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Java implementation of the Ethereum protocol adapted to use for Hedera Smart Contract Service
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/*
 * Copyright (c) [2016] [  ]
 * This file is part of the ethereumJ library.
 *
 * The ethereumJ library is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * The ethereumJ library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with the ethereumJ library. If not, see .
 */
package org.ethereum.util;

import java.util.*;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.Function;
import java.util.function.Predicate;

/**
 * @author Mikhail Kalinin
 * @since 14.07.2015
 */
public class CollectionUtils {
    public static  List truncate(final List items, int limit) {
        if(limit > items.size()) {
            return new ArrayList<>(items);
        }
        List truncated = new ArrayList<>(limit);
        for(T item : items) {
            truncated.add(item);
            if(truncated.size() == limit) {
                break;
            }
        }
        return truncated;
    }

    public static  List truncateRand(final List items, int limit) {
        if(limit > items.size()) {
            return new ArrayList<>(items);
        }
        List truncated = new ArrayList<>(limit);

        LinkedList index = new LinkedList<>();
        for (int i = 0; i < items.size(); ++i) {
            index.add(i);
        }

        if (limit * 2 < items.size()) {
            // Limit is very small comparing to items.size()
            Set smallIndex = new HashSet<>();
            for (int i = 0; i < limit; ++i) {
                int randomNum = ThreadLocalRandom.current().nextInt(0, index.size());
                smallIndex.add(index.remove(randomNum));
            }
            smallIndex.forEach(i -> truncated.add(items.get(i)));
        } else {
            // Limit is compared to items.size()
            for (int i = 0; i < items.size() - limit; ++i) {
                int randomNum = ThreadLocalRandom.current().nextInt(0, index.size());
                index.remove(randomNum);
            }
            index.forEach(i -> truncated.add(items.get(i)));
        }

        return truncated;
    }
}