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Redberry is an open source computer algebra system designed for tensor manipulation. It implements basic computer algebra system routines as well as complex tools for real computations in physics. This is Redberry core, which contains the implementation of the basic computer algebra routines and general-purpose transformations.

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/*
 * Redberry: symbolic tensor computations.
 *
 * Copyright (c) 2010-2013:
 *   Stanislav Poslavsky   
 *   Bolotin Dmitriy       
 *
 * This file is part of Redberry.
 *
 * Redberry 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 3 of the License, or
 * (at your option) any later version.
 *
 * Redberry 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 Redberry. If not, see .
 */
package cc.redberry.core.parser;

import cc.redberry.core.context.CC;
import cc.redberry.core.indices.IndicesFactory;
import cc.redberry.core.indices.SimpleIndices;
import cc.redberry.core.utils.IntArrayList;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Parser for indices of tensors.
 *
 * @author Dmitry Bolotin
 * @author Stanislav Poslavsky
 * @since 1.0
 */
public final class ParserIndices {
    /**
     * Parses string expression an returns indices.
     *
     * @param expression string representation of indices
     * @return integer indices
     * @throws IllegalArgumentException if string does not represent correct indices object.
     */
    public static SimpleIndices parseSimple(String expression) {
        return IndicesFactory.createSimple(null, parse(expression));
    }

    /**
     * Parses string expression an return integer array of indices.
     *
     * @param expression string representation of indices
     * @return integer array of indices
     * @throws IllegalArgumentException if string does not represent correct indices object.
     */
    public static int[] parse(String expression) {
        if (expression.isEmpty())
            return new int[0];

        IntArrayList indices = new IntArrayList();
        int level = 0;
        int state = 0;
        final char[] expressionChars = expression.toCharArray();
        char c;
        int beginIndex = 0, endIndex = 0;
        for (; endIndex < expressionChars.length; ++endIndex) {
            c = expressionChars[endIndex];
            if (c == '{')
                ++level;
            else if (c == '}')
                --level;
            else if (c == '_' && level == 0) {
                if (endIndex != 0)
                    parse(expression.substring(beginIndex + 1, endIndex), indices, state);
                state = 0;
                beginIndex = endIndex;
            } else if (c == '^') {
                if (level != 0)
                    throw new BracketsError();
                if (endIndex != 0)
                    parse(expression.substring(beginIndex + 1, endIndex), indices, state);
                state = 0x80000000;
                beginIndex = endIndex;
            }
        }
        if (level != 0)
            throw new BracketsError();
        if (beginIndex != endIndex)
            parse(expression.substring(beginIndex + 1, endIndex), indices, state);
        return indices.toArray();
    }

    /**
     * A bit of regex
     */
    public static final Pattern pattern = Pattern.compile("((?>(?>[a-zA-Z])|(?>\\\\[a-zA-Z]*))(?>_(?>(?>[0-9])|(?>[\\{][0-9\\s]*[\\}])))?[']*)");

    static void parse(String expression, IntArrayList indices, int state) {
        Matcher matcher = pattern.matcher(expression);
        String singleIndex;
        while (matcher.find()) {
            singleIndex = matcher.group();
            indices.add(CC.getIndexConverterManager().getCode(singleIndex) | state);
        }
        String remainder = matcher.replaceAll("");
        remainder = remainder.replaceAll("[\\{\\}\\s]*", "");
        if (remainder.length() != 0)
            throw new ParserException();
    }
}