LUCENE-6766: implement STRING sort, using segment-local ordinals

lucene/core/src/java/org/apache/lucene/index/MergeState.java

@@ -213,9 +213,7 @@ public class MergeState {
     //System.out.println("MergeState.maybeSortReaders indexSort=" + indexSort);
 
     for (CodecReader leaf : originalReaders) {
-      if (leaf instanceof SegmentReader) {
+      Sort segmentSort = leaf.getIndexSort();
-        SegmentReader segmentReader = (SegmentReader) leaf;
-        Sort segmentSort = segmentReader.getSegmentInfo().info.getIndexSort();
       //System.out.println("  leaf=" + leaf + " sort=" + segmentSort);
 
       if (segmentSort == null) {
@@ -239,9 +237,6 @@ public class MergeState {
       } else if (segmentSort.equals(indexSort) == false) {
         throw new IllegalArgumentException("index sort mismatch: merged segment has sort=" + indexSort + " but to-be-merged segment has sort=" + segmentSort);
       }
-      } else {
-        throw new IllegalArgumentException("cannot sort index with foreign readers; leaf=" + leaf);
-      }
 
       readers.add(leaf);
     }

lucene/core/src/java/org/apache/lucene/index/MultiSorter.java

@@ -123,7 +123,47 @@ final class MultiSorter {
 
   private static CrossReaderComparator getComparator(List<CodecReader> readers, SortField sortField) throws IOException {
     switch(sortField.getType()) {
-    // ncommit: use segment-local ords for string sort
+
+    case STRING:
+      {
+        // this uses the efficient segment-local ordinal map:
+        MultiReader multiReader = new MultiReader(readers.toArray(new LeafReader[readers.size()]));
+        final SortedDocValues sorted = MultiDocValues.getSortedValues(multiReader, sortField.getField());
+        final int[] docStarts = new int[readers.size()];
+        List<LeafReaderContext> leaves = multiReader.leaves();
+        for(int i=0;i<readers.size();i++) {
+          docStarts[i] = leaves.get(i).docBase;
+        }
+        final int missingOrd;
+        if (sortField.getMissingValue() == SortField.STRING_LAST) {
+          missingOrd = Integer.MIN_VALUE;
+        } else {
+          missingOrd = Integer.MAX_VALUE;
+        }
+
+        final int reverseMul;
+        if (sortField.getReverse()) {
+          reverseMul = -1;
+        } else {
+          reverseMul = 1;
+        }
+
+        return new CrossReaderComparator() {
+          @Override
+          public int compare(int readerIndexA, int docIDA, int readerIndexB, int docIDB) {
+            int ordA = sorted.getOrd(docStarts[readerIndexA] + docIDA);
+            if (ordA == -1) {
+              ordA = missingOrd;
+            }
+            int ordB = sorted.getOrd(docStarts[readerIndexB] + docIDB);
+            if (ordB == -1) {
+              ordB = missingOrd;
+            }
+            return reverseMul * Integer.compare(ordA, ordB);
+          }
+        };
+      }
+
     case INT:
       {
         List<NumericDocValues> values = new ArrayList<>();

lucene/core/src/test/org/apache/lucene/index/TestIndexSorting.java

@@ -79,6 +79,40 @@ import org.junit.BeforeClass;
 
 public class TestIndexSorting extends LuceneTestCase {
 
+  public void testBasicString() throws Exception {
+    Directory dir = newDirectory();
+    IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+    Sort indexSort = new Sort(new SortField("foo", SortField.Type.STRING));
+    iwc.setIndexSort(indexSort);
+    IndexWriter w = new IndexWriter(dir, iwc);
+    Document doc = new Document();
+    doc.add(new SortedDocValuesField("foo", new BytesRef("zzz")));
+    w.addDocument(doc);
+    // so we get more than one segment, so that forceMerge actually does merge, since we only get a sorted segment by merging:
+    w.commit();
+
+    doc = new Document();
+    doc.add(new SortedDocValuesField("foo", new BytesRef("aaa")));
+    w.addDocument(doc);
+    w.commit();
+
+    doc = new Document();
+    doc.add(new SortedDocValuesField("foo", new BytesRef("mmm")));
+    w.addDocument(doc);
+    w.forceMerge(1);
+
+    DirectoryReader r = DirectoryReader.open(w);
+    LeafReader leaf = getOnlyLeafReader(r);
+    assertEquals(3, leaf.maxDoc());
+    SortedDocValues values = leaf.getSortedDocValues("foo");
+    assertEquals("aaa", values.get(0).utf8ToString());
+    assertEquals("mmm", values.get(1).utf8ToString());
+    assertEquals("zzz", values.get(2).utf8ToString());
+    r.close();
+    w.close();
+    dir.close();
+  }
+
   public void testSortOnMerge(boolean withDeletes) throws IOException {
     Directory dir = newDirectory();
     IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));

lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java

@@ -146,6 +146,7 @@ public class MockRandomMergePolicy extends MergePolicy {
 
     @Override
     public CodecReader wrapForMerge(CodecReader reader) throws IOException {
+
       // wrap it (e.g. prevent bulk merge etc)
       // TODO: cut this over to FilterCodecReader api, we can explicitly
       // enable/disable bulk merge for portions of the index we want.