NAME

       HTTP::Negotiate - choose a variant to serve

SYNOPSIS

        use HTTP::Negotiate qw(choose);

        #  ID       QS     Content-Type   Encoding Char-Set        Lang   Size
        $variants =
         [['var1',  1.000, 'text/html',   undef,   'iso-8859-1',   'en',   3000],
          ['var2',  0.950, 'text/plain',  'gzip',  'us-ascii',     'no',    400],
          ['var3',  0.3,   'image/gif',   undef,   undef,          undef, 43555],
         ];

        @preferred = choose($variants, $request_headers);
        $the_one   = choose($variants);

DESCRIPTION

       This module provides a complete implementation of the HTTP content negotiation algorithm
       specified in draft-ietf-http-v11-spec-00.ps chapter 12.  Content negotiation allows for
       the selection of a preferred content representation based upon attributes of the
       negotiable variants and the value of the various Accept* header fields in the request.

       The variants are ordered by preference by calling the function choose().

       The first parameter is reference to an array of the variants to choose among.  Each
       element in this array is an array with the values [$id, $qs, $content_type,
       $content_encoding, $charset, $content_language, $content_length] whose meanings are
       described below. The $content_encoding and $content_language can be either a single scalar
       value or an array reference if there are several values.

       The second optional parameter is either a HTTP::Headers or a HTTP::Request object which is
       searched for "Accept*" headers.  If this parameter is missing, then the accept
       specification is initialized from the CGI environment variables HTTP_ACCEPT,
       HTTP_ACCEPT_CHARSET, HTTP_ACCEPT_ENCODING and HTTP_ACCEPT_LANGUAGE.

       In an array context, choose() returns a list of [variant identifier, calculated quality,
       size] tuples.  The values are sorted by quality, highest quality first.  If the calculated
       quality is the same for two variants, then they are sorted by size (smallest first). E.g.:

         (['var1', 1, 2000], ['var2', 0.3, 512], ['var3', 0.3, 1024]);

       Note that also zero quality variants are included in the return list even if these should
       never be served to the client.

       In a scalar context, it returns the identifier of the variant with the highest score or
       "undef" if none have non-zero quality.

       If the $HTTP::Negotiate::DEBUG variable is set to TRUE, then a lot of noise is generated
       on STDOUT during evaluation of choose().

VARIANTS

       A variant is described by a list of the following values.  If the attribute does not make
       sense or is unknown for a variant, then use "undef" instead.

       identifier
          This is a string that you use as the name for the variant.  This identifier for the
          preferred variants returned by choose().

       qs This is a number between 0.000 and 1.000 that describes the "source quality".  This is
          what draft-ietf-http-v11-spec-00.ps says about this value:

          Source quality is measured by the content provider as representing the amount of
          degradation from the original source.  For example, a picture in JPEG form would have a
          lower qs when translated to the XBM format, and much lower qs when translated to an
          ASCII-art representation.  Note, however, that this is a function of the source - an
          original piece of ASCII-art may degrade in quality if it is captured in JPEG form.  The
          qs values should be assigned to each variant by the content provider; if no qs value
          has been assigned, the default is generally "qs=1".

       content-type
          This is the media type of the variant.  The media type does not include a charset
          attribute, but might contain other parameters.  Examples are:

            text/html
            text/html;version=2.0
            text/plain
            image/gif
            image/jpg

       content-encoding
          This is one or more content encodings that has been applied to the variant.  The
          content encoding is generally used as a modifier to the content media type.  The most
          common content encodings are:

            gzip
            compress

       content-charset
          This is the character set used when the variant contains text.  The charset value
          should generally be "undef" or one of these:

            us-ascii
            iso-8859-1 ... iso-8859-9
            iso-2022-jp
            iso-2022-jp-2
            iso-2022-kr
            unicode-1-1
            unicode-1-1-utf-7
            unicode-1-1-utf-8

       content-language
          This describes one or more languages that are used in the variant.  Language is
          described like this in draft-ietf-http-v11-spec-00.ps: A language is in this context a
          natural language spoken, written, or otherwise conveyed by human beings for
          communication of information to other human beings.  Computer languages are explicitly
          excluded.

          The language tags are defined by RFC 3066.  Examples are:

            no               Norwegian
            en               International English
            en-US            US English
            en-cockney

       content-length
          This is the number of bytes used to represent the content.

ACCEPT HEADERS

       The following Accept* headers can be used for describing content preferences in a request
       (This description is an edited extract from draft-ietf-http-v11-spec-00.ps):

       Accept
          This header can be used to indicate a list of media ranges which are acceptable as a
          response to the request.  The "*" character is used to group media types into ranges,
          with "*/*" indicating all media types and "type/*" indicating all subtypes of that
          type.

          The parameter q is used to indicate the quality factor, which represents the user's
          preference for that range of media types.  The parameter mbx gives the maximum
          acceptable size of the response content. The default values are: q=1 and mbx=infinity.
          If no Accept header is present, then the client accepts all media types with q=1.

          For example:

            Accept: audio/*;q=0.2;mbx=200000, audio/basic

          would mean: "I prefer audio/basic (of any size), but send me any audio type if it is
          the best available after an 80% mark-down in quality and its size is less than 200000
          bytes"

       Accept-Charset
          Used to indicate what character sets are acceptable for the response.  The "us-ascii"
          character set is assumed to be acceptable for all user agents.  If no Accept-Charset
          field is given, the default is that any charset is acceptable.  Example:

            Accept-Charset: iso-8859-1, unicode-1-1

       Accept-Encoding
          Restricts the Content-Encoding values which are acceptable in the response.  If no
          Accept-Encoding field is present, the server may assume that the client will accept any
          content encoding.  An empty Accept-Encoding means that no content encoding is
          acceptable.  Example:

            Accept-Encoding: compress, gzip

       Accept-Language
          This field is similar to Accept, but restricts the set of natural languages that are
          preferred in a response.  Each language may be given an associated quality value which
          represents an estimate of the user's comprehension of that language.  For example:

            Accept-Language: no, en-gb;q=0.8, de;q=0.55

          would mean: "I prefer Norwegian, but will accept British English (with 80%
          comprehension) or German (with 55% comprehension).

COPYRIGHT

       Copyright 1996,2001 Gisle Aas.

       This library is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.

AUTHOR

       Gisle Aas <gisle@aas.no>