pred.hpp

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/*
MIT License
 
Copyright (c) 2025 [Peter Robinson]
 
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*/
 
// Aproximating Prolog predicates in C++ using a continuation-passing style
 
#ifndef PL_SEARCH_PRED_HPP_
#define PL_SEARCH_PRED_HPP_
 
#include "term.hpp"
#include "typedefs.hpp"
 
#include <memory>
#include <stack>
#include <vector>
 
namespace pl_search {
 
class ChoiceIterator;
 
class Pred : public std::enable_shared_from_this<Pred> {
public:
  Pred(Engine *eng) : engine(eng), continuation(nullptr) {}
 
  virtual void initialize_call() {};
 
  virtual bool apply_choice() { return false; }
 
  virtual bool more_choices() { return false; }
 
  PredPtr get_continuation() { return continuation; }
 
  virtual void set_continuation(PredPtr cont) { continuation = cont; }
 
  PredPtr last_pred();
 
  bool is_non_det() { return true; }
 
  void wrap_with_once();
 
  virtual ~Pred() = default;
 
  std::string get_name() { return typeid(this).name(); }
 
protected:
  PredPtr continuation; 
  Engine *engine;
};
 
typedef std::shared_ptr<ChoiceIterator> ChoiceIteratorPtr;
 
class ChoicePred : public Pred {
public:
  ChoiceIteratorPtr choice_iterator; 
 
  ChoicePred(Engine *eng, ChoiceIteratorPtr ch)
      : Pred(eng), choice_iterator(ch) {}
 
  void initialize_call() override {}
  bool apply_choice() override;
  bool more_choices() override;
};
 
class SemiDetPred : public Pred {
public:
  SemiDetPred(Engine *eng) : Pred(eng) {}
 
  bool more_choices() override { return false; }
 
  bool is_non_det() { return false; }
};
 
class DetPred : public SemiDetPred {
public:
  DetPred(Engine *eng) : SemiDetPred(eng) {}
 
  bool apply_choice() override { return true; }
};
 
PredPtr conjunction(std::vector<PredPtr> preds);
 
class DisjPred : public Pred {
public:
  DisjPred(Engine *eng, std::vector<PredPtr> preds)
      : Pred(eng), preds(preds), index(0) {}
 
  void initialize_call() override {
    for (auto it = preds.begin(); it != preds.end(); it++) {
      (*it)->set_continuation(continuation);
    }
  };
 
  bool apply_choice() override {
    PredPtr choice = preds[index++];
    continuation = choice;
    return true;
  }
 
  bool more_choices() override { return index < preds.size(); }
 
protected:
  std::vector<PredPtr> preds; 
  int index;                  
};
 
class Cut : public DetPred {
public:
  Cut(Engine *eng, int index) : DetPred(eng), env_index(index) {}
 
  void initialize_call() override {}
 
  bool apply_choice() override;
 
  void set_cut_point(int cut_point) { env_index = cut_point; }
 
protected:
  int env_index;
};
 
class NotNotEnd : public SemiDetPred {
public:
  NotNotEnd(Engine *eng, bool *succ) : SemiDetPred(eng), succeeded(succ) {}
 
  void initialize_call() override {};
  bool apply_choice() override;
 
protected:
  bool *succeeded;
};
 
class NotNot : public Pred {
public:
  NotNot(Engine *eng, PredPtr p) : Pred(eng), pred(p) {}
 
  void initialize_call() override;
  bool apply_choice() override;
  bool more_choices() override;
 
protected:
  PredPtr pred;
  bool succeeded;
  bool another_choice;
  PredPtr saved_continuation;
};
 
class IfThenElse : public Pred {
public:
  IfThenElse(Engine *eng, PredPtr if_pred, PredPtr then_pred,
             PredPtr else_pred);
 
  void initialize_call() override;
  bool apply_choice() override;
  bool more_choices() override;
  void set_continuation(PredPtr cont);
 
protected:
  PredPtr if_pred;
  PredPtr then_pred;
  PredPtr else_pred;
  PredPtr if_then_pred;
  std::shared_ptr<Cut> cut_pred;
  int choice_number;
};
class LoopBodyFactory {
public:
  Engine *engine;
 
  LoopBodyFactory(Engine *eng) : engine(eng) {}
 
  virtual ~LoopBodyFactory(){};
 
  virtual bool loop_continues() = 0;
 
  virtual PredPtr make_body_pred() = 0;
};
 
class Loop : public DetPred {
public:
  LoopBodyFactory *body_factory;
 
  Loop(Engine *eng, LoopBodyFactory *bf) : DetPred(eng), body_factory(bf) {}
 
  void initialize_call() override;
  bool apply_choice() override;
  void set_continuation(PredPtr cont);
 
protected:
  PredPtr saved_continuation;
};
 
// for testing
std::string repr(PredPtr pred);
 
} // namespace pl_search
 
#endif // PL_SEARCH_PRED_HPP_