WIP backport from master

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

@@ -37,7 +37,7 @@
  * Accumulates stats separated by slot number for the fields with {@link org.apache.lucene.index.DocValues}
  */
 public abstract class DocValuesAcc extends SlotAcc {
-  SchemaField sf;
+  protected SchemaField sf;
 
   public DocValuesAcc(FacetContext fcontext, SchemaField sf) throws IOException {
     super(fcontext);

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

@@ -354,7 +354,7 @@ class FacetComponentState {
   }
 
   // base class for facet functions that can be used in a sort
-  abstract static class FacetSortableMerger extends FacetMerger {
+  public abstract static class FacetSortableMerger extends FacetMerger {
     public void prepareSort() {
     }
 
@@ -370,7 +370,7 @@ public void finish(Context mcontext) {
     public abstract int compareTo(FacetSortableMerger other, FacetRequest.SortDirection direction);
   }
 
-  abstract static class FacetDoubleMerger extends FacetSortableMerger {
+  public abstract static class FacetDoubleMerger extends FacetSortableMerger {
     @Override
     public abstract void merge(Object facetResult, Context mcontext);
 
@@ -408,7 +408,7 @@ public static int compare(double a, double b, FacetRequest.SortDirection directi
     }
   }
 
-  static class FacetLongMerger extends FacetSortableMerger {
+  public static class FacetLongMerger extends FacetSortableMerger {
     long val;
 
     @Override
@@ -429,7 +429,7 @@ public int compareTo(FacetSortableMerger other, FacetRequest.SortDirection direc
 
 
   // base class for facets that create buckets (and can hence have sub-facets)
-  abstract static class FacetBucketMerger<FacetRequestT extends FacetRequest> extends FacetMerger {
+  public abstract static class FacetBucketMerger<FacetRequestT extends FacetRequest> extends FacetMerger {
     FacetRequestT freq;
 
     public FacetBucketMerger(FacetRequestT freq) {
@@ -469,7 +469,7 @@ FacetMerger createFacetMerger(String key, Object val) {
   }
 
 
-  static class FacetQueryMerger extends FacetBucketMerger<FacetQuery> {
+  public static class FacetQueryMerger extends FacetBucketMerger<FacetQuery> {
     FacetBucket bucket;
 
     public FacetQueryMerger(FacetQuery freq) {

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

@@ -20,6 +20,7 @@
 import java.util.ArrayList;
 import java.util.Map;
 import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
 
 import org.apache.solr.common.SolrException;
@@ -34,7 +35,7 @@
 
 import static org.apache.solr.common.params.CommonParams.SORT;
 
-abstract class FacetParser<FacetRequestT extends FacetRequest> {
+public abstract class FacetParser<FacetRequestT extends FacetRequest> {
   protected FacetRequestT facet;
   protected FacetParser<?> parent;
   protected String key;
@@ -133,23 +134,30 @@ public Object parseFacetOrStat(String key, Object o) throws SyntaxError {
     return parseFacetOrStat(key, type, args);
   }
 
+  public interface ParseHandler {
+    Object doParse(FacetParser<?> parent, String key, Object args) throws SyntaxError;
+  }
+
+  private static final Map<String, ParseHandler> REGISTERED_TYPES = new ConcurrentHashMap<>();
+
+  static {
+    ParseHandler fieldParser = (p, k, a) -> new FacetFieldParser(p, k).parse(a);
+    REGISTERED_TYPES.put("field", fieldParser);
+    REGISTERED_TYPES.put("terms", fieldParser);
+    REGISTERED_TYPES.put("query", (p, k, a) -> new FacetQueryParser(p, k).parse(a));
+    REGISTERED_TYPES.put("range", (p, k, a) -> new FacetRangeParser(p, k).parse(a));
+    REGISTERED_TYPES.put("heatmap", (p, k, a) -> new FacetHeatmap.Parser(p, k).parse(a));
+    REGISTERED_TYPES.put("func", (p, k, a) -> p.parseStat(k, a));
+  }
+
+  public static void registerParseHandler(String type, ParseHandler parseHandler) {
+    REGISTERED_TYPES.put(type, parseHandler);
+  }
+
   public Object parseFacetOrStat(String key, String type, Object args) throws SyntaxError {
-    // TODO: a place to register all these facet types?
-
-    switch (type) {
-      case "field":
-      case "terms":
-        return new FacetFieldParser(this, key).parse(args);
-      case "query":
-        return new FacetQueryParser(this, key).parse(args);
-      case "range":
-        return new FacetRangeParser(this, key).parse(args);
-      case "topdocs":
-        return TopDocsAgg.AggParser.parse(args);
-      case "heatmap":
-        return new FacetHeatmap.Parser(this, key).parse(args);
-      case "func":
-        return parseStat(key, args);
+    ParseHandler parseHandler = REGISTERED_TYPES.get(type);
+    if (parseHandler != null) {
+      return parseHandler.doParse(this, key, args);
     }
 
     throw err("Unknown facet or stat. key=" + key + " type=" + type + " args=" + args);

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

@@ -29,7 +29,7 @@
 import org.apache.lucene.util.LongValues;
 import org.apache.solr.schema.SchemaField;
 
-class UniqueMultiDvSlotAcc extends UniqueSlotAcc {
+public class UniqueMultiDvSlotAcc extends UniqueSlotAcc {
   SortedSetDocValues topLevel;
   SortedSetDocValues[] subDvs;
   OrdinalMap ordMap;

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

@@ -25,7 +25,7 @@
 import org.apache.solr.schema.SchemaField;
 import org.apache.solr.search.SolrIndexSearcher;
 
-class UniqueMultivaluedSlotAcc extends UniqueSlotAcc implements UnInvertedField.Callback {
+public class UniqueMultivaluedSlotAcc extends UniqueSlotAcc implements UnInvertedField.Callback {
   private UnInvertedField uif;
   private UnInvertedField.DocToTerm docToTerm;
 

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

@@ -29,7 +29,7 @@
 import org.apache.lucene.util.LongValues;
 import org.apache.solr.schema.SchemaField;
 
-class UniqueSinglevaluedSlotAcc extends UniqueSlotAcc {
+public class UniqueSinglevaluedSlotAcc extends UniqueSlotAcc {
   SortedDocValues topLevel;
   SortedDocValues[] subDvs;
   OrdinalMap ordMap;

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

@@ -28,7 +28,7 @@
 import org.apache.solr.schema.SchemaField;
 import org.apache.solr.util.hll.HLL;
 
-abstract class UniqueSlotAcc extends SlotAcc {
+public abstract class UniqueSlotAcc extends SlotAcc {
   HLLAgg.HLLFactory factory;
   SchemaField field;
   FixedBitSet[] arr;

solr/core/src/test/org/apache/solr/search/function/AggValueSourceTest.java

@@ -24,6 +24,7 @@
 import org.apache.solr.SolrTestCase;
 import org.apache.solr.search.facet.FacetContext;
 import org.apache.solr.search.facet.FacetMerger;
+import org.apache.solr.search.facet.FacetModule;
 import org.apache.solr.search.facet.SimpleAggValueSource;
 import org.apache.solr.search.facet.SlotAcc;
 import org.junit.Test;
@@ -75,7 +76,8 @@ public Object getValue(int slot) {
 
     @Override
     public FacetMerger createFacetMerger(Object prototype) {
-      return new FacetMerger() {
+      // check these inner classes can be referenced by name
+      FacetModule.FacetSortableMerger merger = new FacetModule.FacetDoubleMerger() {
         double total = 0.0D;
 
         @Override
@@ -84,13 +86,11 @@ public void merge(Object facetResult, Context mcontext) {
         }
 
         @Override
-        public void finish(Context mcontext) { }
-
-        @Override
-        public Object getMergedResult() {
+        public double getDouble() {
           return total;
         }
       };
+      return merger;
     }
 
     @Override