Base class for {@link BidiFormatter} and {@link SafeHtmlBidiFormatter} that contains their common implementation.

abstract class BidiFormatterBase {

 bool alwaysSpan;
 Direction contextDir;

 BidiFormatterBase(this.contextDir, this.alwaysSpan);

 /**
  * @see BidiFormatter#spanWrapWithKnownDir(HasDirection.Direction, String, boolean, boolean)
  *
  * @param dir {@code str}'s direction
  * @param str The input string
  * @param isHtml Whether {@code str} is HTML / HTML-escaped
  * @param dirReset Whether to append a trailing unicode bidi mark matching the
  *          context direction, when needed, to prevent the possible garbling
  *          of whatever may follow {@code str}
  * @return Input string after applying the above processing.
  */
 String spanWrapWithKnownDirBase(Direction dir, String str, bool isHtml, bool dirReset) {
   bool dirCondition = dir != Direction.DEFAULT && dir != contextDir;
   String origStr = str;
//    if (!isHtml) {
//      str = SafeHtmlUtils.htmlEscape(str);
//    }

   StringBuffer result = new StringBuffer();
   if (alwaysSpan || dirCondition) {
     result.write("<span");
     if (dirCondition) {
       result.write(" ");
       result.write(dir == Direction.RTL ? "dir=rtl" : "dir=ltr");
     }
     result.write(">");
     result.write(str);
     result.write("</span>");
   } else {
     result.write(str);
   }
   // origStr is passed (more efficient when isHtml is false).
   result.write(dirResetIfNeeded(origStr, dir, isHtml, dirReset));
   return result.toString();
 }

 /**
  * Returns a unicode BiDi mark matching the context direction (LRM or RLM) if
  * {@code dirReset}, and if the overall direction or the exit direction of
  * {@code str} are opposite to the context direction. Otherwise returns the
  * empty string.
  *
  * @param str The input string
  * @param dir {@code str}'s overall direction
  * @param isHtml Whether {@code str} is HTML / HTML-escaped
  * @param dirReset Whether to perform the reset
  * @return A unicode BiDi mark or the empty string.
  */
 String dirResetIfNeeded(String str, Direction dir, bool isHtml, bool dirReset) {
   // endsWithRtl and endsWithLtr are called only if needed (short-circuit).
   if (dirReset
       && ((contextDir == Direction.LTR &&
           (dir == Direction.RTL ||
            BidiUtils.get().endsWithRtl(str, isHtml))) ||
           (contextDir == Direction.RTL &&
           (dir == Direction.LTR ||
            BidiUtils.get().endsWithLtr(str, isHtml))))) {
     return contextDir == Direction.LTR ? Format.LRM_STRING : Format.RLM_STRING;
   } else {
     return "";
   }
 }
}