Track the number of docs left to decode instead of the number of docs…

… decoded. (#14045)

`docCountUpto` tracks the number of documents decoded so far, but it's only
used to compute the number of docs left to decode. So let's track the number of
docs left to decode instead.

lucene/core/src/java/org/apache/lucene/codecs/lucene101/Lucene101PostingsReader.java

@@ -315,7 +315,7 @@ final class BlockPostingsEnum extends ImpactsEnum {
 
     private int singletonDocID; // docid when there is a single pulsed posting, otherwise -1
 
-    private int docCountUpto; // number of docs in or before the current block
+    private int docCountLeft; // number of remaining docs in this postings list
     private int prevDocID; // last doc ID of the previous block
 
     private int docBufferSize;
@@ -539,7 +539,7 @@ public BlockPostingsEnum reset(IntBlockTermState termState, int flags) throws IO
 
       doc = -1;
       prevDocID = -1;
-      docCountUpto = 0;
+      docCountLeft = docFreq;
       freqFP = -1L;
       level0LastDocID = -1;
       if (docFreq < LEVEL1_NUM_DOCS) {
@@ -582,31 +582,31 @@ private void refillFullBlock() throws IOException {
         }
         PForUtil.skip(docIn);
       }
-      docCountUpto += BLOCK_SIZE;
+      docCountLeft -= BLOCK_SIZE;
       prevDocID = docBuffer[BLOCK_SIZE - 1];
       docBufferUpto = 0;
       posDocBufferUpto = 0;
       assert docBuffer[docBufferSize] == NO_MORE_DOCS;
     }
 
     private void refillRemainder() throws IOException {
-      final int left = docFreq - docCountUpto;
-      assert left >= 0 && left < BLOCK_SIZE;
+      assert docCountLeft >= 0 && docCountLeft < BLOCK_SIZE;
       if (docFreq == 1) {
         docBuffer[0] = singletonDocID;
         freqBuffer[0] = (int) totalTermFreq;
         docBuffer[1] = NO_MORE_DOCS;
         assert freqFP == -1;
-        docCountUpto++;
+        docCountLeft = 0;
         docBufferSize = 1;
       } else {
         // Read vInts:
-        PostingsUtil.readVIntBlock(docIn, docBuffer, freqBuffer, left, indexHasFreq, needsFreq);
-        prefixSum(docBuffer, left, prevDocID);
-        docBuffer[left] = NO_MORE_DOCS;
+        PostingsUtil.readVIntBlock(
+            docIn, docBuffer, freqBuffer, docCountLeft, indexHasFreq, needsFreq);
+        prefixSum(docBuffer, docCountLeft, prevDocID);
+        docBuffer[docCountLeft] = NO_MORE_DOCS;
         freqFP = -1L;
-        docCountUpto += left;
-        docBufferSize = left;
+        docBufferSize = docCountLeft;
+        docCountLeft = 0;
       }
       prevDocID = docBuffer[BLOCK_SIZE - 1];
       docBufferUpto = 0;
@@ -615,10 +615,9 @@ private void refillRemainder() throws IOException {
     }
 
     private void refillDocs() throws IOException {
-      final int left = docFreq - docCountUpto;
-      assert left >= 0;
+      assert docCountLeft >= 0;
 
-      if (left >= BLOCK_SIZE) {
+      if (docCountLeft >= BLOCK_SIZE) {
         refillFullBlock();
       } else {
         refillRemainder();
@@ -634,10 +633,10 @@ private void skipLevel1To(int target) throws IOException {
         level0BlockPosUpto = level1BlockPosUpto;
         level0PayEndFP = level1PayEndFP;
         level0BlockPayUpto = level1BlockPayUpto;
-        docCountUpto = level1DocCountUpto;
+        docCountLeft = docFreq - level1DocCountUpto;
         level1DocCountUpto += LEVEL1_NUM_DOCS;
 
-        if (docFreq - docCountUpto < LEVEL1_NUM_DOCS) {
+        if (docCountLeft < LEVEL1_NUM_DOCS) {
           level1LastDocID = NO_MORE_DOCS;
           break;
         }
@@ -690,7 +689,7 @@ private void doMoveToNextLevel0Block() throws IOException {
         }
       }
 
-      if (docFreq - docCountUpto >= BLOCK_SIZE) {
+      if (docCountLeft >= BLOCK_SIZE) {
         docIn.readVLong(); // level0NumBytes
         int docDelta = readVInt15(docIn);
         level0LastDocID += docDelta;
@@ -729,7 +728,7 @@ private void moveToNextLevel0Block() throws IOException {
       // Now advance level 0 skip data
       prevDocID = level0LastDocID;
 
-      if (needsDocsAndFreqsOnly && docFreq - docCountUpto >= BLOCK_SIZE) {
+      if (needsDocsAndFreqsOnly && docCountLeft >= BLOCK_SIZE) {
         // Optimize the common path for exhaustive evaluation
         long level0NumBytes = docIn.readVLong();
         docIn.skipBytes(level0NumBytes);
@@ -782,7 +781,7 @@ private void skipLevel0To(int target) throws IOException {
         payFP = level0PayEndFP;
         payUpto = level0BlockPayUpto;
 
-        if (docFreq - docCountUpto >= BLOCK_SIZE) {
+        if (docCountLeft >= BLOCK_SIZE) {
           long numSkipBytes = docIn.readVLong();
           long skip0End = docIn.getFilePointer() + numSkipBytes;
           int docDelta = readVInt15(docIn);
@@ -816,7 +815,7 @@ private void skipLevel0To(int target) throws IOException {
           }
 
           docIn.seek(level0DocEndFP);
-          docCountUpto += BLOCK_SIZE;
+          docCountLeft -= BLOCK_SIZE;
         } else {
           level0LastDocID = NO_MORE_DOCS;
           break;
@@ -850,7 +849,7 @@ private void doAdvanceShallow(int target) throws IOException {
         skipLevel1To(target);
       } else if (needsRefilling) {
         docIn.seek(level0DocEndFP);
-        docCountUpto += BLOCK_SIZE;
+        docCountLeft -= BLOCK_SIZE;
       }
 
       skipLevel0To(target);