split an iteration

[Robin Becker]

Raymond Hettinger wrote:
> [Robin Becker]
> 
>>This function from texlib in oedipus.sf.net is a real cpu hog and I determined
>>to see if it could be optimized.
>>
>>def add_active_node(self, active_nodes, node):
>>     """Add a node to the active node list.
>>     The node is added so that the list of active nodes is always
>>     sorted by line number, and so that the set of (position, line,
>>     fitness_class) tuples has no repeated values.
>>     """
> 
> 
> If you can change the data structure to be an actual list of tuples, then the
> bisect module can be used directly:

This is a way forward and I think is doable. The original Knuth algo 
used only global integer arrays. The actual insert point depends on the 
line attribute only which would be the first tuple element. So 
apparently we always insert non-identical line breaks at the beginning 
of their line group; I think we can do the insert check and then a bit 
of checking to find the actual insert point using bisect_right handwave 
handwave.....


> 
> insert_index = bisect.bisect_left(active_nodes, node)
> if active_nodes[insert_index] == node:
>     return        # avoid creating a duplicate entry
> active_nodes.insert(insert_index, node)
> 
> If the data structure cannot be changed to tuples, then try adding a custom
> compare operation to the node class:
> 
>     def __cmp__(self, other):
>         return cmp((self.line, self.position, self.fitness_class),
>                            (other.line, other.position, other.fitness_class))
> 
> 
> 
> 
>>     insert_index = nan
>>     for index, a in enumerate(active_nodes):
>>         if a.line>=node_line:
>>             insert_index = index
>>             break
>>     index = insert_index
> 
> 
> This loop can be squeezed a bit more using itertools.imap() and
> operator.attrgetter() for the attribute lookup:
> 
>     for index, aline in enumerate(imap(attrgetter('line'), active_nodes):
>         if aline > node_line:
>             . . .
> 
> 
> 
> 
>>Is there a fast way to get enumerate to operate over a slice of an iterable?
> 
> 
> enumerate(s) is the same as izip(count(), s).
> So, to start from position i, write:
> 
>     for index, value in izip(count(i), s[i:]):
>          . . .
> 
> That being said, your best bet is to eliminate the initial linear search which
> is likely consuming most of the clock cycles.
> 
> Also, I noticed that the code does not reference self.  Accordingly, it is a
> good candidate for being a staticmethod or standalone function.
> 
> 
> 
> Raymond Hettinger
> 
> 


-- 
Robin Becker