/*
 * 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.interactive.form;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

import org.apache.pdfbox.cos.COSArray;
import org.apache.pdfbox.cos.COSBase;
import org.apache.pdfbox.cos.COSString;

/**
 * A set of utility methods to help with common AcroForm form and field related functions.
 */
public final class FieldUtils
{
    
    /**
     * An implementation of a basic key value pair.
     * 
     * This implementation is used to help sorting the content of
     * field option entries with an array of two-element arrays as
     * used by choice fields.
     * 
     */
    static class KeyValue
    {
        private final String key;
        private final String value;
        
        KeyValue(final String theKey, final String theValue)
        {
            this.key = theKey;
            this.value = theValue;
        }
        
        public String getKey()
        {
            return this.key;
        }

        public String getValue()
        {
            return this.value;
        }
        
        @Override
        public String toString()
        {
            return "(" + this.key + ", " + this.value + ")";
        }
    }
    
    /**
     * Comparator to sort KeyValue by key.
     */
    static class KeyValueKeyComparator implements Serializable, Comparator<KeyValue>
    {

        private static final long serialVersionUID = 6715364290007167694L;

        @Override
        public int compare(KeyValue o1, KeyValue o2)
        {
            return o1.key.compareTo(o2.key);
        }
    }

    /**
     * Comparator to sort KeyValue by value.
     */
    static class KeyValueValueComparator implements Serializable, Comparator<KeyValue>
    {

        private static final long serialVersionUID = -3984095679894798265L;

        @Override
        public int compare(KeyValue o1, KeyValue o2)
        {
            return o1.value.compareTo(o2.value);
        }
    }

    /**
     * Constructor.
     */
    private FieldUtils()
    {
    }
    
    /**
     * Return two related lists as a single list with key value pairs.
     * 
     * @param key the key elements
     * @param value the value elements
     * @return a sorted list of KeyValue elements.
     */
    static List<KeyValue> toKeyValueList(List<String> key, List<String> value)
    {
        List<KeyValue> list = new ArrayList<>();
        for(int i =0; i<key.size(); i++)
        {
            list.add(new FieldUtils.KeyValue(key.get(i),value.get(i)));
        }
        return list;
    }    
    
    /**
     * Sort two related lists simultaneously by the elements in the key parameter.
     * 
     * @param pairs a list of KeyValue elements
     */
    static void sortByValue(List<KeyValue> pairs)
    {
        Collections.sort(pairs, new FieldUtils.KeyValueValueComparator());
    }

    /**
     * Sort two related lists simultaneously by the elements in the value parameter.
     * 
     * @param pairs a list of KeyValue elements
     */
    static void sortByKey(List<KeyValue> pairs)
    {
        Collections.sort(pairs, new FieldUtils.KeyValueKeyComparator());
    }
    
    /**
     * Return either one of a list which can have two-element arrays entries.
     * <p>
     * Some entries in a dictionary can either be an array of elements
     * or an array of two-element arrays. This method will either return
     * the elements in the array or in case of two-element arrays, the element
     * designated by the pair index
     * </p>
     * <p>
     * An {@link IllegalArgumentException} will be thrown if the items contain
     * two-element arrays and the index is not 0 or 1.
     * </p>
     * 
     * @param items the array of elements or two-element arrays
     * @param pairIdx the index into the two-element array
     * @return a List of single elements
     */
    static List<String> getPairableItems(COSBase items, int pairIdx)
    {
        if (pairIdx < 0 || pairIdx > 1) 
        {
            throw new IllegalArgumentException("Only 0 and 1 are allowed as an index into two-element arrays");
        }
        
        if (items instanceof COSString)
        {
            List<String> array = new ArrayList<>();
            array.add(((COSString) items).getString());
            return array;
        }
        else if (items instanceof COSArray)
        {
            List<String> entryList = new ArrayList<>(); 
            for (COSBase entry : (COSArray) items)
            {
                if (entry instanceof COSString)
                {
                    entryList.add(((COSString) entry).getString());
                }
                else if (entry instanceof COSArray)
                {
                    COSArray cosArray = (COSArray) entry;
                    if (cosArray.size() >= pairIdx +1 && cosArray.get(pairIdx) instanceof COSString)
                    {
                        entryList.add(((COSString) cosArray.get(pairIdx)).getString());
                    }
                }
            }
            return entryList;
        }
        return Collections.emptyList();
    }
}

@@ -853,37 +853,8 @@
 m;%0A%0A
-import java.io.Serializable;%0A
 impo
@@ -877,16 +877,16 @@
 ayList;%0A
+
 import j
@@ -1209,24 +1209,212 @@
 Utils%0A%7B%0A    
+static Comparator%3CKeyValue%3E BY_VALUE_COMPARATOR = Comparator.comparing(KeyValue::getValue);%0A    static Comparator%3CKeyValue%3E BY_KEY_COMPARATOR = Comparator.comparing(KeyValue::getKey);    %0A
 %0A    /**%0A   
@@ -2231,761 +2231,8 @@
    %7D
-%0A    %0A    /**%0A     * Comparator to sort KeyValue by key.%0A     */%0A    static class KeyValueKeyComparator implements Serializable, Comparator%3CKeyValue%3E%0A    %7B%0A%0A        private static final long serialVersionUID = 6715364290007167694L;%0A%0A        @Override%0A        public int compare(KeyValue o1, KeyValue o2)%0A        %7B%0A            return o1.key.compareTo(o2.key);%0A        %7D%0A    %7D%0A%0A    /**%0A     * Comparator to sort KeyValue by value.%0A     */%0A    static class KeyValueValueComparator implements Serializable, Comparator%3CKeyValue%3E%0A    %7B%0A%0A        private static final long serialVersionUID = -3984095679894798265L;%0A%0A        @Override%0A        public int compare(KeyValue o1, KeyValue o2)%0A        %7B%0A            return o1.value.compareTo(o2.value);%0A        %7D%0A    %7D
 %0A%0A  
@@ -3053,72 +3053,38 @@
     
-Collections.sort(pairs, new FieldUtils.KeyValueValueComparator()
+pairs.sort(BY_VALUE_COMPARATOR
 );%0A 
@@ -3312,70 +3312,36 @@
     
-Collections.sort(pairs, new FieldUtils.KeyValueKeyComparator()
+pairs.sort(BY_KEY_COMPARATOR
 );%0A 

use jdk8 method references for sorting