From a5d3793c6a08ec737cd7eb512049561a046b2d6d Mon Sep 17 00:00:00 2001
From: Ruth Comer <ruth.comer@metoffice.gov.uk>
Date: Tue, 2 Mar 2021 17:20:02 +0000
Subject: [PATCH 1/3] fix intersection out of bounds point

---
 lib/iris/cube.py                      | 21 +++++++++++----------
 lib/iris/tests/unit/cube/test_Cube.py |  8 ++++++++
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index f280891fde..00bf11d2fe 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -3185,18 +3185,19 @@ 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:
+                    # Recalculate the extended minimum.
+                    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])
 
             points = wrap_lons(coord.points, minimum, modulus)
 
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 9fe90f5a4e..fbf76b4260 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -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))

From cf62f66878a5951c375654c1b3db7a19f474c22f Mon Sep 17 00:00:00 2001
From: Ruth Comer <ruth.comer@metoffice.gov.uk>
Date: Wed, 10 Mar 2021 15:00:47 +0000
Subject: [PATCH 2/3] fix intersection out of bounds point take 2

---
 docs/iris/src/whatsnew/3.0.2.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/docs/iris/src/whatsnew/3.0.2.rst b/docs/iris/src/whatsnew/3.0.2.rst
index 90ce970326..769b673a4c 100644
--- a/docs/iris/src/whatsnew/3.0.2.rst
+++ b/docs/iris/src/whatsnew/3.0.2.rst
@@ -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`)
+
    📚 **Documentation**
 
    #. `@bjlittle`_ updated the ``intersphinx_mapping`` and fixed documentation

From 71abfbded884873ae4f618168a4f7d4c6c8da620 Mon Sep 17 00:00:00 2001
From: Ruth Comer <ruth.comer@metoffice.gov.uk>
Date: Thu, 6 May 2021 10:37:45 +0100
Subject: [PATCH 3/3] add referencing comment

---
 lib/iris/cube.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 00bf11d2fe..129542fb21 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -3188,7 +3188,10 @@ def _intersect_modulus(
                 indices = inside_indices[split_cell_indices]
                 cells = bounds[indices]
                 if maximum % modulus not in cells:
-                    # Recalculate the extended minimum.
+                    # 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.