# StackOperators.java

    /*
     * Licensed to the Apache Software Foundation (ASF) under one or more
     * contributor license agreements.  See the NOTICE file distributed with
     * this work for additional information regarding copyright ownership.
     * The ASF licenses this file to You 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 org.apache.pdfbox.pdmodel.common.function.type4;

    import java.util.LinkedList;
    import java.util.List;
    import java.util.Stack;

    /**
     * Provides the stack operators such as "pop" and "dup".
     *
     */
    class StackOperators
    {

        private StackOperators()
        {
            // Private constructor.
        }

        /** Implements the "copy" operator. */
        static class Copy implements Operator
        {
            @Override
            public void execute(ExecutionContext context)
            {
                Stack<Object> stack = context.getStack();
                int n = ((Number)stack.pop()).intValue();
                if (n > 0)
                {
                    int size = stack.size();
                    //Need to copy to a new list to avoid ConcurrentModificationException
                    List<Object> copy = new java.util.ArrayList<>(
                            stack.subList(size - n, size));
                    stack.addAll(copy);
                }
            }

        }

        /** Implements the "dup" operator. */
        static class Dup implements Operator
        {
            @Override
            public void execute(ExecutionContext context)
            {
                Stack<Object> stack = context.getStack();
                stack.push(stack.peek());
            }

        }

        /** Implements the "exch" operator. */
        static class Exch implements Operator
        {
            @Override
            public void execute(ExecutionContext context)
            {
                Stack<Object> stack = context.getStack();
                Object any2 = stack.pop();
                Object any1 = stack.pop();
                stack.push(any2);
                stack.push(any1);
            }

        }

        /** Implements the "index" operator. */
        static class Index implements Operator
        {
            @Override
            public void execute(ExecutionContext context)
            {
                Stack<Object> stack = context.getStack();
                int n = ((Number)stack.pop()).intValue();
                if (n < 0)
                {
                    throw new IllegalArgumentException("rangecheck: " + n);
                }
                int size = stack.size();
                stack.push(stack.get(size - n - 1));
            }

        }

        /** Implements the "pop" operator. */
        static class Pop implements Operator
        {
            @Override
            public void execute(ExecutionContext context)
            {
                Stack<Object> stack = context.getStack();
                stack.pop();
            }

        }

        /** Implements the "roll" operator. */
        static class Roll implements Operator
        {
            @Override
            public void execute(ExecutionContext context)
            {
                Stack<Object> stack = context.getStack();
                int j = ((Number)stack.pop()).intValue();
                int n = ((Number)stack.pop()).intValue();
                if (j == 0)
                {
                    return; //Nothing to do
                }
                if (n < 0)
                {
                    throw new IllegalArgumentException("rangecheck: " + n);
                }

                LinkedList<Object> rolled = new LinkedList<>();
                LinkedList<Object> moved = new LinkedList<>();
                if (j < 0)
                {
                    //negative roll
                    int n1 = n + j;
                    for (int i = 0; i < n1; i++)
                    {
                        moved.addFirst(stack.pop());
                    }
                    for (int i = j; i < 0; i++)
                    {
                        rolled.addFirst(stack.pop());
                    }
                    stack.addAll(moved);
                    stack.addAll(rolled);
                }
                else
                {
                    //positive roll
                    int n1 = n - j;
                    for (int i = j; i > 0; i--)
                    {
                        rolled.addFirst(stack.pop());
                    }
                    for (int i = 0; i < n1; i++)
                    {
                        moved.addFirst(stack.pop());
                    }
                    stack.addAll(rolled);
                    stack.addAll(moved);
                }
            }

        }

    }