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Provides common utility functions

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/*
 * Copyright (c) CQSE GmbH
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.conqat.lib.commons.string;

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

/**
 *
 * Compares two strings lexicographically, except at locations in the strings where numbers should
 * be compared. In this case, the value of the number is compared. Useful for cases such as
 * comparing "foo 5" to "foo 20", where the digit 2 is lexicographically before the digit 5, leading
 * to an undesired sorting result.
 */
public class NumbersAwareStringComparator implements Comparator {

	/** Singleton instance. */
	public static final NumbersAwareStringComparator INSTANCE = new NumbersAwareStringComparator();

	/** {@inheritDoc} */
	@Override
	public int compare(String s1, String s2) {
		if (StringUtils.isEmpty(s1) || StringUtils.isEmpty(s2)) {
			return StringUtils.compare(s1, s2);
		}
		String prefix1 = getPrefix(s1);
		String prefix2 = getPrefix(s2);
		if (prefix1.equals(prefix2)) {
			return compare(StringUtils.stripPrefix(s1, prefix1), StringUtils.stripPrefix(s2, prefix2));
		}
		if (startsWithDigit(prefix1) && startsWithDigit(prefix2)) {
			int comparisonResult = new BigInteger(prefix1).compareTo(new BigInteger(prefix2));
			if (comparisonResult == 0) {
				return compare(StringUtils.stripPrefix(s1, prefix1), StringUtils.stripPrefix(s2, prefix2));
			}
			return comparisonResult;
		}
		return prefix1.compareTo(prefix2);
	}

	/** Tests if the first character is digit. */
	private static boolean startsWithDigit(String s) {
		if (StringUtils.isEmpty(s)) {
			return false;
		}
		char first = s.charAt(0);
		return Character.isDigit(first);
	}

	/**
	 * Returns the prefix, which can be leading digit characters or leading non-digit characters.
	 */
	private static String getPrefix(String s) {
		if (startsWithDigit(s)) {
			return s.replaceAll("[\\D]+(.|\\n)*", "");
		}
		return s.replaceAll("[\\d]+(.|\\n)*", "");
	}
}