Standard

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Introduction

This document intends to trace the guidelines of a User Interface (in short: UI) for editing bidirectional (in short: Bidi) text. It is assumed that the user enters text in logical sequence, and that the Unicode Bidi Algorithm (in short: UBA) is used to reorder the text for presentation. We assume that the readers of this document have a working knowledge of the UBA. The UBA is described in Unicode Technical Report 9 (see http://www.unicode.org/unicode/reports/tr9).

When designing these guidelines, the following objectives were set, in order of decreasing priority:

1. Prevent actions unexpected by the user, particularly when the action is destructive (erases one or more characters).
2. Make the interface efficient.
3. Keep the interface easy to implement.

Terms and Definitions

Abbreviations

Acronym	Meaning
Bidi	bidirectional
LTR	left-to-right
RTL	right-to-left
UBA	Unicode Bidi Algorithm
UI	User Interface

Definitions

Bidi Embedding Levels	The UBA assigns a level to each character in the logical buffer, including neutrals, which determines if it is part of LTR or RTL text, and eventually affects the presentation. Level 0 corresponds to base LTR text. Level 1 corresponds to base RTL text, or to RTL text embedded within level 0 LTR text. Level 2 corresponds to LTR text embedded within level 1 RTL text, itself possibly embedded within level 0 LTR text. And so on for higher levels. Even levels always correspond to LTR text, odd levels always correspond to RTL text.
Caret (aka Text Cursor)	Graphic representation of where actions like text entry or Delete are going to take effect. The caret is often displayed as a vertical bar. The text cursor should not be confused with the mouse cursor. Throughout this document, the term "caret" refers to the text cursor, and the term "cursor" refers to the mouse cursor.
Cursor Level	For the needs of the UI, a Bidi level is assigned also to the cursor. This level reflects the Bidi level which is expected to be assigned to the next character entered (there are cases when the actual level of the entered character will be different). The level of the cursor is manipulated by UI functions, like changing the keyboard language. It may also be affected by all functions which change the position of the cursor.
Keyboard Language	Language of the next character that will be entered from the keyboard.
Logical Buffer	Buffer containing the text data in logical sequence (as opposed to visual sequence).
Paragraph Embedding Level	Bidi level of text belonging to the main language used in a paragraph. This is 0 if the main language is LTR, 1 if the main language is RTL. Note: this is also known as "Base Direction".

Visual and Logical Caret

Throughout this document, there are references to a "visual caret" and a "logical caret". The distinction between the two is an important one. As written above, the caret is a visual indication of where actions take place. For Bidi text, it often happens that a single position in the logical buffer can be interpreted to refer to two (or more) visual positions. Typically, this is a result of whether the caret should be interpreted to be before the next character, or after the previous one. Likewise, a single visual position might, under some circumstances, be interpreted to refer to two positions in the logical buffer.

At the time of this writing, all editors implement a caret that acts as a visual aid to indicate the position in the logical buffer. The caret has a definite and well-defined position in the logical buffer, and its visual position was affected from a variety of considerations. We call such a caret a "logical caret".

This document also uses a new type of caret, called a "visual caret". A visual caret has a definite and well defined visual position. That visual position may, under some circumstances, translate to more than one positions in the logical buffer.

Throughout this document, almost any operation will affect the caret. After each operation the caret will either be a visual caret or a logical caret. In other words, the operation will either leave a well defined logical caret position or a visual caret position. An operations that defines the logical caret position is called a "logical operation", and the caret after the operation is a logical caret. An operations that defines the visual caret position is called a "visual operation", and the caret after it is a visual caret.