Fix annotation core:sample_count requirement

According to spec, core:sample_count for annotation is optional

Author: messybear

sigmf/schema-meta.json

@@ -238,8 +238,7 @@
                         "$id": "#/properties/annotations/items/anyOf/0",
                         "description": "The annotations value is an array of annotation segment objects that describe anything regarding the signal data not part of the Captures and Global objects. It MUST be sorted by the value of each Annotation Segment's `core:sample_start` key, ascending.",
                         "required": [
-                            "core:sample_start",
-                            "core:sample_count"
+                            "core:sample_start"
                         ],
                         "type": "object",
                         "properties": {

sigmf/sigmffile.py

@@ -368,15 +368,17 @@ def get_capture_byte_boundarys(self, index):
             end_byte += (self.get_capture_start(index+1) - self.get_capture_start(index)) * self.get_sample_size() * self.get_num_channels()
         return (start_byte, end_byte)
 
-    def add_annotation(self, start_index, length, metadata=None):
+    def add_annotation(self, start_index, length=None, metadata=None):
         """
-        Insert annotation at start_index with length.
+        Insert annotation at start_index with length (if != None).
         """
         assert start_index >= self._get_start_offset()
-        assert length >= 1
+
         new_annot = metadata or {}
         new_annot[self.START_INDEX_KEY] = start_index
-        new_annot[self.LENGTH_INDEX_KEY] = length
+        if length is not None:
+            assert length >= 1
+            new_annot[self.LENGTH_INDEX_KEY] = length
 
         self._metadata[self.ANNOTATION_KEY] += [new_annot]
         # sort annotations by start_index

tests/test_validation.py

@@ -81,6 +81,16 @@ def test_invalid_annotation_order(self):
         with self.assertRaises(ValidationError):
             SigMFFile(self.metadata).validate()
 
+    def test_annotation_without_sample_count(self):
+        '''annotation without length should be accepted'''
+        self.metadata[SigMFFile.ANNOTATION_KEY] = [
+            {
+                SigMFFile.START_INDEX_KEY: 2
+            }
+        ]
+        SigMFFile(self.metadata).validate()
+
+
     def test_invalid_hash(self):
         _, temp_path = tempfile.mkstemp()
         TEST_FLOAT32_DATA.tofile(temp_path)