Add dask laziness docs and regression tests (#1164) (#1165)

Reference page listing every public function's dask laziness level
(fully lazy / partially lazy / fully materialized) so users can plan
pipelines without reading source.  22 tests assert that functions
expected to stay lazy actually return dask collections.

Author: brendancol

docs/source/reference/dask_laziness.rst

@@ -0,0 +1,250 @@
+.. _reference.dask_laziness:
+
+*********************
+Dask backend behavior
+*********************
+
+When you pass a dask-backed ``DataArray`` to an xarray-spatial function, the
+result *should* also be dask-backed so your pipeline stays lazy until you call
+``.compute()``.  Most functions do this, but some algorithms need random access
+to the full array and have to materialize intermediate results.
+
+This page lists every public function and its laziness level so you can plan
+dask pipelines without reading source code.
+
+Laziness levels
+===============
+
+**Fully lazy** -- the function returns a dask array without triggering any
+computation.  Safe for arbitrarily large out-of-core datasets.
+
+**Partially lazy** -- the function computes small bounded statistics (scalars,
+quartiles, a ~20K sample) during setup, then returns a dask array for the main
+result.  The statistics are cheap; the heavy work stays lazy.
+
+**Fully materialized** -- the algorithm needs the entire array in memory
+(connected-component labeling, A* search, viewshed sweepline, etc.).  The
+result may be re-wrapped as dask, but the function calls ``.compute()``
+internally.  Watch your memory on large inputs.
+
+
+Terrain metrics
+===============
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``slope``
+     - Fully lazy
+     - ``map_overlap``, planar and geodesic
+   * - ``aspect``
+     - Fully lazy
+     - ``map_overlap``, planar and geodesic
+   * - ``curvature``
+     - Fully lazy
+     - ``map_overlap``
+   * - ``hillshade``
+     - Fully lazy
+     - ``map_overlap``
+   * - ``northness``
+     - Fully lazy
+     - Uses ``da.cos`` / ``da.deg2rad`` on aspect output
+   * - ``eastness``
+     - Fully lazy
+     - Uses ``da.sin`` / ``da.deg2rad`` on aspect output
+
+
+Focal operations
+================
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``mean``
+     - Fully lazy
+     - Iterative ``map_overlap``
+   * - ``apply``
+     - Fully lazy
+     - ``map_overlap`` with user kernel
+   * - ``focal_stats``
+     - Fully lazy
+     - Multiple stats via ``map_overlap``, 3D output
+   * - ``hotspots``
+     - Partially lazy
+     - Computes global mean and std, result is dask
+
+
+Classification
+==============
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``binary``
+     - Fully lazy
+     - ``map_blocks``
+   * - ``reclassify``
+     - Fully lazy
+     - ``map_blocks``
+   * - ``quantile``
+     - Partially lazy
+     - Computes percentiles from ~20K sample
+   * - ``natural_breaks``
+     - Partially lazy
+     - Computes Jenks breaks from ~20K sample + scalar max
+   * - ``equal_interval``
+     - Partially lazy
+     - Computes scalar min/max
+   * - ``std_mean``
+     - Partially lazy
+     - Computes scalar mean/std/max
+   * - ``head_tail_breaks``
+     - Partially lazy
+     - Computes O(log N) scalar means
+   * - ``percentiles``
+     - Partially lazy
+     - Computes percentiles from ~20K sample
+   * - ``maximum_breaks``
+     - Partially lazy
+     - Computes breaks from ~20K sample
+   * - ``box_plot``
+     - Partially lazy
+     - Computes scalar quartiles and max
+
+
+Normalization
+=============
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``rescale``
+     - Fully lazy
+     - ``da.nanmin`` / ``da.nanmax`` (lazy reductions)
+   * - ``standardize``
+     - Fully lazy
+     - ``da.nanmean`` / ``da.nanstd`` (lazy reductions)
+
+
+Visibility
+==========
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``viewshed``
+     - Fully materialized
+     - Sweepline algorithm needs random access
+   * - ``line_of_sight``
+     - Fully materialized
+     - Extracts 1D transect via ``.compute()``
+   * - ``cumulative_viewshed``
+     - Fully materialized
+     - Runs multiple viewshed calls
+   * - ``visibility_frequency``
+     - Fully materialized
+     - Wraps ``cumulative_viewshed``
+
+
+Morphology
+==========
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``sieve``
+     - Fully materialized
+     - Connected-component labeling needs the full array; result re-wrapped as dask
+
+
+Proximity
+=========
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``proximity``
+     - Fully materialized
+     - Distance computation needs full array
+   * - ``allocation``
+     - Fully materialized
+     - Nearest-source allocation
+   * - ``direction``
+     - Fully materialized
+     - Direction to nearest source
+
+
+Zonal
+=====
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``zonal_stats`` / ``stats``
+     - Partially lazy
+     - Groupby aggregation via dask dataframe
+   * - ``zonal_crosstab`` / ``crosstab``
+     - Partially lazy
+     - Groupby cross-tabulation
+   * - ``zonal_apply`` / ``apply``
+     - Fully lazy
+     - ``map_blocks`` per zone
+   * - ``regions``
+     - Fully materialized
+     - Connected-component labeling
+   * - ``trim``
+     - Fully lazy
+     - Lazy slicing
+   * - ``crop``
+     - Fully lazy
+     - Lazy slicing
+
+
+Pathfinding
+===========
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 20 50
+
+   * - Function
+     - Laziness
+     - Notes
+   * - ``a_star_search``
+     - Fully materialized
+     - A* needs random access and visited-set tracking
+   * - ``multi_stop_search``
+     - Fully materialized
+     - Iterative A*

docs/source/reference/index.rst

@@ -7,6 +7,7 @@ Reference
 .. toctree::
    :maxdepth: 2
 
+   dask_laziness
    classification
    dasymetric
    diffusion