src/com/android/contacts/common/Collapser.java

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * 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 com.android.contacts.common;

import java.util.Iterator;
import java.util.List;

/**
 * Class used for collapsing data items into groups of similar items. The data items that should be
 * collapsible should implement the Collapsible interface. The class also contains a utility
 * function that takes an ArrayList of items and returns a list of the same items collapsed into
 * groups.
 */
public final class Collapser {

    /*
     * This utility class cannot be instantiated.
     */
    private Collapser() {}

    /*
     * The Collapser uses an n^2 algorithm so we don't want it to run on
     * lists beyond a certain size. This specifies the maximum size to collapse.
     */
    private static final int MAX_LISTSIZE_TO_COLLAPSE = 20;

    /*
     * Interface implemented by data types that can be collapsed into groups of similar data. This
     * can be used for example to collapse similar contact data items into a single item.
     */
    public interface Collapsible<T> {
        public void collapseWith(T t);
        public boolean shouldCollapseWith(T t);
    }

    /**
     * Collapses a list of Collapsible items into a list of collapsed items. Items are collapsed
     * if {@link Collapsible#shouldCollapseWith(Object)} returns true, and are collapsed
     * through the {@Link Collapsible#collapseWith(Object)} function implemented by the data item.
     *
     * @param list List of Objects of type <T extends Collapsible<T>> to be collapsed.
     */
    public static <T extends Collapsible<T>> void collapseList(List<T> list) {

        int listSize = list.size();
        // The algorithm below is n^2 so don't run on long lists
        if (listSize > MAX_LISTSIZE_TO_COLLAPSE) {
            return;
        }

        for (int i = 0; i < listSize; i++) {
            T iItem = list.get(i);
            if (iItem != null) {
                for (int j = i + 1; j < listSize; j++) {
                    T jItem = list.get(j);
                    if (jItem != null) {
                        if (iItem.shouldCollapseWith(jItem)) {
                            iItem.collapseWith(jItem);
                            list.set(j, null);
                        } else if (jItem.shouldCollapseWith(iItem)) {
                            jItem.collapseWith(iItem);
                            list.set(i, null);
                            break;
                        }
                    }
                }
            }
        }

        // Remove the null items
        Iterator<T> itr = list.iterator();
        while (itr.hasNext()) {
            if (itr.next() == null) {
                itr.remove();
            }
        }

    }
}