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shuf: improve randperm overflow checking

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* gl/lib/randperm.c: Include randperm.h first, since it’s the API.
Include stdint.h, count-leading-zeros.h, verify.h.
(floor_lg): Rename from ceil_log (which was not actually
implementing the ceiling!) and implement the floor using
count_leading_zeros.
(randperm_bound): Use floor_lg, not ceil_log.  Use uintmax_t
instead of size_t in case the size gets large on a 32-bit host.
* gl/modules/randperm (Depends-on): Add count-leading-zeros, stdint.
This commit is contained in:
Paul Eggert
2019-10-22 12:58:07 -07:00
parent 565dd395c3
commit 74163ea677
2 changed files with 18 additions and 11 deletions
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27
gl/lib/randperm.c
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@@ -19,24 +19,29 @@
#include <config.h>
#include "hash.h"
#include "randperm.h"
#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include "count-leading-zeros.h"
#include "hash.h"
#include "verify.h"
#include "xalloc.h"
/* Return the ceiling of the log base 2 of N. If N is zero, return
an unspecified value. */
/* Return the floor of the log base 2 of N. If N is zero, return -1. */
static size_t _GL_ATTRIBUTE_CONST
ceil_lg (size_t n)
static int _GL_ATTRIBUTE_CONST
floor_lg (size_t n)
{
size_t b = 0;
for (n--; n != 0; n /= 2)
b++;
return b;
verify (SIZE_WIDTH <= ULLONG_WIDTH);
return (n == 0 ? -1
: SIZE_WIDTH <= UINT_WIDTH
? UINT_WIDTH - 1 - count_leading_zeros (n)
: SIZE_WIDTH <= ULONG_WIDTH
? ULONG_WIDTH - 1 - count_leading_zeros_l (n)
: ULLONG_WIDTH - 1 - count_leading_zeros_ll (n));
}
/* Return an upper bound on the number of random bytes needed to
@@ -48,10 +53,10 @@ 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);
uintmax_t lg_n = floor_lg (n) + 1;
/* Upper bound on number of bits needed to generated the first H elements. */
size_t ar = lg_n * h;
uintmax_t ar = lg_n * h;
/* Convert the bit count to a byte count. */
size_t bound = (ar + CHAR_BIT - 1) / CHAR_BIT;

2
gl/modules/randperm
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@@ -6,7 +6,9 @@ lib/randperm.c
lib/randperm.h
Depends-on:
count-leading-zeros
randint
stdint
xalloc
hash