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package com.link_intersystems.util;

import java.util.AbstractList;
import java.util.List;

import static java.lang.Math.max;
import static java.util.Objects.requireNonNull;

/**
 * A decorator that provides a reversed perspective on a {@link List}.
 * In contrast to {@link java.util.Collections#reverse(List)} this {@link ReversedList} does not affect the
 * source list and does not make a copy of the source list. So when the source list changes this decorator
 * remains valid.
 *
 * @author - René Link {@literal }
 */
public class ReversedList extends AbstractList {

    private List sourceList;

    public ReversedList(List sourceList) {
        this.sourceList = requireNonNull(sourceList);
    }

    @Override
    public E get(int index) {
        int reversedIndex = reverseIndex(index);
        return sourceList.get(reversedIndex);
    }

    private int reverseIndex(int index) {
        return max(size() - index - 1, 0);
    }

    @Override
    public int size() {
        return sourceList.size();
    }
}