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 "trail.hpp"
 
#include <memory>
#include <stack>
#include <vector>
 
namespace pl_search {
 
 
  class Pred : public std::enable_shared_from_this<Pred> {
  public:
    Pred(Trail* trail) : trail(trail), continuation(nullptr) {}
 
    PredPtr get_continuation() { return continuation; }
 
    virtual void set_continuation(PredPtr cont) { continuation = cont; }
 
    PredPtr last_pred();
 
    virtual void call_init() {}
 
 
    virtual bool call() { return false; }
 
    bool call_continuation() {
      if (continuation == nullptr) return true;
      continuation->call_init();
      return continuation->call();
    }
 
    virtual ~Pred() = default;
 
    virtual std::string get_name() { return typeid(*this).name(); }
 
  protected:
    PredPtr continuation; 
    Trail* trail;         
  };
 
 
 
  class FailPred : public Pred {
  public:
    FailPred(Trail* trail) : Pred(trail) {}
    void set_continuation(PredPtr cont) override {
      // FailPred doesn not need a continuation, so we ignore this.
    }
 
    bool call() override { return false; }
 
  };
 
  extern std::shared_ptr<FailPred> fail_pred;
 
 
 
  class ChoicePred : public Pred {
  public:
 
    ChoicePred(Trail* trail)
      : Pred(trail) {
    }
 
    bool call() override;
 
    virtual bool has_next() = 0;
 
    virtual bool make_choice() = 0;
 
    virtual bool test_choice() { return true; }
 
  };
 
  class VarChoicePred : public ChoicePred {
  public:
    VarChoicePred(Trail* trail, PVarPtr v, std::vector<TermPtr> choices)
      : ChoicePred(trail), var(v), choices(choices), index(0) {
    }
 
    void call_init() override { index = 0; }
 
    bool has_next() override { return index < choices.size(); }
 
    bool make_choice() override {
      TermPtr t = choices[index++];
      return unify(var, t, trail) && test_choice();
    }
 
  protected:
    PVarPtr var; 
    std::vector<TermPtr> choices; 
    size_t index; 
 
  };
 
  PredPtr conjunction(std::vector<PredPtr> preds);
 
  class DisjPred : public Pred {
  public:
    DisjPred(Trail* trail, std::vector<PredPtr> preds)
      : Pred(trail), preds(preds) {
    }
 
    bool call() override;
 
    void set_continuation(PredPtr cont) override;
 
  protected:
    std::vector<PredPtr> preds; 
 
  };
 
  class Once : public Pred {
  public:
    Once(Trail* trail, PredPtr pred) : Pred(trail), pred(pred) {};
 
    bool call() override;
 
  protected:
    PredPtr pred;
  };
 
 
  class NotNot : public Pred {
  public:
    NotNot(Trail* trail, PredPtr pred) : Pred(trail), pred(pred) {}
 
    bool call() override;
  protected:
    PredPtr pred;
 
  };
 
  class IfThenElse : public Pred {
  public:
    IfThenElse(Trail* trail, PredPtr if_pred, PredPtr then_pred,
      PredPtr else_pred);
 
    void set_continuation(PredPtr cont) override;
    bool call() override;
 
  protected:
    PredPtr if_pred;
    PredPtr then_pred;
    PredPtr else_pred;
  };
  class LoopBodyFactory {
  public:
    Trail* trail;
 
    LoopBodyFactory(Trail* trail) : trail(trail) {}
 
    virtual ~LoopBodyFactory() {};
 
    virtual bool loop_continues(int depth) { return false; }
 
    virtual PredPtr make_body_pred(int depth) = 0;
 
  };
 
  class Loop : public Pred {
  public:
    LoopBodyFactoryPtr body_factory;
 
    Loop(Trail* trail, LoopBodyFactoryPtr bf) : Pred(trail), body_factory(bf) {
      depth = 0;
    }
    Loop(Trail* trail, LoopBodyFactoryPtr bf, int depth) : Pred(trail), body_factory(bf), depth(depth) {}
 
    bool call() override;
 
  protected:
 
    int depth;
 
  };
 
  // for testing
  std::string repr(PredPtr pred);
 
} // namespace pl_search
 
#endif // PL_SEARCH_PRED_HPP_