Add collating, zipping, reversing methods to FluentIterable, add ZippingIterator.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1682121 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/collections4/FluentIterable.java

@@ -18,6 +18,7 @@ package org.apache.commons.collections4;
 
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Comparator;
 import java.util.Enumeration;
 import java.util.Iterator;
 import java.util.List;
@@ -151,6 +152,55 @@ public class FluentIterable<E> implements Iterable<E> {
         return of(IterableUtils.chainedIterable(iterable, other));
     }
 
+    /**
+     * Returns a new FluentIterable whose iterator will traverse the
+     * elements of the current and provided iterable in natural order.
+     * <p>
+     * Example: natural ordering
+     * <ul>
+     *   <li>this contains elements [1, 3, 5, 7]
+     *   <li>other contains elements [2, 4, 6, 8]
+     * </ul>
+     * <p>
+     * The returned iterable will traverse the elements in the following
+     * order: [1, 2, 3, 4, 5, 6, 7, 8]
+     * <p>
+     * A <code>null</code> iterable will be treated as an empty iterable.
+     *
+     * @param other  the other iterable to collate, may be null
+     * @return a new iterable, collating this iterable with the other in natural order
+     * @see {@link org.apache.commons.collections4.iterators.CollatingIterator CollatingIterator}
+     */
+    public FluentIterable<E> collate(final Iterable<E> other) {
+        return of(IterableUtils.collatedIterable(iterable, other, null));
+    }
+
+    /**
+     * Returns a new FluentIterable whose iterator will traverse the
+     * elements of the current and provided iterable according to the
+     * ordering defined by an comparator.
+     * <p>
+     * Example: descending order
+     * <ul>
+     *   <li>this contains elements [7, 5, 3, 1]
+     *   <li>other contains elements [8, 6, 4, 2]
+     * </ul>
+     * <p>
+     * The returned iterable will traverse the elements in the following
+     * order: [8, 7, 6, 5, 4, 3, 2, 1]
+     * <p>
+     * A <code>null</code> iterable will be treated as an empty iterable.
+     *
+     * @param other  the other iterable to collate, may be null
+     * @param comparator  the comparator to define an ordering, may be null,
+     *   in which case natural ordering will be used
+     * @return a new iterable, collating this iterable with the other in natural order
+     * @see {@link org.apache.commons.collections4.iterators.CollatingIterator CollatingIterator}
+     */
+    public FluentIterable<E> collate(final Iterable<E> other, Comparator<? super E> comparator) {
+        return of(IterableUtils.collatedIterable(iterable, other, comparator));
+    }
+
     /**
      * This method fully traverses an iterator of this iterable and returns
      * a new iterable with the same contents, but without any reference
@@ -202,6 +252,16 @@ public class FluentIterable<E> implements Iterable<E> {
         return of(IterableUtils.loopingIterable(iterable));
     }
 
+    /**
+     * Returns a new FluentIterable whose iterator will traverse the
+     * elements from this iterable in reverse order.
+     *
+     * @return a new iterable, providing a reversed view of this iterable
+     */
+    public FluentIterable<E> reverse() {
+        return of(IterableUtils.reversedIterable(iterable));
+    }
+
     /**
      * Returns a new FluentIterable whose iterator will skip the first
      * N elements from this iterable.
@@ -237,6 +297,35 @@ public class FluentIterable<E> implements Iterable<E> {
         return of(IterableUtils.uniqueIterable(iterable));
     }
 
+    /**
+     * Returns a new FluentIterable whose iterator will traverse
+     * the elements of this iterable and the provided elements in
+     * alternating order.
+     *
+     * @param elements  the elements to interleave
+     * @return a new iterable, interleaving this iterable with the elements
+     */
+    @SuppressWarnings("unchecked")
+    public FluentIterable<E> zip(final E... elements) {
+        return zip(Arrays.asList(elements));
+    }
+
+    /**
+     * Returns a new FluentIterable whose iterator will traverse
+     * the elements of this iterable and the other iterable in
+     * alternating order.
+     *
+     * @param other  the other iterable to interleave
+     * @return a new iterable, interleaving this iterable with others
+     */
+    public FluentIterable<E> zip(final Iterable<E>... others) {
+        @SuppressWarnings("unchecked")
+        Iterable<E>[] iterables = new Iterable[1 + others.length];
+        iterables[0] = iterable;
+        System.arraycopy(others, 0, iterables, 1, others.length);
+        return of(IterableUtils.zippingIterable(iterables));
+    }
+
     // convenience methods
     // ----------------------------------------------------------------------
 
@@ -357,10 +446,10 @@ public class FluentIterable<E> implements Iterable<E> {
     }
 
     /**
-     * Returns a list containing all elements of this iterable by traversing
+     * Returns a list containing all elements of this iterable by
-     * its iterator.
+     * traversing its iterator.
      * <p>
-     * The returned list is mutable.
+     * The returned list is guaranteed to be mutable.
      *
      * @return a list of the iterable contents
      */

src/main/java/org/apache/commons/collections4/IterableUtils.java

@@ -17,11 +17,14 @@
 package org.apache.commons.collections4;
 
 import java.util.Collection;
+import java.util.Comparator;
 import java.util.Iterator;
 import java.util.List;
 
 import org.apache.commons.collections4.iterators.LazyIteratorChain;
+import org.apache.commons.collections4.iterators.ReverseListIterator;
 import org.apache.commons.collections4.iterators.UniqueFilterIterator;
+import org.apache.commons.collections4.iterators.ZippingIterator;
 
 /**
  * Provides utility methods and decorators for {@link Iterable} instances.
@@ -136,6 +139,37 @@ public class IterableUtils {
         };
     }
 
+    // Collated
+    // ----------------------------------------------------------------------
+
+    /**
+     * Combines the two provided iterables into an ordered iterable using the
+     * provided comparator. If the comparator is null, natural ordering will be
+     * used.
+     * <p>
+     * The returned iterable's iterator supports {@code remove()} when the corresponding
+     * input iterator supports it.
+     *
+     * @param <E>  the element type
+     * @param a  the first iterable, may be null
+     * @param b  the second iterable, may be null
+     * @param comparator  the comparator defining an ordering over the elements,
+     *   may be null, in which case natural ordering will be used
+     * @return a filtered view on the specified iterable
+     */
+    public static <E> Iterable<E> collatedIterable(final Iterable<E> a,
+                                                   final Iterable<E> b,
+                                                   final Comparator<? super E> comparator) {
+        return new FluentIterable<E>() {
+            @Override
+            public Iterator<E> iterator() {
+                return IteratorUtils.collatedIterator(comparator,
+                                                      emptyIteratorIfNull(a),
+                                                      emptyIteratorIfNull(b));
+            }
+        };
+    }
+
     // Filtered
     // ----------------------------------------------------------------------
 
@@ -151,7 +185,8 @@ public class IterableUtils {
      * @return a filtered view on the specified iterable
      * @throws NullPointerException if predicate is null
      */
-    public static <E> Iterable<E> filteredIterable(final Iterable<E> iterable, final Predicate<? super E> predicate) {
+    public static <E> Iterable<E> filteredIterable(final Iterable<E> iterable,
+                                                   final Predicate<? super E> predicate) {
         if (predicate == null) {
             throw new NullPointerException("predicate must not be null.");
         }
@@ -232,6 +267,38 @@ public class IterableUtils {
         };
     }
 
+    // Reversed
+    // ----------------------------------------------------------------------
+
+    /**
+     * Returns a reversed view of the given iterable.
+     * <p>
+     * In case the provided iterable is a {@link List} instance, a 
+     * {@link ReverseListIterator} will be used to reverse the traversal
+     * order, otherwise an intermediate {@link List} needs to be
+     * created.
+     * <p>
+     * The returned iterable's iterator supports {@code remove()} if the
+     * provided iterable is a {@link List} instance.
+     *
+     * @param <E>  the element type
+     * @param iterable  the iterable to use, may be null
+     * @return a reversed view of the specified iterable
+     * @see ReverseListIterator
+     */
+    public static <E> Iterable<E> reversedIterable(final Iterable<E> iterable) {
+        return new FluentIterable<E>() {
+            @Override
+            public Iterator<E> iterator() {
+                final List<E> list = (iterable instanceof List<?>) ?
+                        (List<E>) iterable :
+                        IteratorUtils.toList(emptyIteratorIfNull(iterable));
+
+                return new ReverseListIterator<E>(list);
+            }
+        };
+    }
+
     // Skipping
     // ----------------------------------------------------------------------
 
@@ -276,7 +343,8 @@ public class IterableUtils {
      * @return a transformed view of the specified iterable
      * @throws NullPointerException if transformer is null
      */
-    public static <I, O> Iterable<O> transformedIterable(final Iterable<I> iterable, final Transformer<? super I, ? extends O> transformer) {
+    public static <I, O> Iterable<O> transformedIterable(final Iterable<I> iterable,
+                                                         final Transformer<? super I, ? extends O> transformer) {
         if (transformer == null) {
             throw new NullPointerException("transformer must not be null.");
         }
@@ -310,6 +378,58 @@ public class IterableUtils {
         };
     }
 
+    // Zipping
+    // ----------------------------------------------------------------------
+
+    /**
+     * Interleaves two iterables into a single iterable.
+     * <p>
+     * The returned iterable has an iterator that traverses the elements in {@code a}
+     * and {@code b} in alternating order. The source iterators are not polled until
+     * necessary.
+     * <p>
+     * The returned iterable's iterator supports {@code remove()} when the corresponding
+     * input iterator supports it.
+     *
+     * @param <E>  the element type
+     * @param a  the first iterable
+     * @param b  the second iterable
+     * @return a new iterable, interleaving the provided iterables
+     */
+    @SuppressWarnings("unchecked")
+    public static <E> Iterable<E> zippingIterable(final Iterable<E> a, final Iterable<E> b) {
+        return zippingIterable(new Iterable[] {a, b});
+    }
+
+    /**
+     * Interleaves two iterables into a single iterable.
+     * <p>
+     * The returned iterable has an iterator that traverses the elements in {@code a}
+     * and {@code b} in alternating order. The source iterators are not polled until
+     * necessary.
+     * <p>
+     * The returned iterable's iterator supports {@code remove()} when the corresponding
+     * input iterator supports it.
+     *
+     * @param <E>  the element type
+     * @param a  the first iterable
+     * @param b  the second iterable
+     * @return a new iterable, interleaving the provided iterables
+     */
+    public static <E> Iterable<E> zippingIterable(final Iterable<E>... iterables) {
+        return new FluentIterable<E>() {
+            @Override
+            public Iterator<E> iterator() {
+                @SuppressWarnings("unchecked")
+                Iterator<E>[] iterators = new Iterator[iterables.length];
+                for (int i = 0; i < iterables.length; i++) {
+                    iterators[i] = emptyIteratorIfNull(iterables[i]);
+                }
+                return new ZippingIterator<E>(iterators);
+            }            
+        };
+    }
+
     // Utility methods
     // ----------------------------------------------------------------------
 

src/main/java/org/apache/commons/collections4/iterators/ZippingIterator.java

@@ -0,0 +1,141 @@
+/*
+ * 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.commons.collections4.iterators;
+
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+import org.apache.commons.collections4.FluentIterable;
+
+/**
+ * Provides an interleaved iteration over the elements contained in a
+ * collection of Iterators.
+ * <p>
+ * Given two {@link Iterator} instances <code>A</code> and
+ * <code>B</code>, the {@link #next} method on this iterator will
+ * alternate between <code>A.next()</code> and <code>B.next()</code>.
+ *
+ * @since 4.1
+ * @version $Id$
+ */
+public class ZippingIterator<E> implements Iterator<E> {
+
+    /** The {@link Iterator}s to evaluate. */
+    private final Iterator<Iterator<? extends E>> iterators;
+    /** The next iterator to use for next(). */
+    private Iterator<? extends E> nextIterator = null;
+    /** The last iterator which was used for next(). */
+    private Iterator<? extends E> lastReturned = null;
+
+    // Constructors
+    // ----------------------------------------------------------------------
+
+    /**
+     * Constructs a new <code>ZippingIterator</code> that will provide
+     * interleaved iteration over the two given iterators.
+     *
+     * @param a the first child iterator
+     * @param b the second child iterator
+     * @throws NullPointerException if either iterator is null
+     */
+    @SuppressWarnings("unchecked")
+    public ZippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b) {
+        this(new Iterator[] {a, b});
+    }
+
+    /**
+     * Constructs a new <code>ZippingIterator</code> that will use the
+     * specified comparator to provide ordered iteration over the array of
+     * iterators.
+     *
+     * @param iterators the array of iterators
+     * @throws NullPointerException if iterators array is or contains null
+     */
+    public ZippingIterator(final Iterator<? extends E>... iterators) {
+        // create a mutable list
+        final List<Iterator<? extends E>> list = new ArrayList<Iterator<? extends E>>();
+        for (Iterator<? extends E> iterator : iterators) {
+            if (iterator == null) {
+                throw new NullPointerException("Iterator must not be null");
+            }
+            list.add(iterator);
+        }
+        this.iterators = FluentIterable.of(list).loop().iterator();
+    }
+
+    // Iterator Methods
+    // -------------------------------------------------------------------
+
+    /**
+     * Returns <code>true</code> if any child iterator has remaining elements.
+     *
+     * @return true if this iterator has remaining elements
+     */
+    public boolean hasNext() {
+        // the next iterator has already been determined
+        // this might happen if hasNext() was called multiple
+        if (nextIterator != null) {
+            return true;
+        }
+
+        while(iterators.hasNext()) {
+            final Iterator<? extends E> iterator = iterators.next();
+            if (iterator.hasNext()) {
+                nextIterator = iterator;
+                return true;
+            } else {
+                // iterator is exhausted, remove it
+                iterators.remove();
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Returns the next element from a child iterator.
+     *
+     * @return the next interleaved element
+     * @throws NoSuchElementException if no child iterator has any more elements
+     */
+    public E next() throws NoSuchElementException {
+        if (!hasNext()) {
+            throw new NoSuchElementException();
+        }
+
+        final E val = nextIterator.next();
+        lastReturned = nextIterator;
+        nextIterator = null;
+        return val;
+    }
+
+    /**
+     * Removes the last returned element from the child iterator that produced it.
+     *
+     * @throws IllegalStateException if there is no last returned element, or if
+     *   the last returned element has already been removed
+     */
+    public void remove() {
+        if (lastReturned == null) {
+            throw new IllegalStateException("No value can be removed at present");
+        }
+        lastReturned.remove();
+        lastReturned = null;
+    }
+
+}