[flint-devel] fmpz_poly_evaluate bug in 1.2.2

[Burcin E. <bu...@er...>]

Hi,

While looking through fmpz_poly.c in 1.2.2, I noticed that the first
lines of fmpz_poly_evaluate are as follows:

void fmpz_poly_evaluate(fmpz_t output, fmpz_poly_t poly, fmpz_t value)
{
   fmpz_t val;
	
	if ((poly->length == 0) || (value[0] == 0)) 
	{
      output[0] = 0L;
		return;
	}

This seems to be a bug, since it always returns 0 if value is 0, when
it should return the constant coefficient of poly.

I didn't produce a fix, since I wasn't sure if just using
fmpz_set(output,value) was enough when poly->length == 1 or value == 0.


If you wonder why I was reading that file, I need to evaluate
polynomials in ZZ[x] modulo p as part of some linear algebra routines
I'm working on. I attached a patch for a function which implements
evaluation modulo p. Is it possible to include some approximation of
that in future versions of FLINT?


I also need functions to translate (compose with polynomials of degree
1) zmod_poly_t's, and fmpz_poly_t's modulo some p. I implemented
Horner's method in cython to have a reasonably quick way of doing this.
Do you think it's worth putting these in FLINT?

BTW, in the first part of this article

http://doi.acm.org/10.1145/258726.258745

von zur Gathen and Gerhard measure the performance of different
algorithms for this task. It might be of interest if we consider making
this fast in FLINT.


Cheers,
Burcin