diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 9c96a6e99ff..f121058667c 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -136,6 +136,14 @@ New Features
   to return BytesRef[] instead of Term[].  (Robert Muir, Mike
   McCandless)
 
+Optimizations
+
+* LUCENE-4839: SorterTemplate.merge can now be overridden in order to replace
+  the default implementation which merges in-place by a faster implementation
+  that could require fewer swaps at the expense of some extra memory.
+  ArrayUtil and CollectionUtil override it so that their mergeSort and timSort
+  methods are faster but only require up to 1% of extra memory. (Adrien Grand)
+
 API Changes
 
 * LUCENE-4844: removed TaxonomyReader.getParent(), you should use
diff --git a/lucene/core/src/java/org/apache/lucene/util/ArrayUtil.java b/lucene/core/src/java/org/apache/lucene/util/ArrayUtil.java
index d31a2e43f33..f793e4ba14d 100644
--- a/lucene/core/src/java/org/apache/lucene/util/ArrayUtil.java
+++ b/lucene/core/src/java/org/apache/lucene/util/ArrayUtil.java
@@ -28,6 +28,11 @@ import java.util.Comparator;
 
 public final class ArrayUtil {
 
+  // affordable memory overhead to merge sorted arrays
+  static final float MERGE_OVERHEAD_RATIO = 0.01f;
+  // arrays below this size will always be sorted in-place
+  static final int MERGE_EXTRA_MEMORY_THRESHOLD = (int) (15 / MERGE_OVERHEAD_RATIO);
+
   private ArrayUtil() {} // no instance
 
   /*
@@ -604,65 +609,145 @@ public final class ArrayUtil {
 
     return result;
   }
-  
+
+  private static abstract class ArraySorterTemplate<T> extends SorterTemplate {
+
+    protected final T[] a;
+
+    ArraySorterTemplate(T[] a) {
+      this.a = a;
+    }
+
+    protected abstract int compare(T a, T b);
+
+    @Override
+    protected void swap(int i, int j) {
+      final T o = a[i];
+      a[i] = a[j];
+      a[j] = o;
+    }
+
+    @Override
+    protected int compare(int i, int j) {
+      return compare(a[i], a[j]);
+    }
+
+    @Override
+    protected void setPivot(int i) {
+      pivot = a[i];
+    }
+
+    @Override
+    protected int comparePivot(int j) {
+      return compare(pivot, a[j]);
+    }
+
+    private T pivot;
+
+  }
+
+  // a template for merge-based sorts which uses extra memory to speed up merging
+  private static abstract class ArrayMergeSorterTemplate<T> extends ArraySorterTemplate<T> {
+
+    private final int threshold; // maximum length of a merge that can be made using extra memory
+    private final T[] tmp;
+
+    ArrayMergeSorterTemplate(T[] a, float overheadRatio) {
+      super(a);
+      this.threshold = (int) (a.length * overheadRatio);
+      @SuppressWarnings("unchecked")
+      final T[] tmpBuf = (T[]) new Object[threshold];
+      this.tmp = tmpBuf;
+    }
+
+    private void mergeWithExtraMemory(int lo, int pivot, int hi, int len1, int len2) {
+      System.arraycopy(a, lo, tmp, 0, len1);
+      int i = 0, j = pivot, dest = lo;
+      while (i < len1 && j < hi) {
+        if (compare(tmp[i], a[j]) <= 0) {
+          a[dest++] = tmp[i++];
+        } else {
+          a[dest++] = a[j++];
+        }
+      }
+      while (i < len1) {
+        a[dest++] = tmp[i++];
+      }
+      while (j < hi) {
+        a[dest++] = a[j++];
+      }
+      assert dest == hi;
+    }
+
+    @Override
+    protected void merge(int lo, int pivot, int hi, int len1, int len2) {
+      if (len1 <= threshold) {
+        mergeWithExtraMemory(lo, pivot, hi, len1, len2);
+      } else {
+        // since this method recurses to run merge on smaller arrays, it will
+        // end up using mergeWithExtraMemory
+        super.merge(lo, pivot, hi, len1, len2);
+      }
+    }
+
+  }
+
   /** SorterTemplate with custom {@link Comparator} */
   private static <T> SorterTemplate getSorter(final T[] a, final Comparator<? super T> comp) {
-    return new SorterTemplate() {
-      @Override
-      protected void swap(int i, int j) {
-        final T o = a[i];
-        a[i] = a[j];
-        a[j] = o;
-      }
-      
-      @Override
-      protected int compare(int i, int j) {
-        return comp.compare(a[i], a[j]);
-      }
+    return new ArraySorterTemplate<T>(a) {
 
       @Override
-      protected void setPivot(int i) {
-        pivot = a[i];
+      protected int compare(T a, T b) {
+        return comp.compare(a, b);
       }
-  
-      @Override
-      protected int comparePivot(int j) {
-        return comp.compare(pivot, a[j]);
-      }
-      
-      private T pivot;
+
     };
   }
-  
+
   /** Natural SorterTemplate */
   private static <T extends Comparable<? super T>> SorterTemplate getSorter(final T[] a) {
-    return new SorterTemplate() {
-      @Override
-      protected void swap(int i, int j) {
-        final T o = a[i];
-        a[i] = a[j];
-        a[j] = o;
-      }
-      
-      @Override
-      protected int compare(int i, int j) {
-        return a[i].compareTo(a[j]);
-      }
+    return new ArraySorterTemplate<T>(a) {
 
       @Override
-      protected void setPivot(int i) {
-        pivot = a[i];
+      protected int compare(T a, T b) {
+        return a.compareTo(b);
       }
-  
-      @Override
-      protected int comparePivot(int j) {
-        return pivot.compareTo(a[j]);
-      }
-      
-      private T pivot;
+
     };
   }
 
+  /** SorterTemplate with custom {@link Comparator} for merge-based sorts. */
+  private static <T> SorterTemplate getMergeSorter(final T[] a, final Comparator<? super T> comp) {
+    if (a.length < MERGE_EXTRA_MEMORY_THRESHOLD) {
+      return getSorter(a, comp);
+    } else {
+      return new ArrayMergeSorterTemplate<T>(a, MERGE_OVERHEAD_RATIO) {
+
+        @Override
+        protected int compare(T a, T b) {
+          return comp.compare(a, b);
+        }
+
+      };
+    }
+  }
+
+  /** Natural SorterTemplate for merge-based sorts. */
+  private static <T extends Comparable<? super T>> SorterTemplate getMergeSorter(final T[] a) {
+    if (a.length < MERGE_EXTRA_MEMORY_THRESHOLD) {
+      return getSorter(a);
+    } else {
+      return new ArrayMergeSorterTemplate<T>(a, MERGE_OVERHEAD_RATIO) {
+
+        @Override
+        protected int compare(T a, T b) {
+          return a.compareTo(b);
+        }
+
+      };
+    }
+  }
+
   // quickSorts (endindex is exclusive!):
   
   /**
@@ -714,7 +799,7 @@ public final class ArrayUtil {
   public static <T> void mergeSort(T[] a, int fromIndex, int toIndex, Comparator<? super T> comp) {
     if (toIndex-fromIndex <= 1) return;
     //System.out.println("SORT: " + (toIndex-fromIndex));
-    getSorter(a, comp).mergeSort(fromIndex, toIndex-1);
+    getMergeSorter(a, comp).mergeSort(fromIndex, toIndex-1);
   }
   
   /**
@@ -733,7 +818,7 @@ public final class ArrayUtil {
    */
   public static <T extends Comparable<? super T>> void mergeSort(T[] a, int fromIndex, int toIndex) {
     if (toIndex-fromIndex <= 1) return;
-    getSorter(a).mergeSort(fromIndex, toIndex-1);
+    getMergeSorter(a).mergeSort(fromIndex, toIndex-1);
   }
   
   /**
@@ -754,7 +839,7 @@ public final class ArrayUtil {
    */
   public static <T> void timSort(T[] a, int fromIndex, int toIndex, Comparator<? super T> comp) {
     if (toIndex-fromIndex <= 1) return;
-    getSorter(a, comp).timSort(fromIndex, toIndex-1);
+    getMergeSorter(a, comp).timSort(fromIndex, toIndex-1);
   }
   
   /**
@@ -773,7 +858,7 @@ public final class ArrayUtil {
    */
   public static <T extends Comparable<? super T>> void timSort(T[] a, int fromIndex, int toIndex) {
     if (toIndex-fromIndex <= 1) return;
-    getSorter(a).timSort(fromIndex, toIndex-1);
+    getMergeSorter(a).timSort(fromIndex, toIndex-1);
   }
   
   /**
diff --git a/lucene/core/src/java/org/apache/lucene/util/CollectionUtil.java b/lucene/core/src/java/org/apache/lucene/util/CollectionUtil.java
index 486a5a365d5..36d4252be6d 100644
--- a/lucene/core/src/java/org/apache/lucene/util/CollectionUtil.java
+++ b/lucene/core/src/java/org/apache/lucene/util/CollectionUtil.java
@@ -17,8 +17,11 @@ package org.apache.lucene.util;
  * limitations under the License.
  */
 
-import java.util.Comparator;
+import static org.apache.lucene.util.ArrayUtil.MERGE_EXTRA_MEMORY_THRESHOLD;
+import static org.apache.lucene.util.ArrayUtil.MERGE_OVERHEAD_RATIO;
+
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.List;
 import java.util.RandomAccess;
 
@@ -34,33 +37,100 @@ import java.util.RandomAccess;
 public final class CollectionUtil {
 
   private CollectionUtil() {} // no instance
-  
+
+  private static abstract class ListSorterTemplate<T> extends SorterTemplate {
+
+    protected final List<T> list;
+
+    ListSorterTemplate(List<T> list) {
+      this.list = list;
+    }
+
+    protected abstract int compare(T a, T b);
+
+    @Override
+    protected void swap(int i, int j) {
+      Collections.swap(list, i, j);
+    }
+
+    @Override
+    protected int compare(int i, int j) {
+      return compare(list.get(i), list.get(j));
+    }
+
+    @Override
+    protected void setPivot(int i) {
+      pivot = list.get(i);
+    }
+
+    @Override
+    protected int comparePivot(int j) {
+      return compare(pivot, list.get(j));
+    }
+
+    private T pivot;
+
+  }
+
+  // a template for merge-based sorts which uses extra memory to speed up merging
+  private static abstract class ListMergeSorterTemplate<T> extends ListSorterTemplate<T> {
+
+    private final int threshold; // maximum length of a merge that can be made using extra memory
+    private final T[] tmp;
+
+    ListMergeSorterTemplate(List<T> list, float overheadRatio) {
+      super(list);
+      this.threshold = (int) (list.size() * overheadRatio);
+      @SuppressWarnings("unchecked")
+      final T[] tmpBuf = (T[]) new Object[threshold];
+      this.tmp = tmpBuf;
+    }
+
+    private void mergeWithExtraMemory(int lo, int pivot, int hi, int len1, int len2) {
+      for (int i = 0; i < len1; ++i) {
+        tmp[i] = list.get(lo + i);
+      }
+      int i = 0, j = pivot, dest = lo;
+      while (i < len1 && j < hi) {
+        if (compare(tmp[i], list.get(j)) <= 0) {
+          list.set(dest++, tmp[i++]);
+        } else {
+          list.set(dest++, list.get(j++));
+        }
+      }
+      while (i < len1) {
+        list.set(dest++, tmp[i++]);
+      }
+      while (j < hi) {
+        list.set(dest++, list.get(j++));
+      }
+      assert dest == hi;
+    }
+
+    @Override
+    protected void merge(int lo, int pivot, int hi, int len1, int len2) {
+      if (len1 <= threshold) {
+        mergeWithExtraMemory(lo, pivot, hi, len1, len2);
+      } else {
+        // since this method recurses to run merge on smaller arrays, it will
+        // end up using mergeWithExtraMemory
+        super.merge(lo, pivot, hi, len1, len2);
+      }
+    }
+
+  }
+
   /** SorterTemplate with custom {@link Comparator} */
   private static <T> SorterTemplate getSorter(final List<T> list, final Comparator<? super T> comp) {
     if (!(list instanceof RandomAccess))
       throw new IllegalArgumentException("CollectionUtil can only sort random access lists in-place.");
-    return new SorterTemplate() {
-      @Override
-      protected void swap(int i, int j) {
-        Collections.swap(list, i, j);
-      }
-      
-      @Override
-      protected int compare(int i, int j) {
-        return comp.compare(list.get(i), list.get(j));
-      }
+    return new ListSorterTemplate<T>(list) {
 
       @Override
-      protected void setPivot(int i) {
-        pivot = list.get(i);
+      protected int compare(T a, T b) {
+        return comp.compare(a, b);
       }
-  
-      @Override
-      protected int comparePivot(int j) {
-        return comp.compare(pivot, list.get(j));
-      }
-      
-      private T pivot;
+
     };
   }
   
@@ -68,31 +138,52 @@ public final class CollectionUtil {
   private static <T extends Comparable<? super T>> SorterTemplate getSorter(final List<T> list) {
     if (!(list instanceof RandomAccess))
       throw new IllegalArgumentException("CollectionUtil can only sort random access lists in-place.");
-    return new SorterTemplate() {
-      @Override
-      protected void swap(int i, int j) {
-        Collections.swap(list, i, j);
-      }
-      
-      @Override
-      protected int compare(int i, int j) {
-        return list.get(i).compareTo(list.get(j));
-      }
+    return new ListSorterTemplate<T>(list) {
 
       @Override
-      protected void setPivot(int i) {
-        pivot = list.get(i);
+      protected int compare(T a, T b) {
+        return a.compareTo(b);
       }
-  
-      @Override
-      protected int comparePivot(int j) {
-        return pivot.compareTo(list.get(j));
-      }
-      
-      private T pivot;
+
     };
   }
 
+  /** SorterTemplate with custom {@link Comparator} for merge-based sorts. */
+  private static <T> SorterTemplate getMergeSorter(final List<T> list, final Comparator<? super T> comp) {
+    if (!(list instanceof RandomAccess))
+      throw new IllegalArgumentException("CollectionUtil can only sort random access lists in-place.");
+    if (list.size() < MERGE_EXTRA_MEMORY_THRESHOLD) {
+      return getSorter(list, comp);
+    } else {
+      return new ListMergeSorterTemplate<T>(list, MERGE_OVERHEAD_RATIO) {
+
+        @Override
+        protected int compare(T a, T b) {
+          return comp.compare(a, b);
+        }
+
+      };
+    }
+  }
+  
+  /** Natural SorterTemplate for merge-based sorts. */
+  private static <T extends Comparable<? super T>> SorterTemplate getMergeSorter(final List<T> list) {
+    if (!(list instanceof RandomAccess))
+      throw new IllegalArgumentException("CollectionUtil can only sort random access lists in-place.");
+    if (list.size() < MERGE_EXTRA_MEMORY_THRESHOLD) {
+      return getSorter(list);
+    } else {
+      return new ListMergeSorterTemplate<T>(list, MERGE_OVERHEAD_RATIO) {
+
+        @Override
+        protected int compare(T a, T b) {
+          return a.compareTo(b);
+        }
+
+      };
+    }
+  }
+
   /**
    * Sorts the given random access {@link List} using the {@link Comparator}.
    * The list must implement {@link RandomAccess}. This method uses the quick sort
@@ -128,7 +219,7 @@ public final class CollectionUtil {
   public static <T> void mergeSort(List<T> list, Comparator<? super T> comp) {
     final int size = list.size();
     if (size <= 1) return;
-    getSorter(list, comp).mergeSort(0, size-1);
+    getMergeSorter(list, comp).mergeSort(0, size-1);
   }
   
   /**
@@ -140,7 +231,7 @@ public final class CollectionUtil {
   public static <T extends Comparable<? super T>> void mergeSort(List<T> list) {
     final int size = list.size();
     if (size <= 1) return;
-    getSorter(list).mergeSort(0, size-1);
+    getMergeSorter(list).mergeSort(0, size-1);
   }
 
   // timSorts:
@@ -154,7 +245,7 @@ public final class CollectionUtil {
   public static <T> void timSort(List<T> list, Comparator<? super T> comp) {
     final int size = list.size();
     if (size <= 1) return;
-    getSorter(list, comp).timSort(0, size-1);
+    getMergeSorter(list, comp).timSort(0, size-1);
   }
   
   /**
@@ -166,7 +257,7 @@ public final class CollectionUtil {
   public static <T extends Comparable<? super T>> void timSort(List<T> list) {
     final int size = list.size();
     if (size <= 1) return;
-    getSorter(list).timSort(0, size-1);
+    getMergeSorter(list).timSort(0, size-1);
   }
 
   // insertionSorts:
diff --git a/lucene/core/src/java/org/apache/lucene/util/SorterTemplate.java b/lucene/core/src/java/org/apache/lucene/util/SorterTemplate.java
index 55024bb5977..b74342435ad 100644
--- a/lucene/core/src/java/org/apache/lucene/util/SorterTemplate.java
+++ b/lucene/core/src/java/org/apache/lucene/util/SorterTemplate.java
@@ -343,8 +343,10 @@ public abstract class SorterTemplate {
     merge(lo, mid, hi, mid - lo, hi - mid);
   }
 
-  // pkg-protected for access from TimSort class
-  void merge(int lo, int pivot, int hi, int len1, int len2) {
+  /** Merge the slices [lo-pivot[ (of length len1) and [pivot-hi[ (of length
+   *  len2) which are already sorted. This method merges in-place but can be
+   *  extended to provide a faster implementation using extra memory. */
+  protected void merge(int lo, int pivot, int hi, int len1, int len2) {
     if (len1 == 0 || len2 == 0) {
       return;
     }
diff --git a/lucene/core/src/test/org/apache/lucene/util/TestArrayUtil.java b/lucene/core/src/test/org/apache/lucene/util/TestArrayUtil.java
index 6b9d0a476be..f59af0e06dc 100644
--- a/lucene/core/src/test/org/apache/lucene/util/TestArrayUtil.java
+++ b/lucene/core/src/test/org/apache/lucene/util/TestArrayUtil.java
@@ -131,12 +131,12 @@ public class TestArrayUtil extends LuceneTestCase {
   public void testQuickSort() {
     int num = atLeast(50);
     for (int i = 0; i < num; i++) {
-      Integer[] a1 = createRandomArray(1000), a2 = a1.clone();
+      Integer[] a1 = createRandomArray(2000), a2 = a1.clone();
       ArrayUtil.quickSort(a1);
       Arrays.sort(a2);
       assertArrayEquals(a2, a1);
       
-      a1 = createRandomArray(1000);
+      a1 = createRandomArray(2000);
       a2 = a1.clone();
       ArrayUtil.quickSort(a1, Collections.reverseOrder());
       Arrays.sort(a2, Collections.reverseOrder());
@@ -171,12 +171,12 @@ public class TestArrayUtil extends LuceneTestCase {
   public void testMergeSort() {
     int num = atLeast(50);
     for (int i = 0; i < num; i++) {
-      Integer[] a1 = createRandomArray(1000), a2 = a1.clone();
+      Integer[] a1 = createRandomArray(2000), a2 = a1.clone();
       ArrayUtil.mergeSort(a1);
       Arrays.sort(a2);
       assertArrayEquals(a2, a1);
       
-      a1 = createRandomArray(1000);
+      a1 = createRandomArray(2000);
       a2 = a1.clone();
       ArrayUtil.mergeSort(a1, Collections.reverseOrder());
       Arrays.sort(a2, Collections.reverseOrder());
@@ -191,12 +191,12 @@ public class TestArrayUtil extends LuceneTestCase {
   public void testTimSort() {
     int num = atLeast(65);
     for (int i = 0; i < num; i++) {
-      Integer[] a1 = createRandomArray(1000), a2 = a1.clone();
+      Integer[] a1 = createRandomArray(2000), a2 = a1.clone();
       ArrayUtil.timSort(a1);
       Arrays.sort(a2);
       assertArrayEquals(a2, a1);
       
-      a1 = createRandomArray(1000);
+      a1 = createRandomArray(2000);
       a2 = a1.clone();
       ArrayUtil.timSort(a1, Collections.reverseOrder());
       Arrays.sort(a2, Collections.reverseOrder());
diff --git a/lucene/core/src/test/org/apache/lucene/util/TestCollectionUtil.java b/lucene/core/src/test/org/apache/lucene/util/TestCollectionUtil.java
index a3cdc3bebd2..c45130f5b4b 100644
--- a/lucene/core/src/test/org/apache/lucene/util/TestCollectionUtil.java
+++ b/lucene/core/src/test/org/apache/lucene/util/TestCollectionUtil.java
@@ -37,12 +37,12 @@ public class TestCollectionUtil extends LuceneTestCase {
   
   public void testQuickSort() {
     for (int i = 0, c = atLeast(500); i < c; i++) {
-      List<Integer> list1 = createRandomList(1000), list2 = new ArrayList<Integer>(list1);
+      List<Integer> list1 = createRandomList(2000), list2 = new ArrayList<Integer>(list1);
       CollectionUtil.quickSort(list1);
       Collections.sort(list2);
       assertEquals(list2, list1);
       
-      list1 = createRandomList(1000);
+      list1 = createRandomList(2000);
       list2 = new ArrayList<Integer>(list1);
       CollectionUtil.quickSort(list1, Collections.reverseOrder());
       Collections.sort(list2, Collections.reverseOrder());
@@ -56,12 +56,12 @@ public class TestCollectionUtil extends LuceneTestCase {
   
   public void testMergeSort() {
     for (int i = 0, c = atLeast(500); i < c; i++) {
-      List<Integer> list1 = createRandomList(1000), list2 = new ArrayList<Integer>(list1);
+      List<Integer> list1 = createRandomList(2000), list2 = new ArrayList<Integer>(list1);
       CollectionUtil.mergeSort(list1);
       Collections.sort(list2);
       assertEquals(list2, list1);
       
-      list1 = createRandomList(1000);
+      list1 = createRandomList(2000);
       list2 = new ArrayList<Integer>(list1);
       CollectionUtil.mergeSort(list1, Collections.reverseOrder());
       Collections.sort(list2, Collections.reverseOrder());
@@ -75,12 +75,12 @@ public class TestCollectionUtil extends LuceneTestCase {
 
   public void testTimSort() {
     for (int i = 0, c = atLeast(500); i < c; i++) {
-      List<Integer> list1 = createRandomList(1000), list2 = new ArrayList<Integer>(list1);
+      List<Integer> list1 = createRandomList(2000), list2 = new ArrayList<Integer>(list1);
       CollectionUtil.timSort(list1);
       Collections.sort(list2);
       assertEquals(list2, list1);
       
-      list1 = createRandomList(1000);
+      list1 = createRandomList(2000);
       list2 = new ArrayList<Integer>(list1);
       CollectionUtil.timSort(list1, Collections.reverseOrder());
       Collections.sort(list2, Collections.reverseOrder());