Use simpler function in {first|last}_run (#1530)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #1405
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] CHANGES.rst has been updated (with summary of main changes)
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### What kind of change does this PR introduce?

* Use an intermediate step instead of `rle` in first/last run
computations

### Does this PR introduce a breaking change?
No

### Other information:

CHANGES.rst

@@ -4,7 +4,7 @@ Changelog
 
 v0.47.0 (unreleased)
 --------------------
-Contributors to this version: Juliette Lavoie (:user:`juliettelavoie`), Pascal Bourgault (:user:`aulemahal`), Trevor James Smith (:user:`Zeitsperre`), David Huard (:user:`huard`).
+Contributors to this version: Juliette Lavoie (:user:`juliettelavoie`), Pascal Bourgault (:user:`aulemahal`), Trevor James Smith (:user:`Zeitsperre`), David Huard (:user:`huard`), Éric Dupuis (:user:`coxipi`).
 
 Announcements
 ^^^^^^^^^^^^^
@@ -20,14 +20,15 @@ Bug fixes
 ^^^^^^^^^
 * Fixed a bug with ``n_escore=-1`` in ``xclim.sdba.adjustment.NpdfTransform``. (:issue:`1515`, :pull:`1515`).
 * In the documentation, fix the tooltips in the indicator search results. (:issue:`1524`, :pull:`1527`).
-* If chunked inputs are passed to indicators ``mean_radiant_temperature``  and ``potential_evapotranspiration``, sub-calculations of the solar angle will also use the same chunks, instead of a single one of the same size as the data. (:issue:`1536`, :pull:`1542`).
+* If chunked inputs are passed to indicators ``mean_radiant_temperature`` and ``potential_evapotranspiration``, sub-calculations of the solar angle will also use the same chunks, instead of a single one of the same size as the data. (:issue:`1536`, :pull:`1542`).
 
 Internal changes
 ^^^^^^^^^^^^^^^^
 * Pinned `cf-xarray` below v0.8.5 in Python3.8 installation to further extend legacy support. (:pull:`1519`).
 * `pip check` in conda builds in GitHub workflows have been temporarily set to always pass. (:pull:`1531`).
 * Configure RtD search rankings to emphasize notebooks and indicators over indices and raw source code. (:pull:`1526`).
 * Addressed around 100 very basic `mypy` typing errors and call signature errors. (:pull:`1532`).
+* Use the intermediate step ``_cumsum_reset_on_zero`` instead of ``rle`` which is sufficient in ``_boundary_run``. (:issue:`1405`, :pull:`1530`).
 
 v0.46.0 (2023-10-24)
 --------------------

xclim/indices/run_length.py

@@ -416,7 +416,7 @@ def _boundary_run(
     ufunc_1dim: str | bool,
     position: str,
 ) -> xr.DataArray:
-    """Return the index of the first item of the first run of at least a given length.
+    """Return the index of the first item of the first or last run of at least a given length.
 
     Parameters
     ----------
@@ -493,9 +493,10 @@ def find_boundary_run(runs, position):
         out = coord_transform(out, da)
 
     else:
-        # for "first" run, return "first" element in the run (and conversely for "last" run)
-        d = rle(da, dim=dim, index=position)
+        # _cusum_reset_on_zero() is an intermediate step in rle, which is sufficient here
+        d = _cumsum_reset_on_zero(da, dim=dim, index=position)
         d = xr.where(d >= window, 1, 0)
+        # for "first" run, return "first" element in the run (and conversely for "last" run)
         if freq is not None:
             out = d.resample({dim: freq}).map(find_boundary_run, position=position)
         else: