account for sparse tensors without dimensions

1 files changed, 11 insertions, 6 deletions

diff --git a/eval/src/vespa/eval/tensor/serialization/format.txt b/eval/src/vespa/eval/tensor/serialization/format.txt
index 9d0a387c36a..02db6114ab2 100644
--- a/eval/src/vespa/eval/tensor/serialization/format.txt
+++ b/eval/src/vespa/eval/tensor/serialization/format.txt
@@ -4,10 +4,9 @@ interpreted as a single unified binary format. The description below
 uses data types defined by document serialization (nbostream) combined
 with some comments and python-inspired flow-control. The mixed[3]
 binary format is defined in such a way that it overlays as
-effortlessly as possible with both existing formats. The only thing
-needed to go from sparse[1] or dense[2] binary formats to the mixed[3]
-format for a specific tensor is to add a single byte indicating there
-are no dimensions of the other kind (mapped/indexed).
+effortlessly as possible with both existing formats.
+
+//-----------------------------------------------------------------------------
 
 byte: type (1:sparse, 2:dense, 3:mixed)
   bit 0 -> 'sparse'
@@ -15,7 +14,7 @@ byte: type (1:sparse, 2:dense, 3:mixed)
   (mixed tensors are tagged as both 'sparse' and 'dense')
 
 if ('sparse'):
-  1_4_int: number of mapped dimensions -> ''n_mapped'
+  1_4_int: number of mapped dimensions -> 'n_mapped'
   'n_mapped' times: (sorted by dimension name)
     small_string: dimension name
 
@@ -25,7 +24,7 @@ if ('dense'):
     small_string: dimensions name
     1_4_int: dimensions size (must be at least 1) -> 'size_i'
 
-if ('n_mapped > 0'):
+if ('n_mapped > 0' || !'dense'):
   1_4_int: number of named dense sub-spaces -> 'n_blocks'
 else:
   'n_blocks' = 1 (a single dense space)
@@ -35,3 +34,9 @@ else:
     small_string: dimension label (same order as dimension names)
   prod('size_i') times: (product of all indexed dimension sizes)
     double: cell value (last indexed dimension is nested innermost)
+
+//-----------------------------------------------------------------------------
+
+Note: A tensor with no dimensions should not be serialized as
+sparse[1], but when it is, it will contain an integer indicating the
+number of cells.