dist/src/random_deviate.c

/* random_deviate routines for mpfr_erandom and mpfr_nrandom.

Copyright 2013-2020 Free Software Foundation, Inc.
Contributed by Charles Karney <charles@karney.com>, SRI International.

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 3 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.LESSER.  If not, see
https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

/*
 * A mpfr_random_deviate represents the initial portion e bits of a random
 * deviate uniformly distributed in (0,1) as
 *
 *  typedef struct {
 *    unsigned long e;            // bits in the fraction
 *    unsigned long h;            // the high W bits of the fraction
 *    mpz_t f;                    // the rest of the fraction
 *  } mpfr_random_deviate_t[1];
 *
 * e is always a multiple of RANDOM_CHUNK.  The first RANDOM_CHUNK bits, the
 * high fraction, are held in an unsigned long, h, and the rest are held in an
 * mpz_t, f.  The data in h is undefined if e == 0 and, similarly the data in f
 * is undefined if e <= RANDOM_CHUNK.
 */

#define MPFR_NEED_LONGLONG_H
#include "random_deviate.h"

/*
 * RANDOM_CHUNK can be picked in the range 1 <= RANDOM_CHUNK <= 64.  Low values
 * of RANDOM_CHUNK are good for testing, since they are more likely to make
 * bugs obvious.  For portability, pick RANDOM_CHUNK <= 32 (since an unsigned
 * long may only hold 32 bits).  For reproducibility across platforms,
 * standardize on RANDOM_CHUNK = 32.
 *
 * When RANDOM_CHUNK = 32, this representation largely avoids manipulating
 * mpz's (until the final cast to an mpfr is done).  In addition
 * mpfr_random_deviate_less usually entails just a single comparison of
 * unsigned longs.  In this way, we can stick with the published interface for
 * extracting portions of an mpz (namely through mpz_tstbit) without hurting
 * efficiency.
 */
#if !defined(RANDOM_CHUNK)
/* note: for MPFR, we could use RANDOM_CHUNK = 32 or 64 according to the
   number of bits per limb, but we use 32 everywhere to get reproducible
   results on 32-bit and 64-bit computers */
#define RANDOM_CHUNK 32     /* Require 1 <= RANDOM_CHUNK <= 32; recommend 32 */
#endif

#define W RANDOM_CHUNK         /* W is just an shorter name for RANDOM_CHUNK */

/* allocate and set to (0,1) */
void
mpfr_random_deviate_init (mpfr_random_deviate_t x)
{
  mpz_init (x->f);
  x->e = 0;
}

/* reset to (0,1) */
void
mpfr_random_deviate_reset (mpfr_random_deviate_t x)
{
  x->e = 0;
}

/* deallocate */
void
mpfr_random_deviate_clear (mpfr_random_deviate_t x)
{
  mpz_clear (x->f);
}

/* swap two random deviates */
void
mpfr_random_deviate_swap (mpfr_random_deviate_t x, mpfr_random_deviate_t y)
{
  mpfr_random_size_t s;
  unsigned long t;

  /* swap x->e and y->e */
  s = x->e;
  x->e = y->e;
  y->e = s;

  /* swap x->h and y->h */
  t = x->h;
  x->h = y->h;
  y->h = t;

  /* swap x->f and y->f */
  mpz_swap (x->f, y->f);
}

/* ensure x has at least k bits */
static void
random_deviate_generate (mpfr_random_deviate_t x, mpfr_random_size_t k,
                         gmp_randstate_t r, mpz_t t)
{
  /* Various compile time checks on mpfr_random_deviate_t */

  /* Check that the h field of a mpfr_random_deviate_t can hold W bits */
  MPFR_STAT_STATIC_ASSERT (W > 0 && W <= sizeof (unsigned long) * CHAR_BIT);

  /* Check mpfr_random_size_t can hold 32 bits and a mpfr_uprec_t.  This
   * ensures that max(mpfr_random_size_t) exceeds MPFR_PREC_MAX by at least
   * 2^31 because mpfr_prec_t is a signed version of mpfr_uprec_t.  This allows
   * random deviates with many leading zeros in the fraction to be handled
   * correctly. */
  MPFR_STAT_STATIC_ASSERT (sizeof (mpfr_random_size_t) * CHAR_BIT >= 32 &&
                           sizeof (mpfr_random_size_t) >=
                           sizeof (mpfr_uprec_t));

  /* Finally, at run time, check that k is not too big.  e is set to ceil(k/W)*W
   * and we require that this allows x->e + 1 in random_deviate_leading_bit to
   * be computed without overflow. */
  MPFR_ASSERTN (k <= (mpfr_random_size_t)(-((int) W + 1)));

  /* if t is non-null, it is used as a temporary */
  if (x->e >= k)
    return;

  if (x->e == 0)
    {
      x->h = gmp_urandomb_ui (r, W); /* Generate the high fraction */
      x->e = W;
      if (x->e >= k)
        return;    /* Maybe that's it? */
    }

  if (t)
    {
      /* passed a mpz_t so compute needed bits in one call to mpz_urandomb */
      k = ((k + (W-1)) / W) * W;  /* Round up to multiple of W */
      k -= x->e;                  /* The number of new bits */
      mpz_urandomb (x->e == W ? x->f : t, r, k); /* Copy directly to x->f? */
      if (x->e > W)
        {
          mpz_mul_2exp (x->f, x->f, k);
          mpz_add (x->f, x->f, t);
        }
      x->e += k;
    }
  else
    {
      /* no mpz_t so compute the bits W at a time via gmp_urandomb_ui */
      while (x->e < k)
        {
          unsigned long w = gmp_urandomb_ui (r, W);
          if (x->e == W)
            mpz_set_ui (x->f, w);
          else
            {
              mpz_mul_2exp (x->f, x->f, W);
              mpz_add_ui (x->f, x->f, w);
            }
          x->e += W;
        }
    }
}

#ifndef MPFR_LONG_WITHIN_LIMB /* a long does not fit in a mp_limb_t */
/*
 * return index [0..127] of highest bit set.  Return 0 if x = 1, 2 if x = 4,
 * etc. Assume x > 0. (From Algorithms for programmers by Joerg Arndt.)
 */
static int
highest_bit_idx (unsigned long x)
{
  unsigned long y;
  int r = 0;

  MPFR_ASSERTD(x > 0);
  MPFR_STAT_STATIC_ASSERT (sizeof (unsigned long) * CHAR_BIT <= 128);

  /* A compiler with VRP (like GCC) will optimize and not generate any code
     for the following lines if unsigned long has at most 64 values bits. */
  y = ((x >> 16) >> 24) >> 24;  /* portable x >> 64 */
  if (y != 0)
    {
      x = y;
      r += 64;
    }

  if (x & ~0xffffffffUL) { x >>= 16; x >>= 16; r +=32; }
  if (x &  0xffff0000UL) { x >>= 16; r += 16; }
  if (x &  0x0000ff00UL) { x >>=  8; r +=  8; }
  if (x &  0x000000f0UL) { x >>=  4; r +=  4; }
  if (x &  0x0000000cUL) { x >>=  2; r +=  2; }
  if (x &  0x00000002UL) {           r +=  1; }
  return r;
}
#else /* a long fits in a mp_limb_t */
/*
 * return index [0..63] of highest bit set. Assume x > 0.
 * Return 0 if x = 1, 63 is if x = ~0 (for 64-bit unsigned long).
 * See alternate code above too.
 */
static int
highest_bit_idx (unsigned long x)
{
  int cnt;

  MPFR_ASSERTD(x > 0);
  count_leading_zeros (cnt, (mp_limb_t) x);
  MPFR_ASSERTD (cnt <= GMP_NUMB_BITS - 1);
  return GMP_NUMB_BITS - 1 - cnt;
}
#endif /* MPFR_LONG_WITHIN_LIMB */

/* return position of leading bit, counting from 1 */
static mpfr_random_size_t
random_deviate_leading_bit (mpfr_random_deviate_t x, gmp_randstate_t r)
{
  mpfr_random_size_t l;
  random_deviate_generate (x, W, r, 0);
  if (x->h)
    return W - highest_bit_idx (x->h);
  random_deviate_generate (x, 2 * W, r, 0);
  while (mpz_sgn (x->f) == 0)
    random_deviate_generate (x, x->e + 1, r, 0);
  l = x->e + 1 - mpz_sizeinbase (x->f, 2);
  /* Guard against a ridiculously long string of leading zeros in the fraction;
   * probability of this happening is 2^(-2^31).  In particular ensure that
   * p + 1 + l in mpfr_random_deviate_value doesn't overflow with p =
   * MPFR_PREC_MAX. */
  MPFR_ASSERTN (l + 1 < (mpfr_random_size_t)(-MPFR_PREC_MAX));
  return l;
}

/* return kth bit of fraction, representing 2^-k */
int
mpfr_random_deviate_tstbit (mpfr_random_deviate_t x, mpfr_random_size_t k,
                            gmp_randstate_t r)
{
  if (k == 0)
    return 0;
  random_deviate_generate (x, k, r, 0);
  if (k <= W)
    return (x->h >> (W - k)) & 1UL;
  return mpz_tstbit (x->f, x->e - k);
}

/* compare two random deviates, x < y */
int
mpfr_random_deviate_less (mpfr_random_deviate_t x, mpfr_random_deviate_t y,
                          gmp_randstate_t r)
{
  mpfr_random_size_t k = 1;

  if (x == y)
    return 0;
  random_deviate_generate (x, W, r, 0);
  random_deviate_generate (y, W, r, 0);
  if (x->h != y->h)
    return x->h < y->h; /* Compare the high fractions */
  k += W;
  for (; ; ++k)
    {             /* Compare the rest of the fraction bit by bit */
      int a = mpfr_random_deviate_tstbit (x, k, r);
      int b = mpfr_random_deviate_tstbit (y, k, r);
      if (a != b)
        return a < b;
    }
}

/* set mpfr_t z = (neg ? -1 : 1) * (n + x) */
int
mpfr_random_deviate_value (int neg, unsigned long n,
                           mpfr_random_deviate_t x, mpfr_t z,
                           gmp_randstate_t r, mpfr_rnd_t rnd)
{
  /* r is used to add as many bits as necessary to match the precision of z */
  int s;
  mpfr_random_size_t l;                     /* The leading bit is 2^(s*l) */
  mpfr_random_size_t p = mpfr_get_prec (z); /* Number of bits in result */
  mpz_t t;
  int inex;

  if (n == 0)
    {
      s = -1;
      l = random_deviate_leading_bit (x, r); /* l > 0 */
    }
  else
    {
      s = 1;
      l = highest_bit_idx (n); /* l >= 0 */
    }

  /*
   * Leading bit is 2^(s*l); thus the trailing bit in result is 2^(s*l-p+1) =
   * 2^-(p-1-s*l).  For the sake of illustration, take l = 0 and p = 4, thus
   * bits through the 1/8 position need to be generated; assume that these bits
   * are 1.010 = 10/8 which represents a deviate in the range (10,11)/8.
   *
   * If the rounding mode is one of RNDZ, RNDU, RNDD, RNDA, we add a 1 bit to
   * the result to give 1.0101 = (10+1/2)/8.  When this is converted to a MPFR
   * the result is rounded to 10/8, 11/8, 10/8, 11/8, respectively, and the
   * inexact flag is set to -1, 1, -1, 1.
   *
   * If the rounding mode is RNDN, an additional random bit must be generated
   * to determine if the result is in (10,10+1/2)/8 or (10+1/2,11)/8.  Assume
   * that this random bit is 0, so the result is 1.0100 = (10+0/2)/8.  Then an
   * additional 1 bit is added to give 1.010101 = (10+1/4)/8.  This last bit
   * avoids the "round ties to even rule" (because there are no ties) and sets
   * the inexact flag so that the result is 10/8 with the inexact flag = 1.
   *
   * Here we always generate at least 2 additional random bits, so that bit
   * position 2^-(p+1-s*l) is generated.  (The result often contains more
   * random bits than this because random bits are added in batches of W and
   * because additional bits may have been required in the process of
   * generating the random deviate.)  The integer and all the bits in the
   * fraction are then copied into an mpz, the least significant bit is
   * unconditionally set to 1, the sign is set, and the result together with
   * the exponent -x->e is used to generate an mpfr using mpfr_set_z_2exp.
   *
   * If random bits were very expensive, we would only need to generate to the
   * 2^-(p-1-s*l) bit (no extra bits) for the RNDZ, RNDU, RNDD, RNDA modes and
   * to the 2^-(p-s*l) bit (1 extra bit) for RNDN.  By always generating 2 bits
   * we save on some bit shuffling when formed the mpz to be converted to an
   * mpfr.  The implementation of the RandomNumber class in RandomLib
   * illustrates the more parsimonious approach (which was taken to allow
   * accurate counts of the number of random digits to be made).
   */
  mpz_init (t);
  /*
   * This is the only call to random_deviate_generate where a mpz_t is passed
   * (because an arbitrarily large number of bits may need to be generated).
   */
  if ((s > 0 && p + 1 > l) ||
      (s < 0 && p + 1 + l > 0))
    random_deviate_generate (x, s > 0 ? p + 1 - l : p + 1 + l, r, t);
  if (n == 0)
    {
      /* Since the minimum prec is 2 we know that x->h has been generated. */
      mpz_set_ui (t, x->h);        /* Set high fraction */
    }
  else
    {
      mpz_set_ui (t, n);           /* The integer part */
      if (x->e > 0)
        {
          mpz_mul_2exp (t, t, W);    /* Shift to allow for high fraction */
          mpz_add_ui (t, t, x->h);   /* Add high fraction */
        }
    }
  if (x->e > W)
    {
      mpz_mul_2exp (t, t, x->e - W); /* Shift to allow for low fraction */
      mpz_add (t, t, x->f);          /* Add low fraction */
    }
  /*
   * We could trim off any excess bits here by shifting rightward.  This is an
   * unnecessary complication.
   */
  mpz_setbit (t, 0);     /* Set the trailing bit so result is always inexact */
  if (neg)
    mpz_neg (t, t);
  /* Is -x->e representable as a mpfr_exp_t? */
  MPFR_ASSERTN (x->e <= (mpfr_uexp_t)(-1) >> 1);
  /*
   * Let mpfr_set_z_2exp do all the work of rounding to the requested
   * precision, setting overflow/underflow flags, and returning the right
   * inexact value.
   */
  inex = mpfr_set_z_2exp (z, t, -x->e, rnd);
  mpz_clear (t);
  return inex;
}