Fix an issue with empty preallocation (#113)

* Fix issue with NumPy chain and preallocation=0

Original logic in `grow_append` was to extend data by 10% of its length.
This is a problem with the original data length is 0, since it then
never extends.
This commit amends `grow_append` to always extend by at least 10 elements.

* [NumPyBackend] Prevent ``preallocate=0`` from creating object arrays.

`grow_append` cannot know if ``preallocate = 0`` was used: it only
looks at the `rigid` value to determine how to append.
Because of this, will always fail when we use `preallocate = 0` with
tensor variables, since then the shapes of `target` and `extension`
don’t match.
A simple fix is to simply deactivate the special behavior for
`preallocate = 0`.

- This commit extends `test_growing` with a `preallocate` parameter,
  so that we test both cases where it is 0 and positive.
- We also fix `test_growing` to match the new behavior of `grow_append`
  introduced in #ea812b0, where data arrays always grow by at least 10.

* Remove trailing whitespace

---------

Co-authored-by: Alexandre René <[email protected]>
Co-authored-by: Michael Osthege <[email protected]>

mcbackend/backends/numpy.py

@@ -23,7 +23,7 @@ def grow_append(
         length = len(target)
         if length == draw_idx:
             # Grow the array by 10 %
-            ngrow = math.ceil(0.1 * length)
+            ngrow = max(10, math.ceil(0.1 * length))
             if rigid[vn]:
                 extension = numpy.empty((ngrow,) + numpy.shape(v))
             else:
@@ -52,7 +52,7 @@ def __init__(self, cmeta: ChainMeta, rmeta: RunMeta, *, preallocate: int) -> Non
             and grow the allocated memory by 10 % when needed.
             Exceptions are variables with non-rigid shapes (indicated by 0 in the shape tuple)
             where the correct amount of memory cannot be pre-allocated.
-            In these cases, and when ``preallocate == 0`` object arrays are used.
+            In these cases object arrays are used.
         """
         self._var_is_rigid: Dict[str, bool] = {}
         self._samples: Dict[str, numpy.ndarray] = {}
@@ -68,11 +68,11 @@ def __init__(self, cmeta: ChainMeta, rmeta: RunMeta, *, preallocate: int) -> Non
             for var in variables:
                 rigid = is_rigid(var.shape) and not var.undefined_ndim and var.dtype != "str"
                 rigid_dict[var.name] = rigid
-                if preallocate > 0 and rigid:
+                if rigid:
                     reserve = (preallocate, *var.shape)
                     target_dict[var.name] = numpy.empty(reserve, var.dtype)
                 else:
-                    target_dict[var.name] = numpy.array([None] * preallocate)
+                    target_dict[var.name] = numpy.array([None] * preallocate, dtype=object)
 
         super().__init__(cmeta, rmeta)
 

mcbackend/test_backend_numpy.py

@@ -2,6 +2,7 @@
 
 import hagelkorn
 import numpy
+import pytest
 
 from mcbackend.backends.numpy import NumPyBackend, NumPyChain, NumPyRun
 from mcbackend.core import RunMeta
@@ -43,8 +44,9 @@ def test_targets(self):
         assert chain._samples["changeling"].dtype == object
         pass
 
-    def test_growing(self):
-        imb = NumPyBackend(preallocate=15)
+    @pytest.mark.parametrize("preallocate", [0, 75])
+    def test_growing(self, preallocate):
+        imb = NumPyBackend(preallocate=preallocate)
         rm = RunMeta(
             rid=hagelkorn.random(),
             variables=[
@@ -62,19 +64,27 @@ def test_growing(self):
         )
         run = imb.init_run(rm)
         chain = run.init_chain(0)
-        assert chain._samples["A"].shape == (15, 2)
-        assert chain._samples["B"].shape == (15,)
-        for _ in range(22):
+        assert chain._samples["A"].shape == (preallocate, 2)
+        assert chain._samples["B"].shape == (preallocate,)
+        for _ in range(130):
             draw = {
                 "A": numpy.random.uniform(size=(2,)),
                 "B": numpy.random.uniform(size=(random.randint(0, 10),)),
             }
             chain.append(draw)
-        # Growth: 15 → 17 → 19 → 21 → 24
-        assert chain._samples["A"].shape == (24, 2)
-        assert chain._samples["B"].shape == (24,)
-        assert chain.get_draws("A").shape == (22, 2)
-        assert chain.get_draws("B").shape == (22,)
+        # NB: Growth algorithm adds max(10, ceil(0.1*length))
+        if preallocate == 75:
+            # 75 → 85 → 95 → 105 → 116 → 128 → 141
+            assert chain._samples["A"].shape == (141, 2)
+            assert chain._samples["B"].shape == (141,)
+        elif preallocate == 0:
+            # 10 → 20 → ... → 90 → 100 → 110 → 121 → 134
+            assert chain._samples["A"].shape == (134, 2)
+            assert chain._samples["B"].shape == (134,)
+        else:
+            assert False, f"Missing test for {preallocate=}"
+        assert chain.get_draws("A").shape == (130, 2)
+        assert chain.get_draws("B").shape == (130,)
         pass