Iterate once to create two iterators in partition

src/main/java/io/vavr/collection/AbstractMultimap.java

@@ -472,8 +472,12 @@ public M orElse(Supplier<? extends Iterable<? extends Tuple2<K, V>>> supplier) {
     @Override
     public Tuple2<M, M> partition(Predicate<? super Tuple2<K, V>> predicate) {
         Objects.requireNonNull(predicate, "predicate is null");
-        final Tuple2<Iterator<Tuple2<K, V>>, Iterator<Tuple2<K, V>>> p = iterator().partition(predicate);
-        return Tuple.of((M) createFromEntries(p._1), (M) createFromEntries(p._2));
+        final java.util.List<Tuple2<K, V>> left = new java.util.ArrayList<>();
+        final java.util.List<Tuple2<K, V>> right = new java.util.ArrayList<>();
+        for (Tuple2<K, V> entry : this) {
+            (predicate.test(entry) ? left : right).add(entry);
+        }
+        return Tuple.of((M) createFromEntries(left), (M) createFromEntries(right));
     }
 
     @SuppressWarnings("unchecked")

src/main/java/io/vavr/collection/BitSet.java

@@ -831,8 +831,7 @@ public BitSet<T> scan(T zero, BiFunction<? super T, ? super T, ? extends T> oper
 
         @Override
         public Tuple2<BitSet<T>, BitSet<T>> partition(Predicate<? super T> predicate) {
-            Objects.requireNonNull(predicate, "predicate is null");
-            return iterator().partition(predicate).map(this::createFromAll, this::createFromAll);
+            return Collections.partition(this, this::createFromAll, predicate);
         }
 
         @Override

src/main/java/io/vavr/collection/Collections.java

@@ -18,6 +18,8 @@
  */
 package io.vavr.collection;
 
+import io.vavr.Tuple;
+import io.vavr.Tuple2;
 import io.vavr.collection.JavaConverters.ChangePolicy;
 import io.vavr.collection.JavaConverters.ListView;
 import io.vavr.control.Option;
@@ -294,6 +296,17 @@ static <K, V, K2, U extends Map<K2, V>> U mapKeys(Map<K, V> source, U zero, Func
         });
     }
 
+    static <C extends Traversable<T>, T> Tuple2<C, C> partition(C collection, Function<Iterable<T>, C> creator,
+                                                                Predicate<? super T> predicate) {
+        Objects.requireNonNull(predicate, "predicate is null");
+        final java.util.List<T> left = new java.util.ArrayList<>();
+        final java.util.List<T> right = new java.util.ArrayList<>();
+        for (T element : collection) {
+            (predicate.test(element) ? left : right).add(element);
+        }
+        return Tuple.of(creator.apply(left), creator.apply(right));
+    }
+
     @SuppressWarnings("unchecked")
     static <C extends Traversable<T>, T> C removeAll(C source, Iterable<? extends T> elements) {
         Objects.requireNonNull(elements, "elements is null");
@@ -556,7 +569,7 @@ private static <T> IterableWithSize<T> withSizeTraversable(Iterable<? extends T>
             return new IterableWithSize<>(iterable, ((Traversable<?>) iterable).size());
         }
     }
-    
+
     static class IterableWithSize<T> {
         private final Iterable<? extends T> iterable;
         private final int size;

src/main/java/io/vavr/collection/HashSet.java

@@ -668,7 +668,7 @@ public boolean isAsync() {
     public boolean isEmpty() {
         return tree.isEmpty();
     }
-    
+
     /**
      * A {@code HashSet} is computed eagerly.
      *
@@ -730,9 +730,7 @@ public HashSet<T> orElse(Supplier<? extends Iterable<? extends T>> supplier) {
 
     @Override
     public Tuple2<HashSet<T>, HashSet<T>> partition(Predicate<? super T> predicate) {
-        Objects.requireNonNull(predicate, "predicate is null");
-        final Tuple2<Iterator<T>, Iterator<T>> p = iterator().partition(predicate);
-        return Tuple.of(HashSet.ofAll(p._1), HashSet.ofAll(p._2));
+        return Collections.partition(this, HashSet::ofAll, predicate);
     }
 
     @Override

src/main/java/io/vavr/collection/Iterator.java

@@ -23,6 +23,7 @@
 
 import java.math.BigDecimal;
 import java.util.*;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.*;
 
 import static io.vavr.collection.BigDecimalHelper.areEqual;
@@ -1722,10 +1723,8 @@ default Tuple2<Iterator<T>, Iterator<T>> partition(Predicate<? super T> predicat
         if (!hasNext()) {
             return Tuple.of(empty(), empty());
         } else {
-            final Stream<T> that = Stream.ofAll(this);
-            final Iterator<T> first = that.iterator().filter(predicate);
-            final Iterator<T> second = that.iterator().filter(predicate.negate());
-            return Tuple.of(first, second);
+            final Tuple2<Iterator<T>, Iterator<T>> dup = IteratorModule.duplicate(this);
+            return Tuple.of(dup._1.filter(predicate), dup._2.filterNot(predicate));
         }
     }
 
@@ -1952,6 +1951,7 @@ default Tuple2<Iterator<T>, Iterator<T>> span(Predicate<? super T> predicate) {
         }
     }
 
+
     @Override
     default String stringPrefix() {
         return "Iterator";
@@ -2182,6 +2182,46 @@ public String toString() {
     }
 }
 
+interface IteratorModule {
+    /**
+     * Creates two new iterators that both iterates over the same elements as
+     * this iterator and in the same order. The duplicate iterators are
+     * considered equal if they are positioned at the same element.
+     * <p>
+     * Given that most methods on iterators will make the original iterator
+     * unfit for further use, this methods provides a reliable way of calling
+     * multiple such methods on an iterator.
+     *
+     * @return a pair of iterators
+     */
+    static <T> Tuple2<Iterator<T>, Iterator<T>> duplicate(Iterator<T> iterator) {
+        final java.util.Queue<T> gap = new java.util.LinkedList<>();
+        final AtomicReference<Iterator<T>> ahead = new AtomicReference<>();
+        class Partner implements Iterator<T> {
+
+            @Override
+            public boolean hasNext() {
+                return (this != ahead.get() && !gap.isEmpty()) || iterator.hasNext();
+            }
+
+            @Override
+            public T next() {
+                if (gap.isEmpty()) {
+                    ahead.set(this);
+                }
+                if (this == ahead.get()) {
+                    final T element = iterator.next();
+                    gap.add(element);
+                    return element;
+                } else {
+                    return gap.poll();
+                }
+            }
+        }
+        return Tuple.of(new Partner(), new Partner());
+    }
+}
+
 final class ConcatIterator<T> implements Iterator<T> {
 
     private static final class Iterators<T> {

src/main/java/io/vavr/collection/LinkedHashSet.java

@@ -751,9 +751,7 @@ public LinkedHashSet<T> orElse(Supplier<? extends Iterable<? extends T>> supplie
 
     @Override
     public Tuple2<LinkedHashSet<T>, LinkedHashSet<T>> partition(Predicate<? super T> predicate) {
-        Objects.requireNonNull(predicate, "predicate is null");
-        final Tuple2<Iterator<T>, Iterator<T>> p = iterator().partition(predicate);
-        return Tuple.of(LinkedHashSet.ofAll(p._1), LinkedHashSet.ofAll(p._2));
+        return Collections.partition(this, LinkedHashSet::ofAll, predicate);
     }
 
     @Override

src/main/java/io/vavr/collection/Maps.java

@@ -201,8 +201,12 @@ static <T, K, V, M extends Map<K, V>> M ofStream(M map, java.util.stream.Stream<
     static <K, V, M extends Map<K, V>> Tuple2<M, M> partition(M map, OfEntries<K, V, M> ofEntries,
             Predicate<? super Tuple2<K, V>> predicate) {
         Objects.requireNonNull(predicate, "predicate is null");
-        final Tuple2<Iterator<Tuple2<K, V>>, Iterator<Tuple2<K, V>>> p = map.iterator().partition(predicate);
-        return Tuple.of(ofEntries.apply(p._1), ofEntries.apply(p._2));
+        final java.util.List<Tuple2<K, V>> left = new java.util.ArrayList<>();
+        final java.util.List<Tuple2<K, V>> right = new java.util.ArrayList<>();
+        for (Tuple2<K, V> entry : map) {
+            (predicate.test(entry) ? left : right).add(entry);
+        }
+        return Tuple.of(ofEntries.apply(left), ofEntries.apply(right));
     }
 
     static <K, V, M extends Map<K, V>> M peek(M map, Consumer<? super Tuple2<K, V>> action) {

src/main/java/io/vavr/collection/Traversable.java

@@ -164,7 +164,7 @@ public interface Traversable<T> extends Iterable<T>, Foldable<T>, io.vavr.Value<
     static <T> Traversable<T> narrow(Traversable<? extends T> traversable) {
         return (Traversable<T>) traversable;
     }
-    
+
     /**
      * Matches each element with a unique key that you extract from it.
      * If the same key is present twice, the function will return {@code None}.
@@ -223,7 +223,7 @@ default Option<Double> average() {
             throw new UnsupportedOperationException("not numeric", x);
         }
     }
-    
+
     /**
      * Collects all elements that are in the domain of the given {@code partialFunction} by mapping the elements to type {@code R}.
      * <p>
@@ -341,7 +341,7 @@ default int count(Predicate<? super T> predicate) {
     Traversable<T> dropUntil(Predicate<? super T> predicate);
 
     /**
-     * Drops elements while the predicate holds for the current element.                                                                            
+     * Drops elements while the predicate holds for the current element.
      * <p>
      * Note: This is essentially the same as {@code dropUntil(predicate.negate())}.
      * It is intended to be used with method references, which cannot be negated directly.
@@ -370,7 +370,7 @@ default int count(Predicate<? super T> predicate) {
      * <li>contain the same elements</li>
      * <li>have the same element order, if the collections are of type Seq</li>
      * </ul>
-     * 
+     *
      * Two Map/Multimap elements, resp. entries, (key1, value1) and (key2, value2) are equal,
      * if the keys are equal and the values are equal.
      * <p>
@@ -685,7 +685,7 @@ default T get() {
     default Option<T> headOption() {
         return isEmpty() ? Option.none() : Option.some(head());
     }
-    
+
     /**
      * Returns the hash code of this collection.
      * <br>
@@ -1055,7 +1055,7 @@ default <U extends Comparable<? super U>> Option<T> minBy(Function<? super T, ?
             return Option.some(tm);
         }
     }
-    
+
     /**
      * Joins the elements of this by concatenating their string representations.
      * <p>
@@ -1372,7 +1372,7 @@ default Option<T> reduceRightOption(BiFunction<? super T, ? super T, ? extends T
      * @throws NullPointerException if {@code operation} is null.
      */
     <U> Traversable<U> scanRight(U zero, BiFunction<? super T, ? super U, ? extends U> operation);
-    
+
     /**
      * Returns the single element of this Traversable or throws, if this is empty or contains more than one element.
      *
@@ -1536,7 +1536,7 @@ default Number sum() {
             }
         }
     }
-    
+
     /**
      * Drops the first element of a non-empty Traversable.
      *
@@ -1666,7 +1666,7 @@ default Number sum() {
      * @throws NullPointerException if {@code that} is null
      */
     <U> Traversable<Tuple2<T, U>> zipAll(Iterable<? extends U> that, T thisElem, U thatElem);
-    
+
     /**
      * Returns a traversable formed from this traversable and another Iterable collection by mapping elements.
      * If one of the two iterables is longer than the other, its remaining elements are ignored.

src/main/java/io/vavr/collection/TreeSet.java

@@ -800,9 +800,7 @@ public TreeSet<T> orElse(Supplier<? extends Iterable<? extends T>> supplier) {
 
     @Override
     public Tuple2<TreeSet<T>, TreeSet<T>> partition(Predicate<? super T> predicate) {
-        Objects.requireNonNull(predicate, "predicate is null");
-        return iterator().partition(predicate).map(i1 -> TreeSet.ofAll(tree.comparator(), i1),
-                i2 -> TreeSet.ofAll(tree.comparator(), i2));
+        return Collections.partition(this, values -> TreeSet.ofAll(tree.comparator(), values), predicate);
     }
 
     @Override

src/test/java/io/vavr/collection/AbstractMapTest.java

@@ -1461,6 +1461,21 @@ public void shouldReturnDefaultValue() {
         assertThat(map.getOrElse("3", "3")).isEqualTo("3");
     }
 
+    // -- partition
+
+    @Test
+    public void shouldPartitionInOneIteration() {
+        final AtomicInteger count = new AtomicInteger(0);
+        final Map<String, Integer> map = mapOf("1", 1, "2", 2, "3", 3);
+        final Tuple2<? extends Map<String, Integer>, ? extends Map<String, Integer>> results = map.partition(entry -> {
+            count.incrementAndGet();
+            return true;
+        });
+        assertThat(results._1).isEqualTo(mapOf("1", 1, "2", 2, "3", 3));
+        assertThat(results._2).isEmpty();
+        assertThat(count.get()).isEqualTo(3);
+    }
+
     // -- spliterator
 
     @Test

src/test/java/io/vavr/collection/AbstractMultimapTest.java

@@ -809,6 +809,20 @@ public void shouldPartitionIntsInOddAndEvenHavingOddAndEvenNumbers() {
                         mapOfTuples(Tuple.of(1, 2), Tuple.of(3, 4))));
     }
 
+    @Test
+    @SuppressWarnings("unchecked")
+    public void shouldPartitionInOneIteration() {
+        final AtomicInteger count = new AtomicInteger(0);
+        final Multimap<String, Integer> map = mapOfTuples(Tuple.of("1", 1), Tuple.of("2", 2), Tuple.of("3", 3));
+        final Tuple2<? extends Multimap<String, Integer>, ? extends Multimap<String, Integer>> results = map.partition(entry -> {
+            count.incrementAndGet();
+            return true;
+        });
+        assertThat(results._1).isEqualTo(mapOfTuples(Tuple.of("1", 1), Tuple.of("2", 2), Tuple.of("3", 3)));
+        assertThat(results._2).isEmpty();
+        assertThat(count.get()).isEqualTo(3);
+    }
+
     // -- put
 
     @Test

src/test/java/io/vavr/collection/AbstractSetTest.java

@@ -18,10 +18,12 @@
  */
 package io.vavr.collection;
 
+import io.vavr.Tuple2;
 import org.junit.Test;
 
 import java.math.BigDecimal;
 import java.util.Spliterator;
+import java.util.concurrent.atomic.AtomicInteger;
 
 public abstract class AbstractSetTest extends AbstractTraversableRangeTest {
 
@@ -189,6 +191,20 @@ public void shouldRemoveElement() {
         assertThat(empty().remove(5)).isEqualTo(empty());
     }
 
+    // -- partition
+
+    @Test
+    public void shouldPartitionInOneIteration() {
+        final AtomicInteger count = new AtomicInteger(0);
+        final Tuple2<? extends Set<Integer>, ? extends Set<Integer>> results = of(1, 2, 3).partition(i -> {
+            count.incrementAndGet();
+            return true;
+        });
+        assertThat(results._1).isEqualTo(of(1, 2, 3));
+        assertThat(results._2).isEqualTo(of());
+        assertThat(count.get()).isEqualTo(3);
+    }
+
     // -- removeAll
 
     @Test
@@ -227,7 +243,7 @@ public void shouldReturnSameSetWhenEmptyUnionNonEmpty() {
             assertThat(empty().union(set)).isSameAs(set);
         }
     }
-    
+
     @Test
     public void shouldReturnSameSetWhenNonEmptyUnionEmpty() {
         final Set<Integer> set = of(1, 2);

src/test/java/io/vavr/collection/ArrayTest.java

@@ -27,6 +27,7 @@
 
 import java.math.BigDecimal;
 import java.util.ArrayList;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Function;
 import java.util.function.Supplier;
 import java.util.stream.Collector;
@@ -243,6 +244,20 @@ public void shouldThrowExceptionWhenGetIndexEqualToLength() {
             .isInstanceOf(IndexOutOfBoundsException.class).hasMessage("get(1)");
     }
 
+    // -- partition
+
+    @Test
+    public void shouldPartitionInOneIteration() {
+        final AtomicInteger count = new AtomicInteger(0);
+        final Tuple2<Array<Integer>, Array<Integer>> results = of(1, 2, 3).partition(i -> {
+            count.incrementAndGet();
+            return true;
+        });
+        assertThat(results._1).isEqualTo(of(1, 2, 3));
+        assertThat(results._2).isEqualTo(of());
+        assertThat(count.get()).isEqualTo(3);
+    }
+
     // -- transform()
 
     @Test