/*****************************************************************************
 * trynext.cc                                                                *
 *                                                                           *
 * Description: Contains the tryNext() functions for the EntList descen-     *
 *              dants.  Their purpose is to search for additional branches   *
 *              of OrList's to try the next available untried solution.  AND *
 *              and ANDOR iterate through their children looking for OR's.   *
 *              OR tries the next branch of its children if more remain.     *
 *                                                                           *
 * Created by:  David Rosenfeld                                              *
 * Date:        10/24/96                                                     *
 *****************************************************************************/

#include "complexSupport.h"

// Local function prototypes:
static EntList * firstCandidate( EntList * );
static EntList * nextCandidate( EntList * );

MatchType MultList::tryNext( EntNode * ents )
/*
 * Loops backwards through the children of this, recursively searching for
 * alternate solutions (i.e., OR's which have alternate paths we haven't
 * tried yet).  We loop through all MultList type children which may have
 * an OR with additional choices.  (Which children to try is determined in
 * a local function.  We loop backwards because this is the reverse of the
 * order we set the OR's.  Later choices must be redone before earlier
 * (reasons discussed in notes, 10/17).  This function is the tryNext()
 * for AND and ANDOR; the OR version is redefined.
 */
{
    MatchType retval;
    EntList * child = getLast();

    child = firstCandidate( child );
    while( child != NULL ) {
        if( ( retval = ( ( MultList * )child )->tryNext( ents ) ) == MATCHALL ) {
            // We're done - a good solution was found.
            return MATCHALL;
        }
        if( retval == NEWCHOICE ) {
            // If a new viable choice was found below, we must now reset all
            // later OR's to their first choice.  (That's what acceptChoice()
            // does when choice = LISTEND.)  This is necessary so that we can
            // exhaust every possibility with this new choice.  To do this we
            // first reset all our children, and then return NEWCHOICE so that
            // our parent (if exists) will also know to reset all its later OR
            // children.
            while( ( child = nextCandidate( child ) ) != NULL ) {
                if( child->acceptChoice( ents ) && ents->allMarked() ) {
                    return MATCHALL;
                }
            }
            return NEWCHOICE;
        }
        child = firstCandidate( child->prev );
    }
    // If we got here, we didn't find any new OR choices:
    return NOMORE;
}

static EntList * firstCandidate( EntList * child )
/*
 * Finds an EntList from child's list which may have an OR with more
 * choices below it.  The acceptable choices are described in commenting
 * below.
 */
{
    EntList * ent = child->lastNot( SIMPLE );

    while( ent != NULL ) {
        if( ent->viableVal() >= MATCHSOME ) {
            // Return any non-SIMPLE ent where viable >= MATCHSOME.  We even
            // want to check an OR where numChoices = 1, because it may have
            // an OR descendant with more choices.
            return ent;
        }
        ent = ent->prevNot( SIMPLE );
    }
    return ent;
}

static EntList * nextCandidate( EntList * child )
/*
 * Same as prev function, searches forwards from ent after child.
 */
{
    EntList * ent = child->nextNot( SIMPLE );

    while( ent != NULL ) {
        if( ent->viableVal() >= MATCHSOME ) {
            return ent;
        }
        ent = ent->nextNot( SIMPLE );
    }
    return ent;
}

MatchType OrList::tryNext( EntNode * ents )
/*
 * Tries out the next choice of this.  Basic algorithm is to first recurse
 * to check for other solutions in the descendants of the current choice,
 * and then to try our next choice.
 */
{
    MatchType retval;
    EntList * child;


    if( choice == LISTEND ) {
        // if we've already exhausted all the choices in this OR,
        return NOMORE;
    }

    // First try other choices of descendants of current choice:
    child = getChild( choice );
    if( child->multiple() ) {
        // I.e., if there are (or may be) more choices within the current
        // choice, try those first.  We must be sure to exhaust all choices in
        // our descendants before moving on.
        retval = ( ( MultList * )child )->tryNext( ents );
        if( retval == MATCHALL ) {
            return MATCHALL;
        }
        if( retval == NEWCHOICE ) {
            // I.e., we found a next choice to go to, return so that the
            // EntLists on the higher levels (if there are) can retry all the
            // later choices with the new choice we just found.  Otherwise,
            // we'll continue below looking into our next choice.
            return NEWCHOICE;
        }
    }

    // No other choices among our descendants.  Look for new choice at our
    // level:
    child->unmarkAll( ents );
    if( choiceCount == 1 ) {
        // Quick way to determine that there won't be any more choices here.
        // (Also, it's nec. to unmark now, as we did above before returning and
        // before the calling tryNext() tries earlier OR's - see notes, 11/12.)
        choice = LISTEND;
        return NOMORE;
    }

    // Otherwise, try our next:
    if( acceptNextChoice( ents ) ) {
        if( ents->allMarked() ) {
            return MATCHALL;
        }
        return NEWCHOICE;
    } else {
        // Must have been no next choice (or none which mark anything new).
        // acceptNextChoice() has set choice to LISTEND.  We leave this OR
        // unset and return NOMORE.  If our calling function finds another
        // choice elsewhere, it will reloop forwards and set all later OR's to
        // their first choices, so that we'll be able to test every possibility
        // with the new choice.  At that time, this OrList will be reset to its
        // first choice too.
        return NOMORE;
    }
}
