pvar.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
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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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*/
 
#ifndef PL_SEARCH_PVAR_HPP
#define PL_SEARCH_PVAR_HPP
 
#include "term.hpp"
#include "typedefs.hpp"
 
namespace pl_search {
 
class PVar : public Term {
 
public:
  static int id; 
 
  TermPtr value; 
 
  PVar() : value(nullptr), var_id(id++) {}
 
  TermPtr dereference() override;
 
  virtual bool bind(const TermPtr &t);
 
  void reset(const TermPtr &t);
 
  bool is_var() override;
 
  bool isLessThan(Term &other) const override;
 
  int getVarId() const { return var_id; }
 
  std::string repr() const override {
    if (value) {
      return value->dereference()->repr();
    }
    return "X" + std::to_string(var_id);
  }
 
protected:
  virtual bool isEqualTo(Term &other) const override {
    if (PVar *v = dynamic_cast<PVar *>(&other)) {
 
      return getVarId() == v->getVarId();
    }
    return false;
  }
 
  Term *deref_term() override;
 
  int var_id; 
};
 
/*
 * This is typically used to store (part of) the
 * state in a way that can be backtracked over. For example, after
 * binding a variable and then making some deductions the available choices
 * for another variable might have been reduced and we can use an UpdatableVar
 * to store that value as the computation moves forward but on backtracking
 * the old value will be restored.
 */
class UpdatablePVar : public PVar {
 
public:
  UpdatablePVar(TermPtr t) : PVar() { value = t; }
 
  // As the intention is to update the value of the variable, we need to
  // simply return this pointer so that a following bind will update the value.
  // If we wanted to do a full dereference for an updatable variable, v, we
  // would use v->PVar::dereference() or v->getValue()->dereference()
  TermPtr dereference() override { return shared_from_this(); }
 
protected:
  Term *deref_term() override { return this; }
};
 
} // namespace pl_search
 
#endif // PL_SEARCH_PVAR_HPP