/* mpfr_zeta_ui -- compute the Riemann Zeta function for integer argument.

Copyright 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao projects, INRIA.

This file is part of the GNU MPFR Library.

The GNU MPFR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The GNU MPFR Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MPFR Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
MA 02110-1301, USA. */

#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"

int
mpfr_zeta_ui (mpfr_ptr z, unsigned long m, mp_rnd_t r)
{
  MPFR_ZIV_DECL (loop);

  if (m == 0)
    {
      mpfr_set_ui (z, 1, r);
      mpfr_div_2ui (z, z, 1, r);
      MPFR_CHANGE_SIGN (z);
      MPFR_RET (0);
    }
  else if (m == 1)
    {
      MPFR_SET_INF (z);
      MPFR_SET_POS (z);
      return 0;
    }
  else /* m >= 2 */
    {
      mp_prec_t p = MPFR_PREC(z);
      unsigned long n, k, err, kbits;
      mpz_t d, t, s, q;
      mpfr_t y;
      int inex;

      if (m >= p) /* 2^(-m) < ulp(1) = 2^(1-p). This means that
                     2^(-m) <= 1/2*ulp(1). We have 3^(-m)+4^(-m)+... < 2^(-m)
                     i.e. zeta(m) < 1+2*2^(-m) for m >= 3 */

        {
          if (m == 2) /* necessarily p=2 */
            return mpfr_set_ui_2exp (z, 13, -3, r);
          else if (r == GMP_RNDZ || r == GMP_RNDD || (r == GMP_RNDN && m > p))
            {
              mpfr_set_ui (z, 1, r);
              return -1;
            }
          else
            {
              mpfr_set_ui (z, 1, r);
              mpfr_nextabove (z);
              return 1;
            }
        }

      /* now treat also the case where zeta(m) - (1+1/2^m) < 1/2*ulp(1),
         and the result is either 1+2^(-m) or 1+2^(-m)+2^(1-p). */
      mpfr_init2 (y, 31);

      if (m >= p / 2) /* otherwise 4^(-m) > 2^(-p) */
        {
          /* the following is a lower bound for log(3)/log(2) */
          mpfr_set_str_binary (y, "1.100101011100000000011010001110");
          mpfr_mul_ui (y, y, m, GMP_RNDZ); /* lower bound for log2(3^m) */
          if (mpfr_cmp_ui (y, p + 2) >= 0)
            {
              mpfr_clear (y);
              mpfr_set_ui (z, 1, GMP_RNDZ);
              mpfr_div_2ui (z, z, m, GMP_RNDZ);
              mpfr_add_ui (z, z, 1, GMP_RNDZ);
              if (r != GMP_RNDU)
                return -1;
              mpfr_nextabove (z);
              return 1;
            }
        }

      mpz_init (s);
      mpz_init (d);
      mpz_init (t);
      mpz_init (q);

      p += MPFR_INT_CEIL_LOG2(p); /* account of the n term in the error */

      p += MPFR_INT_CEIL_LOG2(p) + 15; /* initial value */

      MPFR_ZIV_INIT (loop, p);
      for(;;)
        {
          /* 0.39321985067869744 = log(2)/log(3+sqrt(8)) */
          n = 1 + (unsigned long) (0.39321985067869744 * (double) p);
          err = n + 4;

          mpfr_set_prec (y, p);

          /* computation of the d[k] */
          mpz_set_ui (s, 0);
          mpz_set_ui (t, 1);
          mpz_mul_2exp (t, t, 2 * n - 1); /* t[n] */
          mpz_set (d, t);
          for (k = n; k > 0; k--)
            {
              count_leading_zeros (kbits, k);
              kbits = BITS_PER_MP_LIMB - kbits;
              /* if k^m is too large, use mpz_tdiv_q */
              if (m * kbits > 2 * BITS_PER_MP_LIMB)
                {
                  /* if we know in advance that k^m > d, then floor(d/k^m) will
                     be zero below, so there is no need to compute k^m */
                  kbits = (kbits - 1) * m + 1;
                  /* k^m has at least kbits bits */
                  if (kbits > mpz_sizeinbase (d, 2))
                    mpz_set_ui (q, 0);
                  else
                    {
                      mpz_ui_pow_ui (q, k, m);
                      mpz_tdiv_q (q, d, q);
                    }
                }
              else /* use several mpz_tdiv_q_ui calls */
                {
                  unsigned long km = k, mm = m - 1;
                  while (mm > 0 && km < ULONG_MAX / k)
                    {
                      km *= k;
                      mm --;
                    }
                  mpz_tdiv_q_ui (q, d, km);
                  while (mm > 0)
                    {
                      km = k;
                      mm --;
                      while (mm > 0 && km < ULONG_MAX / k)
                        {
                          km *= k;
                          mm --;
                        }
                      mpz_tdiv_q_ui (q, q, km);
                    }
                }
              if (k % 2)
                mpz_add (s, s, q);
              else
                mpz_sub (s, s, q);

              /* we have d[k] = sum(t[i], i=k+1..n)
                 with t[i] = n*(n+i-1)!*4^i/(n-i)!/(2i)!
                 t[k-1]/t[k] = k*(2k-1)/(n-k+1)/(n+k-1)/2 */
#if (BITS_PER_MP_LIMB == 32)
#define KMAX 46341 /* max k such that k*(2k-1) < 2^32 */
#elif (BITS_PER_MP_LIMB == 64)
#define KMAX 3037000500
#endif
#ifdef KMAX
              if (k <= KMAX)
                mpz_mul_ui (t, t, k * (2 * k - 1));
              else
#endif
                {
                  mpz_mul_ui (t, t, k);
                  mpz_mul_ui (t, t, 2 * k - 1);
                }
              mpz_div_2exp (t, t, 1);
              if (n < 1UL << (BITS_PER_MP_LIMB / 2))
                /* (n - k + 1) * (n + k - 1) < n^2 */
                mpz_divexact_ui (t, t, (n - k + 1) * (n + k - 1));
              else
                {
                  mpz_divexact_ui (t, t, n - k + 1);
                  mpz_divexact_ui (t, t, n + k - 1);
                }
              mpz_add (d, d, t);
            }

          /* multiply by 1/(1-2^(1-m)) = 1 + 2^(1-m) + 2^(2-m) + ... */
          mpz_div_2exp (t, s, m - 1);
          do
            {
              err ++;
              mpz_add (s, s, t);
              mpz_div_2exp (t, t, m - 1);
            }
          while (mpz_cmp_ui (t, 0) > 0);

          /* divide by d[n] */
          mpz_mul_2exp (s, s, p);
          mpz_tdiv_q (s, s, d);
          mpfr_set_z (y, s, GMP_RNDN);
          mpfr_div_2ui (y, y, p, GMP_RNDN);

          err = MPFR_INT_CEIL_LOG2 (err);

          if (MPFR_LIKELY(MPFR_CAN_ROUND (y, p - err, MPFR_PREC(z), r)))
            break;

          MPFR_ZIV_NEXT (loop, p);
        }
      MPFR_ZIV_FREE (loop);

      mpz_clear (d);
      mpz_clear (t);
      mpz_clear (q);
      mpz_clear (s);
      inex = mpfr_set (z, y, r);
      mpfr_clear (y);
      return inex;
    }
}