engine.hpp

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/*
MIT License
 
Copyright (c) 2025 [Peter Robinson]
 
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
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SOFTWARE.
*/
 
#ifndef PL_SEARCH_ENGINE_HPP_
#define PL_SEARCH_ENGINE_HPP_
 
#include "clist.hpp"
#include "pred.hpp"
#include "pvar.hpp"
#include "term.hpp"
#include "typedefs.hpp"
 
#include <memory>
#include <stack>
 
class EngineTest;
 
namespace pl_search {
 
struct trail_entry {
  PVarPtr var;   
  TermPtr value; 
};
 
struct env_entry {
  PredPtr pred;    
  int trail_index; 
};
 
class Engine {
public:
  Engine(){};
 
  bool unify(TermPtr t1, TermPtr t2);
 
  bool execute(PredPtr p, bool unbind);
 
  friend class Cut;
  friend void Pred::wrap_with_once();
  friend class DisjPred;
  friend class IfThenElse;
 
  // for testing
  friend class ::EngineTest;
 
private:
  std::stack<trail_entry *> trail_stack; 
  std::stack<env_entry *> env_stack;     
 
  void trail(PVarPtr v);
 
  void backtrack();
 
  bool call_predicate(PredPtr p);
 
  bool retry_predicate(PredPtr p);
 
  inline bool make_choice_and_continue(PredPtr p) {
    return p->apply_choice() && call_predicate(p->get_continuation());
  }
 
  void push(PredPtr p);
 
  void pop_pred_call();
 
  void cut_to_choice_point(int env_index);
 
  void clear_stacks();
};
 
} // namespace pl_search
 
#endif // PL_SEARCH_ENGINE_HPP_