find_with_equal_func

Checks whether needle exists in haystack, using the given equal_func.

If the element is found, true is returned and the element’s index is returned in index_ (if non-`NULL`). Otherwise, false is returned and index_ is undefined. If needle exists multiple times in haystack, the index of the first instance is returned.

equal_func is called with the element from the array as its first parameter, and needle as its second parameter. If equal_func is `NULL`, pointer equality is used.

Parameters:

needle	the pointer to look for
equal_func	the function to call for each element, which should return true when the desired element is found; or `NULL` to use pointer equality
haystack	the pointer array to be searched
index_	the return location for the index of the element, if found

Returns:

true if needle is one of the elements of haystack; false otherwise