Improvements to the 'highest peaks' example

[plepe]

The 'highest peaks' example does not scale well with higher distances, resp. more peaks, as more and more distances have to be calculated. A different approach would be to order all peaks in the current view port by elevation descending and then process them one by one:

1. the highest peak will be shown in any case
2. the next highest peak only, if none of the already shown peaks is in the given range
3. continue with 2.

This is currently not possible -> it requires (at least):

• global variables
• (quasi-) parallel execution of code
• how to order objects for execution?

How could/should this code look like? Ideas?

[plepe]

• IDEA: Pseudo class to process elements in specified order:

node[natural=peak].order-numerical(ele) {
...
}

Possible pseudo classes: .order-numerical(), .order-alphabetical(), .order-natural(). Expression in brackets is either a tag key or an eval expression. An optional second argument could specify order direction, e.g. 'asc' or 'desc'. -> .order-numerical(ele, desc).

• About collecting added highest peaks: We could collect those peaks in a geometry (a "global" variable) and always add the current node via a geometric union.

[plepe]

Pseudo classes for sure are prepended by ':', so the correct list should be :order-numerical(), :order-alphabetical(), :order-natural().

A third parameter could define the default value (if the first parameter evaluates to none) -> :order-numerical(ele, desc, 0).