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/* help detect directory cycles efficiently
Copyright (C) 2003-2006, 2009-2023 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 <https://www.gnu.org/licenses/>. */
/* Written by Jim Meyering */
#include <config.h>
#include "cycle-check.h"
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include "assure.h"
#define CC_MAGIC 9827862
/* Return true if I is a power of 2, or is zero. */
static bool
is_zero_or_power_of_two (uintmax_t i)
{
return (i & (i - 1)) == 0;
}
void
cycle_check_init (struct cycle_check_state *state)
{
state->chdir_counter = 0;
state->magic = CC_MAGIC;
}
/* In traversing a directory hierarchy, call this function once for each
descending chdir call, with SB corresponding to the chdir operand.
If SB corresponds to a directory that has already been seen,
return true to indicate that there is a directory cycle.
Note that this is done "lazily", which means that some of
the directories in the cycle may be processed twice before
the cycle is detected. */
bool
cycle_check (struct cycle_check_state *state, struct stat const *sb)
{
assure (state->magic == CC_MAGIC);
/* If the current directory ever happens to be the same
as the one we last recorded for the cycle detection,
then it's obviously part of a cycle. */
if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
return true;
/* If the number of "descending" chdir calls is a power of two,
record the dev/ino of the current directory. */
if (is_zero_or_power_of_two (++(state->chdir_counter)))
{
/* On all architectures that we know about, if the counter
overflows then there is a directory cycle here somewhere,
even if we haven't detected it yet. Typically this happens
only after the counter is incremented 2**64 times, so it's a
fairly theoretical point. */
if (state->chdir_counter == 0)
return true;
state->dev_ino.st_dev = sb->st_dev;
state->dev_ino.st_ino = sb->st_ino;
}
return false;
}
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