Small fixes to DiskChopper.from_nexus

src/scippneutron/chopper/disk_chopper.py

@@ -308,11 +308,16 @@ def from_nexus(
 
         Keys in the input correspond to the fields of `NXdisk_chopper
         <https://manual.nexusformat.org/classes/base_classes/NXdisk_chopper.html>`_.
-        The values have to be post-processed, e.g., with
-        :func:`~scippneutron.chopper.nexus_chopper.post_process_disk_chopper`.
-        See its documentation for the required steps.
-        Also, note the class docs of :class:`DiskChopper`
-        about time-dependent fields.
+
+        The input data group must contain a `rotation_speed_setpoint`. If this is not
+        available, a single (scalar) value must be computed from processing the
+        time-dependent `rotation_speed` data. This can be done with
+        :func:`~scippneutron.chopper.nexus_chopper.post_process_disk_chopper`
+        (see its documentation for the required steps). The resulting single value
+        can then be passed as `rotation_speed_setpoint`.
+
+        If a `phase` field is not present, it is computed from the `delay` field
+        and the `rotation_speed_setpoint` as `phase = 2*pi*frequency*delay`.
 
         Parameters
         ----------
@@ -332,11 +337,28 @@ def from_nexus(
                 'Class DiskChopper only supports single choppers,'
                 f'got chopper type {typ}'
             )
+
+        if "rotation_speed_setpoint" in chopper:
+            # `rotation_speed_setpoint` is preferred if available (ESS files)
+            frequency = _get_0d_variable(chopper, 'rotation_speed_setpoint')
+        else:
+            # Fallback to `rotation_speed` (standard NeXus)
+            frequency = _get_0d_variable(chopper, 'rotation_speed')
+        if "phase" in chopper:
+            phase = _get_0d_variable(chopper, 'phase')
+        else:
+            if "delay" not in chopper:
+                raise ValueError(
+                    "DiskChopper.from_nexus: Chopper field 'phase' is missing and "
+                    "cannot be computed because field 'delay' is also missing."
+                )
+            omega = 2 * sc.constants.pi * frequency * sc.scalar(1.0, unit='rad')
+            phase = (chopper["delay"].to(dtype=float) * omega).to(unit='rad')
         return DiskChopper(
             axle_position=chopper['position'],
-            frequency=_get_1d_variable(chopper, 'rotation_speed'),
-            beam_position=_get_1d_variable(chopper, 'beam_position'),
-            phase=_get_1d_variable(chopper, 'phase'),
+            frequency=frequency,
+            beam_position=_get_0d_variable(chopper, 'beam_position'),
+            phase=phase,
             slit_height=chopper.get('slit_height'),
             radius=chopper.get('radius'),
             **_get_edges_from_nexus(chopper),
@@ -557,6 +579,10 @@ def _source_phase_factor(self, pulse_frequency: sc.Variable) -> int:
         # of the slits, so use `max` here:
         return round(max(quot.value, 1))
 
+    def as_dict(self) -> dict[str, Any]:
+        """Return the DiskChopper fields as a dictionary."""
+        return dataclasses.asdict(self)
+
 
 def _field_eq(a: Any, b: Any) -> bool:
     if isinstance(a, sc.Variable | sc.DataArray):
@@ -627,13 +653,7 @@ def _get_edges_from_nexus(
     }
 
 
-def _len_or_1(x: sc.Variable) -> int:
-    if x.ndim == 0:
-        return 1
-    return len(x)
-
-
-def _get_1d_variable(
+def _get_0d_variable(
     dg: Mapping[str, sc.Variable | sc.DataArray], name: str
 ) -> sc.Variable:
     if (val := dg.get(name)) is None:
@@ -642,9 +662,11 @@ def _get_1d_variable(
     msg = (
         "Chopper field '{name}' must be a scalar variable, {got}. "
         "See the chopper user-guide for more information: "
-        "https://scipp.github.io/scippneutron/user-guide/chopper/pre-processing.html"
+        "https://scipp.github.io/scippneutron/user-guide/chopper/processing-nexus-choppers.html"
     )
 
+    if isinstance(val, sc.DataArray):
+        val = val.data
     if not isinstance(val, sc.Variable):
         raise TypeError(msg.format(name=name, got=f'got a {type(val)}'))
     if val.ndim != 0:

tests/chopper/disk_chopper_test.py

@@ -18,13 +18,14 @@ def deg_angle_to_time_factor(frequency: sc.Variable) -> sc.Variable:
     return sc.to_unit(to_rad / angular_frequency, 's/deg')
 
 
-@pytest.fixture
-def nexus_chopper():
+@pytest.fixture(params=['rotation_speed', 'rotation_speed_setpoint'])
+def nexus_chopper(request):
+    rotation_speed_key = request.param
     return sc.DataGroup(
         {
             'type': DiskChopperType.single,
             'position': sc.vector([0.0, 0.0, 2.0], unit='m'),
-            'rotation_speed': sc.scalar(12.0, unit='Hz'),
+            rotation_speed_key: sc.scalar(12.0, unit='Hz'),
             'beam_position': sc.scalar(45.0, unit='deg'),
             'phase': sc.scalar(-20.0, unit='deg'),
             'slit_edges': sc.array(
@@ -38,6 +39,14 @@ def nexus_chopper():
     )
 
 
+def _get_rotation_speed_key(nexus_chopper):
+    return (
+        'rotation_speed_setpoint'
+        if 'rotation_speed_setpoint' in nexus_chopper
+        else 'rotation_speed'
+    )
+
+
 @pytest.mark.parametrize(
     'typ',
     [
@@ -54,7 +63,7 @@ def test_chopper_supports_only_single(nexus_chopper, typ):
 
 
 def test_frequency_must_be_frequency(nexus_chopper):
-    nexus_chopper['rotation_speed'] = sc.scalar(1.0, unit='m/s')
+    nexus_chopper[_get_rotation_speed_key(nexus_chopper)] = sc.scalar(1.0, unit='m/s')
     with pytest.raises(sc.UnitError):
         DiskChopper.from_nexus(nexus_chopper)
 
@@ -78,6 +87,11 @@ def test_eq(nexus_chopper):
 )
 def test_neq(nexus_chopper, replacement):
     ch1 = DiskChopper.from_nexus(nexus_chopper)
+    if (
+        replacement[0] == 'rotation_speed'
+        and 'rotation_speed_setpoint' in nexus_chopper
+    ):
+        replacement = ('rotation_speed_setpoint', replacement[1])
     ch2 = DiskChopper.from_nexus({**nexus_chopper, replacement[0]: replacement[1]})
     assert ch1 != ch2
 
@@ -212,7 +226,7 @@ def test_time_offset_angle_at_beam_no_phase_zero_beam_pos_clockwise_single_angle
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit=beam_position_unit),
             'phase': sc.scalar(0.0, unit=phase_unit),
-            'rotation_speed': sc.scalar(-2.3, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-2.3, unit='Hz'),
         }
     )
     omega = 2 * pi * 2.3
@@ -250,7 +264,7 @@ def test_time_offset_angle_at_beam_no_phase_zero_beam_pos_anti_clockwise_single_
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit=beam_position_unit),
             'phase': sc.scalar(0.0, unit=phase_unit),
-            'rotation_speed': sc.scalar(2.3, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(2.3, unit='Hz'),
         }
     )
     omega = 2 * pi * 2.3
@@ -287,7 +301,7 @@ def test_time_offset_angle_at_beam_no_phase_zero_beam_pos_clockwise_multi_angle(
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit=beam_position_unit),
             'phase': sc.scalar(0.0, unit=phase_unit),
-            'rotation_speed': sc.scalar(-4.4, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-4.4, unit='Hz'),
         }
     )
     omega = 2 * pi * 4.4
@@ -312,7 +326,7 @@ def test_time_offset_angle_at_beam_no_phase_zero_beam_pos_anti_clockwise_multi_a
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit=beam_position_unit),
             'phase': sc.scalar(0.0, unit=phase_unit),
-            'rotation_speed': sc.scalar(4.4, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(4.4, unit='Hz'),
         }
     )
     omega = 2 * pi * 4.4
@@ -335,7 +349,7 @@ def test_time_offset_angle_at_beam_no_phase_with_beam_pos_clockwise(
             **nexus_chopper,
             'beam_position': sc.scalar(1.8, unit='rad'),
             'phase': sc.scalar(0.0, unit=phase_unit),
-            'rotation_speed': sc.scalar(-2.3, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-2.3, unit='Hz'),
         }
     )
     omega = 2 * pi * 2.3
@@ -368,7 +382,7 @@ def test_time_offset_angle_at_beam_no_phase_with_beam_pos_anti_clockwise(
             **nexus_chopper,
             'beam_position': sc.scalar(1.8, unit='rad'),
             'phase': sc.scalar(0.0, unit=phase_unit),
-            'rotation_speed': sc.scalar(2.3, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(2.3, unit='Hz'),
         }
     )
     omega = 2 * pi * 2.3
@@ -401,7 +415,7 @@ def test_time_offset_angle_at_beam_with_phase_zero_beam_pos_clockwise(
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit=beam_position_unit),
             'phase': sc.scalar(-0.7, unit='rad'),
-            'rotation_speed': sc.scalar(-1.1, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-1.1, unit='Hz'),
         }
     )
     omega = 2 * pi * 1.1
@@ -435,7 +449,7 @@ def test_time_offset_angle_at_beam_with_phase_zero_beam_pos_anti_clockwise(
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit=beam_position_unit),
             'phase': sc.scalar(-0.7, unit='rad'),
-            'rotation_speed': sc.scalar(1.1, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(1.1, unit='Hz'),
         }
     )
     omega = 2 * pi * 1.1
@@ -492,7 +506,9 @@ def test_time_offset_open_close_only_slit(nexus_chopper, rotation_speed):
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit='rad'),
             'phase': sc.scalar(0.0, unit='rad'),
-            'rotation_speed': sc.scalar(rotation_speed, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(
+                rotation_speed, unit='Hz'
+            ),
             'slit_edges': sc.array(
                 dims=['slit'],
                 values=[0.0, 360.0],
@@ -538,7 +554,7 @@ def test_time_offset_open_close_one_slit_clockwise(nexus_chopper, phase, beam_po
             **nexus_chopper,
             'beam_position': beam_position,
             'phase': phase,
-            'rotation_speed': sc.scalar(-7.21, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-7.21, unit='Hz'),
             'slit_edges': sc.array(dims=['slit'], values=[87.0, 177.0], unit='deg'),
         }
     )
@@ -584,7 +600,7 @@ def test_time_offset_open_close_one_slit_anticlockwise(
             **nexus_chopper,
             'beam_position': beam_position,
             'phase': phase,
-            'rotation_speed': sc.scalar(7.21, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(7.21, unit='Hz'),
             'slit_edges': sc.array(dims=['slit'], values=[87.0, 177.0], unit='deg'),
         }
     )
@@ -630,7 +646,7 @@ def test_time_offset_open_close_one_slit_across_tdc_clockwise(
             **nexus_chopper,
             'beam_position': beam_position,
             'phase': phase,
-            'rotation_speed': sc.scalar(-7.21, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-7.21, unit='Hz'),
             'slit_edges': sc.array(dims=['slit'], values=[330.0, 380.0], unit='deg'),
         }
     )
@@ -676,7 +692,7 @@ def test_time_offset_open_close_one_slit_across_tdc_anticlockwise(
             **nexus_chopper,
             'beam_position': beam_position,
             'phase': phase,
-            'rotation_speed': sc.scalar(7.21, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(7.21, unit='Hz'),
             'slit_edges': sc.array(dims=['slit'], values=[330.0, 380.0], unit='deg'),
         }
     )
@@ -704,7 +720,7 @@ def test_time_offset_open_close_two_slits_clockwise(nexus_chopper):
             **nexus_chopper,
             'beam_position': sc.scalar(-32.0, unit='deg'),
             'phase': sc.scalar(0.0, unit='deg'),
-            'rotation_speed': sc.scalar(-11.2, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-11.2, unit='Hz'),
             'slit_edges': sc.array(
                 dims=['slit'],
                 values=[87.0, 177.0, 280.0, 342.0],
@@ -743,7 +759,7 @@ def test_time_offset_open_close_two_slits_anticlockwise(nexus_chopper):
             **nexus_chopper,
             'beam_position': sc.scalar(-32.0, unit='deg'),
             'phase': sc.scalar(0.0, unit='deg'),
-            'rotation_speed': sc.scalar(11.2, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(11.2, unit='Hz'),
             'slit_edges': sc.array(
                 dims=['slit'],
                 values=[87.0, 177.0, 280.0, 342.0],
@@ -782,7 +798,7 @@ def test_time_offset_open_close_two_slits_clockwise_two_pulses(nexus_chopper):
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit='deg'),
             'phase': sc.scalar(60.0, unit='deg'),
-            'rotation_speed': sc.scalar(-14.0, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-14.0, unit='Hz'),
             'slit_edges': sc.array(
                 dims=['slit'],
                 values=[87.0, 177.0, 200.0, 240.0],
@@ -837,7 +853,7 @@ def test_time_offset_open_close_two_slits_anticlockwise_two_pulses(nexus_chopper
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit='deg'),
             'phase': sc.scalar(60.0, unit='deg'),
-            'rotation_speed': sc.scalar(14.0, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(14.0, unit='Hz'),
             'slit_edges': sc.array(
                 dims=['slit'],
                 values=[87.0, 177.0, 200.0, 240.0],
@@ -892,7 +908,7 @@ def test_time_offset_open_close_two_slits_clockwise_half_pulse(nexus_chopper):
             **nexus_chopper,
             'beam_position': sc.scalar(0.0, unit='deg'),
             'phase': sc.scalar(60.0, unit='deg'),
-            'rotation_speed': sc.scalar(-3.5, unit='Hz'),
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(-3.5, unit='Hz'),
             'slit_edges': sc.array(
                 dims=['slit'],
                 values=[87.0, 177.0, 200.0, 240.0],
@@ -928,7 +944,10 @@ def test_time_offset_open_close_two_slits_clockwise_half_pulse(nexus_chopper):
 
 def test_time_offset_open_close_source_frequency_not_multiple_of_chopper(nexus_chopper):
     ch = DiskChopper.from_nexus(
-        {**nexus_chopper, 'rotation_speed': sc.scalar(4.52, unit='Hz')}
+        {
+            **nexus_chopper,
+            _get_rotation_speed_key(nexus_chopper): sc.scalar(4.52, unit='Hz'),
+        }
     )
     with pytest.raises(ValueError, match='out of phase'):
         ch.time_offset_open(pulse_frequency=sc.scalar(4.3, unit='Hz'))
@@ -945,3 +964,40 @@ def test_disk_chopper_svg_custom_dim_names(nexus_chopper):
     nexus_chopper['slit_edges'] = nexus_chopper['slit_edges'].rename_dims(slit='dim_0')
     ch = DiskChopper.from_nexus(nexus_chopper)
     assert ch.make_svg()
+
+
+def test_phase_derived_from_delay_and_frequency(nexus_chopper):
+    delay = sc.scalar(5e7, unit='ns')
+    frequency = sc.scalar(14.0, unit='Hz')
+    expected_phase = (2 * pi * frequency * delay * sc.scalar(1.0, unit='rad')).to(
+        unit='deg'
+    )
+
+    chopper_params = nexus_chopper.copy()
+    del chopper_params['phase']
+
+    ch = DiskChopper.from_nexus(
+        {
+            **chopper_params,
+            'delay': delay,
+            _get_rotation_speed_key(nexus_chopper): frequency,
+        }
+    )
+    assert sc.allclose(ch.phase.to(unit='deg'), expected_phase)
+
+
+def test_missing_both_phase_and_delay_raises(nexus_chopper):
+    chopper_params = nexus_chopper.copy()
+    del chopper_params['phase']
+
+    with pytest.raises(
+        ValueError,
+        match="DiskChopper.from_nexus: Chopper field 'phase' is missing and "
+        "cannot be computed because field 'delay' is also missing.",
+    ):
+        DiskChopper.from_nexus(
+            {
+                **chopper_params,
+                _get_rotation_speed_key(nexus_chopper): sc.scalar(14.0, unit='Hz'),
+            }
+        )