/*
 * Copyright (C) 2018 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.role.controller.util;

import android.util.ArraySet;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

/**
 * Utility methods for dealing with {@link java.util.Collection}s.
 */
public final class CollectionUtils {

    private CollectionUtils() {}

    /**
     * Return the first element of a list, or {@code null} if the list is {@code null} or empty.
     *
     * @param <T> the class of the elements of the list
     * @param list the list to get the first element
     *
     * @return the first element of the list, or {@code null} if the list is {@code null} or empty
     */
    @Nullable
    public static <T> T firstOrNull(@Nullable List<T> list) {
        if (list == null || list.isEmpty()) {
            return null;
        }
        return list.get(0);
    }

    /**
     * Remove all values in the array set that do <b>not</b> exist in the given collection.
     *
     * @param <T> the class of the elements to retain and of the {@code ArraySet}
     * @param arraySet the {@code ArraySet} whose elements are to be removed or retained
     * @param valuesToRetain the values to be used to determine which elements to retain
     *
     * @return {@code true} if any values were removed from the array set, {@code false} otherwise.
     *
     * @see ArraySet#retainAll(java.util.Collection)
     */
    @SafeVarargs
    public static <T> boolean retainAll(ArraySet<T> arraySet, T... valuesToRetain) {
        boolean removed = false;
        List<T> valuesToRetainList = Arrays.asList(valuesToRetain);
        for (int i = arraySet.size() - 1; i >= 0; i--) {
            if (!valuesToRetainList.contains(arraySet.valueAt(i))) {
                arraySet.removeAt(i);
                removed = true;
            }
        }
        return removed;
    }

    /**
     * Return a singleton list containing the element, or an empty list if the element is
     * {@code null}.
     *
     * @param <T> the class of the element
     * @param element the element to be put into the list
     *
     * @return a singleton list containing the element, or an empty list if the element is
     *         {@code null}.
     */
    @NonNull
    public static <T> List<T> singletonOrEmpty(@Nullable T element) {
        return element != null ? Collections.singletonList(element) : Collections.emptyList();
    }
}