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Date: -- (:)
From: Thomas Fischbacher <tf@f...>
Subject: Re: [Caml-list] Bigarray question
Xavier Leroy wrote:

(on whether it is permissible to dynamically change the dim and data
pointers in a ML bigarray):

>>as far as I can tell, it is easy (but not explicitly allowed in the
>>documentation) to allocate a Caml bigarray and change the data
>>pointer inside that bigarray afterwards.
>>There is no problem in implementing this, technically speaking.
>>But this would require a change to the ML documentation stating
>>that it is explicitly considered as permissible to change the
>>data pointer in a bigarray. Can I get that, please?
> 
> 
> I see no reason why this would cause problems, as long as the data
> pointer points to C data of the right shape and dimensions.  (Of
> course, you could update the dimensions of the bigarray as
> appropriate, if needed.)  I don't think I would document this, as I
> wouldn't quite know how to word it in the docs, but you have my
> encouragements to try and report problems if any.


As a matter of fact, considering the two C functions below which I use
in our "nsim" abstract field theory/multiphysics simulation engine to
speed up some internal computation, the first variant would be desirable
but crashes. The second works. What this actually is about is to
systematically go through the rows of a sparse matrix, handled by the
PETSc library. (So, basically, I am using a sparse matrix as sort-of a
hash of row index to entries here.)

CAMLprim value caml_petsc_matrix_call_on_rows_raw(value ml_mx,
						  value ml_fun,
						  value ml_start_row,
						  value ml_end_row
						  )
{
   CAMLparam4(ml_mx,ml_fun,ml_start_row,ml_end_row);
   Mat mx;
   int start_row,end_row,start_own,end_own;
   int i,ncols=0;
   const int *cols=0;
   const PetscScalar *vals=0;
   CAMLlocal2(ba_indices,ba_vals);

   petsc_checkinit();
   petsc_check_mat(ml_mx);

   mx=(Mat)Field(ml_mx,1);

   MatGetOwnershipRange(mx,&start_own,&end_own);

   start_row=Int_val(ml_start_row);
   end_row=Int_val(ml_end_row);

   if(start_row == -1)start_row=start_own;
   if(end_row == -1)end_row=end_own;

   /* We would like to do things this way, but while one may guess that 
it should work,
      this (1) is not covered by what is permitted according to the 
OCaml documentation,
      and (2) indeed produces crashes.
   */
   ba_indices=alloc_bigarray_dims(BIGARRAY_NATIVE_INT | BIGARRAY_C_LAYOUT,
				 1, cols, ncols);

   ba_vals=alloc_bigarray_dims(BIGARRAY_FLOAT64 | BIGARRAY_C_LAYOUT,
			      1, vals, ncols);


   for(i=start_row;i<end_row;i++)
     {
       MatGetRow(mx,i,&ncols,&cols,&vals);

       Bigarray_val(ba_indices)->dim[0]=ncols;
       Bigarray_val(ba_vals)->dim[0]=ncols;

       Bigarray_val(ba_indices)->data=(void*)cols;
       Bigarray_val(ba_vals)->data=(void*)vals;

       callback3(ml_fun,Val_int(i),ba_indices,ba_vals);

       MatRestoreRow(mx,i,&ncols,&cols,&vals);
     }

   CAMLreturn(Val_unit);
}



CAMLprim value caml_petsc_matrix_call_on_rows_raw_defensive(value ml_mx,
							    value ml_fun,
							    value ml_start_row,
							    value ml_end_row
							    )
{
   CAMLparam4(ml_mx,ml_fun,ml_start_row,ml_end_row);
   Mat mx;
   int start_row,end_row,start_own,end_own;
   int i,ncols=0;
   const int *cols=0;
   const PetscScalar *vals=0;
   CAMLlocal2(ba_indices,ba_vals);

   petsc_checkinit();
   petsc_check_mat(ml_mx);

   mx=(Mat)Field(ml_mx,1);

   MatGetOwnershipRange(mx,&start_own,&end_own);

   start_row=Int_val(ml_start_row);
   end_row=Int_val(ml_end_row);

   if(start_row == -1)start_row=start_own;
   if(end_row == -1)end_row=end_own;


   for(i=start_row;i<end_row;i++)
     {
       MatGetRow(mx,i,&ncols,&cols,&vals);

       ba_indices=alloc_bigarray_dims(BIGARRAY_NATIVE_INT | 
BIGARRAY_C_LAYOUT,
				     1, cols, ncols);

       ba_vals=alloc_bigarray_dims(BIGARRAY_FLOAT64 | BIGARRAY_C_LAYOUT,
				  1, vals, ncols);

       callback3(ml_fun,Val_int(i),ba_indices,ba_vals);

       MatRestoreRow(mx,i,&ncols,&cols,&vals);
     }

   CAMLreturn(Val_unit);
}


-- 
best regards,
Thomas Fischbacher
tf@functionality.de