Avoid reload block when seeking backward in SegmentTermsEnum.

lucene/core/src/java/org/apache/lucene/codecs/lucene90/blocktree/SegmentTermsEnum.java

@@ -309,6 +309,7 @@ private boolean setEOF() {
   @Override
   public boolean seekExact(BytesRef target) throws IOException {
 
+    int originEndCount = -1;
     if (fr.index == null) {
       throw new IllegalStateException("terms index was not loaded");
     }
@@ -435,7 +436,9 @@ public boolean seekExact(BytesRef target) throws IOException {
         // rewind frame ord=" + lastFrame.ord);
         // }
         currentFrame = lastFrame;
-        currentFrame.rewind();
+        originEndCount = currentFrame.entCount;
+        currentFrame.rewind2();
+
       } else {
         // Target is exactly the same as current term
         assert term.length() == target.length;
@@ -519,6 +522,9 @@ public boolean seekExact(BytesRef target) throws IOException {
         currentFrame.loadBlock();
 
         final SeekStatus result = currentFrame.scanToTerm(target, true);
+        if (originEndCount != -1) {
+          currentFrame.entCount = originEndCount;
+        }
         if (result == SeekStatus.FOUND) {
           // if (DEBUG) {
           //   System.out.println("  return FOUND term=" + term.utf8ToString() + " " + term);
@@ -574,6 +580,9 @@ public boolean seekExact(BytesRef target) throws IOException {
     currentFrame.loadBlock();
 
     final SeekStatus result = currentFrame.scanToTerm(target, true);
+    if (originEndCount != -1) {
+      currentFrame.entCount = originEndCount;
+    }
     if (result == SeekStatus.FOUND) {
       // if (DEBUG) {
       //   System.out.println("  return FOUND term=" + term.utf8ToString() + " " + term);

lucene/core/src/java/org/apache/lucene/codecs/lucene90/blocktree/SegmentTermsEnumFrame.java

@@ -284,6 +284,61 @@ void rewind() {
     */
   }
 
+  // Only rewind, don't force reload block.
+  // Reset reader position, don't read, decompress.
+  // Current term greater than target, reduce endCount.
+  void rewind2() {
+
+    // Force reload:
+    fp = fpOrig;
+    // TODO: Reset entCount after this seek.
+    entCount = nextEnt;
+    nextEnt = 0;
+    hasTerms = hasTermsOrig;
+    if (isFloor) {
+      floorDataReader.setPosition(rewindPos);
+      numFollowFloorBlocks = floorDataReader.readVInt();
+      assert numFollowFloorBlocks > 0;
+      nextFloorLabel = floorDataReader.readByte() & 0xff;
+    }
+
+    suffixesReader.setPosition(0);
+    suffixLengthsReader.setPosition(0);
+    /*
+    //System.out.println("rewind");
+    // Keeps the block loaded, but rewinds its state:
+    if (nextEnt > 0 || fp != fpOrig) {
+    if (DEBUG) {
+    System.out.println("      rewind frame ord=" + ord + " fpOrig=" + fpOrig + " fp=" + fp + " hasTerms?=" + hasTerms + " isFloor?=" + isFloor + " nextEnt=" + nextEnt + " prefixLen=" + prefix);
+    }
+    if (fp != fpOrig) {
+    fp = fpOrig;
+    nextEnt = -1;
+    } else {
+    nextEnt = 0;
+    }
+    hasTerms = hasTermsOrig;
+    if (isFloor) {
+    floorDataReader.rewind();
+    numFollowFloorBlocks = floorDataReader.readVInt();
+    nextFloorLabel = floorDataReader.readByte() & 0xff;
+    }
+    assert suffixBytes != null;
+    suffixesReader.rewind();
+    assert statBytes != null;
+    statsReader.rewind();
+    metaDataUpto = 0;
+    state.termBlockOrd = 0;
+    // TODO: skip this if !hasTerms?  Then postings
+    // impl wouldn't have to write useless 0 byte
+    postingsReader.resetTermsBlock(fieldInfo, state);
+    lastSubFP = -1;
+    } else if (DEBUG) {
+    System.out.println("      skip rewind fp=" + fp + " fpOrig=" + fpOrig + " nextEnt=" + nextEnt + " ord=" + ord);
+    }
+    */
+  }
+
   // Decodes next entry; returns true if it's a sub-block
   public boolean next() throws IOException {
     if (isLeafBlock) {