/* Generate random permutations. Copyright (C) 2006, 2007 Free Software Foundation, Inc. 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, see <http://www.gnu.org/licenses/>. */ /* Written by Paul Eggert. */ #include <config.h> #include "randperm.h" #include <limits.h> #include "xalloc.h" /* Return the ceiling of the log base 2 of N. If N is zero, return an unspecified value. */ static size_t ceil_lg (size_t n) { size_t b = 0; for (n--; n != 0; n /= 2) b++; return b; } /* Return an upper bound on the number of random bytes needed to generate the first H elements of a random permutation of N elements. H must not exceed N. */ size_t randperm_bound (size_t h, size_t n) { /* Upper bound on number of bits needed to generate the first number of the permutation. */ size_t lg_n = ceil_lg (n); /* Upper bound on number of bits needed to generated the first H elements. */ size_t ar = lg_n * h; /* Convert the bit count to a byte count. */ size_t bound = (ar + CHAR_BIT - 1) / CHAR_BIT; return bound; } /* From R, allocate and return a malloc'd array of the first H elements of a random permutation of N elements. H must not exceed N. Return NULL if H is zero. */ size_t * randperm_new (struct randint_source *r, size_t h, size_t n) { size_t *v; switch (h) { case 0: v = NULL; break; case 1: v = xmalloc (sizeof *v); v[0] = randint_choose (r, n); break; default: { size_t i; v = xnmalloc (n, sizeof *v); for (i = 0; i < n; i++) v[i] = i; for (i = 0; i < h; i++) { size_t j = i + randint_choose (r, n - i); size_t t = v[i]; v[i] = v[j]; v[j] = t; } v = xnrealloc (v, h, sizeof *v); } break; } return v; }

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