code: private File findFile(File base, String path, boolean cs) {
          if (FileUtil.isAbsolutePath(path)) {
              if (base == null) {
                  String[] s = FileUtil.dissect(path);
                  base = new File(s[0]);
                  path = s[1];
              } else {
                  File f = FileUtil.normalize(path);
                  String s = FileUtil.removeLeadingPath(base, f);
                  if (s.equals(f.getAbsolutePath())) {
                      //removing base from path yields no change; path not child of base
                      return null;
                  }
                  path = s;
              }
          }
          return findFile(base, SelectorUtils.tokenizePath(path), cs);
      }
docs: From <code>base</code> traverse the filesystem in order to find
  a file that matches the given name.
  
  @param base base File (dir).
  @param path file path.
  @param cs whether to scan case-sensitively.
  @return File object that points to the file in question or null.
  
  @since Ant 1.6.3