Introduce singleton for PackedInts.NullReader of size 256 (#13232)

Size 256 is very common here throguh the monotonic long values default
page size. In ES we're seing many MB O(10M) of duplicate instances of this
size relatively quickly.
=> adding a singleton for it to save some heap

lucene/CHANGES.txt

@@ -62,6 +62,8 @@ Optimizations
 
 * GITHUB#13224: Use singleton for all-zeros DirectMonotonicReader.Meta (Armin Braun)
 
+* GITHUB#13232 : Introduce singleton for PackedInts.NullReader of size 256 (Armin Braun)
+
 Bug Fixes
 ---------------------
 

lucene/core/src/java/org/apache/lucene/util/packed/PackedInts.java

@@ -566,10 +566,20 @@ public String toString() {
   /** A {@link Reader} which has all its values equal to 0 (bitsPerValue = 0). */
   public static final class NullReader extends Reader {
 
+    private static final NullReader DEFAULT_PACKED_LONG_VALUES_PAGE_SIZE =
+        new NullReader(PackedLongValues.DEFAULT_PAGE_SIZE);
+
     private final int valueCount;
 
+    public static NullReader forCount(int valueCount) {
+      if (valueCount == PackedLongValues.DEFAULT_PAGE_SIZE) {
+        return DEFAULT_PACKED_LONG_VALUES_PAGE_SIZE;
+      }
+      return new NullReader(valueCount);
+    }
+
     /** Sole constructor. */
-    public NullReader(int valueCount) {
+    private NullReader(int valueCount) {
       this.valueCount = valueCount;
     }
 

lucene/core/src/java/org/apache/lucene/util/packed/PackedLongValues.java

@@ -276,7 +276,7 @@ void pack(long[] values, int numValues, int block, float acceptableOverheadRatio
 
       // build a new packed reader
       if (minValue == 0 && maxValue == 0) {
-        this.values[block] = new PackedInts.NullReader(numValues);
+        this.values[block] = PackedInts.NullReader.forCount(numValues);
       } else {
         final int bitsRequired = minValue < 0 ? 64 : PackedInts.bitsRequired(maxValue);
         final PackedInts.Mutable mutable =

lucene/core/src/test/org/apache/lucene/util/packed/TestPackedInts.java

@@ -502,7 +502,7 @@ public void testFill() {
   public void testPackedIntsNull() {
     // must be > 10 for the bulk reads below
     int size = TestUtil.nextInt(random(), 11, 256);
-    Reader packedInts = new PackedInts.NullReader(size);
+    Reader packedInts = PackedInts.NullReader.forCount(size);
     assertEquals(0, packedInts.get(TestUtil.nextInt(random(), 0, size - 1)));
     long[] arr = new long[size + 10];
     int r;