source: trunk/third/gmp/mpz/fac_ui.c @ 18191

/* mpz_fac_ui(result, n) -- Set RESULT to N!.

Copyright 1991, 1993, 1994, 1995, 2000, 2001, 2002 Free Software Foundation,
Inc.

This file is part of the GNU MP Library.

The GNU MP 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 MP 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 MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */

#include "gmp.h"
#include "gmp-impl.h"
#include "longlong.h"


/* Enhancements:

   Data tables could be used for results up to 3 or 4 limbs to avoid
   fiddling around with small quantities.

   The product accumulation might be worth splitting out into something that
   could be used elsewhere too.

   The tree of partial products should be done with TMP_ALLOC, not mpz_init.
   It should be possible to know a maximum size at each level.

   Factors of two could be stripped from k to save some multiplying (but not
   very much).  The same could be done with factors of 3, perhaps.

   The prime factorization of n! is easy to determine, it might be worth
   using that rather than a simple 1 to n.  The powering of primes could do
   some squaring instead of multiplying.  There's probably other ways to use
   some squaring too.  */


/* for single non-zero limb */
#define MPZ_SET_1_NZ(z,n)       \
  do {                          \
    mpz_ptr  __z = (z);         \
    ASSERT ((n) != 0);          \
    PTR(__z)[0] = (n);          \
    SIZ(__z) = 1;               \
  } while (0)

/* for single non-zero limb */
#define MPZ_INIT_SET_1_NZ(z,n)                  \
  do {                                          \
    mpz_ptr  __iz = (z);                        \
    ALLOC(__iz) = 1;                            \
    PTR(__iz) = __GMP_ALLOCATE_FUNC_LIMBS (1);  \
    MPZ_SET_1_NZ (__iz, n);                     \
  } while (0)

/* for src>0 and n>0 */
#define MPZ_MUL_1_POS(dst,src,n)                        \
  do {                                                  \
    mpz_ptr    __dst = (dst);                           \
    mpz_srcptr __src = (src);                           \
    mp_size_t  __size = SIZ(__src);                     \
    mp_ptr     __dst_p;                                 \
    mp_limb_t  __c;                                     \
                                                        \
    ASSERT (__size > 0);                                \
    ASSERT ((n) != 0);                                  \
                                                        \
    MPZ_REALLOC (__dst, __size+1);                      \
    __dst_p = PTR(__dst);                               \
                                                        \
    __c = mpn_mul_1 (__dst_p, PTR(__src), __size, n);   \
    __dst_p[__size] = __c;                              \
    SIZ(__dst) = __size + (__c != 0);                   \
                                                        \
  } while (0)


void
mpz_fac_ui (mpz_ptr result, unsigned long int n)
{
#if SIMPLE_FAC
  /* Be silly.  Just multiply the numbers in ascending order.  O(n**2).  */
  unsigned long int k;
  mpz_set_ui (result, 1L);
  for (k = 2; k <= n; k++)
    mpz_mul_ui (result, result, k);
#else

  /* Be smarter.  Multiply groups of numbers in ascending order until the
     product doesn't fit in a limb.  Multiply these partial product in a
     balanced binary tree fashion, to make the operand have as equal sizes
     as possible.  When the operands have about the same size, mpn_mul
     becomes faster.  */

  unsigned long  k;
  mp_limb_t      p, p1, p0;

  /* Stack of partial products, used to make the computation balanced
     (i.e. make the sizes of the multiplication operands equal).  The
     topmost position of MP_STACK will contain a one-limb partial product,
     the second topmost will contain a two-limb partial product, and so
     on.  MP_STACK[0] will contain a partial product with 2**t limbs.
     To compute n! MP_STACK needs to be less than
     log(n)**2/log(BITS_PER_MP_LIMB), so 30 is surely enough.  */
#define MP_STACK_SIZE 30
  mpz_t mp_stack[MP_STACK_SIZE];

  /* TOP is an index into MP_STACK, giving the topmost element.
     TOP_LIMIT_SO_FAR is the largets value it has taken so far.  */
  int top, top_limit_so_far;

  /* Count of the total number of limbs put on MP_STACK so far.  This
     variable plays an essential role in making the compututation balanced.
     See below.  */
  unsigned int tree_cnt;

  /* for testing with small limbs */
  if (MP_LIMB_T_MAX < ULONG_MAX)
    ASSERT_ALWAYS (n <= MP_LIMB_T_MAX);

  top = top_limit_so_far = -1;
  tree_cnt = 0;
  p = 1;
  for (k = 2; k <= n; k++)
    {
      /* Multiply the partial product in P with K.  */
      umul_ppmm (p1, p0, p, (mp_limb_t) k);

#if GMP_NAIL_BITS == 0
#define OVERFLOW (p1 != 0)
#else
#define OVERFLOW ((p1 | (p0 >> GMP_NUMB_BITS)) != 0)
#endif
      /* Did we get overflow into the high limb, i.e. is the partial
         product now more than one limb?  */
      if OVERFLOW
        {
          tree_cnt++;

          if (tree_cnt % 2 == 0)
            {
              mp_size_t i;

              /* TREE_CNT is even (i.e. we have generated an even number of
                 one-limb partial products), which means that we have a
                 single-limb product on the top of MP_STACK.  */

              MPZ_MUL_1_POS (mp_stack[top], mp_stack[top], p);

              /* If TREE_CNT is divisable by 4, 8,..., we have two
                 similar-sized partial products with 2, 4,... limbs at
                 the topmost two positions of MP_STACK.  Multiply them
                 to form a new partial product with 4, 8,... limbs.  */
              for (i = 4; (tree_cnt & (i - 1)) == 0; i <<= 1)
                {
                  mpz_mul (mp_stack[top - 1],
                           mp_stack[top], mp_stack[top - 1]);
                  top--;
                }
            }
          else
            {
              /* Put the single-limb partial product in P on the stack.
                 (The next time we get a single-limb product, we will
                 multiply the two together.)  */
              top++;
              if (top > top_limit_so_far)
                {
                  if (top > MP_STACK_SIZE)
                    abort();
                  /* The stack is now bigger than ever, initialize the top
                     element.  */
                  MPZ_INIT_SET_1_NZ (mp_stack[top], p);
                  top_limit_so_far++;
                }
              else
                MPZ_SET_1_NZ (mp_stack[top], p);
            }

          /* We ignored the last result from umul_ppmm.  Put K in P as the
             first component of the next single-limb partial product.  */
          p = k;
        }
      else
        /* We didn't get overflow in umul_ppmm.  Put p0 in P and try
           with one more value of K.  */
        p = p0;
    }

  /* We have partial products in mp_stack[0..top], in descending order.
     We also have a small partial product in p.
     Their product is the final result.  */
  if (top < 0)
    MPZ_SET_1_NZ (result, p);
  else
    MPZ_MUL_1_POS (result, mp_stack[top--], p);
  while (top >= 0)
    mpz_mul (result, result, mp_stack[top--]);

  /* Free the storage allocated for MP_STACK.  */
  for (top = top_limit_so_far; top >= 0; top--)
    mpz_clear (mp_stack[top]);
#endif
}