/* randist/shuffle.c

 * 

 * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 James Theiler, Brian Gough

 * 

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License as published by

 * the Free Software Foundation; either version 3 of the License, or (at

 * your option) any later version.

 * 

 * This program 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

 * General Public License for more details.

 * 

 * You should have received a copy of the GNU General Public License

 * along with this program; if not, write to the Free Software

 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.

 */

#include <config.h>

#include <stdlib.h>

#include <math.h>

#include <gsl/gsl_rng.h>

#include <gsl/gsl_randist.h>

/* Inline swap and copy functions for moving objects around */

static inline 

void swap (void * base, size_t size, size_t i, size_t j)

{

  register char * a = size * i + (char *) base ;

  register char * b = size * j + (char *) base ;

  register size_t s = size ;

  if (i == j)

    return ;

  do                                            

    {                                           

      char tmp = *a;                            

      *a++ = *b;                                

      *b++ = tmp;                               

    } 

  while (--s > 0);                              

}

static inline void 

copy (void * dest, size_t i, void * src, size_t j, size_t size)

{

  register char * a = size * i + (char *) dest ;

  register char * b = size * j + (char *) src ;

  register size_t s = size ;

  do                                            

    {                                           

      *a++ = *b++;                              

    } 

  while (--s > 0);                              

}

/* Randomly permute (shuffle) N indices

   Supply an array x[N] with nmemb members, each of size size and on

   return it will be shuffled into a random order.  The algorithm is

   from Knuth, SemiNumerical Algorithms, v2, p139, who cites Moses and

   Oakford, and Durstenfeld */

void

gsl_ran_shuffle (const gsl_rng * r, void * base, size_t n, size_t size)

{

  size_t i ;

  for (i = n - 1; i > 0; i--)

    {

      size_t j = gsl_rng_uniform_int(r, i+1); /* originally (i + 1) * gsl_rng_uniform (r) */

      swap (base, size, i, j) ;

    }

}

int

gsl_ran_choose (const gsl_rng * r, void * dest, size_t k, void * src, 

                 size_t n, size_t size)

{

  size_t i, j = 0;

  /* Choose k out of n items, return an array x[] of the k items.

     These items will prevserve the relative order of the original

     input -- you can use shuffle() to randomize the output if you

     wish */

  if (k > n)

    {

      GSL_ERROR ("k is greater than n, cannot sample more than n items",

                 GSL_EINVAL) ;

    }

  for (i = 0; i < n && j < k; i++)

    {

      if ((n - i) * gsl_rng_uniform (r) < k - j)

        {

          copy (dest, j, src, i, size) ;

          j++ ;

        }

    }

  return GSL_SUCCESS;

}

void

gsl_ran_sample (const gsl_rng * r, void * dest, size_t k, void * src, 

                size_t n, size_t size)

{

  size_t i, j = 0;

  /* Choose k out of n items, with replacement */

  for (i = 0; i < k; i++)

    {

      j = gsl_rng_uniform_int (r, n);  /* originally n * gsl_rng_uniform (r) */

      copy (dest, i, src, j, size) ;

    }

}