Compute the doc range more efficiently when flushing doc block (#14447)

Author: bugmakerrrrrr

lucene/CHANGES.txt

@@ -37,6 +37,8 @@ Optimizations
 
 * GITHUB#14333: Introduce a specialized trie for block tree index, instead of FST. (Guo Feng)
 
+* GITHUB#14447: Compute the doc range more efficiently when flushing doc block. (Pan Guixin)
+
 Bug Fixes
 ---------------------
 (No changes)

lucene/core/src/java/org/apache/lucene/codecs/lucene103/Lucene103PostingsWriter.java

@@ -429,12 +429,13 @@ private void flushDocBlock(boolean finishTerm) throws IOException {
       // storage-efficient than the next number of bits per value (which effectively slightly biases
       // towards the bit set approach).
       int bitsPerValue = forDeltaUtil.bitsRequired(docDeltaBuffer);
-      int sum = Math.toIntExact(Arrays.stream(docDeltaBuffer).sum());
-      int numBitSetLongs = FixedBitSet.bits2words(sum);
+      int docRange = lastDocID - level0LastDocID;
+      assert docRange == Arrays.stream(docDeltaBuffer).sum();
+      int numBitSetLongs = FixedBitSet.bits2words(docRange);
       int numBitsNextBitsPerValue = Math.min(Integer.SIZE, bitsPerValue + 1) * BLOCK_SIZE;
-      if (sum == BLOCK_SIZE) {
+      if (docRange == BLOCK_SIZE) {
         level0Output.writeByte((byte) 0);
-      } else if (numBitsNextBitsPerValue <= sum) {
+      } else if (numBitsNextBitsPerValue <= docRange) {
         level0Output.writeByte((byte) bitsPerValue);
         forDeltaUtil.encodeDeltas(bitsPerValue, docDeltaBuffer, level0Output);
       } else {