sdk/pkg/intl/lib/bidi_utils.dart

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of intl;

/**
 * Bidi stands for Bi-directional text.
 * According to http://en.wikipedia.org/wiki/Bi-directional_text:
 * Bi-directional text is text containing text in both text directionalities,
 * both right-to-left (RTL) and left-to-right (LTR). It generally involves text
 * containing different types of alphabets, but may also refer to boustrophedon,
 * which is changing text directionality in each row.
 *
 * This file provides some utility classes for determining directionality of
 * text, switching CSS layout from LTR to RTL, and other normalizing utilities
 * needed when switching between RTL and LTR formatting.
 *
 * It defines the TextDirection class which is used to represent directionality
 * of text,
 * In most cases, it is preferable to use bidi_formatter.dart, which provides
 * bidi functionality in the given directional context, instead of using
 * bidi_utils.dart directly.
 */
class TextDirection {
  static const LTR = const TextDirection._('LTR', 'ltr');
  static const RTL = const TextDirection._('RTL', 'rtl');
  // If the directionality of the text cannot be determined and we are not using
  // the context direction (or if the context direction is unknown), then the
  // text falls back on the more common ltr direction.
  static const UNKNOWN = const TextDirection._('UNKNOWN', 'ltr');

  /**
   * Textual representation of the directionality constant. One of
   * 'LTR', 'RTL', or 'UNKNOWN'.
   */
  final String value;

  /** Textual representation of the directionality when used in span tag. */
  final String spanText;

  const TextDirection._(this.value, this.spanText);

  /**
   * Returns true if [otherDirection] is known to be different from this
   * direction.
   */
  bool isDirectionChange(TextDirection otherDirection) {
    return otherDirection != TextDirection.UNKNOWN && this != otherDirection;
  }
}

/**
 * This provides utility methods for working with bidirectional text. All
 * of the methods are static, and are organized into a class primarily to
 * group them together for documentation and discoverability.
 */
class Bidi {

  /** Unicode "Left-To-Right Embedding" (LRE) character. */
  static const LRE = '\u202A';

  /** Unicode "Right-To-Left Embedding" (RLE) character. */
  static const RLE = '\u202B';

  /** Unicode "Pop Directional Formatting" (PDF) character. */
  static const PDF = '\u202C';

  /** Unicode "Left-To-Right Mark" (LRM) character. */
  static const LRM = '\u200E';

  /** Unicode "Right-To-Left Mark" (RLM) character. */
  static const RLM = '\u200F';

  /** Constant to define the threshold of RTL directionality. */
  static num _RTL_DETECTION_THRESHOLD = 0.40;

  /**
   * Practical patterns to identify strong LTR and RTL characters, respectively.
   * These patterns are not completely correct according to the Unicode
   * standard. They are simplified for performance and small code size.
   */
  static const String _LTR_CHARS =
      r'A-Za-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02B8\u0300-\u0590'
      r'\u0800-\u1FFF\u2C00-\uFB1C\uFDFE-\uFE6F\uFEFD-\uFFFF';
  static const String _RTL_CHARS = r'\u0591-\u07FF\uFB1D-\uFDFD\uFE70-\uFEFC';

  /**
   * Returns the input [text] with spaces instead of HTML tags or HTML escapes,
   * which is helpful for text directionality estimation.
   * Note: This function should not be used in other contexts.
   * It does not deal well with many things: comments, script,
   * elements, style elements, dir attribute,`>` in quoted attribute values,
   * etc. But it does handle well enough the most common use cases.
   * Since the worst that can happen as a result of these shortcomings is that
   * the wrong directionality will be estimated, we have not invested in
   * improving this.
   */
  static String stripHtmlIfNeeded(String text) {
    // The regular expression is simplified for an HTML tag (opening or
    // closing) or an HTML escape. We might want to skip over such expressions
    // when estimating the text directionality.
    return text.replaceAll(new RegExp(r'<[^>]*>|&[^;]+;'), ' ');
  }

  /**
   * Determines if the first character in [text] with strong directionality is
   * LTR. If [isHtml] is true, the text is HTML or HTML-escaped.
   */
  static bool startsWithLtr(String text, [isHtml=false]) {
    return new RegExp('^[^$_RTL_CHARS]*[$_LTR_CHARS]').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * Determines if the first character in [text] with strong directionality is
   * RTL. If [isHtml] is true, the text is HTML or HTML-escaped.
   */
  static bool startsWithRtl(String text, [isHtml=false]) {
    return new RegExp('^[^$_LTR_CHARS]*[$_RTL_CHARS]').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * Determines if the exit directionality (ie, the last strongly-directional
   * character in [text] is LTR. If [isHtml] is true, the text is HTML or
   * HTML-escaped.
   */
  static bool endsWithLtr(String text, [isHtml=false]) {
    return new RegExp('[$_LTR_CHARS][^$_RTL_CHARS]*\$').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * Determines if the exit directionality (ie, the last strongly-directional
   * character in [text] is RTL. If [isHtml] is true, the text is HTML or
   * HTML-escaped.
   */
  static bool endsWithRtl(String text, [isHtml=false]) {
    return new RegExp('[$_RTL_CHARS][^$_LTR_CHARS]*\$').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * Determines if the given [text] has any LTR characters in it.
   * If [isHtml] is true, the text is HTML or HTML-escaped.
   */
  static bool hasAnyLtr(String text, [isHtml=false]) {
    return new RegExp(r'[' '$_LTR_CHARS' r']').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * Determines if the given [text] has any RTL characters in it.
   * If [isHtml] is true, the text is HTML or HTML-escaped.
   */
  static bool hasAnyRtl(String text, [isHtml=false]) {
    return new RegExp(r'[' '$_RTL_CHARS' r']').hasMatch(
        isHtml? stripHtmlIfNeeded(text) : text);
  }

  /**
   * Check if a BCP 47 / III [languageString] indicates an RTL language.
   *
   * i.e. either:
   * - a language code explicitly specifying one of the right-to-left scripts,
   *   e.g. "az-Arab", or
   * - a language code specifying one of the languages normally written in a
   *   right-to-left script, e.g. "fa" (Farsi), except ones explicitly
   *   specifying Latin or Cyrillic script (which are the usual LTR
   *   alternatives).
   *
   * The list of right-to-left scripts appears in the 100-199 range in
   * http://www.unicode.org/iso15924/iso15924-num.html, of which Arabic and
   * Hebrew are by far the most widely used. We also recognize Thaana, N'Ko, and
   * Tifinagh, which also have significant modern usage. The rest (Syriac,
   * Samaritan, Mandaic, etc.) seem to have extremely limited or no modern usage
   * and are not recognized.
   * The languages usually written in a right-to-left script are taken as those
   * with Suppress-Script: Hebr|Arab|Thaa|Nkoo|Tfng  in
   * http://www.iana.org/assignments/language-subtag-registry,
   * as well as Sindhi (sd) and Uyghur (ug).
   * The presence of other subtags of the language code, e.g. regions like EG
   * (Egypt), is ignored.
   */
  static bool isRtlLanguage(String languageString) {
    return new RegExp(r'^(ar|dv|he|iw|fa|nqo|ps|sd|ug|ur|yi|.*[-_]'
        r'(Arab|Hebr|Thaa|Nkoo|Tfng))(?!.*[-_](Latn|Cyrl)($|-|_))'
        r'($|-|_)', caseSensitive: false).hasMatch(languageString);
  }

  /**
   * Enforce the [html] snippet in RTL directionality regardless of overall
   * context. If the html piece was enclosed by a tag, the direction will be
   * applied to existing tag, otherwise a span tag will be added as wrapper.
   * For this reason, if html snippet start with with tag, this tag must enclose
   * the whole piece. If the tag already has a direction specified, this new one
   * will override existing one in behavior (should work on Chrome, FF, and IE
   * since this was ported directly from the Closure version).
   */
  static String enforceRtlInHtml(String html) {
    return _enforceInHtmlHelper(html, 'rtl');
  }

  /**
   * Enforce RTL on both end of the given [text] using unicode BiDi formatting
   * characters RLE and PDF.
   */
  static String enforceRtlInText(String text) {
    return '$RLE$text$PDF';
  }

  /**
   * Enforce the [html] snippet in LTR directionality regardless of overall
   * context. If the html piece was enclosed by a tag, the direction will be
   * applied to existing tag, otherwise a span tag will be added as wrapper.
   * For this reason, if html snippet start with with tag, this tag must enclose
   * the whole piece. If the tag already has a direction specified, this new one
   * will override existing one in behavior (tested on FF and IE).
   */
  static String enforceLtrInHtml(String html) {
    return _enforceInHtmlHelper(html, 'ltr');
  }

  /**
   * Enforce LTR on both end of the given [text] using unicode BiDi formatting
   * characters LRE and PDF.
   */
  static String enforceLtrInText(String text) {
    return '$LRE$text$PDF';
  }

  /**
   * Enforce the [html] snippet in the desired [direction] regardless of overall
   * context. If the html piece was enclosed by a tag, the direction will be
   * applied to existing tag, otherwise a span tag will be added as wrapper.
   * For this reason, if html snippet start with with tag, this tag must enclose
   * the whole piece. If the tag already has a direction specified, this new one
   * will override existing one in behavior (tested on FF and IE).
   */
  static String _enforceInHtmlHelper(String html, String direction) {
    if (html.startsWith('<')) {
      StringBuffer buffer = new StringBuffer();
      var startIndex = 0;
      Match match = new RegExp('<\\w+').firstMatch(html);
      if (match != null) {
        buffer..add(html.substring(startIndex, match.end))
              ..add(' dir=$direction');
        startIndex = match.end;
      }
      return (buffer..add(html.substring(startIndex))).toString();
    }
    // '\n' is important for FF so that it won't incorrectly merge span groups.
    return '\n<span dir=$direction>$html</span>';
  }

  /**
   * Apply bracket guard to [str] using html span tag. This is to address the
   * problem of messy bracket display that frequently happens in RTL layout.
   * If [isRtlContext] is true, then we explicitly want to wrap in a span of RTL
   * directionality, regardless of the estimated directionality.
   */
  static String guardBracketInHtml(String str, [bool isRtlContext]) {
    var useRtl = isRtlContext == null ? hasAnyRtl(str) : isRtlContext;
    RegExp matchingBrackets =
        new RegExp(r'(\(.*?\)+)|(\[.*?\]+)|(\{.*?\}+)|(&lt;.*?(&gt;)+)');
    return _guardBracketHelper(str, matchingBrackets,
        '<span dir=${useRtl? "rtl" : "ltr"}>', '</span>');
  }

  /**
   * Apply bracket guard to [str] using LRM and RLM. This is to address the
   * problem of messy bracket display that frequently happens in RTL layout.
   * This version works for both plain text and html, but in some cases is not
   * as good as guardBracketInHtml.
   * If [isRtlContext] is true, then we explicitly want to wrap in a span of RTL
   * directionality, regardless of the estimated directionality.
   */
  static String guardBracketInText(String str, [bool isRtlContext]) {
    var useRtl = isRtlContext == null ? hasAnyRtl(str) : isRtlContext;
    var mark = useRtl ? RLM : LRM;
    return _guardBracketHelper(str,
        new RegExp(r'(\(.*?\)+)|(\[.*?\]+)|(\{.*?\}+)|(<.*?>+)'), mark, mark);
  }

  /**
   * (Mostly) reimplements the $& functionality of "replace" in JavaScript.
   * Given a [str] and the [regexp] to match with, optionally supply a string to
   * be inserted [before] the match and/or [after]. For example,
   * `_guardBracketHelper('firetruck', new RegExp('truck'), 'hydrant', '!')`
   * would return 'firehydrant!'.
   */
  // TODO(efortuna): Get rid of this once this is implemented in Dart.
  // See Issue 2979.
  static String _guardBracketHelper(String str, RegExp regexp, [String before,
      String after]) {
    StringBuffer buffer = new StringBuffer();
    var startIndex = 0;
    Iterable matches = regexp.allMatches(str);
    for (Match match in matches) {
      buffer..add(str.substring(startIndex, match.start))
            ..add(before)
            ..add(str.substring(match.start, match.end))
            ..add(after);
      startIndex = match.end;
    }
    return (buffer..add(str.substring(startIndex))).toString();
  }

  /**
   * Estimates the directionality of [text] using the best known
   * general-purpose method (using relative word counts). A
   * TextDirection.UNKNOWN return value indicates completely neutral input.
   * [isHtml] is true if [text] HTML or HTML-escaped.
   *
   * If the number of RTL words is above a certain percentage of the total
   * number of strongly directional words, returns RTL.
   * Otherwise, if any words are strongly or weakly LTR, returns LTR.
   * Otherwise, returns UNKNOWN, which is used to mean `neutral`.
   * Numbers and URLs are counted as weakly LTR.
   */
  static TextDirection estimateDirectionOfText(String text,
                                               {bool isHtml: false}) {
    text = isHtml? stripHtmlIfNeeded(text) : text;
    var rtlCount = 0;
    var total = 0;
    var hasWeaklyLtr = false;
    // Split a string into 'words' for directionality estimation based on
    // relative word counts.
    for (String token in text.split(new RegExp(r'\s+'))) {
      if (startsWithRtl(token)) {
        rtlCount++;
        total++;
      } else if (new RegExp(r'^http://').hasMatch(token)) {
        // Checked if token looks like something that must always be LTR even in
        // RTL text, such as a URL.
        hasWeaklyLtr = true;
      } else if (hasAnyLtr(token)) {
        total++;
      } else if (new RegExp(r'\d').hasMatch(token)) {
        // Checked if token contains any numerals.
        hasWeaklyLtr = true;
      }
    }

    if (total == 0) {
      return hasWeaklyLtr ? TextDirection.LTR : TextDirection.UNKNOWN;
    } else if (rtlCount > _RTL_DETECTION_THRESHOLD * total) {
      return TextDirection.RTL;
    } else {
      return TextDirection.LTR;
    }
  }

  /**
   * Replace the double and single quote directly after a Hebrew character in
   * [str] with GERESH and GERSHAYIM. This is most likely the user's intention.
   */
  static String normalizeHebrewQuote(String str) {
    StringBuffer buf = new StringBuffer();
    if (str.length > 0) {
      buf.add(str.substring(0, 1));
    }
    // Start at 1 because we're looking for the patterns [\u0591-\u05f2])" or
    // [\u0591-\u05f2]'.
    for (int i = 1; i < str.length; i++) {
      if (str.substring(i, i+1) == '"'
          && new RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i-1, i))) {
        buf.add('\u05f4');
      } else if (str.substring(i, i+1) == "'"
          && new RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i-1, i))) {
        buf.add('\u05f3');
      } else {
        buf.add(str.substring(i, i+1));
      }
    }
    return buf.toString();
  }

  /**
   * Check the estimated directionality of [str], return true if the piece of
   * text should be laid out in RTL direction. If [isHtml] is true, the string
   * is HTML or HTML-escaped.
   */
  static bool detectRtlDirectionality(String str, {bool isHtml: false}) {
    return estimateDirectionOfText(str, isHtml: isHtml) == TextDirection.RTL;
  }
}