Efficient XML Interchange (EXI) Extended String

1. Introduction

The EXI String datatype representation is a length prefixed sequence of characters. The length usually indicates the number of characters in the string.

Moreover, EXI uses a string table to assign "compact identifiers" to some string values (e.g., value content items). Each value content item is assigned to two partitions, a "local" value partition and the global value partition. When a string value is found in the global or "local" partition, it is represented using a compact identifier. In the case of a string table the length prefix gets a slightly extended meaning.

• When a string value is found in the "local" value partition, the String value may be represented as zero (0) encoded as an Unsigned Integer followed by the compact identifier of the String value in the "local" value partition.

• When a string value is found in the global value partition, the String value may be represented as one (1) encoded as an Unsigned Integer followed by the compact identifier of the String value in the global value partition.

• When a string value S is not found in the global or "local" value partition, its string literal is encoded as a String with the length L + 2 (incremented by two) where L is the number of characters in the string value.

In many use cases the String representation is sufficient. Some use cases, though, require an extended mechanism to represent string values by

• leveraging well-known values from XML schema (or respectively EXI grammars)

• exchanging or pre-populating initial string values

  Note:

  Partitions containing value items are initially empty.

• splitting longer string values in consecutive strings to increase likelyhood of string table hits

This document provides an extended String representation by specifying an user-defined datatype representation namely exi:estring by extending the range of pre-defined String length identifiers.

2. Concept

Table 2-1 describes the meaning of the length identifier for the user-defined datatype representation exi:estring.

Length identifiers zero (0) and one (1) remain as is and refer to "local" and global value partition. What changes is that when a string value S is not found in the global or "local" value partition, its string literal is encoded as a String with the length L + 6 (incremented by six).

Length identifier two (2) is described in 3. Grammar String, length identifier three (3) is described in 4. Shared String, and length identifier four (4) is described in 5. Split String

3. Grammar String

When the CH event type that matched to process a string value is schema-informed and the production's event code is of length 1 (one), grammar strings may be available for representing the string value. One or more grammar strings can be associated with each schema datatype^XS2. Given a schema datatype^XS2 in effect at the time of processing a string that matched such a CH event type, it is the grammar strings (if any) associated with the schema datatype^XS2 that can be used for representing the string value.

In XML Schema, a datatype^XS2 definition can be given a list of grammar strings inside its annotation element in the form shown below. The variable m is the number of grammar strings defined for the datatype^XS2, and S _i is the i-th grammar string.

When a grammar string is used to represent a string value, the value is encoded as n-bit Unsigned Integers where n = ⌈ log ₂ m ⌉ and m is the number of items in the list of grammar strings associated with the schema datatype^XS2 in effect.

4. Shared String

This section describes how well-known values can be leveraged for string value encoding.

@TODO describe how the fixed list of entries is created.

<header>
 <lesscommon>
   <uncommon>
    <string>foo</string>
    <string>bla</string>
   </uncommon>
 </lesscommon>
</header>

5. Split String

This section defines how a string can be split in multiple consecutive strings. One the one hand it MAY reduce memory and/or buffer requierements and on the other hand it MAY increase the likelyhood of string table value hits.

Similar to the List datatype string values are encoded as a length prefixed sequence of string values. The length is encoded as an Unsigned Integer and each value is encoded according to the exi:estring datatype.