LUCENE-10408: Write doc IDs of KNN vectors as ints rather than vints. (#708)

Since doc IDs with a vector are loaded as an int[] in memory, this changes the
on-disk format of vectors to align with the in-memory representation by using
ints instead of vints to represent doc IDs. This might make vectors a bit
larger on disk, but also a bit faster to open.

I made the same change to how we encode nodes on levels for the same reason.

lucene/core/src/java/org/apache/lucene/codecs/lucene91/Lucene91HnswVectorsFormat.java

@@ -69,13 +69,13 @@ import org.apache.lucene.util.hnsw.HnswGraph;
  *   <li><b>[int]</b> the number of documents having values for this field
  *   <li><b>[int8]</b> if equals to -1, dense – all documents have values for a field. If equals to
  *       0, sparse – some documents missing values.
- *   <li><b>array[vint]</b> for sparse case, the docids of documents having vectors, in order
+ *   <li><b>array[int]</b> for sparse case, the docids of documents having vectors, in order
  *   <li><b>[int]</b> the maximum number of connections (neigbours) that each node can have
  *   <li><b>[int]</b> number of levels in the graph
  *   <li>Graph nodes by level. For each level
  *       <ul>
  *         <li><b>[int]</b> the number of nodes on this level
- *         <li><b>array[vint]</b> for levels greater than 0 list of nodes on this level, stored as
+ *         <li><b>array[int]</b> for levels greater than 0 list of nodes on this level, stored as
  *             the the level 0th nodes ordinals.
  *       </ul>
  * </ul>

lucene/core/src/java/org/apache/lucene/codecs/lucene91/Lucene91HnswVectorsReader.java

@@ -347,7 +347,7 @@ public final class Lucene91HnswVectorsReader extends KnnVectorsReader {
         // as not all docs have vector values, fill a mapping from dense vector ordinals to docIds
         ordToDoc = new int[size];
         for (int i = 0; i < size; i++) {
-          int doc = input.readVInt();
+          int doc = input.readInt();
           ordToDoc[i] = doc;
         }
       }
@@ -366,7 +366,7 @@ public final class Lucene91HnswVectorsReader extends KnnVectorsReader {
         } else {
           nodesByLevel[level] = new int[numNodesOnLevel];
           for (int i = 0; i < numNodesOnLevel; i++) {
-            nodesByLevel[level][i] = input.readVInt();
+            nodesByLevel[level][i] = input.readInt();
           }
         }
       }

lucene/core/src/java/org/apache/lucene/codecs/lucene91/Lucene91HnswVectorsWriter.java

@@ -213,7 +213,7 @@ public final class Lucene91HnswVectorsWriter extends KnnVectorsWriter {
       meta.writeByte((byte) 0); // sparse marker, some documents don't have vector values
       DocIdSetIterator iter = docsWithField.iterator();
       for (int doc = iter.nextDoc(); doc != DocIdSetIterator.NO_MORE_DOCS; doc = iter.nextDoc()) {
-        meta.writeVInt(doc);
+        meta.writeInt(doc);
       }
     }
 
@@ -229,7 +229,7 @@ public final class Lucene91HnswVectorsWriter extends KnnVectorsWriter {
         if (level > 0) {
           while (nodesOnLevel.hasNext()) {
             int node = nodesOnLevel.nextInt();
-            meta.writeVInt(node); // list of nodes on a level
+            meta.writeInt(node); // list of nodes on a level
           }
         }
       }