Don't include hidden arrays when counting max_total_source_arrays

cubed/core/optimization.py

@@ -109,6 +109,19 @@ def is_fusable(node_dict):
     return is_primitive_op(node_dict) and node_dict["primitive_op"].fusable
 
 
+def num_source_arrays(dag, name):
+    """Return the number of (non-hidden) arrays that are inputs to an op.
+
+    Hidden arrays are used for internal bookkeeping, are very small virtual arrays
+    (empty, or offsets for example), and are not shown on the plan visualization.
+    For these reasons they shouldn't count towards ``max_total_source_arrays``.
+    """
+    nodes = dict(dag.nodes(data=True))
+    return sum(
+        not nodes[array]["hidden"] for array in predecessors_unordered(dag, name)
+    )
+
+
 def can_fuse_predecessors(
     dag,
     name,
@@ -145,9 +158,7 @@ def can_fuse_predecessors(
     # the fused function would be more than an allowed maximum, then don't fuse
     if len(list(predecessor_ops(dag, name))) > 1:
         total_source_arrays = sum(
-            len(list(predecessors_unordered(dag, pre)))
-            if is_primitive_op(nodes[pre])
-            else 1
+            num_source_arrays(dag, pre) if is_primitive_op(nodes[pre]) else 1
             for pre in predecessor_ops(dag, name)
         )
         if total_source_arrays > max_total_source_arrays: