--- jsr166/src/main/java/util/ArrayDeque.java	2016/10/30 16:32:40	1.99
+++ jsr166/src/main/java/util/ArrayDeque.java	2016/11/20 06:42:41	1.117
@@ -60,22 +60,40 @@ import java.util.function.UnaryOperator;
 public class ArrayDeque<E> extends AbstractCollection<E>
                            implements Deque<E>, Cloneable, Serializable
 {
+    /*
+     * VMs excel at optimizing simple array loops where indices are
+     * incrementing or decrementing over a valid slice, e.g.
+     *
+     * for (int i = start; i < end; i++) ... elements[i]
+     *
+     * Because in a circular array, elements are in general stored in
+     * two disjoint such slices, we help the VM by writing unusual
+     * nested loops for all traversals over the elements.  Having only
+     * one hot inner loop body instead of two or three eases human
+     * maintenance and encourages VM loop inlining into the caller.
+     */
+
     /**
      * The array in which the elements of the deque are stored.
-     * We guarantee that all array cells not holding deque elements
-     * are always null.
+     * All array cells not holding deque elements are always null.
+     * The array always has at least one null slot (at tail).
      */
     transient Object[] elements;
 
     /**
      * The index of the element at the head of the deque (which is the
      * element that would be removed by remove() or pop()); or an
-     * arbitrary number 0 <= head < elements.length if the deque is empty.
+     * arbitrary number 0 <= head < elements.length equal to tail if
+     * the deque is empty.
      */
     transient int head;
 
-    /** Number of elements in this collection. */
-    transient int size;
+    /**
+     * The index at which the next element would be added to the tail
+     * of the deque (via addLast(E), add(E), or push(E));
+     * elements[tail] is always null.
+     */
+    transient int tail;
 
     /**
      * The maximum size of array to allocate.
@@ -92,16 +110,16 @@ public class ArrayDeque<E> extends Abstr
      */
     private void grow(int needed) {
         // overflow-conscious code
-        // checkInvariants();
         final int oldCapacity = elements.length;
         int newCapacity;
-        // Double size if small; else grow by 50%
+        // Double capacity if small; else grow by 50%
         int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);
         if (jump < needed
             || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0)
             newCapacity = newCapacity(needed, jump);
         elements = Arrays.copyOf(elements, newCapacity);
-        if (oldCapacity - head < size) {
+        // Exceptionally, here tail == head needs to be disambiguated
+        if (tail < head || (tail == head && elements[head] != null)) {
             // wrap around; slide first leg forward to end of array
             int newSpace = newCapacity - oldCapacity;
             System.arraycopy(elements, head,
@@ -136,8 +154,9 @@ public class ArrayDeque<E> extends Abstr
      * @since TBD
      */
     /* public */ void ensureCapacity(int minCapacity) {
-        if (minCapacity > elements.length)
-            grow(minCapacity - elements.length);
+        int needed;
+        if ((needed = (minCapacity + 1 - elements.length)) > 0)
+            grow(needed);
         // checkInvariants();
     }
 
@@ -147,9 +166,11 @@ public class ArrayDeque<E> extends Abstr
      * @since TBD
      */
     /* public */ void trimToSize() {
-        if (size < elements.length) {
-            elements = toArray();
+        int size;
+        if ((size = size()) + 1 < elements.length) {
+            elements = toArray(new Object[size + 1]);
             head = 0;
+            tail = size;
         }
         // checkInvariants();
     }
@@ -169,7 +190,10 @@ public class ArrayDeque<E> extends Abstr
      * @param numElements lower bound on initial capacity of the deque
      */
     public ArrayDeque(int numElements) {
-        elements = new Object[numElements];
+        elements =
+            new Object[(numElements < 1) ? 1 :
+                       (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE :
+                       numElements + 1];
     }
 
     /**
@@ -183,15 +207,8 @@ public class ArrayDeque<E> extends Abstr
      * @throws NullPointerException if the specified collection is null
      */
     public ArrayDeque(Collection<? extends E> c) {
-        Object[] es = c.toArray();
-        // defend against c.toArray (incorrectly) not returning Object[]
-        // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
-        if (es.getClass() != Object[].class)
-            es = Arrays.copyOf(es, es.length, Object[].class);
-        for (Object obj : es)
-            Objects.requireNonNull(obj);
-        this.elements = es;
-        this.size = es.length;
+        this(c.size());
+        addAll(c);
     }
 
     /**
@@ -213,35 +230,40 @@ public class ArrayDeque<E> extends Abstr
     }
 
     /**
-     * Adds i and j, mod modulus.
-     * Precondition and postcondition: 0 <= i < modulus, 0 <= j <= modulus.
+     * Circularly adds the given distance to index i, mod modulus.
+     * Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
+     * @return index 0 <= i < modulus
      */
-    static final int add(int i, int j, int modulus) {
-        if ((i += j) - modulus >= 0) i -= modulus;
+    static final int add(int i, int distance, int modulus) {
+        if ((i += distance) - modulus >= 0) distance -= modulus;
         return i;
     }
 
     /**
-     * Returns the array index of the last element.
-     * May return invalid index -1 if there are no elements.
+     * Subtracts j from i, mod modulus.
+     * Index i must be logically ahead of index j.
+     * Precondition: 0 <= i < modulus, 0 <= j < modulus.
+     * @return the "circular distance" from j to i; corner case i == j
+     * is diambiguated to "empty", returning 0.
      */
-    final int tail() {
-        return add(head, size - 1, elements.length);
+    static final int sub(int i, int j, int modulus) {
+        if ((i -= j) < 0) i += modulus;
+        return i;
     }
 
     /**
      * Returns element at array index i.
+     * This is a slight abuse of generics, accepted by javac.
      */
     @SuppressWarnings("unchecked")
-    private E elementAt(int i) {
-        return (E) elements[i];
+    static final <E> E elementAt(Object[] es, int i) {
+        return (E) es[i];
     }
 
     /**
      * A version of elementAt that checks for null elements.
      * This check doesn't catch all possible comodifications,
-     * but does catch ones that corrupt traversal.  It's a little
-     * surprising that javac allows this abuse of generics.
+     * but does catch ones that corrupt traversal.
      */
     static final <E> E nonNullElementAt(Object[] es, int i) {
         @SuppressWarnings("unchecked") E e = (E) es[i];
@@ -261,18 +283,12 @@ public class ArrayDeque<E> extends Abstr
      * @throws NullPointerException if the specified element is null
      */
     public void addFirst(E e) {
-        // checkInvariants();
-        Objects.requireNonNull(e);
-        Object[] es;
-        int capacity, h;
-        final int s;
-        if ((s = size) == (capacity = (es = elements).length)) {
+        if (e == null)
+            throw new NullPointerException();
+        final Object[] es = elements;
+        es[head = dec(head, es.length)] = e;
+        if (head == tail)
             grow(1);
-            capacity = (es = elements).length;
-        }
-        if ((h = head - 1) < 0) h = capacity - 1;
-        es[head = h] = e;
-        size = s + 1;
         // checkInvariants();
     }
 
@@ -285,17 +301,12 @@ public class ArrayDeque<E> extends Abstr
      * @throws NullPointerException if the specified element is null
      */
     public void addLast(E e) {
-        // checkInvariants();
-        Objects.requireNonNull(e);
-        Object[] es;
-        int capacity;
-        final int s;
-        if ((s = size) == (capacity = (es = elements).length)) {
+        if (e == null)
+            throw new NullPointerException();
+        final Object[] es = elements;
+        es[tail] = e;
+        if (head == (tail = inc(tail, es.length)))
             grow(1);
-            capacity = (es = elements).length;
-        }
-        es[add(head, s, capacity)] = e;
-        size = s + 1;
         // checkInvariants();
     }
 
@@ -311,12 +322,12 @@ public class ArrayDeque<E> extends Abstr
      *         of its elements are null
      */
     public boolean addAll(Collection<? extends E> c) {
-        final int s = size, needed = c.size() - (elements.length - s);
-        if (needed > 0)
+        final int s, needed;
+        if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
             grow(needed);
-        c.forEach((e) -> addLast(e));
+        c.forEach(this::addLast);
         // checkInvariants();
-        return size > s;
+        return size() > s;
     }
 
     /**
@@ -347,10 +358,10 @@ public class ArrayDeque<E> extends Abstr
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E removeFirst() {
-        // checkInvariants();
         E e = pollFirst();
         if (e == null)
             throw new NoSuchElementException();
+        // checkInvariants();
         return e;
     }
 
@@ -358,37 +369,32 @@ public class ArrayDeque<E> extends Abstr
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E removeLast() {
-        // checkInvariants();
         E e = pollLast();
         if (e == null)
             throw new NoSuchElementException();
+        // checkInvariants();
         return e;
     }
 
     public E pollFirst() {
+        final Object[] es;
+        final int h;
+        E e = elementAt(es = elements, h = head);
+        if (e != null) {
+            es[h] = null;
+            head = inc(h, es.length);
+        }
         // checkInvariants();
-        int s, h;
-        if ((s = size) <= 0)
-            return null;
-        final Object[] es = elements;
-        @SuppressWarnings("unchecked") E e = (E) es[h = head];
-        es[h] = null;
-        if (++h >= es.length) h = 0;
-        head = h;
-        size = s - 1;
         return e;
     }
 
     public E pollLast() {
+        final Object[] es;
+        final int t;
+        E e = elementAt(es = elements, t = dec(tail, es.length));
+        if (e != null)
+            es[tail = t] = null;
         // checkInvariants();
-        final int s, tail;
-        if ((s = size) <= 0)
-            return null;
-        final Object[] es = elements;
-        @SuppressWarnings("unchecked")
-        E e = (E) es[tail = add(head, s - 1, es.length)];
-        es[tail] = null;
-        size = s - 1;
         return e;
     }
 
@@ -396,35 +402,34 @@ public class ArrayDeque<E> extends Abstr
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E getFirst() {
+        E e = elementAt(elements, head);
+        if (e == null)
+            throw new NoSuchElementException();
         // checkInvariants();
-        if (size <= 0) throw new NoSuchElementException();
-        return elementAt(head);
+        return e;
     }
 
     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
-    @SuppressWarnings("unchecked")
     public E getLast() {
-        // checkInvariants();
-        final int s;
-        if ((s = size) <= 0) throw new NoSuchElementException();
         final Object[] es = elements;
-        return (E) es[add(head, s - 1, es.length)];
+        E e = elementAt(es, dec(tail, es.length));
+        if (e == null)
+            throw new NoSuchElementException();
+        // checkInvariants();
+        return e;
     }
 
     public E peekFirst() {
         // checkInvariants();
-        return (size <= 0) ? null : elementAt(head);
+        return elementAt(elements, head);
     }
 
-    @SuppressWarnings("unchecked")
     public E peekLast() {
         // checkInvariants();
-        final int s;
-        if ((s = size) <= 0) return null;
-        final Object[] es = elements;
-        return (E) es[add(head, s - 1, es.length)];
+        final Object[] es;
+        return elementAt(es = elements, dec(tail, es.length));
     }
 
     /**
@@ -442,16 +447,14 @@ public class ArrayDeque<E> extends Abstr
     public boolean removeFirstOccurrence(Object o) {
         if (o != null) {
             final Object[] es = elements;
-            int i, end, to, todo;
-            todo = (end = (i = head) + size)
-                - (to = (es.length - end >= 0) ? end : es.length);
-            for (;; to = todo, i = 0, todo = 0) {
+            for (int i = head, end = tail, to = (i <= end) ? end : es.length;
+                 ; i = 0, to = end) {
                 for (; i < to; i++)
                     if (o.equals(es[i])) {
                         delete(i);
                         return true;
                     }
-                if (todo == 0) break;
+                if (to == end) break;
             }
         }
         return false;
@@ -472,15 +475,14 @@ public class ArrayDeque<E> extends Abstr
     public boolean removeLastOccurrence(Object o) {
         if (o != null) {
             final Object[] es = elements;
-            int i, to, end, todo;
-            todo = (to = ((end = (i = tail()) - size) >= -1) ? end : -1) - end;
-            for (;; to = (i = es.length - 1) - todo, todo = 0) {
-                for (; i > to; i--)
+            for (int i = tail, end = head, to = (i >= end) ? end : 0;
+                 ; i = es.length, to = end) {
+                for (i--; i > to - 1; i--)
                     if (o.equals(es[i])) {
                         delete(i);
                         return true;
                     }
-                if (todo == 0) break;
+                if (to == end) break;
             }
         }
         return false;
@@ -608,16 +610,16 @@ public class ArrayDeque<E> extends Abstr
      * <p>This method is called delete rather than remove to emphasize
      * that its semantics differ from those of {@link List#remove(int)}.
      *
-     * @return true if elements moved backwards
+     * @return true if elements near tail moved backwards
      */
     boolean delete(int i) {
         // checkInvariants();
         final Object[] es = elements;
         final int capacity = es.length;
         final int h = head;
-        int front;              // number of elements before to-be-deleted elt
-        if ((front = i - h) < 0) front += capacity;
-        final int back = size - front - 1; // number of elements after
+        // number of elements before to-be-deleted elt
+        final int front = sub(i, h, capacity);
+        final int back = size() - front - 1; // number of elements after
         if (front < back) {
             // move front elements forwards
             if (h <= i) {
@@ -628,13 +630,12 @@ public class ArrayDeque<E> extends Abstr
                 System.arraycopy(es, h, es, h + 1, front - (i + 1));
             }
             es[h] = null;
-            if ((head = (h + 1)) >= capacity) head = 0;
-            size--;
+            head = inc(h, capacity);
             // checkInvariants();
             return false;
         } else {
             // move back elements backwards
-            int tail = tail();
+            tail = dec(tail, capacity);
             if (i <= tail) {
                 System.arraycopy(es, i + 1, es, i, back);
             } else { // Wrap around
@@ -644,7 +645,6 @@ public class ArrayDeque<E> extends Abstr
                 System.arraycopy(es, 1, es, 0, back - firstLeg - 1);
             }
             es[tail] = null;
-            size--;
             // checkInvariants();
             return true;
         }
@@ -658,7 +658,7 @@ public class ArrayDeque<E> extends Abstr
      * @return the number of elements in this deque
      */
     public int size() {
-        return size;
+        return sub(tail, head, elements.length);
     }
 
     /**
@@ -667,7 +667,7 @@ public class ArrayDeque<E> extends Abstr
      * @return {@code true} if this deque contains no elements
      */
     public boolean isEmpty() {
-        return size == 0;
+        return head == tail;
     }
 
     /**
@@ -691,7 +691,7 @@ public class ArrayDeque<E> extends Abstr
         int cursor;
 
         /** Number of elements yet to be returned. */
-        int remaining = size;
+        int remaining = size();
 
         /**
          * Index of element returned by most recent call to next.
@@ -711,14 +711,14 @@ public class ArrayDeque<E> extends Abstr
             final Object[] es = elements;
             E e = nonNullElementAt(es, cursor);
             lastRet = cursor;
-            if (++cursor >= es.length) cursor = 0;
+            cursor = inc(cursor, es.length);
             remaining--;
             return e;
         }
 
         void postDelete(boolean leftShifted) {
             if (leftShifted)
-                if (--cursor < 0) cursor = elements.length - 1;
+                cursor = dec(cursor, elements.length);
         }
 
         public final void remove() {
@@ -730,18 +730,29 @@ public class ArrayDeque<E> extends Abstr
 
         public void forEachRemaining(Consumer<? super E> action) {
             Objects.requireNonNull(action);
-            final int k;
-            if ((k = remaining) > 0) {
-                remaining = 0;
-                ArrayDeque.forEachRemaining(action, elements, cursor, k);
-                if ((lastRet = cursor + k - 1) >= elements.length)
-                    lastRet -= elements.length;
+            int r;
+            if ((r = remaining) <= 0)
+                return;
+            remaining = 0;
+            final Object[] es = elements;
+            if (es[cursor] == null || sub(tail, cursor, es.length) != r)
+                throw new ConcurrentModificationException();
+            for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
+                 ; i = 0, to = end) {
+                for (; i < to; i++)
+                    action.accept(elementAt(es, i));
+                if (to == end) {
+                    if (end != tail)
+                        throw new ConcurrentModificationException();
+                    lastRet = dec(end, es.length);
+                    break;
+                }
             }
         }
     }
 
     private class DescendingIterator extends DeqIterator {
-        DescendingIterator() { cursor = tail(); }
+        DescendingIterator() { cursor = dec(tail, elements.length); }
 
         public final E next() {
             if (remaining <= 0)
@@ -749,31 +760,36 @@ public class ArrayDeque<E> extends Abstr
             final Object[] es = elements;
             E e = nonNullElementAt(es, cursor);
             lastRet = cursor;
-            if (--cursor < 0) cursor = es.length - 1;
+            cursor = dec(cursor, es.length);
             remaining--;
             return e;
         }
 
         void postDelete(boolean leftShifted) {
             if (!leftShifted)
-                if (++cursor >= elements.length) cursor = 0;
+                cursor = inc(cursor, elements.length);
         }
 
         public final void forEachRemaining(Consumer<? super E> action) {
             Objects.requireNonNull(action);
-            final int k;
-            if ((k = remaining) > 0) {
-                remaining = 0;
-                final Object[] es = elements;
-                int i, end, to, todo;
-                todo = (to = ((end = (i = cursor) - k) >= -1) ? end : -1) - end;
-                for (;; to = (i = es.length - 1) - todo, todo = 0) {
-                    for (; i > to; i--)
-                        action.accept(nonNullElementAt(es, i));
-                    if (todo == 0) break;
+            int r;
+            if ((r = remaining) <= 0)
+                return;
+            remaining = 0;
+            final Object[] es = elements;
+            if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
+                throw new ConcurrentModificationException();
+            for (int i = cursor, end = head, to = (i >= end) ? end : 0;
+                 ; i = es.length - 1, to = end) {
+                // hotspot generates faster code than for: i >= to !
+                for (; i > to - 1; i--)
+                    action.accept(elementAt(es, i));
+                if (to == end) {
+                    if (end != head)
+                        throw new ConcurrentModificationException();
+                    lastRet = end;
+                    break;
                 }
-                if ((lastRet = cursor - (k - 1)) < 0)
-                    lastRet += es.length;
             }
         }
     }
@@ -792,64 +808,75 @@ public class ArrayDeque<E> extends Abstr
      * @since 1.8
      */
     public Spliterator<E> spliterator() {
-        return new ArrayDequeSpliterator();
+        return new DeqSpliterator();
     }
 
-    final class ArrayDequeSpliterator implements Spliterator<E> {
-        private int cursor;
-        private int remaining; // -1 until late-binding first use
+    final class DeqSpliterator implements Spliterator<E> {
+        private int fence;      // -1 until first use
+        private int cursor;     // current index, modified on traverse/split
 
         /** Constructs late-binding spliterator over all elements. */
-        ArrayDequeSpliterator() {
-            this.remaining = -1;
+        DeqSpliterator() {
+            this.fence = -1;
         }
 
-        /** Constructs spliterator over the given slice. */
-        ArrayDequeSpliterator(int cursor, int count) {
-            this.cursor = cursor;
-            this.remaining = count;
+        /** Constructs spliterator over the given range. */
+        DeqSpliterator(int origin, int fence) {
+            this.cursor = origin;
+            this.fence = fence;
         }
 
-        /** Ensures late-binding initialization; then returns remaining. */
-        private int remaining() {
-            if (remaining < 0) {
+        /** Ensures late-binding initialization; then returns fence. */
+        private int getFence() { // force initialization
+            int t;
+            if ((t = fence) < 0) {
+                t = fence = tail;
                 cursor = head;
-                remaining = size;
             }
-            return remaining;
+            return t;
         }
 
-        public ArrayDequeSpliterator trySplit() {
-            final int mid;
-            if ((mid = remaining() >> 1) > 0) {
-                int oldCursor = cursor;
-                cursor = add(cursor, mid, elements.length);
-                remaining -= mid;
-                return new ArrayDequeSpliterator(oldCursor, mid);
-            }
-            return null;
+        public DeqSpliterator trySplit() {
+            final Object[] es = elements;
+            final int i, n;
+            return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
+                ? null
+                : new DeqSpliterator(i, cursor = add(i, n, es.length));
         }
 
         public void forEachRemaining(Consumer<? super E> action) {
-            Objects.requireNonNull(action);
-            final int k = remaining(); // side effect!
-            remaining = 0;
-            ArrayDeque.forEachRemaining(action, elements, cursor, k);
+            if (action == null)
+                throw new NullPointerException();
+            final int end = getFence(), cursor = this.cursor;
+            final Object[] es = elements;
+            if (cursor != end) {
+                this.cursor = end;
+                // null check at both ends of range is sufficient
+                if (es[cursor] == null || es[dec(end, es.length)] == null)
+                    throw new ConcurrentModificationException();
+                for (int i = cursor, to = (i <= end) ? end : es.length;
+                     ; i = 0, to = end) {
+                    for (; i < to; i++)
+                        action.accept(elementAt(es, i));
+                    if (to == end) break;
+                }
+            }
         }
 
         public boolean tryAdvance(Consumer<? super E> action) {
-            Objects.requireNonNull(action);
-            final int k;
-            if ((k = remaining()) <= 0)
+            if (action == null)
+                throw new NullPointerException();
+            final int t, i;
+            if ((t = getFence()) == (i = cursor))
                 return false;
-            action.accept(nonNullElementAt(elements, cursor));
-            if (++cursor >= elements.length) cursor = 0;
-            remaining = k - 1;
+            final Object[] es = elements;
+            cursor = inc(i, es.length);
+            action.accept(nonNullElementAt(es, i));
             return true;
         }
 
         public long estimateSize() {
-            return remaining();
+            return sub(getFence(), cursor, elements.length);
         }
 
         public int characteristics() {
@@ -860,56 +887,39 @@ public class ArrayDeque<E> extends Abstr
         }
     }
 
-    @SuppressWarnings("unchecked")
     public void forEach(Consumer<? super E> action) {
         Objects.requireNonNull(action);
         final Object[] es = elements;
-        int i, end, to, todo;
-        todo = (end = (i = head) + size)
-            - (to = (es.length - end >= 0) ? end : es.length);
-        for (;; to = todo, i = 0, todo = 0) {
+        for (int i = head, end = tail, to = (i <= end) ? end : es.length;
+             ; i = 0, to = end) {
             for (; i < to; i++)
-                action.accept((E) es[i]);
-            if (todo == 0) break;
+                action.accept(elementAt(es, i));
+            if (to == end) {
+                if (end != tail) throw new ConcurrentModificationException();
+                break;
+            }
         }
         // checkInvariants();
     }
 
     /**
-     * Calls action on remaining elements, starting at index i and
-     * traversing in ascending order.  A variant of forEach that also
-     * checks for concurrent modification, for use in iterators.
-     */
-    static <E> void forEachRemaining(
-        Consumer<? super E> action, Object[] es, int i, int remaining) {
-        int end, to, todo;
-        todo = (end = i + remaining)
-            - (to = (es.length - end >= 0) ? end : es.length);
-        for (;; to = todo, i = 0, todo = 0) {
-            for (; i < to; i++)
-                action.accept(nonNullElementAt(es, i));
-            if (todo == 0) break;
-        }
-    }
-
-    /**
      * Replaces each element of this deque with the result of applying the
      * operator to that element, as specified by {@link List#replaceAll}.
      *
      * @param operator the operator to apply to each element
      * @since TBD
      */
-    @SuppressWarnings("unchecked")
     /* public */ void replaceAll(UnaryOperator<E> operator) {
         Objects.requireNonNull(operator);
         final Object[] es = elements;
-        int i, end, to, todo;
-        todo = (end = (i = head) + size)
-            - (to = (es.length - end >= 0) ? end : es.length);
-        for (;; to = todo, i = 0, todo = 0) {
+        for (int i = head, end = tail, to = (i <= end) ? end : es.length;
+             ; i = 0, to = end) {
             for (; i < to; i++)
-                es[i] = operator.apply((E) es[i]);
-            if (todo == 0) break;
+                es[i] = operator.apply(elementAt(es, i));
+            if (to == end) {
+                if (end != tail) throw new ConcurrentModificationException();
+                break;
+            }
         }
         // checkInvariants();
     }
@@ -942,34 +952,76 @@ public class ArrayDeque<E> extends Abstr
     private boolean bulkRemove(Predicate<? super E> filter) {
         // checkInvariants();
         final Object[] es = elements;
+        // Optimize for initial run of survivors
+        for (int i = head, end = tail, to = (i <= end) ? end : es.length;
+             ; i = 0, to = end) {
+            for (; i < to; i++)
+                if (filter.test(elementAt(es, i)))
+                    return bulkRemoveModified(filter, i);
+            if (to == end) {
+                if (end != tail) throw new ConcurrentModificationException();
+                break;
+            }
+        }
+        return false;
+    }
+
+    // A tiny bit set implementation
+
+    private static long[] nBits(int n) {
+        return new long[((n - 1) >> 6) + 1];
+    }
+    private static void setBit(long[] bits, int i) {
+        bits[i >> 6] |= 1L << i;
+    }
+    private static boolean isClear(long[] bits, int i) {
+        return (bits[i >> 6] & (1L << i)) == 0;
+    }
+
+    /**
+     * Helper for bulkRemove, in case of at least one deletion.
+     * Tolerate predicates that reentrantly access the collection for
+     * read (but writers still get CME), so traverse once to find
+     * elements to delete, a second pass to physically expunge.
+     *
+     * @param beg valid index of first element to be deleted
+     */
+    private boolean bulkRemoveModified(
+        Predicate<? super E> filter, final int beg) {
+        final Object[] es = elements;
         final int capacity = es.length;
-        int i = head, j = i, remaining = size, deleted = 0;
-        try {
-            for (; remaining > 0; remaining--) {
-                @SuppressWarnings("unchecked") E e = (E) es[i];
-                if (filter.test(e))
-                    deleted++;
-                else {
-                    if (j != i)
-                        es[j] = e;
-                    if (++j >= capacity) j = 0;
-                }
-                if (++i >= capacity) i = 0;
+        final int end = tail;
+        final long[] deathRow = nBits(sub(end, beg, capacity));
+        deathRow[0] = 1L;   // set bit 0
+        for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
+             ; i = 0, to = end, k -= capacity) {
+            for (; i < to; i++)
+                if (filter.test(elementAt(es, i)))
+                    setBit(deathRow, i - k);
+            if (to == end) break;
+        }
+        // a two-finger traversal, with hare i reading, tortoise w writing
+        int w = beg;
+        for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
+             ; w = 0) { // w rejoins i on second leg
+            // In this loop, i and w are on the same leg, with i > w
+            for (; i < to; i++)
+                if (isClear(deathRow, i - k))
+                    es[w++] = es[i];
+            if (to == end) break;
+            // In this loop, w is on the first leg, i on the second
+            for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
+                if (isClear(deathRow, i - k))
+                    es[w++] = es[i];
+            if (i >= to) {
+                if (w == capacity) w = 0; // "corner" case
+                break;
             }
-            return deleted > 0;
-        } catch (Throwable ex) {
-            if (deleted > 0)
-                for (; remaining > 0; remaining--) {
-                    es[j] = es[i];
-                    if (++i >= capacity) i = 0;
-                    if (++j >= capacity) j = 0;
-                }
-            throw ex;
-        } finally {
-            size -= deleted;
-            clearSlice(es, j, deleted);
-            // checkInvariants();
         }
+        if (end != tail) throw new ConcurrentModificationException();
+        circularClear(es, tail = w, end);
+        // checkInvariants();
+        return true;
     }
 
     /**
@@ -983,14 +1035,12 @@ public class ArrayDeque<E> extends Abstr
     public boolean contains(Object o) {
         if (o != null) {
             final Object[] es = elements;
-            int i, end, to, todo;
-            todo = (end = (i = head) + size)
-                - (to = (es.length - end >= 0) ? end : es.length);
-            for (;; to = todo, i = 0, todo = 0) {
+            for (int i = head, end = tail, to = (i <= end) ? end : es.length;
+                 ; i = 0, to = end) {
                 for (; i < to; i++)
                     if (o.equals(es[i]))
                         return true;
-                if (todo == 0) break;
+                if (to == end) break;
             }
         }
         return false;
@@ -1018,21 +1068,19 @@ public class ArrayDeque<E> extends Abstr
      * The deque will be empty after this call returns.
      */
     public void clear() {
-        clearSlice(elements, head, size);
-        size = head = 0;
+        circularClear(elements, head, tail);
+        head = tail = 0;
         // checkInvariants();
     }
 
     /**
-     * Nulls out count elements, starting at array index i.
+     * Nulls out slots starting at array index i, upto index end.
      */
-    private static void clearSlice(Object[] es, int i, int count) {
-        int end, to, todo;
-        todo = (end = i + count)
-            - (to = (es.length - end >= 0) ? end : es.length);
-        for (;; to = todo, i = 0, todo = 0) {
+    private static void circularClear(Object[] es, int i, int end) {
+        for (int to = (i <= end) ? end : es.length;
+             ; i = 0, to = end) {
             Arrays.fill(es, i, to, null);
-            if (todo == 0) break;
+            if (to == end) break;
         }
     }
 
@@ -1055,18 +1103,18 @@ public class ArrayDeque<E> extends Abstr
 
     private <T> T[] toArray(Class<T[]> klazz) {
         final Object[] es = elements;
-        final int capacity = es.length;
-        final int head = this.head, end = head + size;
         final T[] a;
-        if (end >= 0) {
+        final int size = size(), head = this.head, end;
+        final int len = Math.min(size, es.length - head);
+        if ((end = head + size) >= 0) {
             a = Arrays.copyOfRange(es, head, end, klazz);
         } else {
             // integer overflow!
             a = Arrays.copyOfRange(es, 0, size, klazz);
-            System.arraycopy(es, head, a, 0, capacity - head);
+            System.arraycopy(es, head, a, 0, len);
         }
-        if (end - capacity > 0)
-            System.arraycopy(es, 0, a, capacity - head, end - capacity);
+        if (tail < head)
+            System.arraycopy(es, 0, a, len, tail);
         return a;
     }
 
@@ -1109,14 +1157,14 @@ public class ArrayDeque<E> extends Abstr
     @SuppressWarnings("unchecked")
     public <T> T[] toArray(T[] a) {
         final int size;
-        if ((size = this.size) > a.length)
+        if ((size = size()) > a.length)
             return toArray((Class<T[]>) a.getClass());
         final Object[] es = elements;
-        final int head = this.head, end = head + size;
-        final int front = (es.length - end >= 0) ? size : es.length - head;
-        System.arraycopy(es, head, a, 0, front);
-        if (front < size)
-            System.arraycopy(es, 0, a, front, size - front);
+        for (int i = head, j = 0, len = Math.min(size, es.length - i);
+             ; i = 0, len = tail) {
+            System.arraycopy(es, i, a, j, len);
+            if ((j += len) == size) break;
+        }
         if (size < a.length)
             a[size] = null;
         return a;
@@ -1156,17 +1204,15 @@ public class ArrayDeque<E> extends Abstr
         s.defaultWriteObject();
 
         // Write out size
-        s.writeInt(size);
+        s.writeInt(size());
 
         // Write out elements in order.
         final Object[] es = elements;
-        int i, end, to, todo;
-        todo = (end = (i = head) + size)
-            - (to = (es.length - end >= 0) ? end : es.length);
-        for (;; to = todo, i = 0, todo = 0) {
+        for (int i = head, end = tail, to = (i <= end) ? end : es.length;
+             ; i = 0, to = end) {
             for (; i < to; i++)
                 s.writeObject(es[i]);
-            if (todo == 0) break;
+            if (to == end) break;
         }
     }
 
@@ -1182,7 +1228,9 @@ public class ArrayDeque<E> extends Abstr
         s.defaultReadObject();
 
         // Read in size and allocate array
-        elements = new Object[size = s.readInt()];
+        int size = s.readInt();
+        elements = new Object[size + 1];
+        this.tail = size;
 
         // Read in all elements in the proper order.
         for (int i = 0; i < size; i++)
@@ -1191,18 +1239,20 @@ public class ArrayDeque<E> extends Abstr
 
     /** debugging */
     void checkInvariants() {
+        // Use head and tail fields with empty slot at tail strategy.
+        // head == tail disambiguates to "empty".
         try {
             int capacity = elements.length;
-            // assert size >= 0 && size <= capacity;
-            // assert head >= 0;
-            // assert capacity == 0 || head < capacity;
-            // assert size == 0 || elements[head] != null;
-            // assert size == 0 || elements[tail()] != null;
-            // assert size == capacity || elements[dec(head, capacity)] == null;
-            // assert size == capacity || elements[inc(tail(), capacity)] == null;
+            // assert head >= 0 && head < capacity;
+            // assert tail >= 0 && tail < capacity;
+            // assert capacity > 0;
+            // assert size() < capacity;
+            // assert head == tail || elements[head] != null;
+            // assert elements[tail] == null;
+            // assert head == tail || elements[dec(tail, capacity)] != null;
         } catch (Throwable t) {
-            System.err.printf("head=%d size=%d capacity=%d%n",
-                              head, size, elements.length);
+            System.err.printf("head=%d tail=%d capacity=%d%n",
+                              head, tail, elements.length);
             System.err.printf("elements=%s%n",
                               Arrays.toString(elements));
             throw t;