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 <chapter id="fields-and-charsets">
  <title>Field Structure and Character Sets
  </title>
  
  <para>
   In order to provide a flexible approach to national character set
   handling, &zebra; allows the administrator to configure the set up the
   system to handle any 8-bit character set &mdash; including sets that
   require multi-octet diacritics or other multi-octet characters. The
   definition of a character set includes a specification of the
   permissible values, their sort order (this affects the display in the
   SCAN function), and relationships between upper- and lowercase
   characters. Finally, the definition includes the specification of
   space characters for the set.
  </para>

  <para>
   The operator can define different character sets for different fields,
   typical examples being standard text fields, numerical fields, and
   special-purpose fields such as WWW-style linkages (URx).
  </para>

  <para>
   Zebra 1.3 and Zebra versions 2.0.18 and earlier required that the field
   type is a single character, e.g. <literal>w</literal> (for word), and
   <literal>p</literal> for phrase. Zebra 2.0.20 and later allow field types 
   to be any string. This allows for greater flexibility - in particular
   per-locale (language) fields can be defined.
  </para>

  <para>
   Version 2.1 of Zebra can also be configured - per field - to use the
   <ulink url="&url.icu;">ICU</ulink> library to perform tokenization and
   normalization of strings. This is an alternative to the "charmap"
   files which has been part of Zebra since its first release.
  </para>

  <section id="default-idx-file">
   <title>The default.idx file</title>
   <para>
    The field types, and hence character sets, are associated with data
    elements by the indexing rules (say <literal>title:w</literal>) in the
    various filters. Fields are defined in a field definition file which,
    by default, is called <filename>default.idx</filename>. 
    This file provides the association between field type codes 
    and the character map files (with the .chr suffix). The format
    of the .idx file is as follows
   </para>
   
   <para>
    <variablelist>

     <varlistentry>
      <term>index <replaceable>field type code</replaceable></term>
      <listitem>
       <para>
        This directive introduces a new search index code.
        The argument is a one-character code to be used in the
        .abs files to select this particular index type. An index, roughly,
        corresponds to a particular structure attribute during search. Refer
        to <xref linkend="zebrasrv-search"/>.
       </para>
      </listitem></varlistentry>
     <varlistentry>
      <term>sort <replaceable>field code type</replaceable></term>
      <listitem>
       <para>
        This directive introduces a 
        sort index. The argument is a one-character code to be used in the
        .abs fie to select this particular index type. The corresponding
        use attribute must be used in the sort request to refer to this
        particular sort index. The corresponding character map (see below)
        is used in the sort process.
       </para>
      </listitem></varlistentry>
     <varlistentry>
      <term>completeness <replaceable>boolean</replaceable></term>
      <listitem>
       <para>
        This directive enables or disables complete field indexing.
        The value of the <replaceable>boolean</replaceable> should be 0
        (disable) or 1. If completeness is enabled, the index entry will
        contain the complete contents of the field (up to a limit), with words
        (non-space characters) separated by single space characters
        (normalized to " " on display). When completeness is
        disabled, each word is indexed as a separate entry. Complete subfield
        indexing is most useful for fields which are typically browsed (eg.
        titles, authors, or subjects), or instances where a match on a
        complete subfield is essential (eg. exact title searching). For fields
        where completeness is disabled, the search engine will interpret a
        search containing space characters as a word proximity search.
       </para>
      </listitem></varlistentry>

     <varlistentry id="default.idx.firstinfield">
      <term>firstinfield <replaceable>boolean</replaceable></term>
      <listitem>
       <para>
        This directive enables or disables first-in-field indexing.
        The value of the <replaceable>boolean</replaceable> should be 0
        (disable) or 1. 
       </para>
      </listitem></varlistentry>

     <varlistentry id="default.idx.alwaysmatches">
      <term>alwaysmatches <replaceable>boolean</replaceable></term>
      <listitem>
       <para>
        This directive enables or disables alwaysmatches indexing.
        The value of the <replaceable>boolean</replaceable> should be 0
        (disable) or 1. 
       </para>
      </listitem></varlistentry>

     <varlistentry>
      <term>charmap <replaceable>filename</replaceable></term>
      <listitem>
       <para>
        This is the filename of the character
        map to be used for this index for field type.
        See <xref linkend="character-map-files"/> for details.
       </para>
      </listitem></varlistentry>
     
     <varlistentry>
      <term>icuchain <replaceable>filename</replaceable></term>
      <listitem>
       <para>
        Specifies the filename with ICU tokenization and
        normalization rules. 
        See <xref linkend="icuchain-files"/> for details.
        Using icuchain for a field type is an alternative to
        charmap. It does not make sense to define both
        icuchain and charmap for the same field type.
       </para>
      </listitem></varlistentry>
    </variablelist>
   </para>
   <example id="field-types">
    <title>Field types</title>
    <para>
     Following are three excerpts of the standard
     <filename>tab/default.idx</filename> configuration file. Notice
     that the <literal>index</literal> and <literal>sort</literal>
     are grouping directives, which bind all other following directives
     to them:
     <screen>
     # Traditional word index
     # Used if completenss is 'incomplete field' (@attr 6=1) and
     # structure is word/phrase/word-list/free-form-text/document-text
     index w
     completeness 0
     position 1
     alwaysmatches 1
     firstinfield 1
     charmap string.chr

     ...

     # Null map index (no mapping at all)
     # Used if structure=key (@attr 4=3)
     index 0
     completeness 0
     position 1
     charmap @

     ...

     # Sort register
     sort s
     completeness 1
     charmap string.chr
     </screen>
    </para>
   </example>
  </section>

  <section id="character-map-files">
   <title>Charmap Files</title>
   <para>
    The character map files are used to define the word tokenization
    and character normalization performed before inserting text into
    the inverse indexes. &zebra; ships with the predefined character map
    files <filename>tab/*.chr</filename>. Users are allowed to add
    and/or modify maps according to their needs.  
   </para>

   <table id="character-map-table" frame="top">
     <title>Character maps predefined in &zebra;</title>
      <tgroup cols="3">
       <thead>
        <row>
         <entry>File name</entry>
         <entry>Intended type</entry>
         <entry>Description</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry><literal>numeric.chr</literal></entry>
         <entry><literal>:n</literal></entry>
         <entry>Numeric digit tokenization and normalization map. All
         characters not in the set <literal>-{0-9}.,</literal> will be
         suppressed. Note that floating point numbers are processed
         fine, but scientific exponential numbers are trashed.</entry>
        </row>
        <row>
         <entry><literal>scan.chr</literal></entry>
         <entry><literal>:w or :p</literal></entry>
         <entry>Word tokenization char map for Scandinavian
         languages. This one resembles the generic word tokenization
         character map <literal>tab/string.chr</literal>, the main
         differences are sorting of the special characters 
        <literal>üzæäøöå</literal> and equivalence maps according to
         Scandinavian language rules.</entry>
        </row>
        <row>
         <entry><literal>string.chr</literal></entry>
         <entry><literal>:w or :p</literal></entry>
         <entry>General word tokenization and normalization character
         map, mostly useful for English texts. Use this to derive your
         own language tokenization and normalization derivatives.</entry>
        </row>
        <row>
         <entry><literal>urx.chr</literal></entry>
         <entry><literal>:u</literal></entry>
         <entry>URL parsing and tokenization character map.</entry>
        </row>
        <row>
         <entry><literal>@</literal></entry>
         <entry><literal>:0</literal></entry>
         <entry>Do-nothing character map used for literal binary
         indexing. There is no existing file associated to it, and
         there is no normalization or tokenization performed at all.</entry>
        </row>
      </tbody>
     </tgroup>
   </table>

   <para>
    The contents of the character map files are structured as follows:
    <variablelist>
     <varlistentry>
      <term>encoding <replaceable>encoding-name</replaceable></term>
      <listitem>
       <para>
        This directive must be at the very beginning of the file, and it
        specifies the character encoding used in the entire file. If
        omitted, the encoding <literal>ISO-8859-1</literal> is assumed.
       </para>
       <para>
        For example, one of the test files found at  
          <literal>test/rusmarc/tab/string.chr</literal> contains the following
        encoding directive:
        <screen>
         encoding koi8-r
        </screen>
          and the test file
          <literal>test/charmap/string.utf8.chr</literal> is encoded
          in UTF-8:
        <screen>
         encoding utf-8
        </screen>
       </para>
      </listitem></varlistentry>

     <varlistentry>
      <term>lowercase <replaceable>value-set</replaceable></term>
      <listitem>
       <para>
        This directive introduces the basic value set of the field type.
        The format is an ordered list (without spaces) of the
        characters which may occur in "words" of the given type.
        The order of the entries in the list determines the
        sort order of the index. In addition to single characters, the
        following combinations are legal:
       </para>

       <para>

        <itemizedlist>
         <listitem>
          <para>
           Backslashes may be used to introduce three-digit octal, or
           two-digit hex representations of single characters
           (preceded by <literal>x</literal>).
           In addition, the combinations
           \\, \\r, \\n, \\t, \\s (space &mdash; remember that real
           space-characters may not occur in the value definition), and
           \\ are recognized, with their usual interpretation.
          </para>
         </listitem>

         <listitem>
          <para>
           Curly braces {} may be used to enclose ranges of single
           characters (possibly using the escape convention described in the
           preceding point), eg. {a-z} to introduce the
           standard range of ASCII characters.
           Note that the interpretation of such a range depends on
           the concrete representation in your local, physical character set.
          </para>
         </listitem>

         <listitem>
          <para>
           paranthesises () may be used to enclose multi-byte characters -
           eg. diacritics or special national combinations (eg. Spanish
           "ll"). When found in the input stream (or a search term),
           these characters are viewed and sorted as a single character, with a
           sorting value depending on the position of the group in the value
           statement.
          </para>
         </listitem>

        </itemizedlist>

       </para>
       <para>
        For example, <literal>scan.chr</literal> contains the following
        lowercase normalization and sorting order:
        <screen>
         lowercase {0-9}{a-y}üzæäøöå
        </screen>
       </para>
      </listitem></varlistentry>
     <varlistentry>
      <term>uppercase <replaceable>value-set</replaceable></term>
      <listitem>
       <para>
        This directive introduces the
        upper-case equivalences to the value set (if any). The number and
        order of the entries in the list should be the same as in the
        <literal>lowercase</literal> directive.
       </para>
       <para>
        For example, <literal>scan.chr</literal> contains the following
        uppercase equivalent:
        <screen>
         uppercase {0-9}{A-Y}ÜZÆÄØÖÅ
        </screen>
       </para>
      </listitem></varlistentry>
     <varlistentry>
      <term>space <replaceable>value-set</replaceable></term>
      <listitem>
       <para>
        This directive introduces the character
        which separate words in the input stream. Depending on the
        completeness mode of the field in question, these characters either
        terminate an index entry, or delimit individual "words" in
        the input stream. The order of the elements is not significant &mdash;
        otherwise the representation is the same as for the
        <literal>uppercase</literal> and <literal>lowercase</literal>
        directives.
       </para>
       <para>
        For example, <literal>scan.chr</literal> contains the following
        space instruction:
        <screen><![CDATA[
         space {\001-\040}!"#$%&'\()*+,-./:;<=>?@\[\\]^_`\{|}~
        ]]></screen>
       </para>
      </listitem></varlistentry>
     <varlistentry>
      <term>map <replaceable>value-set</replaceable>
       <replaceable>target</replaceable></term>
      <listitem>
       <para>
        This directive introduces a mapping between each of the
        members of the value-set on the left to the character on the
        right. The character on the right must occur in the value
        set (the <literal>lowercase</literal> directive) of the
        character set, but it may be a parenthesis-enclosed
        multi-octet character. This directive may be used to map
        diacritics to their base characters, or to map HTML-style
        character-representations to their natural form, etc. The
        map directive can also be used to ignore leading articles in
        searching and/or sorting, and to perform other special
        transformations.
       </para>
       <para>
        For example, <literal>scan.chr</literal> contains the following
        map instructions among others, to make sure that HTML entity
        encoded  Danish special characters are mapped to the
        equivalent Latin-1 characters:
        <screen><![CDATA[
         map (&aelig;)      æ
         map (&oslash;)     ø
         map (&aring;)      å
        ]]></screen>
        </para>
       <para>
        In addition to specifying sort orders, space (blank) handling,
        and upper/lowercase folding, you can also use the character map
        files to make &zebra; ignore leading articles in sorting records,
        or when doing complete field searching.
       </para>
       <para>
        This is done using the <literal>map</literal> directive in the
        character map file. In a nutshell, what you do is map certain
        sequences of characters, when they occur <emphasis> in the
         beginning of a field</emphasis>, to a space. Assuming that the
        character "@" is defined as a space character in your file, you
        can do:
        <screen>
         map (^The\s) @
         map (^the\s) @
        </screen>
        The effect of these directives is to map either 'the' or 'The',
        followed by a space character, to a space. The hat ^ character
        denotes beginning-of-field only when complete-subfield indexing
        or sort indexing is taking place; otherwise, it is treated just
        as any other character.
       </para>
       <para>
        Because the <literal>default.idx</literal> file can be used to
        associate different character maps with different indexing types
        -- and you can create additional indexing types, should the need
        arise -- it is possible to specify that leading articles should
        be ignored either in sorting, in complete-field searching, or
        both.
       </para>
       <para>
        If you ignore certain prefixes in sorting, then these will be
        eliminated from the index, and sorting will take place as if
        they weren't there. However, if you set the system up to ignore
        certain prefixes in <emphasis>searching</emphasis>, then these
        are deleted both from the indexes and from query terms, when the
        client specifies complete-field searching. This has the effect
        that a search for 'the science journal' and 'science journal'
        would both produce the same results.
       </para>
      </listitem></varlistentry>
     <varlistentry>
      <term>equivalent <replaceable>value-set</replaceable></term>
      <listitem>
       <para>
        This directive introduces equivalence classes of characters
        and/or strings for sorting purposes only. It resembles the map
        directive, but does not affect search and retrieval indexing,
        but only sorting order under present requests. 
       </para>
       <para>
        For example, <literal>scan.chr</literal> contains the following
        equivalent sorting instructions, which can be uncommented:
        <screen><![CDATA[
         # equivalent æä(ae)
         # equivalent øö(oe)
         # equivalent å(aa)
         # equivalent uü
        ]]></screen>
       </para>
      </listitem></varlistentry>
    </variablelist>
   </para>
  </section>

  <section id="icuchain-files">
   <title>ICU Chain Files</title>
   <para>
    The <ulink url="&url.icu;">ICU</ulink> chain files defines a 
    <emphasis>chain</emphasis> of rules
    which specify the conversion process to be carried out for each
    record string for indexing.
   </para>
   <para>
    Both searching and sorting is based on the <emphasis>sort</emphasis>
    normalization that ICU provides. This means that scan and sort will
    return terms in the sort order given by ICU.
   </para>
   <para>
    Zebra is using YAZ' ICU wrapper. Refer to the 
    <ulink url="&url.yaz.yaz-icu;">yaz-icu man page</ulink> for
    documentation about the ICU chain rules.
   </para>
   <tip>
    <para>
     Use the yaz-icu program to test your icuchain rules.
    </para>
   </tip>
   <example id="indexing-greek-example"><title>Indexing Greek text</title>
    <para>
     Consider a system where all "regular" text is to be indexed
     using as Greek (locale: EL).
     We would have to change our index type file - to read
     <screen>
      # Index greek words
      index w
      completeness 0
      position 1
      alwaysmatches 1
      firstinfield 1
      icuahain greek.xml
      ..
     </screen>
     The ICU chain file <filename>greek.xml</filename> could look
     as follows:
     <screen><![CDATA[
      <icu_chain locale="el">
      <transform rule="[:Control:] Any-Remove"/>
      <tokenize rule="l"/>
      <transform rule="[[:WhiteSpace:][:Punctuation:]] Remove"/>
      <display/>
      <casemap rule="l"/>
     </icu_chain>
     ]]></screen>
    </para>
   </example>
   <para>
    Zebra is shipped with a field types file <filename>icu.idx</filename>
    which is an ICU chain version of <filename>default.idx</filename>.
   </para>

   <example id="indexing-marcxml-example"><title>MARCXML indexing using ICU</title>
    <para>
     The directory <filename>examples/marcxml</filename> includes
     a complete sample with MARCXML recordst that are DOM XML indexed 
     using ICU chain rules. Study the
     <filename>README</filename> in the <filename>marcxml</filename>
     directory for details.
    </para>
   </example>
  </section>

 </chapter>
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