fix bug of incorrect cost after upgradeToBitSet in DocIdSetBuilder class (#11939)

Author: maosuhan

lucene/CHANGES.txt

@@ -11,6 +11,9 @@ Bug Fixes
  This addresses a bug that was introduced in 9.2.0 where having many vectors is not handled well
  in the vector connections reader.
 
+* GITHUB#11939: Fix incorrect cost calculation in DocIdSetBuilder after upgradeToBitSet when doc list is growing.
+  This addresses a bug where the cost of TermRangeQuery/TermInSetQuery and some other queries will be highly underestimated.
+
 Improvements
 ---------------------
 * GITHUB#11912, GITHUB#11918: Port generic exception handling from MemorySegmentIndexInput

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

@@ -162,12 +162,12 @@ public DocIdSetBuilder(int maxDoc, PointValues values, String field) throws IOEx
    * RoaringDocIdSet.Builder}.
    */
   public void add(DocIdSetIterator iter) throws IOException {
+    int cost = (int) Math.min(Integer.MAX_VALUE, iter.cost());
+    BulkAdder adder = grow(cost);
     if (bitSet != null) {
       bitSet.or(iter);
       return;
     }
-    int cost = (int) Math.min(Integer.MAX_VALUE, iter.cost());
-    BulkAdder adder = grow(cost);
     for (int i = 0; i < cost; ++i) {
       int doc = iter.nextDoc();
       if (doc == DocIdSetIterator.NO_MORE_DOCS) {

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

@@ -243,6 +243,18 @@ public void testLeverageStats() throws IOException {
     assertTrue(builder.multivalued);
   }
 
+  public void testCostIsCorrectAfterBitsetUpgrade() throws IOException {
+    final int maxDoc = 1000000;
+    DocIdSetBuilder builder = new DocIdSetBuilder(maxDoc);
+    // 1000000 >> 6 is greater than DocIdSetBuilder.threshold which is 1000000 >> 7
+    for (int i = 0; i < 1000000 >> 6; ++i) {
+      builder.add(DocIdSetIterator.range(i, i + 1));
+    }
+    DocIdSet result = builder.build();
+    assertTrue(result instanceof BitDocIdSet);
+    assertEquals(1000000 >> 6, result.iterator().cost());
+  }
+
   private static class DummyTerms extends Terms {
 
     private final int docCount;