1 files changed, 63 insertions, 0 deletions
diff --git a/lib/cycle-check.c b/lib/cycle-check.c
new file mode 100644
index 000000000..403dc1615
--- /dev/null
+++ b/lib/cycle-check.c
@@ -0,0 +1,63 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <assert.h>
+#include <stdlib.h>
+
+#if HAVE_STDBOOL_H
+# include <stdbool.h>
+#else
+typedef enum {false = 0, true = 1} bool;
+#endif
+
+#include "cycle-check.h"
+#include "xalloc.h"
+
+#define SAME_INODE(Stat_buf_1, Stat_buf_2) \
+  ((Stat_buf_1).st_ino == (Stat_buf_2).st_ino \
+   && (Stat_buf_1).st_dev == (Stat_buf_2).st_dev)
+
+#define CC_MAGIC 9827862
+
+static inline bool
+is_power_of_two (unsigned int 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)
+{
+  assert (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_power_of_two (++(state->chdir_counter)))
+    {
+      state->dev_ino.st_dev = sb->st_dev;
+      state->dev_ino.st_ino = sb->st_ino;
+    }
+
+  return false;
+}