std.parallelism and multidimensional arrays

[Vlad Levenfeld via Digitalmars-d-learn]

On Friday, 22 May 2015 at 10:54:36 UTC, Stefan Frijters wrote:
> I have a code which does a lot of work on 2D/3D arrays, for 
> which I use the 2.066 multidimensional slicing syntax through a 
> fork of the Unstandard package [1].
> Many times the order of operations doesn't matter and I thought 
> I would give the parallelism module a try to try and get some 
> easy speedups (I also use MPI, but that has some additional 
> overhead).
>
> The way I currently have my foreach loops set up, p is a 
> size_t[2], the payload of the array v is double[9] and the 
> array is indexed directly with a size_t[2] array and all works 
> fine:
>
> foreach(immutable p, ref v, arr) { double[9] stuff; arr[p] = 
> stuff; }
>
> If I naively try
>
> foreach(immutable p, ref v, parallel(arr)) { ... }
>
> I first get errors of the type "Error: foreach: cannot make v 
> ref". I do not understand where that particular problem comes 
> from, but I can possibly live without the ref, so I went for
>
> foreach(immutable p, v, parallel(arr)) { ... }
>
> Which gets me "Error: no [] operator overload for type 
> (complicated templated type of some wrapper struct I have for 
> arr)". I'm guessing it doesn't like that there is no such thing 
> as a simple one-dimensional slicing operation for a 
> multidimensional array?
> Should I define an opSlice function that takes the usual two 
> size_t arguments for the upper and lower bounds and doesn't 
> require a dimension template argument and somehow map this to 
> my underlying two-dimensional array? Will it need an opIndex 
> function that takes only takes a single size_t as well?
> Or is this just taking the simple parallel(...) too far and 
> should I try to put something together myself using lower-level 
> constructs?
>
> Any hints would be appreciated!
>
> [1] http://code.dlang.org/packages/unstandard

I'd define a "flatten range" adaptor that presents n-dimensional 
ranges as 1d range that traverses the original array indices 
lexicographically, if that makes sense for your app.