LUCENE-7299: Speed up BytesRefHash.sort.

lucene/CHANGES.txt

@@ -86,6 +86,8 @@ Optimizations
   (which is used by TermsQuery, multi-term queries and several point queries).
   (Adrien Grand, Jeff Wartes, David Smiley)
 
+* LUCENE-7299: Speed up BytesRefHash.sort(). (Adrien Grand)
+
 Bug Fixes
 
 * LUCENE-7127: Fix corner case bugs in GeoPointDistanceQuery. (Robert Muir)

lucene/core/src/java/org/apache/lucene/util/BytesRefHash.java

@@ -158,40 +158,23 @@ public final class BytesRefHash {
    */
   public int[] sort() {
     final int[] compact = compact();
-    new IntroSorter() {
+    new StringMSBRadixSorter() {
+
+      BytesRef scratch = new BytesRef();
+
       @Override
       protected void swap(int i, int j) {
-        final int o = compact[i];
+        int tmp = compact[i];
         compact[i] = compact[j];
-        compact[j] = o;
+        compact[j] = tmp;
       }
 
       @Override
-      protected int compare(int i, int j) {
-        final int id1 = compact[i], id2 = compact[j];
-        assert bytesStart.length > id1 && bytesStart.length > id2;
-        pool.setBytesRef(scratch1, bytesStart[id1]);
-        pool.setBytesRef(scratch2, bytesStart[id2]);
-        return scratch1.compareTo(scratch2);
+      protected BytesRef get(int i) {
+        pool.setBytesRef(scratch, bytesStart[compact[i]]);
+        return scratch;
       }
 
-      @Override
-      protected void setPivot(int i) {
-        final int id = compact[i];
-        assert bytesStart.length > id;
-        pool.setBytesRef(pivot, bytesStart[id]);
-      }
-  
-      @Override
-      protected int comparePivot(int j) {
-        final int id = compact[j];
-        assert bytesStart.length > id;
-        pool.setBytesRef(scratch2, bytesStart[id]);
-        return pivot.compareTo(scratch2);
-      }
-      
-      private final BytesRef pivot = new BytesRef(),
-        scratch1 = new BytesRef(), scratch2 = new BytesRef();
     }.sort(0, count);
     return compact;
   }

lucene/core/src/java/org/apache/lucene/util/StringMSBRadixSorter.java

@@ -0,0 +1,187 @@
+/*
+ * 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.lucene.util;
+
+import java.util.Arrays;
+
+/** Radix sorter for variable-length strings. This class sorts based on the most
+ *  significant byte first and falls back to {@link IntroSorter} when the size
+ *  of the buckets to sort becomes small. It is <b>NOT</b> stable.
+ *  Worst-case memory usage is about {@code 2.3 KB} */
+abstract class StringMSBRadixSorter extends Sorter {
+
+  // after that many levels of recursion we fall back to introsort anyway
+  private static final int LEVEL_THRESHOLD = 8;
+  // size of histograms: 256 + 1 to indicate that the string is finished
+  private static final int HISTOGRAM_SIZE = 257;
+  // buckets below this size will be sorted with introsort
+  private static final int LENGTH_THRESHOLD = 100;
+
+  // we store one histogram per recursion level
+  private final int[][] histograms = new int[LEVEL_THRESHOLD][];
+  private final int[] endOffsets = new int[HISTOGRAM_SIZE];
+
+  /** Get a {@link BytesRef} for the given index. */
+  protected abstract BytesRef get(int i);
+
+  /** Store bytes for the given index into {@code dest}, without the first k bytes. */
+  private void get(int i, int k, BytesRef dest) {
+    BytesRef ref = get(i);
+    assert ref.length >= k;
+    dest.bytes = ref.bytes;
+    dest.offset = ref.offset + k;
+    dest.length = ref.length - k;
+  }
+
+  @Override
+  protected final int compare(int i, int j) {
+    throw new UnsupportedOperationException("unused: not a comparison-based sort");
+  }
+
+  @Override
+  public void sort(int from, int to) {
+    checkRange(from, to);
+    sort(from, to, 0);
+  }
+
+  private void sort(int from, int to, int k) {
+    if (to - from <= LENGTH_THRESHOLD || k >= LEVEL_THRESHOLD) {
+      introSort(from, to, k);
+    } else {
+      radixSort(from, to, k);
+    }
+  }
+
+  private void introSort(int from, int to, int k) {
+    new IntroSorter() {
+      @Override
+      protected void swap(int i, int j) {
+        StringMSBRadixSorter.this.swap(i, j);
+      }
+
+      @Override
+      protected int compare(int i, int j) {
+        get(i, k, scratch1);
+        get(j, k, scratch2);
+        return scratch1.compareTo(scratch2);
+      }
+
+      @Override
+      protected void setPivot(int i) {
+        get(i, k, pivot);
+      }
+
+      @Override
+      protected int comparePivot(int j) {
+        get(j, k, scratch2);
+        return pivot.compareTo(scratch2);
+      }
+
+      private final BytesRef pivot = new BytesRef(),
+        scratch1 = new BytesRef(), scratch2 = new BytesRef();
+    }.sort(from, to);
+  }
+
+  private void radixSort(int from, int to, int k) {
+    int[] histogram = histograms[k];
+    if (histogram == null) {
+      histogram = histograms[k] = new int[HISTOGRAM_SIZE];
+    } else {
+      Arrays.fill(histogram, 0);
+    }
+
+    buildHistogram(from, to, k, histogram);
+
+    // short-circuit: if all keys have the same byte at offset k, then recurse directly
+    for (int i = 0; i < HISTOGRAM_SIZE; ++i) {
+      if (histogram[i] == to - from) {
+        // everything is in the same bucket, recurse
+        if (i > 0) {
+          sort(from, to, k + 1);
+        }
+        return;
+      } else if (histogram[i] != 0) {
+        break;
+      }
+    }
+
+    int[] startOffsets = histogram;
+    int[] endOffsets = this.endOffsets;
+    sumHistogram(histogram, endOffsets);
+    reorder(from, to, startOffsets, endOffsets, k);
+    endOffsets = startOffsets;
+
+    // recurse on all but the first bucket since all keys are equals in this
+    // bucket (we already compared all bytes)
+    for (int prev = endOffsets[0], i = 1; i < HISTOGRAM_SIZE; ++i) {
+      int h = endOffsets[i];
+      final int bucketLen = h - prev;
+      if (bucketLen > 1) {
+        sort(from + prev, from + h, k + 1);
+      }
+      prev = h;
+    }
+  }
+
+  /** Return a number for the k-th character between 0 and {@link #HISTOGRAM_SIZE}. */
+  private int getBucket(int id, int k) {
+    BytesRef ref = get(id);
+    if (ref.length <= k) {
+      return 0;
+    }
+    final int b = ref.bytes[ref.offset + k] & 0xff;
+    return b + 1;
+  }
+
+  /** Build a histogram of the number of values per {@link #getBucket(int, int) bucket}. */
+  private int[] buildHistogram(int from, int to, int k, int[] histogram) {
+    for (int i = from; i < to; ++i) {
+      histogram[getBucket(i, k)]++;
+    }
+    return histogram;
+  }
+
+  /** Accumulate values of the histogram so that it does not store counts but
+   *  start offsets. {@code endOffsets} will store the end offsets. */
+  private static void sumHistogram(int[] histogram, int[] endOffsets) {
+    int accum = 0;
+    for (int i = 0; i < HISTOGRAM_SIZE; ++i) {
+      final int count = histogram[i];
+      histogram[i] = accum;
+      accum += count;
+      endOffsets[i] = accum;
+    }
+  }
+
+  /**
+   * Reorder based on start/end offsets for each bucket. When this method
+   * returns, startOffsets and endOffsets are equal.
+   * @param startOffsets start offsets per bucket
+   * @param endOffsets end offsets per bucket
+   */
+  private void reorder(int from, int to, int[] startOffsets, int[] endOffsets, int k) {
+    // reorder in place, like the dutch flag problem
+    for (int i = 0; i < HISTOGRAM_SIZE; ++i) {
+      final int limit = endOffsets[i];
+      for (int h1 = startOffsets[i]; h1 < limit; h1 = startOffsets[i]) {
+        final int b = getBucket(from + h1, k);
+        final int h2 = startOffsets[b]++;
+        swap(from + h1, from + h2);
+      }
+    }
+  }
+}

lucene/core/src/test/org/apache/lucene/util/TestStringMSBRadixSorter.java

@@ -0,0 +1,97 @@
+/*
+ * 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.lucene.util;
+
+import java.util.Arrays;
+
+public class TestStringMSBRadixSorter extends LuceneTestCase {
+
+  private void test(BytesRef[] refs, int len) {
+    BytesRef[] expected = Arrays.copyOf(refs, len);
+    Arrays.sort(expected);
+
+    new StringMSBRadixSorter() {
+
+      @Override
+      protected BytesRef get(int i) {
+        return refs[i];
+      }
+
+      @Override
+      protected void swap(int i, int j) {
+        BytesRef tmp = refs[i];
+        refs[i] = refs[j];
+        refs[j] = tmp;
+      }
+    }.sort(0, len);
+    BytesRef[] actual = Arrays.copyOf(refs, len);
+    assertArrayEquals(expected, actual);
+  }
+
+  public void testEmpty() {
+    test(new BytesRef[random().nextInt(5)], 0);
+  }
+
+  public void testOneValue() {
+    BytesRef bytes = new BytesRef(TestUtil.randomSimpleString(random()));
+    test(new BytesRef[] { bytes }, 1);
+  }
+
+  public void testTwoValues() {
+    BytesRef bytes1 = new BytesRef(TestUtil.randomSimpleString(random()));
+    BytesRef bytes2 = new BytesRef(TestUtil.randomSimpleString(random()));
+    test(new BytesRef[] { bytes1, bytes2 }, 2);
+  }
+
+  private void testRandom(int commonPrefixLen, int maxLen) {
+    byte[] commonPrefix = new byte[commonPrefixLen];
+    random().nextBytes(commonPrefix);
+    final int len = random().nextInt(100000);
+    BytesRef[] bytes = new BytesRef[len + random().nextInt(50)];
+    for (int i = 0; i < len; ++i) {
+      byte[] b = new byte[commonPrefixLen + random().nextInt(maxLen)];
+      random().nextBytes(b);
+      System.arraycopy(commonPrefix, 0, b, 0, commonPrefixLen);
+      bytes[i] = new BytesRef(b);
+    }
+    test(bytes, len);
+  }
+
+  public void testRandom() {
+    for (int iter = 0; iter < 10; ++iter) {
+      testRandom(0, 10);
+    }
+  }
+
+  public void testRandomWithLotsOfDuplicates() {
+    for (int iter = 0; iter < 10; ++iter) {
+      testRandom(0, 2);
+    }
+  }
+
+  public void testRandomWithSharedPrefix() {
+    for (int iter = 0; iter < 10; ++iter) {
+      testRandom(TestUtil.nextInt(random(), 1, 30), 10);
+    }
+  }
+
+  public void testRandomWithSharedPrefixAndLotsOfDuplicates() {
+    for (int iter = 0; iter < 10; ++iter) {
+      testRandom(TestUtil.nextInt(random(), 1, 30), 2);
+    }
+  }
+}