unify.h

/*
    IL : Intlog Language
    Object Oriented Prolog Project
    Copyright (C) 1992-2021 - Ing. Capelli Carlo

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
*/


#ifndef UNIFY_H_
#define UNIFY_H_

#include <stdexcept>
#include "qdata.h"

//////////////////////////
// Unification data stack
//

#define MaxUnifyStack 128

class UnifyStack {
public:

    // access to vars
    UnifyStack(BindStack *, TrailStack *);

    // a unified pair
    struct termPair {
        Term    t1, t2;
        stkpos  i1, i2;
    };

    // use this to initialize
    termPair* push();

    // carry on evaluation (note: variables binded!)
    int work(Term, stkpos, Term, stkpos);

    // empty storage
    void clear();

    // on overflow abort execution
    void check_overflow() const;

private:
    termPair v[MaxUnifyStack];
    int free;

    BindStack *vs;
    TrailStack *ts;
};

inline UnifyStack::termPair *UnifyStack::push() {
    return v + free++;
}

inline void UnifyStack::clear() {
    free = 0;
}

inline void UnifyStack::check_overflow() const {
    if (free >= MaxUnifyStack)
        throw std::range_error("unification stack overflow");
}

#endif