Add BiDirectional Text formatting utilites to the i18n library.

lib/i18n/bidi_formatter.dart

Index: lib/i18n/bidi_formatter.dart
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--- lib/i18n/bidi_formatter.dart	(revision 0)
+++ lib/i18n/bidi_formatter.dart	(revision 0)
@@ -0,0 +1,188 @@
+// 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_formatter');
+
+#import('bidi_utils.dart');
+
+/**
+ * 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.
+ *
+ * Utility class for formatting display text in a potentially
+ * opposite-directionality context without garbling layout issues.
+ * Mostly a very "slimmed-down" and dart-ified port of the Closure Birectional
+ * formatting libary. If there is a utility in the Closure library (or ICU, or
+ * elsewhere) that you would like this formatter to make available, please
+ * contact the Dart team.
+ *
+ * Provides the following functionality:
+ *
+ * 1. *BiDi Wrapping*
+ * When text in one language is mixed into a document in another, opposite-
+ * directionality language, e.g. when an English business name is embedded in a
+ * Hebrew web page, both the inserted string and the text following it may be
+ * displayed incorrectly unless the inserted string is explicitly separated
+ * from the surrounding text in a "wrapper" that declares its directionality at
+ * the start and then resets it back at the end. This wrapping can be done in
+ * HTML mark-up (e.g. a 'span dir=rtl' tag) or - only in contexts where mark-up
+ * can not be used - in Unicode BiDi formatting codes (LRE|RLE and PDF).
+ * Providing such wrapping services is the basic purpose of the BiDi formatter.
+ *
+ * 2. *Directionality estimation*
+ * How does one know whether a string about to be inserted into surrounding
+ * text has the same directionality? Well, in many cases, one knows that this
+ * must be the case when writing the code doing the insertion, e.g. when a
+ * localized message is inserted into a localized page. In such cases there is
+ * no need to involve the BiDi formatter at all. In the remaining cases, e.g.
+ * when the string is user-entered or comes from a database, the language of
+ * the string (and thus its directionality) is not known a priori, and must be
+ * estimated at run-time. The BiDi formatter does this automatically.
+ *
+ * 3. *Escaping*
+ * When wrapping plain text - i.e. text that is not already HTML or HTML-
+ * escaped - in HTML mark-up, the text must first be HTML-escaped to prevent XSS
+ * attacks and other nasty business. This of course is always true, but the
+ * escaping cannot be done after the string has already been wrapped in
+ * mark-up, so the BiDi formatter also serves as a last chance and includes
+ * escaping services.
+ *
+ * Thus, in a single call, the formatter will escape the input string as
+ * specified, determine its directionality, and wrap it as necessary. It is
+ * then up to the caller to insert the return value in the output.
+ */
+
+#import('dart:web');
+
+class BidiFormatter {
+
+  /** The direction of the surrounding text (the context). */
+  TextDirection contextDirection;
+
+  /**
+   * Indicates if we should always wrap the formatted text in a <span<,.
+   */
+  bool _alwaysSpan;
+
+  /**
+   * Create a formatting object with a direction. If [alwaysSpan] is true we
+   * should always use a `span` tag, even when the input directionality is
+   * neutral or matches the context, so that the DOM structure of the output
+   * does not depend on the combination of directionalities.
+   */
+  BidiFormatter.LTR([alwaysSpan=false]) : contextDirection = TextDirection.LTR,
+      _alwaysSpan = alwaysSpan;
+  BidiFormatter.RTL([alwaysSpan=false]) : contextDirection = TextDirection.RTL,
+      _alwaysSpan = alwaysSpan;
+  BidiFormatter.UNKNOWN([alwaysSpan=false]) : 
+      contextDirection = TextDirection.UNKNOWN, _alwaysSpan = alwaysSpan;
+
+  /** Is true if the context direction for this formatter is RTL. */
+  bool get isRTL() => contextDirection == TextDirection.RTL;
+
+  /**
+   * Formats a string of a given (or estimated, if not provided)
+   * [direction] for use in HTML output of the context directionality, so
+   * an opposite-directionality string is neither garbled nor garbles what
+   * follows it.
+   * If the input string's directionality doesn't match the context
+   * directionality, we wrap it with a `span` tag and add a `dir` attribute
+   * (either "dir=rtl" or "dir=ltr").
+   * If alwaysSpan was true when constructing the formatter, the input is always
+   * wrapped with `span` tag, skipping the dir attribute when it's not needed.
+   *
+   * If [resetDir] is true and the overall directionality or the exit
+   * directionality of [text] is opposite to the context directionality,
+   * a trailing unicode BiDi mark matching the context directionality is
+   * appended (LRM or RLM). If [isHtml] is false, we HTML-escape the [text].
+   */
+  String wrapWithSpan(String text, [bool isHtml=false, bool resetDir=true,
+                  TextDirection direction]) {
+    if (direction == null) direction = estimateDirection(text, isHtml);
+    var result;
+    if (!isHtml) text = htmlEscape(text);
+    var directionChange = contextDirection.isDirectionChange(direction);
+    if (_alwaysSpan || directionChange) {
+      var spanDirection = '';
+      if (directionChange) {
+        spanDirection = ' dir=${direction.spanText}';
+      }
+      result= '<span$spanDirection>$text</span>';
+    } else {
+      result = text;
+    }
+    return result.concat(resetDir? _resetDir(text, direction, isHtml) : '');
+  }
+
+  /**
+   * Format [text] of a known (if specified) or estimated [direction] for use 
+   * in *plain-text* output of the context directionality, so an
+   * opposite-directionality text is neither garbled nor garbles what follows
+   * it. Unlike wrapWithSpan, this makes use of unicode BiDi formatting
+   * characters instead of spans for wrapping. The returned string would be
+   * RLE+text+PDF for RTL text, or LRE+text+PDF for LTR text.
+   *
+   * If [resetDir] is true, and if the overall directionality or the exit
+   * directionality of text are opposite to the context directionality,
+   * a trailing unicode BiDi mark matching the context directionality is
+   * appended (LRM or RLM).
+   *
+   * In HTML, the *only* valid use of this function is inside of elements that
+   * do not allow markup, e.g. an 'option' tag.
+   * This function does *not* do HTML-escaping regardless of the value of
+   * [isHtml]. [isHtml] is used to designate if the text contains HTML (escaped
+   * or unescaped).
+   */
+  String wrapWithUnicode(String text, [bool isHtml=false, bool resetDir=true,
+                     TextDirection direction]) {
+    if (direction == null) direction = estimateDirection(text, isHtml);
+    var result = text;
+    if (contextDirection.isDirectionChange(direction)) {
+      result = '''${direction == TextDirection.RTL ?
+                      BidiUtils.RLE :
+                      BidiUtils.LRE}$text${BidiUtils.PDF}''';
+    }
+    return result.concat(resetDir? _resetDir(text, direction, isHtml) : '');
+  }
+
+  /**
+   * 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.
+   */
+  TextDirection estimateDirection(String text, [bool isHtml=false]) {
+    return BidiUtils.estimateDirection(text, isHtml);
+  }
+
+  /**
+   * Returns a unicode BiDi mark matching the surrounding context's [direction]
+   * (not necessarily the direction of [text]). The function returns an LRM or
+   * RLM if the overall directionality or the exit directionality of [text] is
+   * opposite the context directionality. Otherwise
+   * return the empty string. [isHtml] is true if [text] is HTML or 
+   * HTML-escaped.
+   */
+  String _resetDir(String text, TextDirection direction, bool isHtml) {
+    // endsWithRtl and endsWithLtr are called only if needed (short-circuit).
+    if ((contextDirection == TextDirection.LTR &&
+          (direction == TextDirection.RTL ||
+           BidiUtils.endsWithRtl(text, isHtml))) ||
+        (contextDirection == TextDirection.RTL &&
+          (direction == TextDirection.LTR ||
+           BidiUtils.endsWithLtr(text, isHtml)))) {
+      if (contextDirection == TextDirection.LTR) {
+        return BidiUtils.LRM;
+      } else {
+        return BidiUtils.RLM;
+      }
+    } else {
+      return '';
+    }
+  }
+}