIteration<Distance>> iterations;  
 
  IDAStarResult() = default;
};
 
namespace ida_star_detail {
 
template <typename Distance>
[[nodiscard]] constexpr Distance distance_unreachable() noexcept
{
  if constexpr (std::is_floating_point_v<Distance>)
    return std::numeric_limits<Distance>::infinity();
  else
    return std::numeric_limits<Distance>::max();
}
 
template <typename Distance>
struct DFSResult
{
  bool found = false;
  Distance next_bound = distance_unreachable<Distance>();
};
 
template <IDAStarDomain Domain, typename Solution, typename OnSolution>
  requires SearchSolutionVisitor<OnSolution, Solution>
DFSResult<typename Domain::Distance> dfs(Domain &domain,
                                         typename Domain::State &state,
                                         SearchPath<typename Domain::Move> &path,
                                         const typename Domain::Distance g,
                                         const typename Domain::Distance threshold,
                                         const size_t depth,
                                         IDAStarResult<Solution, typename Domain::Distance> &result,
                                         OnSolution &on_solution)
{
  using Distance = typename Domain::Distance;
 
  ++result.stats.visited_states;
  if (depth > result.stats.max_depth_reached)
    result.stats.max_depth_reached = depth;
 
  const Distance h = domain.heuristic(state);
  const Distance f = g + h;
 
  if (f > threshold)
    return {false, f};
 
  if (domain.is_goal(state))
    {
      ++result.stats.solutions_found;
      Solution solution{state, path, depth};
      if (result.best_solution.consider(solution))
        result.total_cost = g;
      const bool keep_searching = on_solution(solution);
      if (not keep_searching)
        result.status = SearchStatus::StoppedOnSolution;
      return {true, threshold};
    }
 
  if (search_engine_detail::is_terminal_state(domain, state))
    {
      ++result.stats.terminal_states;
      return {false, distance_unreachable<Distance>()};
    }
 
  if (search_engine_detail::should_prune_state(domain, state, depth))
    {
      ++result.stats.pruned_by_domain;
      return {false, distance_unreachable<Distance>()};
    }
 
  if (depth >= result.limits.max_depth)
    {
      ++result.stats.pruned_by_depth;
      return {false, distance_unreachable<Distance>()};
    }
 
  if (result.stats.expanded_states >= result.limits.max_expansions)
    {
      ++result.stats.limit_hits;
      result.status = SearchStatus::LimitReached;
      return {false, distance_unreachable<Distance>()};
    }
 
  ++result.stats.expanded_states;
 
  using Move = typename Domain::Move;
  DFSResult<Distance> out{false, distance_unreachable<Distance>()};
 
  (void) domain.for_each_successor(state,
                                   [&](const Move &move) -> bool
  {
    if (result.status == SearchStatus::LimitReached
        or result.status == SearchStatus::StoppedOnSolution)
      return false;
 
    ++result.stats.generated_successors;
    const Distance step = domain.cost(state, move);
    bool applied = false;
    bool path_appended = false;
    DFSResult<Distance> child;
    try
      {
        domain.apply(state, move);
        applied = true;
        path.append(move);
        path_appended = true;
        child = dfs(domain, state, path, g + step, threshold, depth + 1, result, on_solution);
      }
    catch (...)
      {
        if (path_appended)
          (void) path.remove_last();
        if (applied)
          domain.undo(state, move);
        throw;
      }
 
    (void) path.remove_last();
    domain.undo(state, move);
 
    if (child.found)
      {
        out.found = true;
        return false;
      }
 
    if (result.status == SearchStatus::LimitReached
        or result.status == SearchStatus::StoppedOnSolution)
      return false;
 
    if (child.next_bound < out.next_bound)
      out.next_bound = child.next_bound;
 
    return true;
  });
 
  return out;
}
 
}  // end namespace ida_star_detail
 
template <IDAStarDomain Domain>
class IDA_Star_State_Search
{
public:
  using Domain_Type = Domain;
  using State = typename Domain::State;
  using Move = typename Domain::Move;
  using Distance = typename Domain::Distance;
  using Solution = SearchSolution<State, Move>;
  using Result = IDAStarResult<Solution, Distance>;
 
  static constexpr bool supports_best_first = false;
 
  explicit IDA_Star_State_Search(Domain domain, ExplorationPolicy policy = {},
                                 const SearchLimits &limits = {})
    : domain_(std::move(domain)), policy_(policy), limits_(limits)
  {
    // 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)
      << "IDA_Star_State_Search only supports depth-first strategy";
    ah_invalid_argument_if(limits_.max_solutions == 0)
      << "SearchLimits::max_solutions must be positive or Search_Unlimited";
 
    Result result;
    result.policy = policy_;
    result.limits = limits_;
 
    const auto start_time = SearchClock::now();
 
    SearchPath<Move> path;
    reserve_search_path(path, limits_);
 
    Distance threshold = domain_.heuristic(initial_state);
 
    for (;;)
      {
        IDAStarIteration<Distance> iteration;
        iteration.threshold = threshold;
 
        const size_t stats_before = result.stats.visited_states;
 
        auto [found, next_bound] = ida_star_detail::dfs(
          domain_, initial_state, path, Distance{}, threshold, 0, result, on_solution);
 
        iteration.visited_states = result.stats.visited_states - stats_before;
        iteration.found_solution = found;
        iteration.next_threshold = found ? threshold : next_bound;
        result.iterations.append(iteration);
 
        if (result.status == SearchStatus::StoppedOnSolution)
          break;
 
        if (found)
          {
            if (result.status != SearchStatus::LimitReached)
              result.status = SearchStatus::StoppedOnSolution;
            break;
          }
 
        if (result.status == SearchStatus::LimitReached)
          break;
 
        if (next_bound >= ida_star_detail::distance_unreachable<Distance>())
          {
            result.status = SearchStatus::Exhausted;
            break;
          }
 
        threshold = next_bound;
      }
 
    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_;
};
 
template <IDAStarDomain Domain>
[[nodiscard]] auto ida_star_search(Domain domain,
                                   typename Domain::State initial_state,
                                   ExplorationPolicy policy = {},
                                   SearchLimits limits = {})
{
  IDA_Star_State_Search<Domain> engine(std::move(domain), policy, limits);
  return engine.search(std::move(initial_state));
}
 
template <IDAStarDomain Domain, typename OnSolution>
  requires SearchSolutionVisitor<OnSolution, SearchSolution<typename Domain::State, typename Domain::Move>>
[[nodiscard]] auto ida_star_search(Domain domain,
                                   typename Domain::State initial_state,
                                   OnSolution &on_solution,
                                   ExplorationPolicy policy = {},
                                   SearchLimits limits = {})
{
  IDA_Star_State_Search<Domain> engine(std::move(domain), policy, limits);
  return engine.search(std::move(initial_state), on_solution);
}
 
}  // end namespace Aleph
 
# endif  // STATE_SEARCH_IDA_STAR_H