bitmap agg to get freq-of-freq of int32 vals

solr/core/src/java/org/apache/solr/search/ValueSourceParser.java

@@ -59,6 +59,7 @@
 import org.apache.solr.search.facet.BitmapCollectorAgg;
 import org.apache.solr.search.facet.BitmapFrequencyAgg;
 import org.apache.solr.search.facet.CountAgg;
+import org.apache.solr.search.facet.FrequencyOfFrequenciesAgg;
 import org.apache.solr.search.facet.HLLAgg;
 import org.apache.solr.search.facet.MinMaxAgg;
 import org.apache.solr.search.facet.PercentileAgg;
@@ -1065,6 +1066,8 @@ public ValueSource parse(FunctionQParser fp) throws SyntaxError {
 
     addParser("agg_bitmapfreq", new BitmapFrequencyAgg.Parser());
 
+    addParser("agg_bitmapfreqfreq", new FrequencyOfFrequenciesAgg.Parser());
+
     addParser("childfield", new ChildFieldValueSourceParser());
   }
 

solr/core/src/java/org/apache/solr/search/facet/FrequencyOfFrequenciesAgg.java

@@ -0,0 +1,97 @@
+package org.apache.solr.search.facet;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import org.apache.lucene.queries.function.ValueSource;
+import org.apache.solr.common.util.SimpleOrderedMap;
+import org.apache.solr.search.FunctionQParser;
+import org.apache.solr.search.SyntaxError;
+import org.apache.solr.search.ValueSourceParser;
+
+/**
+ * Calculates the frequency-of-frequencies (number of values occurring x times) of ordinal values.
+ *
+ * The response is a map where the keys are frequencies (x = number of times a value occurred), and the values are
+ * the frequency-of-frequencies (number of values which occurred x times).
+ *
+ * Lacking a coherent definition of magnitude other than the raw count, this aggregate cannot be used for sorting.
+ */
+public class FrequencyOfFrequenciesAgg extends SimpleAggValueSource {
+  private final int size;
+
+  public FrequencyOfFrequenciesAgg(ValueSource vs, Integer size) {
+    super("bitmapfreqfreq", vs);
+
+    this.size = size;
+  }
+
+  @Override
+  public SlotAcc createSlotAcc(FacetContext fcontext, int numDocs, int numSlots) {
+    return new BitmapFrequencySlotAcc(getArg(), fcontext, numSlots, size);
+  }
+
+  @Override
+  public FacetMerger createFacetMerger(Object prototype) {
+    return new Merger(size);
+  }
+
+  public static class Parser extends ValueSourceParser {
+    @Override
+    public ValueSource parse(FunctionQParser fp) throws SyntaxError {
+      ValueSource valueSource = fp.parseValueSource();
+
+      int size = 8;
+      if (fp.hasMoreArguments()) {
+        size = fp.parseInt();
+      }
+
+      return new FrequencyOfFrequenciesAgg(valueSource, size);
+    }
+  }
+
+  private static class Merger extends FacetMerger {
+    private final int size;
+    private BitmapFrequencyCounter result;
+
+    public Merger(int size) {
+      this.size = size;
+      this.result = new BitmapFrequencyCounter(size);
+    }
+
+    @Override
+    public void merge(Object facetResult, Context mcontext) {
+      if (facetResult instanceof SimpleOrderedMap) {
+        BitmapFrequencyCounter deserialized = new BitmapFrequencyCounter(size);
+        deserialized.deserialize((SimpleOrderedMap<Object>) facetResult);
+
+        result = result.merge(deserialized);
+      }
+    }
+
+    @Override
+    public void finish(Context mcontext) {
+      // never called
+    }
+
+    @Override
+    public Object getMergedResult() {
+      Map<Integer, Integer> map = new LinkedHashMap<>();
+
+      result.normalize();
+
+      int[] lowFrequencies = result.decode();
+      for (int i = 0; i < lowFrequencies.length; i++) {
+        int value = lowFrequencies[i];
+        if (value > 0) {
+          map.put(i, value);
+        }
+      }
+
+      result.getOverflow()
+        .forEach((value, freq) -> map.merge(freq, 1, Integer::sum));
+
+      return map;
+    }
+  }
+}