fix: n_to_one repartition

src/dask_awkward/lib/structure.py

@@ -1412,12 +1412,17 @@ def simple_repartition_layer(
     layer: dict[tuple[str, int], tuple[Any, ...]] = {}
     new_divisions: tuple[Any, ...]
     if n_to_one:
-        for i in range(0, arr.npartitions, n_to_one):
-            layer[(key, i)] = (_subcat,) + tuple(
+        for i0, i in enumerate(range(0, arr.npartitions, n_to_one)):
+            layer[(key, i0)] = (_subcat,) + tuple(
                 (arr.name, part)
                 for part in range(i, min(i + n_to_one, arr.npartitions))
             )
         new_divisions = arr.divisions[::n_to_one]
+        if arr.npartitions % n_to_one:
+            new_divisions = new_divisions + (arr.divisions[-1],)
+            layer[(key, i0 + 1)] = (_subcat,) + tuple(
+                (arr.name, part) for part in range(new_divisions[-2], new_divisions[-1])
+            )
     elif one_to_n:
         for i in range(arr.npartitions):
             for part in range(one_to_n):

tests/test_structure.py

@@ -554,6 +554,22 @@ def test_repartition_one_to_n(daa):
     assert_eq(daa, daa1, check_divisions=False)
 
 
+def test_repartition_n_to_one():
+    daa = dak.from_lists([[[1, 2, 3], [], [4, 5]]] * 52)
+    daa2 = daa.repartition(n_to_one=52)
+    assert daa2.npartitions == 1
+    assert daa.compute().to_list() == daa2.compute().to_list()
+    daa2 = daa.repartition(n_to_one=53)
+    assert daa2.npartitions == 1
+    assert daa.compute().to_list() == daa2.compute().to_list()
+    daa2 = daa.repartition(n_to_one=2)
+    assert daa2.npartitions == 26
+    assert daa.compute().to_list() == daa2.compute().to_list()
+    daa2 = daa.repartition(n_to_one=10)
+    assert daa2.npartitions == 6
+    assert daa.compute().to_list() == daa2.compute().to_list()
+
+
 def test_repartition_no_change(daa):
     daa1 = daa.repartition(divisions=(0, 5, 10, 15))
     assert daa1.npartitions == 3