Allow for 0-length dynamic arrays of simple types

structured/serializers/arrays.py

@@ -235,7 +235,7 @@ def __init__(
     ) -> None:
         self.count_serializer = count_serializer
         self.item_serializer = item_serializer
-        self.num_values = 1
+        self.num_values = 0
         self.size = 0
 
     def with_byte_order(self, byte_order: ByteOrder) -> Self:
@@ -247,7 +247,11 @@ def with_byte_order(self, byte_order: ByteOrder) -> Self:
     def _packer(self, values: tuple[list[T]]) -> tuple[Serializer, list[T]]:
         items = values[0]
         count = len(items)
-        serializer = self.count_serializer + (self.item_serializer * count)
+        if count == 0:
+            # Since we'll be modifying its .num_values, we want a copy
+            serializer = StructSerializer(self.count_serializer.format)
+        else:
+            serializer = self.count_serializer + (self.item_serializer * count)
         serializer.num_values -= 1
         self.size = serializer.size
         return serializer, items

structured/serializers/structs.py

@@ -31,7 +31,7 @@
     Ts,
     Unpack,
 )
-from .api import Serializer
+from .api import NullSerializer, Serializer
 
 
 def noop_action(x: T) -> T:
@@ -187,8 +187,10 @@ def __matmul__(self, other: int) -> Self:
     def _mul_impl(self, other: int, combine_strings: bool = False) -> Self:
         if not isinstance(other, int):
             return NotImplemented
-        elif other <= 0:
-            raise ValueError('count must be positive')
+        elif other == 0:
+            return NullSerializer()
+        elif other < 0:
+            raise ValueError('count must be non-negative')
         elif other == 1:
             return self
         byte_order, fmt = self._split_format

tests/test_arrays.py

@@ -155,6 +155,16 @@ class Compound(Structured):
     standard_tests(target_obj, target_data)
 
 
+def test_zero_length():
+    class EmptyList(Structured):
+        a: array[Header[uint32], uint8]
+
+    target_obj = EmptyList([])
+    target_data = struct.pack('I', 0)
+
+    standard_tests(target_obj, target_data)
+
+
 def test_dynamic_structured(items: list[Item]):
     class Compound(Structured):
         a: array[Header[uint32], Item]

tests/test_base_types.py

@@ -24,7 +24,7 @@ def test_counted() -> None:
     with pytest.raises(TypeError):
         pad['']
     with pytest.raises(ValueError):
-        pad[0]
+        pad[-1]
 
 
 class TestCustomType: