Add BiDirectional Text formatting utilites to the i18n library.

lib/i18n/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.
+
+#library('bidi_utils');
+
+/**
+ * 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 containing constants to represent the directionality of text. */
+class TextDirection {
+  static final LTR = const TextDirection._('LTR', 'ltr');
+  static final 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 final 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;
+  }
+}
+
+class BidiUtils {
+  /** Unicode "Left-To-Right Embedding" (LRE) character. */
+  static final LRE = '\u202A';
+
+  /** Unicode "Right-To-Left Embedding" (RLE) character. */
+  static final RLE = '\u202B';
+
+  /** Unicode "Pop Directional Formatting" (PDF) character. */
+  static final PDF = '\u202C';
+
+  /** Unicode "Left-To-Right Mark" (LRM) character. */
+  static final LRM = '\u200E';
+
+  /** Unicode "Right-To-Left Mark" (RLM) character. */
+  static final 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 final String _LTR_CHARS =
+      @'A-Za-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02B8\u0300-\u0590'
+      @'\u0800-\u1FFF\u2C00-\uFB1C\uFDFE-\uFE6F\uFEFD-\uFFFF';
+  static final String _RTL_CHARS = @'\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(const RegExp(@'<[^>]*>|&[^;]+;'), ' ');
+  }
+
+  /**
+   * 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 const 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 const 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 const 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 const 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 const RegExp(@'[' '$_LTR_CHARS' @']').hasMatch(
+        isHtml? stripHtmlIfNeeded(text) : text);
+  }
+
+  /**
+   * Determines if the given [text] has any RTL characters in it.
+   * If [isHtml] is true, the text is HTML or HTML-escaped.
+   */
+  static bool hasAnyRtl(String text, [isHtml=false]) {
+    return const RegExp(@'[' '$_RTL_CHARS' @']').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 const RegExp(@'^(ar|dv|he|iw|fa|nqo|ps|sd|ug|ur|yi|.*[-_]'
+        @'(Arab|Hebr|Thaa|Nkoo|Tfng))(?!.*[-_](Latn|Cyrl)($|-|_))'
+        @'($|-|_)', ignoreCase : true).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 = const 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 = 
+        const RegExp(@'(\(.*?\)+)|(\[.*?\]+)|(\{.*?\}+)|(&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,
+        const RegExp(@'(\(.*?\)+)|(\[.*?\]+)|(\{.*?\}+)|(<.*?>+)'), 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', const 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);
+      buffer.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 estimateDirection(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(const RegExp(@'\s+'))) {
+      if (BidiUtils.startsWithRtl(token)) {
+        rtlCount++;
+        total++;
+      } else if (const RegExp(@'^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 (BidiUtils.hasAnyLtr(token)) {
+        total++;
+      } else if (const RegExp(@'\d').hasMatch(token)) {
+        // Checked if token contains any numerals.
+        hasWeaklyLtr = true;
+      }
+    }
+
+    if (total == 0) {
+      return hasWeaklyLtr ? TextDirection.LTR : TextDirection.UNKNOWN;
+    } else if (rtlCount > BidiUtils._RTL_DETECTION_THRESHOLD * total) {
+      return TextDirection.RTL;
+    } else {
+      return TextDirection.LTR;
+    }
+  }
+
+  /**
+   * Find the first index in [str] of the first closing parenthesis that does
+   * not match an opening parenthesis.
+   */
+  static int _unmatchedParenIndex(String str) {
+    int sum = 0;
+    int index = 0;
+    while (sum >= 0 || index > str.length) {
+      int char = str.charCodeAt(index);
+      if (char == '('.charCodeAt(0)) sum++;
+      else if (char == ')'.charCodeAt(0)) sum--;
+      index++;
+    }
+    return index;
+  }
+
+  /**
+   * 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) == '"'
+          && const RegExp('[\u0591-\u05f2]').hasMatch(str.substring(i-1, i))) {
+        buf.add('\u05f4');
+      } else if (str.substring(i, i+1) == "'"
+          && const 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 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]) {
+    return estimateDirection(str, isHtml) == TextDirection.RTL;
+  }
+}