From: Ville Sipola <ville_sipola@...>
Date: 2011-05-07T04:47:05+09:00
Subject: Re: Fast way to move C array to ruby (rb_ary_new4)?

Ville Sipola wrote:
> Now I only have to figure out how to use that.

Now that I have, I thought it a good idea to describe how it was done,
so that others reading the post might be able to use the information.

So this was my method.

The writing part is a bit of a hack. Masahiro Tanaka himself advices
against this approach in here: https://www.ruby-forum.com/topic/83484,
but I'm yiet to find a better solution. If there is a function for
writing c data in an narray in a safe way, I'd like to hear about it.

First, in Ruby I did something like

@x=NArray.float(@number_of_samples).fill(some_value)

to make sure that the narray is of correct size.

Then in C:
static VALUE writingfunction(VALUE class,VALUE x)
{
    struct NARRAY *xarray;
    GetNArray(x, xarray);
    double *floatpointerx = xarray->ptr;

    double c_value;

    for (counter = 0; counter < number_of_samples; counter++)
    {
       (do the actual calculation)
       floatpointerx[samplecounter] = c_value; (This is the illegal
hack)
    }
}

To read in C (this I believe is completely ok, please correct if I'm
wrong.)

static VALUE readingfunction(VALUE class, VALUE x)
{
    struct NARRAY *xarray;
    GetNArray(x, xarray);
    double *floatpointerx = xarray->ptr;
    for (counter = 0; counter < xarray->total; counter++)
    {
        a_function_that_takes_c_double(floatpointerx[counter1]);
    }
}

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