Support 'unique' conditions

This is critical for the first release of FEMP 2.

"They should use feedapi_get_settings() do know if they should do site-wide or feed-wide deduping." - I would add this later. At first we can assume that site wide deduping is required. If this is not a performance problem, we should leave it that way for simplicity's sake.

#4: Awesome. Read the patch, here are my observations:

A

feedapi_mapper_unique_toggle() will need to use a token to avoid XSS attacks (I know, minor, but still). We should use FAPI for toggling (i. e. make the toggle link a form submission).

B

If there are no uniques in feedapi_mapper_unique() array_pop() will be executed on an empty array and return NULL.

C

feedapi_mapper_unique() needs better documentation: what are the parameters, what return values to expect, when is this function being used?

2 problems:

A For effective cross feed deduping we need feed_nids in the array. We've talked about this on IM and found that a mapper that implements 'unique' should return a value in the format array([id] => array([feed_id], [feed_id], [feed_id]).

B The function merges all results into one array. This points to a bigger issue where FEMP does not know which mappers react on which processors - I've opened an issue here #541814: Breaks when enabling multiple processors - once this is in, we should add a $processor flag to the function signature of feedapi_mapper_unique() that indicates which mappers to query.

Great.

Springing to the eye:

Array structure:
+ *   array(
+ *    [0] => array(itemid1 => array(feedid1, feedid2))
+ *    [1] => array(itemid2 => array(feedid1, feedid2))

this is not correct anymore.

Otherwise looking very good. Need to review further.

More nitpicking:

1 $url_toggle_unique should be $toggle_unique_path, $url_delete $delete_path - it's a 'path' and the natural order of words makes the variable easier to read. Further it's consistent with $feed_path
2 Don't use the ?: ternary form if the statement doesn't fit neatly on a line
3 There is an indentation problem around "if ($unique_supported == TRUE) {"
4 $unique = 'N/A'; - shouldn't it be t('N/A') ?
5 feedapi_mapper_unique_toggle() should be feedapi_mapper_toggle_unique()
6 "Collects the ids of the duplicated items of $item. Utilized from FeedAPI processors." should be "Collects ids of duplicate items of $item. Most commonly used by FeedAPI processors to determine whether given item is unique."
7 "Feed item to decide about." -> "Feed item to decide on."
8 @return of feedapi_mapper_unique() should be:

@return
  An array containing duplicate items per unique elements, FALSE if no unique elements were defined. The exact format of the duplicate items is up to the implementer of feedapi_mapper($op = 'unique'):

array(
  [0] => array(/* duplicates for unique element 1, e. g. URL - format up to implementer */),
  [1] => array(/* duplicates for unique element 2, e. g. GUID - format up to implementer */),
  // ...
  )

For instance, feedapi_node processor expects arrays where the key is the item's node id and the value is an array of feeds the feed item is associated with:

array(
  [0] => array(itemid1 => array(feedid1, feedid2)),
  [1] => array(itemid2 => array(feedid1, feedid2)),
  )

9 Let's use better names for $fields_ids and $dup_ids: $result and $duplicates
10 There is an indentation problem with both patches to .test files

I really love how neat feedapi_mapper_unique() is now.

"2 Don't use the ?: ternary form if the statement doesn't fit neatly on a line - in which line? FeedAPI Mapper does this elsewhere anyway :)" - we should fix that...

Beautiful. RTBC.