more agg queriers work

Author: normen662

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/combinatorics/EnumeratingIterable.java

@@ -44,6 +44,10 @@ static <T> EnumeratingIterable<T> singleIterable(@Nonnull final T singleElement)
         return new SingleIterable<>(singleElement);
     }
 
+    static <T> EnumeratingIterable<T> emptyOnEmptyIterable() {
+        return new SingleIterable<>();
+    }
+
     /**
      * An implementation of {@link EnumeratingIterable} that is optimized to work for empty
      * input sets.
@@ -84,13 +88,17 @@ public EnumeratingIterator<T> iterator() {
      * @param <T> type
      */
     class SingleIterable<T> implements EnumeratingIterable<T> {
-        @Nonnull
+        @Nullable
         private final T singleElement;
 
         private SingleIterable(@Nonnull final T singleElement) {
             this.singleElement = singleElement;
         }
 
+        private SingleIterable() {
+            this.singleElement = null;
+        }
+
         @Nonnull
         @Override
         public EnumeratingIterator<T> iterator() {
@@ -103,12 +111,12 @@ public EnumeratingIterator<T> iterator() {
      * @param <T> type of the element
      */
     class SingleIterator<T> extends AbstractIterator<List<T>> implements EnumeratingIterator<T> {
-        @Nonnull
+        @Nullable
         private final T singleElement;
 
         boolean atFirst = true;
 
-        private SingleIterator(@Nonnull final T singleElement) {
+        private SingleIterator(@Nullable final T singleElement) {
             this.singleElement = singleElement;
         }
 
@@ -128,6 +136,9 @@ protected List<T> computeNext() {
 
             atFirst = false;
 
+            if (singleElement == null) {
+                return ImmutableList.of();
+            }
             return ImmutableList.of(singleElement);
         }
     }

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/combinatorics/TopologicalSort.java

@@ -537,9 +537,9 @@ public static <T, P> Iterable<List<T>> satisfyingPermutations(@Nonnull final Par
                                                                   @Nonnull final List<P> targetPermutation,
                                                                   @Nonnull final Function<T, P> domainMapper,
                                                                   @Nonnull final Function<List<T>, Integer> satisfiabilityFunction) {
-        if (partiallyOrderedSet.isEmpty()) {
-            return ImmutableList.of();
-        }
+//        if (partiallyOrderedSet.isEmpty()) {
+//            return ImmutableList.of();
+//        }
 
         if (partiallyOrderedSet.size() < targetPermutation.size()) {
             return ImmutableList.of();
@@ -565,9 +565,11 @@ protected Iterator<T> domain(final int t) {
                     }
                 }
             };
-        } else {
-            Verify.verify(partiallyOrderedSet.size() == 1);
+        } else if (partiallyOrderedSet.size() == 1) {
             enumeratingIterator = EnumeratingIterable.singleIterable(Iterables.getOnlyElement(partiallyOrderedSet.getSet())).iterator();
+        } else {
+            Verify.verify(partiallyOrderedSet.isEmpty());
+            enumeratingIterator = EnumeratingIterable.<T>emptyOnEmptyIterable().iterator();
         }
 
         return () -> new AbstractIterator<>() {