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