mod to symmetric range

[Torbjorn Granlund]

Paul Zimmermann <Paul.Zimmermann at loria.fr> writes:

  this is a function I have written maybe ten times (if not more),
  and I would like very much to have it included in GMP.
  
  In my applications b is invariant, thus we might indeed precompute b/2,
  but this is inefficient memory-wise, and the heuristic that compares the
  most significant limbs of r and b should decide in most cases what to do
  (note that b/2 is only needed for the comparison, not for the subtraction r-b
  in case r > b/2).
  
I implemented a function that attempts to compute the following function:

/*
  GMP symmetric mod, i.e., floor((2-|B|)/2) <= A smod B <= floor(|B|/2)

  E.g.:
  B = 1: return  0
  B = 2: return  0 1
  B = 3: return -1 0 1
  B = 4: return -1 0 1 2
  B = 5: return -2 -1 0 1 2
*/

Efficient code at the mpz level is attached.  I'd appreciate if you Paul
could review my code skeptically and if you Roman could test it!

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Torbj?rn