Document map_selection core operation (#617)

* Document `map_selection` core operation.

* Rework dependency tree diagram for core and primitive ops to include `map_selection`

* Update five-layer design diagram

* Add rechunk apidoc

* Add note about `general_blockwise`

* Fix double backticks

cubed/core/ops.py

@@ -775,6 +775,28 @@ def map_selection(
     max_num_input_blocks,
     **kwargs,
 ) -> "Array":
+    """
+    Apply a function to selected subsets of an input array using standard NumPy indexing notation.
+
+    Parameters
+    ----------
+    func : callable
+        Function to apply to every block to produce the output array.
+        Must accept ``block_id`` as a keyword argument (with same meaning as for ``map_blocks``).
+    selection_function : callable
+        A function that maps an output chunk key to one or more selections on the input array.
+    x: Array
+        The input array.
+    shape : tuple
+        Shape of the output array.
+    dtype : np.dtype
+        The ``dtype`` of the output array.
+    chunks : tuple
+        Chunk shape of blocks in the output array.
+    max_num_input_blocks : int
+        The maximum number of input blocks read from the input array.
+    """
+
     def key_function(out_key):
         # compute the selection on x required to get the relevant chunk for out_key
         in_sel = selection_function(out_key)
@@ -1009,6 +1031,18 @@ def wrap(*a, block_id=None, **kw):
 
 
 def rechunk(x, chunks, target_store=None):
+    """Change the chunking of an array without changing its shape or data.
+
+    Parameters
+    ----------
+    chunks : tuple
+        The desired chunks of the array after rechunking.
+
+    Returns
+    -------
+    cubed.Array
+        An array with the desired chunks.
+    """
     if isinstance(chunks, dict):
         chunks = {validate_axis(c, x.ndim): v for c, v in chunks.items()}
         for i in range(x.ndim):

docs/design.md

@@ -4,7 +4,7 @@ Cubed is composed of five layers: from the storage layer at the bottom, to the A
 
 ![Five layer diagram](images/design.svg)
 
-Blue blocks are implemented in Cubed, green in Rechunker, and red in other projects like Zarr and Beam.
+Blue blocks are implemented in Cubed; red blocks in other projects like Zarr and Lithops.
 
 Let's go through the layers from the bottom:
 
@@ -14,7 +14,7 @@ Every _array_ in Cubed is backed by a Zarr array. This means that the array type
 
 ## Runtime
 
-Cubed uses external runtimes for computation. It follows the Rechunker model (and uses its algorithm) to delegate tasks to stateless executors, which include Python (in-process), Lithops, Modal, and Apache Beam.
+Cubed uses external runtimes for computation, delegating tasks to stateless executors, which include Python (in-process), Lithops, Modal, and Apache Beam.
 
 
 ## Primitive operations
@@ -45,8 +45,7 @@ These are built on top of the primitive operations, and provide functions that a
 
     elemwise
     map_blocks
-    map_direct
-    index
+    map_selection
     reduction
     arg_reduction
 ```

docs/images/ops.dot

@@ -11,21 +11,20 @@ digraph {
     // core
     elemwise [style="filled"; fillcolor="#ffd8b1";];
     map_blocks [style="filled"; fillcolor="#ffd8b1";];
-    map_direct [style="filled"; fillcolor="#ffd8b1";];
+    map_selection [style="filled"; fillcolor="#ffd8b1";];
     reduction [style="filled"; fillcolor="#ffd8b1";];
     arg_reduction [style="filled"; fillcolor="#ffd8b1";];
 
     elemwise -> blockwise;
     map_blocks -> blockwise;
-    map_direct -> map_blocks;
+    map_selection -> blockwise;
     reduction -> blockwise;
-    reduction -> rechunk;
     arg_reduction -> reduction;
 
     // array API
 
     // array object
-    __getitem__ -> map_direct
+    __getitem__ -> map_selection
 
     // elementwise
     add -> elemwise
@@ -34,12 +33,11 @@ digraph {
     // linear algebra
     matmul -> blockwise;
     matmul -> reduction;
-    outer -> blockwise;
 
     // manipulation
-    concat -> map_direct;
+    concat -> blockwise;
     reshape -> rechunk;
-    reshape -> map_direct;
+    reshape -> blockwise;
     squeeze -> map_blocks;
 
     // searching
@@ -51,18 +49,17 @@ digraph {
     // utility
     all -> reduction;
 
-
     {
         rank = min;
 
         // fix horizontal placing with invisible edges
         edge[style=invis];
-        add -> negative -> outer -> matmul -> __getitem__ -> concat -> reshape -> squeeze -> argmax -> sum -> all;
+        add -> negative -> squeeze -> __getitem__ -> concat -> matmul -> sum -> all -> argmax -> reshape;
         rankdir = LR;
     }
     {
         rank = same;
-        elemwise; map_blocks; reduction;
+        elemwise; map_blocks; map_selection; reduction;
     }
     {
         rank = max;