# Iterables.java

    /*
    ** Module   : Iterables.java
    ** Abstract : Helper methods which make convenient use of Iterable interface
    **            and foreach loops.
    **
    ** Copyright (c) 2010-2019, Golden Code Development Corporation.
    **
    ** -#- -I- --Date-- --JPRM-- ----------------Description-----------------
    ** 001 SIY 20100804  @       Created initial version
    ** 002 SIY 20110112          Minor cleanups.
    ** 003 HC  20191211          Added reverseFrom and directFrom overloads.
    */
    /*
    ** This program is free software: you can redistribute it and/or modify
    ** it under the terms of the GNU Affero General Public License as
    ** published by the Free Software Foundation, either version 3 of the
    ** License, or (at your option) any later version.
    **
    ** This program 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 Affero General Public License for more details.
    **
    ** You may find a copy of the GNU Affero GPL version 3 at the following
    ** location: https://www.gnu.org/licenses/agpl-3.0.en.html
    **
    ** Additional terms under GNU Affero GPL version 3 section 7:
    **
    **   Under Section 7 of the GNU Affero GPL version 3, the following additional
    **   terms apply to the works covered under the License.  These additional terms
    **   are non-permissive additional terms allowed under Section 7 of the GNU
    **   Affero GPL version 3 and may not be removed by you.
    **
    **   0. Attribution Requirement.
    **
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    **     work or Appropriate Legal Notices displayed by works containing the covered
    **     work.  You may not remove from the covered work any author or developer
    **     credit already included within the covered work.
    **
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    **
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    */

    package com.goldencode.p2j.util;

    import java.util.*;

    /**
     * Set of helper methods for iterables.
     */
    public class Iterables
    {
       /**
        * Static factory method which returns reverse iterable for a given list.
        *
        * @param   <T>
        *          List item type.
        * @param   list
        *          List.
        *
        * @return  Instance of {@link Iterable} which performs iteration through
        *          elements in reverse order.
        */
       public static <T> Iterable<T> reverse(final List<T> list)
       {
          return new RevIterable<T>(list.listIterator(list.size()));
       }

       /**
        * Create new reverse iterable which will traverse list starting from
        * given element.
        *
        * @param   <T>
        *          List item type.
        * @param   list
        *          List.
        * @param   elem
        *          Initial element from which iteration will be started.
        *          Note that the element itself will not be returned.
        *
        * @return  Instance of {@link Iterable} which performs iteration through
        *          elements in reverse order.
        */
       public static <T> Iterable<T> reverseFrom(List<T> list, T elem)
       {
          return reverseFrom(list, elem, false);
       }

       /**
        * Create new reverse iterable which will traverse list starting from
        * given element.
        *
        * @param   <T>
        *          List item type.
        * @param   list
        *          List.
        * @param   elem
        *          Initial element from which iteration will be started.
        * @param   inclusive
        *          When {@code true} the iterator will also include the initial element.
        *
        * @return  Instance of {@link Iterable} which performs iteration through
        *          elements in reverse order.
        */
       public static <T> Iterable<T> reverseFrom(List<T> list, T elem, boolean inclusive)
       {
          int idx = list.indexOf(elem);

          if (idx < 0)
             return null;

          if (inclusive)
          {
             idx++;
          }

          return new RevIterable<T>(list.listIterator(idx));
       }

       /**
        * Return iterable which traverses list in direct order starting from given
        * element.
        *
        * @param   <T>
        *          List item type.
        * @param   list
        *          List.
        * @param   elem
        *          Initial element from which iteration will be started.
        *          Note that the element itself will not be returned.
        *
        * @return  Instance of {@link Iterable} which performs iteration through
        *          elements in direct order.
        */
       public static <T> Iterable<T> directFrom(final List<T> list, T elem)
       {
          return directFrom(list, elem, false);
       }

       /**
        * Return iterable which traverses list in direct order starting from given
        * element.
        *
        * @param   <T>
        *          List item type.
        * @param   list
        *          List.
        * @param   elem
        *          Initial element from which iteration will be started.
        * @param   inclusive
        *          When {@code true} the iterator will also include the initial element.
        *
        * @return  Instance of {@link Iterable} which performs iteration through
        *          elements in direct order.
        */
       public static <T> Iterable<T> directFrom(final List<T> list, T elem, boolean inclusive)
       {
          final int idx = list.indexOf(elem);

          if (idx < 0)
             return null;

          final ListIterator<T> iter = list.listIterator(idx);

          if (!inclusive && iter.hasNext())
             iter.next();

          return new Iterable<T>()
          {
             /**
              * @see java.lang.Iterable#iterator()
              */
             @Override
             public Iterator<T> iterator()
             {
                return iter;
             }
          };
       }

       /**
        * Reverse iterable implementation.
        *
        * @param <T>
        */
       private static class RevIterable<T>
       implements Iterable<T>
       {
          /**
           * Underlying iterator.
           */
          private final ListIterator<T> iter;

          /**
           * Constructor.
           *
           * @param   iter
           */
          RevIterable(ListIterator<T> iter)
          {
             this.iter = iter;
          }

          /**
           * @see java.lang.Iterable#iterator()
           */
          @Override
          public Iterator<T> iterator()
          {
             return new Iterator<T>()
             {
                /**
                 * @see java.util.Iterator#hasNext()
                 */
                @Override
                public boolean hasNext()
                {
                   return iter.hasPrevious();
                }

                /**
                 * @see java.util.Iterator#next()
                 */
                @Override
                public T next()
                {
                   return iter.previous();
                }

                /**
                 * @see java.util.Iterator#remove()
                 */
                @Override
                public void remove()
                {
                   throw new UnsupportedOperationException();
                }

             };
          }
       }
    }