table code types containing unions

Unions have a 2-entry header (8 bytes), plus 1 entry for each field,
plus the entries describing each field.

The maximum number of fields is 255. Each field cannot consist of more
than 255 entries (including header entries and entries describing nested
types).

This completes the list of DSDL types.

dronecan_dsdlc_helpers.py

@@ -295,10 +295,45 @@ def build_coding_table(msg_underscored_name, msg_union, msg_max_bitlen, msg_fiel
     return table_str
 
 def _build_coding_table_core(obj_underscored_name, obj_union, obj_fields, tao):
-    if obj_union:
-        return None # not yet supported
-
     table = []
+
+    if obj_union: # unions are a special case
+        if len(obj_fields) == 0 or len(obj_fields) > 255: # out of range to encode
+            return None
+
+        # entries to describe the union
+        table.append(("CANARD_TABLE_CODING_ENTRIES_UNION", False,
+            f"offsetof(struct {obj_underscored_name}, {obj_fields[0].name})", len(obj_fields),
+            union_msg_tag_bitlen_from_num_fields(len(obj_fields)),
+            f"offsetof(struct {obj_underscored_name}, union_tag)"
+        ))
+        table.append((None, None)) # dummy auxiliary entry for accurate entry count
+
+        field_entries = []
+        for field in obj_fields:
+            t = field.type
+
+            if isinstance(t, dronecan.dsdl.parser.CompoundType):
+                sub = _build_coding_table_core(field.name, t.union, t.fields, tao)
+            else:
+                sub = _build_coding_table_core(field.name, False, [t], tao)
+            if sub is None: return None
+            if len(sub) > 255: # too many entries to encode
+                return None
+
+            # entry to describe this field
+            table.append(("CANARD_TABLE_CODING_ENTRY_UNION_FIELD", True, len(sub)))
+
+            for s_kind, s_relative, *s_params in sub:
+                if s_kind is None:
+                    field_entries.append((None, None))
+                else:
+                    # all entries must have a relative offset
+                    field_entries.append((s_kind, True, *s_params))
+
+        table.extend(field_entries)
+        return table
+
     for field_idx, field in enumerate(obj_fields):
         tao_eligible = tao and field_idx == len(obj_fields)-1
         if isinstance(field, dronecan.dsdl.parser.Type):
@@ -328,7 +363,8 @@ def _build_coding_table_core(obj_underscored_name, obj_union, obj_fields, tao):
                     table.append((None, None))
                 else:
                     if not s_relative: # not relative, needs offset adjusted
-                        if s_kind.startswith("CANARD_TABLE_CODING_ENTRIES_ARRAY_DYNAMIC"): # has a second offset
+                        if s_kind.startswith("CANARD_TABLE_CODING_ENTRIES_ARRAY_DYNAMIC") \
+                                or s_kind == "CANARD_TABLE_CODING_ENTRIES_UNION": # has a second offset
                             s_params = (*s_params[:-1], f"{offset}+{s_params[-1]}")
                         table.append((s_kind, s_relative, f"{offset}+{s_params[0]}", *s_params[1:]))
                     else: