[GitHub] [lucene] mayya-sharipova commented on a diff in pull request #792: LUCENE-10502: Use IndexedDISI to store docIds and DirectMonotonicWriter/Reader to handle ordToDoc

[GitBox]

mayya-sharipova commented on code in PR #792:
URL: https://github.com/apache/lucene/pull/792#discussion_r854526400


##########
lucene/core/src/java/org/apache/lucene/codecs/lucene91/Lucene91HnswVectorsWriter.java:
##########
@@ -207,15 +210,41 @@ private void writeMeta(
     // write docIDs
     int count = docsWithField.cardinality();
     meta.writeInt(count);
-    if (count == maxDoc) {
-      meta.writeByte((byte) -1); // dense marker, each document has a vector 
value
+    if (count == 0) {
+      meta.writeLong(-2); // docsWithFieldOffset
+      meta.writeLong(0L); // docsWithFieldLength
+      meta.writeShort((short) -1); // jumpTableEntryCount
+      meta.writeByte((byte) -1); // denseRankPower
+    } else if (count == maxDoc) {
+      meta.writeLong(-1); // docsWithFieldOffset
+      meta.writeLong(0L); // docsWithFieldLength
+      meta.writeShort((short) -1); // jumpTableEntryCount
+      meta.writeByte((byte) -1); // denseRankPower
     } else {
-      meta.writeByte((byte) 0); // sparse marker, some documents don't have 
vector values
-      DocIdSetIterator iter = docsWithField.iterator();
-      for (int doc = iter.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc 
= iter.nextDoc()) {
-        meta.writeInt(doc);
-      }
+      long offset = vectorData.getFilePointer();
+      meta.writeLong(offset); // docsWithFieldOffset
+      final short jumpTableEntryCount =
+          IndexedDISI.writeBitSet(
+              docsWithField.iterator(), vectorData, 
IndexedDISI.DEFAULT_DENSE_RANK_POWER);
+      meta.writeLong(vectorData.getFilePointer() - offset); // 
docsWithFieldLength
+      meta.writeShort(jumpTableEntryCount);
+      meta.writeByte(IndexedDISI.DEFAULT_DENSE_RANK_POWER);
+    }
+
+    // write ordToDoc mapping

Review Comment:
   Do we need to write `ordToDoc` mapping for the dense case, where is 1-1 
mapping between ord and doc? May be, we can skip it in this case?



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