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Intersection bounds fix (replacement PR) #4059

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merged 3 commits into from May 13, 2021
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Intersection bounds fix (replacement PR) #4059

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a5d3793
fix intersection out of bounds point
rcomer Mar 2, 2021
cf62f66
fix intersection out of bounds point take 2
rcomer Mar 10, 2021
71abfbd
add referencing comment
rcomer May 6, 2021
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4 changes: 4 additions & 0 deletions docs/iris/src/whatsnew/3.0.2.rst
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Expand Up @@ -21,6 +21,10 @@ This document explains the changes made to Iris for this release
#. `@jonseddon`_ handled a malformed ``um_stash_source`` CF variable attribute in
a netCDF file rather than raising a ``ValueError``. (:pull:`4035`)

#. `@rcomer`_ fixed :meth:`~iris.cube.Cube.intersection` for special cases
where one cell's bounds align with the requested maximum and minimum, as
reported in :issue:`3391`. (:pull:`4059`)

馃摎 **Documentation**

#. `@bjlittle`_ updated the ``intersphinx_mapping`` and fixed documentation
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24 changes: 14 additions & 10 deletions lib/iris/cube.py
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Expand Up @@ -3185,18 +3185,22 @@ def _intersect_modulus(
pre_wrap_delta != post_wrap_delta
)

# Recalculate the extended minimum.
indices = inside_indices[split_cell_indices]
cells = bounds[indices]
cells_delta = np.diff(coord.bounds[indices])

# Watch out for ascending/descending bounds
if cells_delta[0, 0] > 0:
cells[:, 0] = cells[:, 1] - cells_delta[:, 0]
minimum = np.min(cells[:, 0])
else:
cells[:, 1] = cells[:, 0] + cells_delta[:, 0]
minimum = np.min(cells[:, 1])
if maximum % modulus not in cells:
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Perhaps a comment here, even referencing the exact problem that this code is aiming to solve.

# Recalculate the extended minimum only if the output bounds
# do not span the requested (minumum, maximum) range. If
# they do span that range, this adjustment would give unexpected
# results (see #3391).
cells_delta = np.diff(coord.bounds[indices])

# Watch out for ascending/descending bounds.
if cells_delta[0, 0] > 0:
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Previously, minimum would have been set by either branch of this if statement on every call. Now that it is contained within if maximum % modulus not in cells it may not be. Should there be an else to the if on L3135?

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It's only been two months but I had completely forgotten the logic here! I think this is fine though: we don't explicitly set minimum here, so it is just left to whatever minimum was originally passed into the method. Note that because of the previous logic

maximum - modulus <= minimum (line 3081)
cells >= minimum (line 3098)

So the only way that maximum - modulus in cells will be True is if maximum - modulus == minimum and minimum is in cells. I.e. the very specific case described in #3391. In that case, it is expected that the bounds on the output coordinate will precisely span the requested (minimum, maximum) range.

Definitely needs a comment though!

cells[:, 0] = cells[:, 1] - cells_delta[:, 0]
minimum = np.min(cells[:, 0])
else:
cells[:, 1] = cells[:, 0] + cells_delta[:, 0]
minimum = np.min(cells[:, 1])

points = wrap_lons(coord.points, minimum, modulus)

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8 changes: 8 additions & 0 deletions lib/iris/tests/unit/cube/test_Cube.py
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Expand Up @@ -1731,6 +1731,14 @@ def test_aligned_exclusive(self):
self.assertEqual(result.data[0, 0, 0], 171)
self.assertEqual(result.data[0, 0, -1], 189)

def test_aligned_bounds_at_modulus(self):
cube = create_cube(-179.5, 180.5, bounds=True)
result = cube.intersection(longitude=(0, 360))
self.assertArrayEqual(result.coord("longitude").bounds[0], [0, 1])
self.assertArrayEqual(result.coord("longitude").bounds[-1], [359, 360])
self.assertEqual(result.data[0, 0, 0], 180)
self.assertEqual(result.data[0, 0, -1], 179)

def test_negative_misaligned_points_inside(self):
cube = create_cube(0, 360, bounds=True)
result = cube.intersection(longitude=(-10.25, 10.25))
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