1 files changed, 85 insertions, 0 deletions
diff --git a/lib/cycle-check.c b/lib/cycle-check.c
new file mode 100644
index 0000000..20e0e1e
--- /dev/null
+++ b/lib/cycle-check.c
@@ -0,0 +1,85 @@
+/* help detect directory cycles efficiently
+
+   Copyright (C) 2003-2006, 2009-2022 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 <stdbool.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;
+}