shellsortlistsorter.java

BOOK:Beginning Algorithms Code Examples

package com.wrox.algorithms.sorting;

import com.wrox.algorithms.lists.List;

/**
 * A {@link ListSorter} that uses an shellsort algorithm.
 *
 */
public class ShellsortListSorter implements ListSorter {
    /** The comparator to control the order of the sorted objects. */
    private final Comparator _comparator;

    /** our sequence of increments for H-sorting the list. */
    private final int[] _increments = {121, 40, 13, 4, 1};

    /**
     * @param comparator the comparator to control the order of the sorted objects.
     */
    public ShellsortListSorter(Comparator comparator) {
        assert comparator != null : "comparator cannot be null";
        _comparator = comparator;
    }

    /**
     * Sorts a list using the shellsort algorithm.
     *
     * @param list The list to sort.
     * @return the original List in sorted order.
     */
    public List sort(List list) {
        assert list != null : "list cannot be null";

        for (int i = 0; i < _increments.length; ++i) {
            int increment = _increments[i];
            hSort(list, increment);
        }

        return list;
    }

    private void hSort(List list, int increment) {
        if (list.size() < (increment * 2)) {
            return;
        }

        for (int i = 0; i < increment; ++i) {
            sortSublist(list, i, increment);
        }
    }

    private void sortSublist(List list, int startIndex, int increment) {
        for (int i = startIndex + increment; i < list.size(); i += increment) {
            Object value = list.get(i);
            int j;
            for (j = i; j > startIndex; j -= increment) {
                Object previousValue = list.get(j - increment);
                if (_comparator.compare(value, previousValue) >= 0) {
                    break;
                }
                list.set(j, previousValue);
            }
            list.set(j, value);
        }
    }
}