Intersection bounds when requested minimum is negative

[rcomer]

🐛 Bug Report

#4059 aimed to fix cube.intersection so if a cell's bounds align with the requested maximum and minimum, the new bounds exactly span that range. This does not work if the requested minimum is negative.

How To Reproduce

import iris
import iris.coords
import iris.cube
import numpy as np

lon = iris.coords.DimCoord(
    np.arange(0.5, 360.5, 1), standard_name="longitude", units='degrees', circular=True)
lon.guess_bounds()
cube = iris.cube.Cube(range(360))
cube.add_dim_coord(lon, 0)

new_cube = cube.intersection(longitude=(-180, 180))
print(new_cube.coord("longitude")[0])
print(new_cube.coord("longitude")[-1])

Output:

DimCoord(array([-180.5]), bounds=array([[-181., -180.]]), standard_name='longitude', units=Unit('degrees'))
DimCoord(array([178.5]), bounds=array([[178., 179.]]), standard_name='longitude', units=Unit('degrees'))

Expected behaviour

Above code should produce:

DimCoord(array([-179.5]), bounds=array([[-180., -179.]]), standard_name='longitude', units=Unit('degrees'))
DimCoord(array([179.5]), bounds=array([[179., 180.]]), standard_name='longitude', units=Unit('degrees'))

Proposed fix

I think this line

iris/lib/iris/cube.py

Line 3190 in 274057b

if maximum % modulus not in cells:

should have been

if maximum - modulus not in cells:

though I have not checked this fix.

Had it been written this way, I do not think it would have triggered the bug highlighted at #4220.

Environment

• OS & Version: RHEL 7.9
• Iris Version: 3.0.2