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A modified version of the GGP-Base library for Alloy.
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package org.ggp.base.util.gdl;

import org.ggp.base.util.symbol.grammar.Symbol;
import org.ggp.base.util.symbol.grammar.SymbolAtom;
import org.ggp.base.util.symbol.grammar.SymbolList;

/**
 * The GdlValidator class implements Gdl validation for the GdlFactory class.
 * Its purpose is to validate whether or not a Symbol can be transformed into a
 * Gdl expression without error.
 */
public final class GdlValidator
{

    /**
     * Validates whether a Symbol can be transformed into a Gdl expression
     * without error using the following process:
     * 
     * Returns true if the Symbol is a SymbolAtom. Otherwise, treats the
     * Symbol as a SymbolList.
     * Checks that the SymbolList contains no sub-elements that are
     * SymbolLists which do not begin with a SymbolAtom.
     * Checks that neither the SymbolList nor its sub-elements contain the
     * deprecated 'or' keyword.
     * 
     * Note that as implemented, this method is incomplete: it only verifies a
     * subset of the correctness properties of well-formed Gdl. A more thorough
     * implementation is advisable.
     *
     * @param symbol
     *            The Symbol to validate.
     * @return True if the Symbol passes validation; false otherwise.
     */
    public boolean validate(Symbol symbol)
    {
        if ( symbol instanceof SymbolAtom )
        {
            return true;
        }
        else if ( containsAnonymousList(symbol) )
        {
            return false;
        }
        else if ( containsOr(symbol) )
        {
            return false;
        }
        else
        {
            return true;
        }
    }

    /**
     * A recursive method that checks whether a Symbol contains SymbolList that
     * does not begin with a SymbolAtom.
     *
     * @param symbol
     *            The Symbol to validate.
     * @return True if the Symbol passes validation; false otherwise.
     */
    private boolean containsAnonymousList(Symbol symbol)
    {
        if ( symbol instanceof SymbolAtom )
        {
            return false;
        }
        else
        {
            if ( symbol instanceof SymbolList )
            {
                return true;
            }
            else
            {
                for ( int i = 1; i < ((SymbolList)symbol).size(); i++ )
                {
                    if ( containsAnonymousList(((SymbolList)symbol).get(i)) )
                    {
                        return true;
                    }
                }

                return false;
            }
        }
    }

    /**
     * A recursive method that checks whether a Symbol contains the deprecated
     * 'or' keyword.
     *
     * @param symbol
     *            The Symbol to validate.
     * @return True if the Symbol passes validation; false otherwise.
     */
    private boolean containsOr(Symbol symbol)
    {
        if ( symbol instanceof SymbolAtom )
        {
            return false;
        }
        else
        {
            if ( symbol.toString().toLowerCase().equals("or") )
            {
                return true;
            }
            else if ( symbol instanceof SymbolList )
            {
                for ( int i = 1; i < ((SymbolList)symbol).size(); i++ )
                {
                    if ( containsOr(((SymbolList)symbol).get(i)) )
                    {
                        return true;
                    }
                }
            }
        }
        return false;
    }

}