mikemccand commented on code in PR #13359:
URL: https://github.com/apache/lucene/pull/13359#discussion_r1609700636


##########
lucene/core/src/java/org/apache/lucene/codecs/lucene90/blocktree/SegmentTermsEnum.java:
##########
@@ -307,6 +309,30 @@ private boolean setEOF() {
     return true;
   }
 
+  @Override
+  public void prepareSeekExact(BytesRef target) throws IOException {
+    if (fr.index == null) {
+      throw new IllegalStateException("terms index was not loaded");
+    }
+
+    if (fr.size() == 0 || target.compareTo(fr.getMin()) < 0 || 
target.compareTo(fr.getMax()) > 0) {
+      return;
+    }
+
+    // TODO: should we try to reuse the current state of this terms enum when 
applicable?
+    BytesRefFSTEnum<BytesRef> indexEnum = new BytesRefFSTEnum<>(fr.index);
+    InputOutput<BytesRef> output = indexEnum.seekFloor(target);
+    if (output != null) { // should never be null since we already checked 
against fr.getMin()?
+      final long code =
+          fr.readVLongOutput(
+              new ByteArrayDataInput(
+                  output.output.bytes, output.output.offset, 
output.output.length));
+      final long fpSeek = code >>> 
Lucene90BlockTreeTermsReader.OUTPUT_FLAGS_NUM_BITS;
+      initIndexInput();
+      in.prefetch(fpSeek, 1); // TODO: could we know the length of the block?

Review Comment:
   Egads, that is really weird -- I would not expect those `fp` to go backwards 
on `.next()` -- I thought the FST index was a depth-first traversal of all 
on-disk (leaf) blocks.  I will need to mull some more about this :)



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