Speed up Lucene912PostingsReader nextDoc() impls. (#13963)

127 times out of 128, nextDoc() returns the next doc ID in the buffer.
Currently, we check if the current doc is equal to the last doc ID in the block
to know if we need to refill. We can do better by comparing the current index
in the block with the block size, which is a bit more efficient since the
latter is a constant.

lucene/CHANGES.txt

@@ -77,6 +77,9 @@ Optimizations
 
 * GITHUB#13961: Replace Map<String,Object> with IntObjectHashMap for DV producer. (Pan Guixin)
 
+* GITHUB#13963: Speed up nextDoc() implementations in Lucene912PostingsReader.
+  (Adrien Grand)
+
 Bug Fixes
 ---------------------
 * GITHUB#13832: Fixed an issue where the DefaultPassageFormatter.format method did not format passages as intended

lucene/core/src/java/org/apache/lucene/codecs/lucene912/Lucene912PostingsReader.java

@@ -580,7 +580,7 @@ private void moveToNextLevel0Block() throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      if (doc == level0LastDocID) { // advance skip data on level 0
+      if (docBufferUpto == BLOCK_SIZE) { // advance skip data on level 0
         moveToNextLevel0Block();
       }
 
@@ -875,7 +875,7 @@ private void moveToNextLevel0Block() throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      if (doc == level0LastDocID) { // advance level 0 skip data
+      if (docBufferUpto == BLOCK_SIZE) { // advance level 0 skip data
         moveToNextLevel0Block();
       }
 
@@ -1417,11 +1417,13 @@ private void moveToNextLevel0Block() throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      if (doc == level0LastDocID) {
-        moveToNextLevel0Block();
-      } else if (needsRefilling) {
-        refillDocs();
-        needsRefilling = false;
+      if (docBufferUpto == BLOCK_SIZE) {
+        if (needsRefilling) {
+          refillDocs();
+          needsRefilling = false;
+        } else {
+          moveToNextLevel0Block();
+        }
       }
 
       return this.doc = (int) docBuffer[docBufferUpto++];
@@ -1644,8 +1646,9 @@ public void advanceShallow(int target) throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      advanceShallow(doc + 1);
-      if (needsRefilling) {
+      if (docBufferUpto == BLOCK_SIZE) {
+        advanceShallow(doc + 1);
+        assert needsRefilling;
         refillDocs();
         needsRefilling = false;
       }