LUCENE-3423: add Terms.getDocCount statistic

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1169452 13f79535-47bb-0310-9956-ffa450edef68

lucene/CHANGES.txt

@@ -492,6 +492,9 @@ New features
 * LUCENE-3376: ReusableAnalyzerBase has been moved from modules/analysis/common
   into lucene/src/java/org/apache/lucene/analysis (Chris Male)
 
+* LUCENE-3423: add Terms.getDocCount(), which returns the number of documents
+  that have at least one term for a field.  (Yonik Seeley, Robert Muir)
+
 Optimizations
 
 * LUCENE-2588: Don't store unnecessary suffixes when writing the terms

lucene/contrib/instantiated/src/java/org/apache/lucene/store/instantiated/InstantiatedIndexReader.java

@@ -401,6 +401,12 @@ public class InstantiatedIndexReader extends IndexReader {
             return -1;
           }
 
+          // TODO: support this?
+          @Override
+          public int getDocCount() throws IOException {
+            return -1;
+          }
+
           @Override
           public Comparator<BytesRef> getComparator() {
             return BytesRef.getUTF8SortedAsUnicodeComparator();

lucene/contrib/memory/src/java/org/apache/lucene/index/memory/MemoryIndex.java

@@ -846,6 +846,13 @@ public class MemoryIndex {
                 // each term has df=1
                 return info.sortedTerms.length;
               }
+
+              @Override
+              public int getDocCount() throws IOException {
+                return info.sortedTerms.length > 0 ? 1 : 0;
+              }
+              
+              
             };
           }
         }

lucene/src/java/org/apache/lucene/index/CheckIndex.java

@@ -45,6 +45,7 @@ import org.apache.lucene.index.values.ValuesEnum;
 import org.apache.lucene.store.FSDirectory;
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.FixedBitSet;
 import org.apache.lucene.util.StringHelper;
 
 /**
@@ -705,6 +706,7 @@ public class CheckIndex {
 
         long sumTotalTermFreq = 0;
         long sumDocFreq = 0;
+        FixedBitSet visitedDocs = new FixedBitSet(reader.maxDoc());
         while(true) {
 
           final BytesRef term = terms.next();
@@ -766,6 +768,7 @@ public class CheckIndex {
             if (doc == DocIdSetIterator.NO_MORE_DOCS) {
               break;
             }
+            visitedDocs.set(doc);
             final int freq = docs2.freq();
             status.totPos += freq;
             totalTermFreq += freq;
@@ -810,6 +813,7 @@ public class CheckIndex {
             docCount = 0;
             totalTermFreq = 0;
             while(docsNoDel.nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
+              visitedDocs.set(docsNoDel.docID());
               docCount++;
               totalTermFreq += docsNoDel.freq();
             }
@@ -919,6 +923,13 @@ public class CheckIndex {
               throw new RuntimeException("sumDocFreq for field " + field + "=" + v + " != recomputed sumDocFreq=" + sumDocFreq);
             }
           }
+        
+        if (fieldTerms != null) {
+          final int v = fieldTerms.getDocCount();
+          if (v != -1 && visitedDocs.cardinality() != v) {
+            throw new RuntimeException("docCount for field " + field + "=" + v + " != recomputed docCount=" + visitedDocs.cardinality());
+          }
+        }
 
           // Test seek to last term:
           if (lastTerm != null) {

lucene/src/java/org/apache/lucene/index/FilterIndexReader.java

@@ -109,6 +109,11 @@ public class FilterIndexReader extends IndexReader {
     public long getSumDocFreq() throws IOException {
       return in.getSumDocFreq();
     }
+
+    @Override
+    public int getDocCount() throws IOException {
+      return in.getDocCount();
+    }
   }
 
   /** Base class for filtering {@link TermsEnum} implementations. */

lucene/src/java/org/apache/lucene/index/FreqProxTermsWriterPerField.java

@@ -29,6 +29,7 @@ import org.apache.lucene.index.codecs.TermStats;
 import org.apache.lucene.index.codecs.TermsConsumer;
 import org.apache.lucene.util.BitVector;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.FixedBitSet;
 import org.apache.lucene.util.RamUsageEstimator;
 
 // TODO: break into separate freq and prox writers as
@@ -261,6 +262,7 @@ final class FreqProxTermsWriterPerField extends TermsHashConsumerPerField implem
     final ByteSliceReader freq = new ByteSliceReader();
     final ByteSliceReader prox = new ByteSliceReader();
 
+    FixedBitSet visitedDocs = new FixedBitSet(state.numDocs);
     long sumTotalTermFreq = 0;
     long sumDocFreq = 0;
 
@@ -347,6 +349,7 @@ final class FreqProxTermsWriterPerField extends TermsHashConsumerPerField implem
         // passes, ie first sweep marks all del docs, and
         // 2nd sweep does the real flush, but I suspect
         // that'd add too much time to flush.
+        visitedDocs.set(docID);
         postingsConsumer.startDoc(docID, termDocFreq);
         if (docID < delDocLimit) {
           // Mark it deleted.  TODO: we could also skip
@@ -408,7 +411,7 @@ final class FreqProxTermsWriterPerField extends TermsHashConsumerPerField implem
       sumDocFreq += numDocs;
     }
 
-    termsConsumer.finish(sumTotalTermFreq, sumDocFreq);
+    termsConsumer.finish(sumTotalTermFreq, sumDocFreq, visitedDocs.cardinality());
   }
 
 }

lucene/src/java/org/apache/lucene/index/MultiTerms.java

@@ -120,6 +120,19 @@ public final class MultiTerms extends Terms {
     }
     return sum;
   }
+  
+  @Override
+  public int getDocCount() throws IOException {
+    int sum = 0;
+    for(Terms terms : subs) {
+      final int v = terms.getDocCount();
+      if (v == -1) {
+        return -1;
+      }
+      sum += v;
+    }
+    return sum;
+  }
 
   @Override
   public Comparator<BytesRef> getComparator() {

lucene/src/java/org/apache/lucene/index/Terms.java

@@ -174,6 +174,13 @@ public abstract class Terms {
    *  into account. */
   public abstract long getSumDocFreq() throws IOException;
 
+  /** Returns the number of documents that have at least one
+   *  term for this field, or -1 if this measure isn't
+   *  stored by the codec.  Note that, just like other term
+   *  measures, this measure does not take deleted documents
+   *  into account. */
+  public abstract int getDocCount() throws IOException;
+  
   /**
    * Returns a thread-private {@link TermsEnum} instance. Obtaining
    * {@link TermsEnum} from this method might be more efficient than using

lucene/src/java/org/apache/lucene/index/codecs/BlockTermsReader.java

@@ -139,8 +139,9 @@ public class BlockTermsReader extends FieldsProducer {
         final FieldInfo fieldInfo = fieldInfos.fieldInfo(field);
         final long sumTotalTermFreq = fieldInfo.indexOptions == IndexOptions.DOCS_ONLY ? -1 : in.readVLong();
         final long sumDocFreq = in.readVLong();
+        final int docCount = in.readVInt();
         assert !fields.containsKey(fieldInfo.name);
-        fields.put(fieldInfo.name, new FieldReader(fieldInfo, numTerms, termsStartPointer, sumTotalTermFreq, sumDocFreq));
+        fields.put(fieldInfo.name, new FieldReader(fieldInfo, numTerms, termsStartPointer, sumTotalTermFreq, sumDocFreq, docCount));
       }
       success = true;
     } finally {
@@ -243,14 +244,16 @@ public class BlockTermsReader extends FieldsProducer {
     final long termsStartPointer;
     final long sumTotalTermFreq;
     final long sumDocFreq;
+    final int docCount;
 
-    FieldReader(FieldInfo fieldInfo, long numTerms, long termsStartPointer, long sumTotalTermFreq, long sumDocFreq) {
+    FieldReader(FieldInfo fieldInfo, long numTerms, long termsStartPointer, long sumTotalTermFreq, long sumDocFreq, int docCount) {
       assert numTerms > 0;
       this.fieldInfo = fieldInfo;
       this.numTerms = numTerms;
       this.termsStartPointer = termsStartPointer;
       this.sumTotalTermFreq = sumTotalTermFreq;
       this.sumDocFreq = sumDocFreq;
+      this.docCount = docCount;
     }
 
     @Override
@@ -283,6 +286,11 @@ public class BlockTermsReader extends FieldsProducer {
       return sumDocFreq;
     }
 
+    @Override
+    public int getDocCount() throws IOException {
+      return docCount;
+    }
+
     // Iterates through terms in this field
     private final class SegmentTermsEnum extends TermsEnum {
       private final IndexInput in;

lucene/src/java/org/apache/lucene/index/codecs/BlockTermsWriter.java

@@ -134,6 +134,7 @@ public class BlockTermsWriter extends FieldsConsumer {
             out.writeVLong(field.sumTotalTermFreq);
           }
           out.writeVLong(field.sumDocFreq);
+          out.writeVInt(field.docCount);
         }
       }
       writeTrailer(dirStart);
@@ -160,6 +161,7 @@ public class BlockTermsWriter extends FieldsConsumer {
     private final TermsIndexWriterBase.FieldWriter fieldIndexWriter;
     long sumTotalTermFreq;
     long sumDocFreq;
+    int docCount;
 
     private TermEntry[] pendingTerms;
 
@@ -234,7 +236,7 @@ public class BlockTermsWriter extends FieldsConsumer {
 
     // Finishes all terms in this field
     @Override
-    public void finish(long sumTotalTermFreq, long sumDocFreq) throws IOException {
+    public void finish(long sumTotalTermFreq, long sumDocFreq, int docCount) throws IOException {
       if (pendingCount > 0) {
         flushBlock();
       }
@@ -243,6 +245,7 @@ public class BlockTermsWriter extends FieldsConsumer {
 
       this.sumTotalTermFreq = sumTotalTermFreq;
       this.sumDocFreq = sumDocFreq;
+      this.docCount = docCount;
       fieldIndexWriter.finish(out.getFilePointer());
     }
 

lucene/src/java/org/apache/lucene/index/codecs/BlockTreeTermsReader.java

@@ -153,9 +153,10 @@ public class BlockTreeTermsReader extends FieldsProducer {
         assert fieldInfo != null: "field=" + field;
         final long sumTotalTermFreq = fieldInfo.indexOptions == IndexOptions.DOCS_ONLY ? -1 : in.readVLong();
         final long sumDocFreq = in.readVLong();
+        final int docCount = in.readVInt();
         final long indexStartFP = indexDivisor != -1 ? indexIn.readVLong() : 0;
         assert !fields.containsKey(fieldInfo.name);
-        fields.put(fieldInfo.name, new FieldReader(fieldInfo, numTerms, rootCode, sumTotalTermFreq, sumDocFreq, indexStartFP, indexIn));
+        fields.put(fieldInfo.name, new FieldReader(fieldInfo, numTerms, rootCode, sumTotalTermFreq, sumDocFreq, docCount, indexStartFP, indexIn));
       }
       success = true;
     } finally {
@@ -399,6 +400,7 @@ public class BlockTreeTermsReader extends FieldsProducer {
     final FieldInfo fieldInfo;
     final long sumTotalTermFreq;
     final long sumDocFreq;
+    final int docCount;
     final long indexStartFP;
     final long rootBlockFP;
     final BytesRef rootCode;
@@ -406,13 +408,14 @@ public class BlockTreeTermsReader extends FieldsProducer {
 
     //private boolean DEBUG;
 
-    FieldReader(FieldInfo fieldInfo, long numTerms, BytesRef rootCode, long sumTotalTermFreq, long sumDocFreq, long indexStartFP, IndexInput indexIn) throws IOException {
+    FieldReader(FieldInfo fieldInfo, long numTerms, BytesRef rootCode, long sumTotalTermFreq, long sumDocFreq, int docCount, long indexStartFP, IndexInput indexIn) throws IOException {
       assert numTerms > 0;
       this.fieldInfo = fieldInfo;
       //DEBUG = BlockTreeTermsReader.DEBUG && fieldInfo.name.equals("id");
       this.numTerms = numTerms;
       this.sumTotalTermFreq = sumTotalTermFreq; 
       this.sumDocFreq = sumDocFreq; 
+      this.docCount = docCount;
       this.indexStartFP = indexStartFP;
       this.rootCode = rootCode;
       // if (DEBUG) {
@@ -475,6 +478,11 @@ public class BlockTreeTermsReader extends FieldsProducer {
       return sumDocFreq;
     }
 
+    @Override
+    public int getDocCount() throws IOException {
+      return docCount;
+    }
+
     @Override
     public TermsEnum intersect(CompiledAutomaton compiled, BytesRef startTerm) throws IOException {
       if (compiled.type != CompiledAutomaton.AUTOMATON_TYPE.NORMAL) {

lucene/src/java/org/apache/lucene/index/codecs/BlockTreeTermsWriter.java

@@ -350,6 +350,7 @@ public class BlockTreeTermsWriter extends FieldsConsumer {
     private long numTerms;
     long sumTotalTermFreq;
     long sumDocFreq;
+    int docCount;
     long indexStartFP;
 
     // Used only to partition terms into the block tree; we
@@ -866,7 +867,7 @@ public class BlockTreeTermsWriter extends FieldsConsumer {
 
     // Finishes all terms in this field
     @Override
-    public void finish(long sumTotalTermFreq, long sumDocFreq) throws IOException {
+    public void finish(long sumTotalTermFreq, long sumDocFreq, int docCount) throws IOException {
       if (numTerms > 0) {
         blockBuilder.finish();
 
@@ -878,6 +879,7 @@ public class BlockTreeTermsWriter extends FieldsConsumer {
 
         this.sumTotalTermFreq = sumTotalTermFreq;
         this.sumDocFreq = sumDocFreq;
+        this.docCount = docCount;
 
         // Write FST to index
         indexStartFP = indexOut.getFilePointer();
@@ -929,6 +931,7 @@ public class BlockTreeTermsWriter extends FieldsConsumer {
             out.writeVLong(field.sumTotalTermFreq);
           }
           out.writeVLong(field.sumDocFreq);
+          out.writeVInt(field.docCount);
           indexOut.writeVLong(field.indexStartFP);
         }
       }

lucene/src/java/org/apache/lucene/index/codecs/PostingsConsumer.java

@@ -24,6 +24,7 @@ import org.apache.lucene.index.DocsEnum;
 import org.apache.lucene.index.FieldInfo.IndexOptions;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.FixedBitSet;
 
 /**
  * @lucene.experimental
@@ -56,7 +57,7 @@ public abstract class PostingsConsumer {
 
   /** Default merge impl: append documents, mapping around
    *  deletes */
-  public TermStats merge(final MergeState mergeState, final DocsEnum postings) throws IOException {
+  public TermStats merge(final MergeState mergeState, final DocsEnum postings, final FixedBitSet visitedDocs) throws IOException {
 
     int df = 0;
     long totTF = 0;
@@ -67,6 +68,7 @@ public abstract class PostingsConsumer {
         if (doc == DocIdSetIterator.NO_MORE_DOCS) {
           break;
         }
+        visitedDocs.set(doc);
         final int freq = postings.freq();
         this.startDoc(doc, freq);
         this.finishDoc();
@@ -80,6 +82,7 @@ public abstract class PostingsConsumer {
         if (doc == DocIdSetIterator.NO_MORE_DOCS) {
           break;
         }
+        visitedDocs.set(doc);
         final int freq = postingsEnum.freq();
         this.startDoc(doc, freq);
         totTF += freq;

lucene/src/java/org/apache/lucene/index/codecs/TermsConsumer.java

@@ -26,6 +26,7 @@ import org.apache.lucene.index.MultiDocsEnum;
 import org.apache.lucene.index.MultiDocsAndPositionsEnum;
 
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.FixedBitSet;
 
 /**
  * @lucene.experimental
@@ -42,7 +43,7 @@ public abstract class TermsConsumer {
   public abstract void finishTerm(BytesRef text, TermStats stats) throws IOException;
 
   /** Called when we are done adding terms to this field */
-  public abstract void finish(long sumTotalTermFreq, long sumDocFreq) throws IOException;
+  public abstract void finish(long sumTotalTermFreq, long sumDocFreq, int docCount) throws IOException;
 
   /** Return the BytesRef Comparator used to sort terms
    *  before feeding to this API. */
@@ -59,6 +60,7 @@ public abstract class TermsConsumer {
     long sumTotalTermFreq = 0;
     long sumDocFreq = 0;
     long sumDFsinceLastAbortCheck = 0;
+    FixedBitSet visitedDocs = new FixedBitSet(mergeState.mergedDocCount);
 
     if (mergeState.fieldInfo.indexOptions != IndexOptions.DOCS_AND_FREQS_AND_POSITIONS) {
       if (docsEnum == null) {
@@ -75,7 +77,7 @@ public abstract class TermsConsumer {
         if (docsEnumIn != null) {
           docsEnum.reset(docsEnumIn);
           final PostingsConsumer postingsConsumer = startTerm(term);
-          final TermStats stats = postingsConsumer.merge(mergeState, docsEnum);
+          final TermStats stats = postingsConsumer.merge(mergeState, docsEnum, visitedDocs);
           if (stats.docFreq > 0) {
             finishTerm(term, stats);
             sumTotalTermFreq += stats.totalTermFreq;
@@ -109,7 +111,7 @@ public abstract class TermsConsumer {
             }
           }
           final PostingsConsumer postingsConsumer = startTerm(term);
-          final TermStats stats = postingsConsumer.merge(mergeState, postingsEnum);
+          final TermStats stats = postingsConsumer.merge(mergeState, postingsEnum, visitedDocs);
           if (stats.docFreq > 0) {
             finishTerm(term, stats);
             sumTotalTermFreq += stats.totalTermFreq;
@@ -124,6 +126,6 @@ public abstract class TermsConsumer {
       }
     }
 
-    finish(sumTotalTermFreq, sumDocFreq);
+    finish(sumTotalTermFreq, sumDocFreq, visitedDocs.cardinality());
   }
 }

lucene/src/java/org/apache/lucene/index/codecs/memory/MemoryCodec.java

@@ -220,7 +220,7 @@ public class MemoryCodec extends Codec {
     }
 
     @Override
-    public void finish(long sumTotalTermFreq, long sumDocFreq) throws IOException {
+    public void finish(long sumTotalTermFreq, long sumDocFreq, int docCount) throws IOException {
       if (termCount > 0) {
         out.writeVInt(termCount);
         out.writeVInt(field.number);
@@ -228,6 +228,7 @@ public class MemoryCodec extends Codec {
           out.writeVLong(sumTotalTermFreq);
         }
         out.writeVLong(sumDocFreq);
+        out.writeVInt(docCount);
         builder.finish().save(out);
         if (VERBOSE) System.out.println("finish field=" + field.name + " fp=" + out.getFilePointer());
       }
@@ -682,6 +683,7 @@ public class MemoryCodec extends Codec {
 
     private final long sumTotalTermFreq;
     private final long sumDocFreq;
+    private final int docCount;
     private FST<BytesRef> fst;
     private final ByteSequenceOutputs outputs = ByteSequenceOutputs.getSingleton();
     private final FieldInfo field;
@@ -695,6 +697,7 @@ public class MemoryCodec extends Codec {
         sumTotalTermFreq = -1;
       }
       sumDocFreq = in.readVLong();
+      docCount = in.readVInt();
       
       fst = new FST<BytesRef>(in, outputs);
     }
@@ -709,6 +712,11 @@ public class MemoryCodec extends Codec {
       return sumDocFreq;
     }
 
+    @Override
+    public int getDocCount() throws IOException {
+      return docCount;
+    }
+
     @Override
     public TermsEnum iterator() {
       return new FSTTermsEnum(field, fst);

lucene/src/java/org/apache/lucene/index/codecs/preflex/PreFlexFields.java

@@ -249,6 +249,11 @@ public class PreFlexFields extends FieldsProducer {
     public long getSumDocFreq() throws IOException {
       return -1;
     }
+
+    @Override
+    public int getDocCount() throws IOException {
+      return -1;
+    }
   }
 
   private class PreTermsEnum extends TermsEnum {

lucene/src/java/org/apache/lucene/index/codecs/simpletext/SimpleTextFieldsReader.java

@@ -31,6 +31,7 @@ import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.CharsRef;
+import org.apache.lucene.util.OpenBitSet;
 import org.apache.lucene.util.UnicodeUtil;
 import org.apache.lucene.util.fst.Builder;
 import org.apache.lucene.util.fst.BytesRefFSTEnum;
@@ -472,9 +473,11 @@ class SimpleTextFieldsReader extends FieldsProducer {
     private final IndexOptions indexOptions;
     private long sumTotalTermFreq;
     private long sumDocFreq;
+    private int docCount;
     private FST<PairOutputs.Pair<Long,PairOutputs.Pair<Long,Long>>> fst;
     private int termCount;
     private final BytesRef scratch = new BytesRef(10);
+    private final CharsRef scratchUTF16 = new CharsRef(10);
 
     public SimpleTextTerms(String field, long termsStart) throws IOException {
       this.termsStart = termsStart;
@@ -494,6 +497,7 @@ class SimpleTextFieldsReader extends FieldsProducer {
       long lastDocsStart = -1;
       int docFreq = 0;
       long totalTermFreq = 0;
+      OpenBitSet visitedDocs = new OpenBitSet();
       while(true) {
         readLine(in, scratch);
         if (scratch.equals(END) || scratch.startsWith(FIELD)) {
@@ -507,6 +511,9 @@ class SimpleTextFieldsReader extends FieldsProducer {
         } else if (scratch.startsWith(DOC)) {
           docFreq++;
           sumDocFreq++;
+          UnicodeUtil.UTF8toUTF16(scratch.bytes, scratch.offset+DOC.length, scratch.length-DOC.length, scratchUTF16);
+          int docID = ArrayUtil.parseInt(scratchUTF16.chars, 0, scratchUTF16.length);
+          visitedDocs.set(docID);
         } else if (scratch.startsWith(POS)) {
           totalTermFreq++;
         } else if (scratch.startsWith(TERM)) {
@@ -528,6 +535,7 @@ class SimpleTextFieldsReader extends FieldsProducer {
           termCount++;
         }
       }
+      docCount = (int) visitedDocs.cardinality();
       fst = b.finish();
       /*
       PrintStream ps = new PrintStream("out.dot");
@@ -566,6 +574,11 @@ class SimpleTextFieldsReader extends FieldsProducer {
     public long getSumDocFreq() throws IOException {
       return sumDocFreq;
     }
+
+    @Override
+    public int getDocCount() throws IOException {
+      return docCount;
+    }
   }
 
   @Override

lucene/src/java/org/apache/lucene/index/codecs/simpletext/SimpleTextFieldsWriter.java

@@ -95,7 +95,7 @@ class SimpleTextFieldsWriter extends FieldsConsumer {
     }
 
     @Override
-    public void finish(long sumTotalTermFreq, long sumDocFreq) throws IOException {
+    public void finish(long sumTotalTermFreq, long sumDocFreq, int docCount) throws IOException {
     }
 
     @Override

lucene/src/test-framework/org/apache/lucene/index/codecs/preflexrw/PreFlexFieldsWriter.java

@@ -196,7 +196,7 @@ class PreFlexFieldsWriter extends FieldsConsumer {
     }
 
     @Override
-    public void finish(long sumTotalTermCount, long sumDocFreq) throws IOException {
+    public void finish(long sumTotalTermCount, long sumDocFreq, int docCount) throws IOException {
     }
 
     @Override

lucene/src/test/org/apache/lucene/TestExternalCodecs.java

@@ -99,6 +99,7 @@ public class TestExternalCodecs extends LuceneTestCase {
       final SortedMap<String,RAMTerm> termToDocs = new TreeMap<String,RAMTerm>();
       long sumTotalTermFreq;
       long sumDocFreq;
+      int docCount;
 
       RAMField(String field) {
         this.field = field;
@@ -118,6 +119,11 @@ public class TestExternalCodecs extends LuceneTestCase {
       public long getSumDocFreq() throws IOException {
         return sumDocFreq;
       }
+      
+      @Override
+      public int getDocCount() throws IOException {
+        return docCount;
+      }
 
       @Override
       public TermsEnum iterator() {
@@ -206,9 +212,10 @@ public class TestExternalCodecs extends LuceneTestCase {
       }
 
       @Override
-      public void finish(long sumTotalTermFreq, long sumDocFreq) {
+      public void finish(long sumTotalTermFreq, long sumDocFreq, int docCount) {
         field.sumTotalTermFreq = sumTotalTermFreq;
         field.sumDocFreq = sumDocFreq;
+        field.docCount = docCount;
       }
     }
 

lucene/src/test/org/apache/lucene/index/TestCodecs.java

@@ -42,6 +42,7 @@ import org.apache.lucene.search.ScoreDoc;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.LuceneTestCase;
+import org.apache.lucene.util.OpenBitSet;
 import org.apache.lucene.util.Version;
 import org.apache.lucene.util._TestUtil;
 import org.junit.BeforeClass;
@@ -108,11 +109,15 @@ public class TestCodecs extends LuceneTestCase {
       final TermsConsumer termsConsumer = consumer.addField(fieldInfo);
       long sumTotalTermCount = 0;
       long sumDF = 0;
+      OpenBitSet visitedDocs = new OpenBitSet();
       for (final TermData term : terms) {
+        for (int i = 0; i < term.docs.length; i++) {
+          visitedDocs.set(term.docs[i]);
+        }
         sumDF += term.docs.length;
         sumTotalTermCount += term.write(termsConsumer);
       }
-      termsConsumer.finish(sumTotalTermCount, sumDF);
+      termsConsumer.finish(sumTotalTermCount, sumDF, (int) visitedDocs.cardinality());
     }
   }
 

lucene/src/test/org/apache/lucene/index/TestDocCount.java

@@ -0,0 +1,83 @@
+package org.apache.lucene.index;
+
+/**
+ * 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.
+ */
+
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.StringField;
+import org.apache.lucene.index.codecs.CodecProvider;
+import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.util.FixedBitSet;
+import org.apache.lucene.util.LuceneTestCase;
+import org.apache.lucene.util._TestUtil;
+
+/**
+ * Tests the Terms.docCount statistic
+ */
+public class TestDocCount extends LuceneTestCase {
+  public void testSimple() throws Exception {
+    assumeFalse("PreFlex codec does not support docCount statistic!", 
+        "PreFlex".equals(CodecProvider.getDefault().getDefaultFieldCodec()));
+    Directory dir = newDirectory();
+    RandomIndexWriter iw = new RandomIndexWriter(random, dir);
+    int numDocs = atLeast(100);
+    for (int i = 0; i < numDocs; i++) {
+      iw.addDocument(doc());
+    }
+    IndexReader ir = iw.getReader();
+    verifyCount(ir);
+    ir.close();
+    iw.optimize();
+    ir = iw.getReader();
+    verifyCount(ir);
+    ir.close();
+    iw.close();
+    dir.close();
+  }
+  
+  private Document doc() {
+    Document doc = new Document();
+    int numFields = _TestUtil.nextInt(random, 1, 10);
+    for (int i = 0; i < numFields; i++) {
+      doc.add(newField("" + _TestUtil.nextInt(random, 'a', 'z'), "" + _TestUtil.nextInt(random, 'a', 'z'), StringField.TYPE_UNSTORED));
+    }
+    return doc;
+  }
+  
+  private void verifyCount(IndexReader ir) throws Exception {
+    Fields fields = MultiFields.getFields(ir);
+    if (fields == null) {
+      return;
+    }
+    FieldsEnum e = fields.iterator();
+    String field;
+    while ((field = e.next()) != null) {
+      Terms terms = fields.terms(field);
+      int docCount = terms.getDocCount();
+      FixedBitSet visited = new FixedBitSet(ir.maxDoc());
+      TermsEnum te = terms.iterator();
+      while (te.next() != null) {
+        DocsEnum de = te.docs(null, null);
+        while (de.nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
+          visited.set(de.docID());
+        }
+      }
+      assertEquals(visited.cardinality(), docCount);
+    }
+  }
+}

lucene/src/test/org/apache/lucene/search/FieldCacheRewriteMethod.java

@@ -134,6 +134,11 @@ public final class FieldCacheRewriteMethod extends MultiTermQuery.RewriteMethod
         public long getSumDocFreq() throws IOException {
           return -1;
         }
+
+        @Override
+        public int getDocCount() throws IOException {
+          return -1;
+        }
       });
       
       assert termsEnum != null;