apacheGH-38655: [C++] "iso_calendar" kernel returns incorrect results…

… for array length > 32 (apache#39360)

### Rationale for this change

When defining `StructArray`'s field builders for `ISOCalendar` we don't pre-allocate memory and then use unsafe append. This causes the resulting array to be at most 32 rows long.

### What changes are included in this PR?

This introduces required memory pre-allocation in the `ISOCalendar` c++ kernel.

### Are these changes tested?

This adds a test for the Python wrapper.

### Are there any user-facing changes?

Fixes the behavior of `iso_calendar` kernel.
* Closes: apache#38655

Lead-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

cpp/src/arrow/compute/kernels/scalar_temporal_unary.cc

@@ -1510,7 +1510,7 @@ struct ISOCalendar {
     for (int i = 0; i < 3; i++) {
       field_builders.push_back(
           checked_cast<BuilderType*>(struct_builder->field_builder(i)));
-      RETURN_NOT_OK(field_builders[i]->Reserve(1));
+      RETURN_NOT_OK(field_builders[i]->Reserve(in.length));
     }
     auto visit_null = [&]() { return struct_builder->AppendNull(); };
     std::function<Status(typename InType::c_type arg)> visit_value;

python/pyarrow/tests/test_compute.py

@@ -2263,6 +2263,19 @@ def test_extract_datetime_components():
             _check_datetime_components(timestamps, timezone)
 
 
+@pytest.mark.parametrize("unit", ["s", "ms", "us", "ns"])
+def test_iso_calendar_longer_array(unit):
+    # https://github.com/apache/arrow/issues/38655
+    # ensure correct result for array length > 32
+    arr = pa.array([datetime.datetime(2022, 1, 2, 9)]*50, pa.timestamp(unit))
+    result = pc.iso_calendar(arr)
+    expected = pa.StructArray.from_arrays(
+        [[2021]*50, [52]*50, [7]*50],
+        names=['iso_year', 'iso_week', 'iso_day_of_week']
+    )
+    assert result.equals(expected)
+
+
 @pytest.mark.pandas
 @pytest.mark.skipif(sys.platform == "win32" and not util.windows_has_tzdata(),
                     reason="Timezone database is not installed on Windows")