Speed up operations that use the Coord.cells method for time coordi…

…nates (#4969)

* Convert all time points at once before generating cells

* Small simplification

* Add a whatnew entry

* Fix link

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

docs/src/whatsnew/latest.rst

@@ -71,6 +71,10 @@ This document explains the changes made to Iris for this release
    :obj:`~iris.analysis.SUM`, :obj:`~iris.analysis.COUNT` and
    :obj:`~iris.analysis.PROPORTION` on real data. (:pull:`4905`)
 
+#. `@bouweandela`_ made :meth:`iris.coords.Coord.cells` faster for time
+   coordinates. This also affects :meth:`iris.cube.Cube.extract`,
+   :meth:`iris.cube.Cube.subset`, and :meth:`iris.coords.Coord.intersect`.
+   (:pull:`4969`)
 
 🔥 Deprecations
 ===============

lib/iris/coords.py

@@ -1895,7 +1895,22 @@ def cells(self):
               ...
 
         """
-        return _CellIterator(self)
+        if self.ndim != 1:
+            raise iris.exceptions.CoordinateMultiDimError(self)
+
+        points = self.points
+        bounds = self.bounds
+        if self.units.is_time_reference():
+            points = self.units.num2date(points)
+            if self.has_bounds():
+                bounds = self.units.num2date(bounds)
+
+        if self.has_bounds():
+            for point, bound in zip(points, bounds):
+                yield Cell(point, bound)
+        else:
+            for point in points:
+                yield Cell(point)
 
     def _sanity_check_bounds(self):
         if self.ndim == 1:
@@ -3137,22 +3152,6 @@ def xml_element(self, doc):
         return cellMethod_xml_element
 
 
-# See Coord.cells() for the description/context.
-class _CellIterator(Iterator):
-    def __init__(self, coord):
-        self._coord = coord
-        if coord.ndim != 1:
-            raise iris.exceptions.CoordinateMultiDimError(coord)
-        self._indices = iter(range(coord.shape[0]))
-
-    def __next__(self):
-        # NB. When self._indices runs out it will raise StopIteration for us.
-        i = next(self._indices)
-        return self._coord.cell(i)
-
-    next = __next__
-
-
 # See ExplicitCoord._group() for the description/context.
 class _GroupIterator(Iterator):
     def __init__(self, points):