[SparseArraysBase] Update README (#1595)

NDTensors/src/lib/SparseArraysBase/README.md

@@ -1,26 +1,83 @@
 # SparseArraysBase
 
-Defines a generic interface for sparse arrays in Julia.
+SparseArraysBase is a package that aims to expand on the sparse array functionality that is currently in Julia Base.
+While SparseArrays.jl is centered mostly around `SparseMatrixCSC` and the SuiteSparse library, here we wish to broaden the scope a bit, and consider generic sparse arrays.
+Abstractly, the mental model can be considered as a storage object that holds the stored values, and a bijection between the array indices and the indices of the storage.
+For now, we focus on providing efficient implementations of Dictionary of Key (DOK) type sparse storage formats, but may expand upon this in the future.
+As a result, for typical linear algebra routines, we still expect `SparseMatrixCSC` to be the object of choice.
+
+The design consists of roughly three components:
+- `AbstractSparseArray` interface functions
+- Overloaded Julia base methods
+- `SparseArrayDOK` struct that implements this
+
+## AbstractSparseArray
+
+The first part consists of typical functions that are useful in the context of sparse arrays.
+The minimal interface, which enables the usage of the rest of this package, consists of the following functions:
+
+| Signature | Description | Default |
+|-----------|-------------|---------|
+| `sparse_storage(a::AbstractArray)` | Returns the storage object of the sparse array | `a` |
+| `storage_index_to_index(a::AbstractArray, I)` | Converts a storage index to an array index | `I` |
+| `index_to_storage_index(a::AbstractArray, I)` | Converts an array index to a storage index | `I` |
+
+Using these primitives, several convenience functions are defined to facilitate the writing of sparse array algorithms.
+
+| Signature | Description | Default |
+|-----------|-------------|---------|
+| `storage_indices(a)` | Returns the indices of the storage | `eachindex(sparse_storage(a))` |
+| `stored_indices(a)` | Returns the indices of the stored values | `Iterators.map(Base.Fix1(storage_index_to_index, a), storage_indices(a))` |
+| `stored_length(a)` | Returns the number of stored values | `length(storage_indices(a))` |
+
+<!-- TODO: `getindex!`, `increaseindex!`, `sparse_map`, expose "zero" functionality?  -->
+
+Interesting to note here is that the design is such that we can define sparse arrays without having to subtype `AbstractSparseArray`.
+To achieve this, each function `f` is defined in terms of `sparse_f`, rather than directly overloading `f`.
+<!--
+TODO:
+In order to opt-in to the sparse array functionality, one needs to dispatch the functions through `sparse_f` instead of `f`.
+For convenience, you can automatically dispatch all functions through `sparse_f` by using the following macro:
 
-The minimal interface is:
-```julia
-nonzeros(a::AbstractArray) = ...
-nonzero_index_to_index(a::AbstractArray, Inz) = ...
-index_to_nonzero_index(a::AbstractArray{<:Any,N}, I::CartesianIndex{N}) where {N} = ...
-Broadcast.BroadcastStyle(arraytype::Type{<:AbstractArray}) = SparseArraysBase.SparseArrayStyle{ndims(arraytype)}()
-```
-Once these are defined, along with Julia AbstractArray interface functions like
-`Base.size` and `Base.similar`, functions like the following will take advantage of sparsity:
 ```julia
-SparseArraysBase.nonzero_length # SparseArrays.nnz
-SparseArraysBase.sparse_getindex
-SparseArraysBase.sparse_setindex!
-SparseArraysBase.sparse_map!
-SparseArraysBase.sparse_copy!
-SparseArraysBase.sparse_copyto!
-SparseArraysBase.sparse_permutedims!
+@abstractsparsearray MySparseArrayType
 ```
-which can be used to define the corresponding `Base` functions.
+-->
+
+## Overloaded Julia base methods
+
+The second part consists of overloading Julia base methods to work with sparse arrays.
+In particular, specialised implementations exist for the following functions:
+
+- `sparse_similar`
+- `sparse_reduce`
+- `sparse_map`
+- `sparse_map!`
+- `sparse_all`
+- `sparse_any`
+- `sparse_isequal`
+- `sparse_fill!`
+- `sparse_zero`, `sparse_zero!`, `sparse_iszero`
+- `sparse_one`, `sparse_one!`, `sparse_isone`
+- `sparse_reshape`, `sparse_reshape!`
+- `sparse_cat`, `sparse_cat!`
+- `sparse_copy!`, `sparse_copyto!`
+- `sparse_permutedims`, `sparse_permutedims!`
+- `sparse_mul!`, `sparse_dot`
+
+## SparseArrayDOK
+
+Finally, the `SparseArrayDOK` struct is provided as a concrete implementation of the `AbstractSparseArray` interface.
+It is a dictionary of keys (DOK) type sparse array, which stores the values in a `Dictionaries.jl` dictionary, and maps the indices to the keys of the dictionary.
+This model is particularly useful for sparse arrays with a small number of non-zero elements, or for arrays that are constructed incrementally, as it boasts fast random accesses and insertions.
+The drawback is that sequential iteration is slower than for other sparse array types, leading to slower linear algebra operations.
+For the purposes of `SparseArraysBase`, this struct will serve as the canonical example of a sparse array, and will be returned by default when new sparse arrays are created.
+
+One particular feature of `SparseArrayDOK` is that it can be used in cases where the non-stored entries have to be constructed in a non-trivial way.
+Typically, sparse arrays use `zero(eltype(a))` to construct the non-stored entries, but this is not always sufficient.
+A concrete example is found in `BlockSparseArrays.jl`, where initialization of the non-stored entries requires the construction of a block of zeros of appropriate size.
+
+<!-- TODO: update TODOs -->
 
 ## TODO
 Still need to implement `Base` functions: