01-original.c

// original

int CheckExtension (char *filename, char *ext)
{
  char *s;
  if (!filename) return(0);
  if (strlen(filename) == 0) return(0);
  s = strrchr(filename,'.');
  if (!s) return(0);
  if (!strcmp(s,ext)) return(1);
  return(0);
}

02-rewritten.c

// my take at readable C

int CheckExtension (char *filename, char *ext)
{
  char *s;
  return filename
      && strlen(filename)
      && (s = strrchr(filename,'.'))
      && strcmp(s,ext);
}

03-high-level.clj

; high level goodness

(defn check-extension [filename extension]
  (= extension
     (second (re-matches (re-pattern (str ".*[.](" extension ")"))
                         filename))))

04-threaded.clj

(defn check-extension 
  [filename extension]
  (-> (str ".*[.](" extension ")")
      re-pattern
      (re-matches filename)
      second
      (= extension)))