ibf_sim.c

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/*
      This file is part of GNUnet
      Copyright (C) 2013 GNUnet e.V.
 
      GNUnet is free software: you can redistribute it and/or modify it
      under the terms of the GNU Affero General Public License as published
      by the Free Software Foundation, either version 3 of the License,
      or (at your option) any later version.
 
      GNUnet 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
      Affero General Public License for more details.
 
      You should have received a copy of the GNU Affero General Public License
      along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
     SPDX-License-Identifier: AGPL3.0-or-later
 */
 
#include "platform.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
 
#define MAX_IBF_DECODE 16
 
/* report average over how many rounds? */
#define ROUNDS 100000
 
/* enable one of the three below */
// simple fix
#define FIX1 0
// possibly slightly better fix for large IBF_DECODE values
#define FIX2 1
 
// SIGCOMM algorithm
#define STRATA 0
 
// print each value?
#define VERBOSE 0
// avoid assembly? (ASM is about 50% faster)
#define SLOW 0
 
int
main (int argc, char **argv)
{
  unsigned int round;
  unsigned int buckets[31]; // max is 2^31 as 'random' returns only between 0 and 2^31
  unsigned int i;
  int j;
  unsigned int r;
  unsigned int ret;
  unsigned long long total;
  unsigned int want;
  double predict;
 
  srandom (time (NULL));
  total = 0;
  want = atoi (argv[1]);
  for (round = 0; round < ROUNDS; round++)
  {
    memset (buckets, 0, sizeof(buckets));
    for (i = 0; i < want; i++)
    {
      /* FIXME: might want to use 'better' PRNG to avoid
         PRNG-induced biases */
      r = random ();
      if (0 == r)
        continue;
#if SLOW
      for (j = 0; (j < 31) && (0 == (r & (1 << j))); j++)
        ;
#else
      /* use assembly / gcc */
      j = __builtin_ffs (r) - 1;
#endif
      buckets[j]++;
    }
    ret = 0;
    predict = 0.0;
    for (j = 31; j >= 0; j--)
    {
#if FIX1
      /* improved algorithm, for 1000 elements with IBF-DECODE 8, I
         get 990/1000 elements on average over 1 million runs; key
         idea being to stop short of the 'last' possible IBF as
         otherwise a "lowball" per-chance would unduly influence the
         result */if ((j > 0) &&
          (buckets[j - 1] > MAX_IBF_DECODE))
      {
        ret *= (1 << (j + 1));
        break;
      }
#endif
#if FIX2
      /* another improvement: don't just always cut off the last one,
         but rather try to predict based on all previous values where
         that "last" one is; additional prediction can only really
         work if MAX_IBF_DECODE is sufficiently high */
      if ((j > 0) &&
          ((buckets[j - 1] > MAX_IBF_DECODE) ||
           (predict > MAX_IBF_DECODE)))
      {
        ret *= (1 << (j + 1));
        break;
      }
#endif
#if STRATA
      /* original algorithm, for 1000 elements with IBF-DECODE 8,
         I get 920/1000 elements on average over 1 million runs */
      if (buckets[j] > MAX_IBF_DECODE)
      {
        ret *= (1 << (j + 1));
        break;
      }
#endif
      ret += buckets[j];
      predict = (buckets[j] + 2.0 * predict) / 2.0;
    }
#if VERBOSE
    fprintf (stderr, "%u ", ret);
#endif
    total += ret;
  }
  fprintf (stderr, "\n");
  fprintf (stdout, "average %llu\n", total / ROUNDS);
  return 0;
}
 
 
/* TODO: should calculate stddev of the results to also be able to
   say something about the stability of the results, outside of
   large-scale averages -- gaining 8% precision at the expense of
   50% additional variance might not be worth it... */