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However, because these right-to-left scriptsuse digits that are written from left to right, the text is actually bidirectional:a mixture of right-to-left and left-to-right text. In addition todigits, embedded words from English and other scripts are also written fromleft to right, also producing bidirectional text. Without a clearspecification, ambiguities can arise in determining the ordering of the displayed characters when thehorizontal direction of the text is not uniform.

This annex describes the algorithm used to determine the directionality for bidirectional Unicode text. The algorithm extends the implicit model currently employed by a number of existing implementations and adds explicit formatting characters for special circumstances. In most cases, there is no need to include additional information with the text to obtain correct display ordering.

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However, in the case of bidirectional text, there are circumstances wherean implicit bidirectional ordering is not sufficient to producecomprehensible text. To deal with these cases, a minimal set of directionalformatting characters is defined to control the ordering of characters whenrendered. This allows exact control of the display ordering for legibleinterchange and ensures that plain text used for simple items like filenames or labels can always be correctly ordered for display.

Each character has an implicit bidirectional type. The bidirectional types left-to-right and right-to-left are called strong types, and characters of those types are called strong directional characters. The bidirectional types associated with numbers are called weak types, and characters of those types are called weak directional characters. With the exception of the directional formatting characters, the remaining bidirectional types and characters are called neutral. The algorithm uses the implicit bidirectional types of the characters in a text to arrive at a reasonable display ordering for text.

The following characters allow the bidirectional character types to be overridden when required for special cases, such as for part numbers. They are to be avoided wherever possible, because of security concerns. For more information, see [UTR36]. Directional overrides can be nested one inside another, and in embeddings and isolates.

In addition to allowing the embedding of strongly directional text without unduly affecting the bidirectional order of its surroundings, one of the isolate formatting characters also offers an extra feature: embedding text while inferring its direction heuristically from its constituent characters.

There is no special mention of the implicit directional marks in the following algorithm. That is because their effect on bidirectional ordering is exactly the same as a corresponding strong directional character; the only difference is that they do not appear in the display.

BD1. The bidirectional character types are values assigned to each Unicode character, including unassigned characters.The formal property name in the Unicode Character Database [UCD]is Bidi_Class.

BD6. The directional override status determines whether thebidirectional type of characters is to be reset. The directional override statusis set by using explicit directional formatting characters. This status has three states, as shown in Table 2.

The normative bidirectional character types for each character are specified in the Unicode Character Database [UCD] and are summarized in Table 4. This is a summary only: there are exceptions to the general scope. For example, certain characters such as U+0CBF KANNADA VOWEL SIGN I are given Type L (instead of NSM) to preserve canonical equivalence.

The body of the Bidirectional Algorithm uses bidirectional character types, explicit formatting characters, and bracket pairs to produce a list of resolved levels. This resolution process consists of the following steps:

An explicit directional override sets the embedding level in the same way the explicit embedding formatting characters do, but also changes the bidirectional character type of affected characters to the override direction.

The explicit embedding levels that have been assigned to the characters by the preceding rules will soon be further adjusted on the basis of the characters' implicit bidirectional types. The adjustment made for a given character will then depend on the characters around it. However, this dependency is limited by logically dividing the paragraph into sub-units, and doing the subsequent implicit processing on each unit independently.

tag_hash_109___ Process bracket pairs in an isolating run sequence sequentially in the logical order of the text positions of the opening paired brackets using the logic given below. Within this scope, bidirectional types EN and AN are treated as R.

The following clauses are the only permissible ways for systems to apply higher-level protocols to the ordering of bidirectional text. Some of the clauses apply to segments of structured text. This refers to the situation where text is interpreted as being structured, whether with explicit markup such as XML or HTML, or internally structured such as in a word processor or spreadsheet. In such a case, a segment is span of text that is distinguished in some way by the structure.

tag_hash_110____ Override P3, and set the paragraph embedding level explicitly. This does tag_hash_111___ apply when deciding how to treat FSI in rule X5c. A higher-level protocol may set any paragraph level. This can be done on the basis of the context, such as on a table cell, paragraph, document, or system level. (P2 may be skipped if P3 is overridden). Note that this does not allow a higher-level protocol to override the limit specified in BD2. A higher-level protocol may apply rules equivalent to P2 and P3 but default to level 1 (RTL) rather than 0 (LTR) to match overall RTL context. A higher-level protocol may use an entirely different algorithm that heuristically auto-detects the paragraph embedding level based on the paragraph text and its context. For example, it could base it on whether there are more RTL characters in the text than LTR. As another example, when the paragraph contains no strong characters, its direction could be determined by the levels of the paragraphs before and after. tag_hash_112____ Override W2, and set EN or AN explicitly. A higher-level protocol may reset characters of type EN to AN, or vice versa, and ignore W2. For example, style sheet or markup information can be used within a span of text to override the setting of EN text to be always be AN, or vice versa. tag_hash_113____ Emulate explicit directional formatting characters. A higher-level protocol can impose a directional embedding, isolate or override on a segment of structured text. The behavior must always be defined by reference to what would happen if the equivalent explicit directional formatting characters as defined in the algorithm were inserted into the text. For example, a style sheet or markup can modify the embedding level on a span of text. tag_hash_114____ Apply the Bidirectional Algorithm to segments. The Bidirectional Algorithm can be applied independently to one or more segments of structured text. For example, when displaying a document consisting of textual data and visible markup in an editor, a higher-level process can handle syntactic elements in the markup separately from the textual data. tag_hash_115____ Provide artificial context. Text can be processed by the Bidirectional Algorithm as if it were preceded by a character of a given type and/or followed by a character of a given type. This allows a piece of text that is extracted from a longer sequence of text to behave as it did in the larger context. tag_hash_116____ Additional mirroring. Certain characters that do not have the Bidi_Mirrored property can also be depicted bya mirrored glyph in specialized contexts. Such contexts include, but arenot limited to, historic scripts and associated punctuation, private-usecharacters, and characters in mathematical expressions. (See Section 7, Mirroring.)These characters are those that fit at least one of the followingconditions:Characters with a resolved directionality of RCharacters with a resolveddirectionality of L and whose bidirectional type is R or AL Clauses HL1 and HL3 are specialized applications of the more general clauses HL4 and HL5. They are provided here explicitly because they directly correspond to common operations.

Note that the various fields of the URL can appear to the user in a jumbled order. Moreover, if any of the fields contain mixed bidi text (including digits), part of the contents of a field may flip around a delimiter, as in the following:

The Unicode Character Database [UCD] includes two files that provide conformance tests for implementations of the Bidirectional Algorithm [Tests9]. One of the test files, BidiTest.txt, comprises exhaustive test sequences of bidirectional types up to a given length, currently 4. The other test file, BidiCharacterTest.txt, contains test sequences of explicit code points, including, for example, bracket pairs. The format of each test file is described in the header of that file.

The reference code is designed to follow the steps of the algorithm without applying any optimizations. An example of an effective optimization is to first test for right-to-left characters and invoke the Bidirectional Algorithm only if they are present. Another example of optimization is in matching bracket pairs. The bidirectional bracket pairs (the characters with Bidi_Paired_Bracket_Type property values Open and Close) constitute a subset of the characters with bidirectional type ON. Conversely, the characters with a bidirectional type distinct from ON have the Bidi_Paired_Bracket_Type property value None. Therefore, lookup of Bidi_Paired_Bracket_Type property values for the identification of bracket pairs can be optimized by restricting the processing to characters whose bidirectional type is ON. e24fc04721