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Collections and collection utilities for Neo4j.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j 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.
 *
 * 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, see .
 */
package org.neo4j.internal.helpers.collection;

import java.math.BigInteger;
import java.util.Comparator;
import java.util.Iterator;

/**
 * Comparator for strings that may, or may not, contain groups of digits representing numbers and where
 * those numbers should be compared for what their numeric values are, not their string representations.
 * This will solve a classic sorting issue that plain string sorting misses:
 * 
 * string-1
 * string-2
 * string-12
 * 
 * Where the above would be sorted as {@code string-1}, {@code string-12}, {@code string-2}, which may be
 * undesirable in scenarios where the number matters. This comparator will sort the strings from the
 * example above as {@code string-1}, {@code string-2}, {@code string-12}.
 */
public final class NumberAwareStringComparator implements Comparator {
    public static final Comparator INSTANCE = new NumberAwareStringComparator();

    private NumberAwareStringComparator() {}

    @SuppressWarnings("unchecked")
    @Override
    public int compare(String o1, String o2) {
        Iterator c1 = comparables(o1);
        Iterator c2 = comparables(o2);

        boolean c1Has = c1.hasNext();
        boolean c2Has = c2.hasNext();
        while (c1Has || c2Has) {
            if (!c1Has) {
                return -1;
            }
            if (!c2Has) {
                return 1;
            }

            int diff = c1.next().compareTo(c2.next());
            if (diff != 0) {
                return diff;
            }
            c1Has = c1.hasNext();
            c2Has = c2.hasNext();
            // else continue
        }
        // All elements are comparable with each other
        return 0;
    }

    private static Iterator comparables(final String string) {
        return new PrefetchingIterator<>() {
            private int index;

            @Override
            protected Comparable fetchNextOrNull() {
                if (index >= string.length()) { // End reached
                    return null;
                }

                int startIndex = index;
                char ch = string.charAt(index);
                boolean isNumber = Character.isDigit(ch);
                while (Character.isDigit(ch) == isNumber && ++index < string.length()) {
                    ch = string.charAt(index);
                }
                String substring = string.substring(startIndex, index);
                return isNumber ? new BigInteger(substring) : substring;
            }
        };
    }
}