1 files changed, 134 insertions, 0 deletions
diff --git a/src/relpath.c b/src/relpath.c
new file mode 100644
index 0000000..bec34f7
--- /dev/null
+++ b/src/relpath.c
@@ -0,0 +1,134 @@
+/* relpath - print the relative path
+   Copyright (C) 2012-2022 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 Pádraig Brady.  */
+
+#include <config.h>
+
+#include "error.h"
+#include "system.h"
+#include "relpath.h"
+
+
+/* Return the length of the longest common prefix
+   of canonical PATH1 and PATH2, ensuring only full path components
+   are matched.  Return 0 on no match.  */
+ATTRIBUTE_PURE
+static int
+path_common_prefix (char const *path1, char const *path2)
+{
+  int i = 0;
+  int ret = 0;
+
+  /* We already know path1[0] and path2[0] are '/'.  Special case
+     '//', which is only present in a canonical name on platforms
+     where it is distinct.  */
+  if ((path1[1] == '/') != (path2[1] == '/'))
+    return 0;
+
+  while (*path1 && *path2)
+    {
+      if (*path1 != *path2)
+        break;
+      if (*path1 == '/')
+        ret = i + 1;
+      path1++;
+      path2++;
+      i++;
+    }
+
+  if ((!*path1 && !*path2)
+      || (!*path1 && *path2 == '/')
+      || (!*path2 && *path1 == '/'))
+    ret = i;
+
+  return ret;
+}
+
+/* Either output STR to stdout or
+   if *PBUF is not NULL then append STR to *PBUF
+   and update *PBUF to point to the end of the buffer
+   and adjust *PLEN to reflect the remaining space.
+   Return TRUE on failure.  */
+static bool
+buffer_or_output (char const *str, char **pbuf, size_t *plen)
+{
+  if (*pbuf)
+    {
+      size_t slen = strlen (str);
+      if (slen >= *plen)
+        return true;
+      memcpy (*pbuf, str, slen + 1);
+      *pbuf += slen;
+      *plen -= slen;
+    }
+  else
+    {
+      fputs (str, stdout);
+    }
+
+  return false;
+}
+
+/* Output the relative representation if possible.
+   If BUF is non-NULL, write to that buffer rather than to stdout.  */
+bool
+relpath (char const *can_fname, char const *can_reldir, char *buf, size_t len)
+{
+  bool buf_err = false;
+
+  /* Skip the prefix common to --relative-to and path.  */
+  int common_index = path_common_prefix (can_reldir, can_fname);
+  if (!common_index)
+    return false;
+
+  char const *relto_suffix = can_reldir + common_index;
+  char const *fname_suffix = can_fname + common_index;
+
+  /* Skip over extraneous '/'.  */
+  if (*relto_suffix == '/')
+    relto_suffix++;
+  if (*fname_suffix == '/')
+    fname_suffix++;
+
+  /* Replace remaining components of --relative-to with '..', to get
+     to a common directory.  Then output the remainder of fname.  */
+  if (*relto_suffix)
+    {
+      buf_err |= buffer_or_output ("..", &buf, &len);
+      for (; *relto_suffix; ++relto_suffix)
+        {
+          if (*relto_suffix == '/')
+            buf_err |= buffer_or_output ("/..", &buf, &len);
+        }
+
+      if (*fname_suffix)
+        {
+          buf_err |= buffer_or_output ("/", &buf, &len);
+          buf_err |= buffer_or_output (fname_suffix, &buf, &len);
+        }
+    }
+  else
+    {
+        buf_err |= buffer_or_output (*fname_suffix ? fname_suffix : ".",
+                                     &buf, &len);
+    }
+
+  if (buf_err)
+    error (0, ENAMETOOLONG, "%s", _("generating relative path"));
+
+  return !buf_err;
+}