[racket] math/matrix <--> FFI/array

When you pass it directly to c-code, maybe you can completely bypass the  
array and convert
matrix -> flarray -> flvector -> cpointer
the last 3 conversions should be blazing fast but i guess that won't  
matter much for 16 elements.
Neil, can you nevertheless tell me more about the expected performance of  
the first one?

Here is a sample code (it was surprisingly easy to integrate ffi into TR)

Tobias


;; --------------------------

#lang typed/racket/base

(require math)
(require/typed ffi/unsafe
                [#:opaque CPointer cpointer?]
                [flvector->cpointer (FlVector -> CPointer)]
                [#:opaque CType ctype?]
                [_double CType]
                [ptr-ref (CPointer CType Exact-Nonnegative-Integer -> Any)])

(define dim 4)
(define m (build-matrix dim dim
                         (lambda: ([i : Integer]
                                   [j : Integer])
                           (->fl (+ (* i dim) j)))))

(define flv (flarray-data (array->flarray m)))
(define cp (flvector->cpointer flv))

(for/list: : (Listof Any) ([i (flvector-length flv)])
   (ptr-ref cp _double i))





On Mon, 21 Jan 2013 20:42:51 +0100, Neil Toronto <neil.toronto at gmail.com>  
wrote:

> On 01/21/2013 02:48 AM, Pierpaolo Bernardi wrote:
>> Hello,
>>
>> what's the most convenient way to convert between math/matrices and
>> FFI/arrays?
>>
>> To reduce the scope of the question, at the moment I'm only interested
>> in conversions from and to the following FFI types:
>>
>>   (_array _double 3 3)
>>   (_array _double 3)
>>   (_array _double 2 3)
>
>  From the FFI docs, it looks like you need to construct either:
>
>   1. A vector of vectors for foreign functions that accept an
>      (_array/vector type m n).
>
>   2. A flat vector with elements in row-major order for foreign functions
>      that accept an (_array type m n) or (_array type m).
>
> For #1, use `matrix->vector*', and for #2, use `matrix->vector'. I don't  
> know whether you can pass the vectors returned from these functions  
> directly to foreign functions. I wouldn't be surprised.
>
> If you're working in Typed Racket and you need this to be fast, do the  
> conversions to and from vectors in Typed Racket. Currently, passing  
> matrices across the contract boundary makes element access very slow. :/
>
> Neil ⊥
>
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users