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=====Generating a unique id=====

>>See also:
>>In a number or different contexts we want to generate HTML elements that have an id attribute. The result will only be valid XHTML if the id is unique on the page.

This is the development page for a new method that can generate id values while making sure they are actually unique. Of course the latter will only work reliably if every bit of Wikka code that (now) generates an id (or would need to) makes use of this method so it can keep track of already-used id values.

====New ##""makeId()""## method====
The method will become part of the [[Docs:WikkaCore Wikka core]], and have public access so user-contributed extensions can (//should//) make use of it.

The following code should be inserted in the ""//MISC"" section of ##wikka.php##, right after the ##""ReturnSafeHTML()""## method:
* Create a unique id for an HTML element.
* Although - given Wikka accepts can use embedded HTML - it cannot be
* guaranteed that an id generated by this method is unique it tries its
* best to make it unique:
* - ids are organized into groups, with the group name used as a prefix
* - if an id is specified it is compared with other ids in the same group;
* if an identical id exists within the same group, a sequence suffix is
* added, otherwise the specified id is accepted and recorded as a member
* of the group
* - if no id is specified (or an invalid one) an id will be generated, and
* given a sequence suffix if needed
* For headings, it is possible to derive an id from the heading content;
* to support this, any embedded whitespace is replaced with underscores
* to generate a recognizable id that will remain (mostly) constant even if
* new headings are inserted in a page. (This is not done for embedded
* HTML.)
* The method supports embedded HTML as well: as long as the formatter
* passes each id found in embedded HTML through this method it can take
* care that the id is valid and unique.
* This works as follows:
* - indicate an 'embedded' id with group 'embed'
* - NO prefix will be added for this reserved group
* - ids will be recorded and checked for uniqueness and validity
* - invalid ids are replaced
* - already-existing ids in the group are given a sequence suffix
* The result is that as long as the already-defined id is valid and
* unique, it will be remain unchanged (but recorded to ensure uniqueness
* overall).
* @author {@link JavaWoman}
* @copyright Copyright © 2005, Marjolein Katsma
* @license GNU Lesser General Public License
* @access public
* @uses ID_LENGTH
* @param string $group required: id group (e.g. form, head); will be
* used as prefix (except for the reserved group
* 'embed' to be used for embedded HTML only)
* @param string $id optional: id to use; if not specified or
* invalid, an id will be generated; if not
* unique, a sequence number will be appended
* @return string resulting id
function makeId($group,$id='')
// initializations
static $aSeq = array(); # group sequences
static $aIds = array(); # used ids

// preparation for group
if (!preg_match('/^[A-Z-a-z]/',$group)) # make sure group starts with a letter
$group = 'g'.$group;
if (!isset($aSeq[$group]))
$aSeq[$group] = 0;
if (!isset($aIds[$group]))
$aIds[$group] = array();
if ('embed' != $group)
$id = preg_replace('/\s+/','_',trim($id)); # replace any whitespace sequence in $id with a single underscore

// validation (full for 'embed', characters only for other groups since we'll add a prefix)
if ('embed' == $group)
$validId = preg_match('/^[A-Za-z][A-Za-z0-9_:.-]*$/',$id); # ref:
$validId = preg_match('/^[A-Za-z0-9_:.-]*$/',$id);

// build or generate id
if ('' == $id || !$validId || in_array($id,$aIds)) # ignore specified id if it is invalid or exists already
$id = substr(md5($group.$id),0,ID_LENGTH); # use group and id as basis for generated id
$idOut = ('embed' == $group) ? $id : $group.'_'.$id; # add group prefix (unless embedded HTML)
if (in_array($id,$aIds[$group]))
$idOut .= '_'.++$aSeq[$group]; # add suffiX to make ID unique

// result
$aIds[$group][] = $id; # keep track of both specified and generated ids (without suffix)
return $idOut;

In order to ensure really unique ids on a page, it's important to actually **use this method wherever an id is needed** (or, in the case of embedded HTML, already used). Even existing ids will then be automatically validated and "recorded" to prevent duplicates from happening.

The method can take two parameters, the first of which, ##$group##, is required. The optional parameter $id can be used to record and validate an existing or proposed id. A few (somewhat simplified) examples:

~-Create an id attribute for a form, given an "id proposal" derived from parameters for the ##""FormOpen()""## method: %%(php)$attrId = ' id="'.$this->makeId('form',$id).'"';%% --- The new [[AdvancedFormOpen FormOpen()]] always uses this so every form in Wikka built using this method to open a form element will have an id in the 'form' group.

~-Create an id for a heading element based on the heading's content: %%(php)$heading = 'This is a nice heading';
$id = $this->makeId('hn',$heading);
$h4 = '<h4 id="'.$id.'">'.$heading.'</h4>';%% --- Provided the id didn't already exist, this will result in: --- <h4 id="hn_This_is_a_nice_heading">This is a nice heading</h4>

~-Generate a new id for a list: %%(php)$id = $this->makeId('list');%% --- If used for several lists, this code will result in similar ids for each, made different from each other with a sequence number.

~-Given an embedded HTML element (##$element##) that already has an id (##$id##), ensure the id is valid and unique: %%(php) $newid = $wakka->makeId('embed',$id);
if ($newid != $id) # replace if we got a different id back
$element = str_replace('id="'.$id.'"','id="'.$newid.'"',$element);
%% --- Note that we use the reserved 'embed' group here: provided the given id is valid and unique, it will remain unchanged; but at the same time, ##""makeId()""## will record the id so others can be compared to ensure uniqueness.

====Supporting code====
As can be seen (and is documented in the docblock) the new ##""makeId()""## method uses a constant that doesn't exist yet in ##wikka.php##.

===##ID_LENGTH## constant===
To avoid excessively long id strings when an id is generated, we take a substring of a shorter length; this length is set via ##ID_LENGTH##. Alternatively, it could be made a configurable value via the configuration file - but for now I've just chosen a reasonable length.

See ArrayToList for the code (for this and some other constants) and where to add it.

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