Combine alphabetical and natural order (aka. User sane sorting)

Question

I thought this would be easy to find premade, but it seems any solution I can find on the web solves only part of the problem.

I want to sort a list of Filenames (and the files have mostly been named after persons and/or addresses) that have been given by users, sometimes in different languages (mostly german, with a bit of french and italian mixed in here and there, and rarely any other western language).

The idea is to present this list ordered in a way that the (german) users generally deem sane. That means the order should follow the java.text.Collator for Locale.GERMAN, but at the same time the expectation is to make an exception for numbers in the string, so "10" comes after "2".

I have found code to do the natural sorting on the web, but it relies on comparing character by character (and Collator doesn't support that). I could hack something with substring, but inside a comparator it doesn't deem me the brightest idea to create multiple substrings on each compare-call.

Any ideas how to implement this efficiently (in both execution time and implementation time), or better yet a tested and ready-to-use implementation?

Answer 1

If you use the Comparator suggested by @millimoose (http://www.davekoelle.com/alphanum.html) modify it to pass the Collator

public class AlphanumComparator implements Comparator
{
private Collator collator;
public AlphanumComparator(Collator collator) {
    this.collator = collator;
}
.....
    public int compare(Object o1, Object o2)
    {
......
result = thisChunk.compareTo(thatChunk); //should become
collator.compare(thisChuck, thatChuck);
....

this code seems to have a problem, for example "01" is grater then "2". But this depends on you preference, if this is important modify it to skip the leading zeros before number compare.

Answer 2

This is the adapted code (based on The Alphanum Algorithm) as in the accepted answer. The code was optimized to reduce garbage creation and to deal with leading zeros (01 < 001 < 2). Also it was generified, and is now more flexible as its no longer limited to java.lang.String, instead it now takes java.lang.CharSequence. Have fun:

import java.text.Collator;
import java.util.Comparator;

/**
 * Comparator for ordering by Collator while treating digits numerically.
 * This provides a "natural" order that humans usually perceive as 'logical'.
 * 
 * It should work reasonably well for western languages (provided you
 * use the proper collator when constructing). For free control over the
 * Collator, use the constructor that takes a Collator as parameter.
 * Configure the Collator using Collator.setDecomposition()/setStrength()
 * to suit your requirements.
 */
public class AlphanumComparator implements Comparator<CharSequence> {

    /**
     * The collator used for comparison of the alpha part
     */
    private final Collator collator;

    /**
     * Create comparator using platform default collator.
     * (equivalent to using Collator.getInstance())
     */
    public AlphanumComparator() {
        this(Collator.getInstance()); 
    }

    /**
     * Create comparator using specified collator
     */
    public AlphanumComparator(final Collator collator) {
        if (collator == null)
            throw new IllegalArgumentException("collator must not be null");
        this.collator = collator;
    }

    /**
     * Ideally this would be generalized to Character.isDigit(), but I have
     * no knowledge about arabic language and other digits, so I treat
     * them as characters...
     */
    private static boolean isDigit(final int character) {
        // code between ASCII '0' and '9'?
        return character >= 48 && character <= 57;
    }

    /**
     * Get subsequence of only characters or only digits, but not mixed
     */
    private static CharSequence getChunk(final CharSequence charSeq, final int start) {
        int index = start;
        final int length = charSeq.length();
        final boolean mode = isDigit(charSeq.charAt(index++));
        while (index < length) {
            if (isDigit(charSeq.charAt(index)) != mode)
                break;
            ++index;
        }
        return charSeq.subSequence(start, index);
    }

    /**
     * Implements Comparator<CharSequence>.compare
     */
    public int compare(final CharSequence charSeq1, final CharSequence charSeq2) {
        final int length1 = charSeq1.length();
        final int length2 = charSeq2.length();
        int index1 = 0;
        int index2 = 0;
        int result = 0;
        while (result == 0 && index1 < length1 && index2 < length2) {
            final CharSequence chunk1 = getChunk(charSeq1, index1);
            index1 += chunk1.length();

            final CharSequence chunk2 = getChunk(charSeq2, index2);
            index2 += chunk2.length();

            if (isDigit(chunk1.charAt(0)) && isDigit(chunk2.charAt(0))) {
                final int clen1 = chunk1.length();
                final int clen2 = chunk2.length();
                // count and skip leading zeros
                int zeros1 = 0;
                while (zeros1 < clen1 && chunk1.charAt(zeros1) == '0')
                    ++zeros1;
                // count and skip leading zeros
                int zeros2 = 0;
                while (zeros2 < clen2 && chunk2.charAt(zeros2) == '0')
                    ++zeros2;
                // the longer run of non-zero digits is greater
                result = (clen1 - zeros1) - (clen2 - zeros2);
                // if the length is the same, the first differing digit decides
                // which one is deemed greater.
                int subi1 = zeros1;
                int subi2 = zeros2;
                while (result == 0 && subi1 < clen1 && subi2 < clen2) {
                    result = chunk1.charAt(subi1++) - chunk2.charAt(subi2++);
                }
                // if still no difference, the longer zeros-prefix is greater
                if (result == 0)
                    result = subi1 - subi2;
            } else {
                // in case we are working with Strings, toString() doesn't create
                // any objects (String.toString() returns the same string itself).
                result = collator.compare(chunk1.toString(), chunk2.toString());
            }
        }
        // if there was no difference at all, let the longer one be the greater one
        if (result == 0)
            result = length1 - length2;
        // limit result to (-1, 0, or 1)
        return Integer.signum(result);
    }


}

Edit 2014-12-01: Fixed version as noted by Konstantin Petrukhnov in the comments.