On 2 April 2018 at 21:30, Olivier S.
Hello,
I'm resending this proposal, which is simplified w.r.t the first one, and where I removed a wrong analysis of a benchmark.
- Proposal I : Optimize the time complexity of (key -> Maybe Vertex) lookups and graph creation when keys are Integral and consecutive.
(The related PR for proposal I is [1], including benchmarks showing the performance improvements.)
Currently, (key -> Maybe Vertex) lookups returned by graphFromEdges consist of a binary search on an array, with a time complexity of O(log V) (I will use V for "Count of vertices", E for "Count of edges").
At the risk of bikeshedding, can you please use |V| and |E| to refer to the order and size of the graph respectively?
When key is Integral, and keys (of nodes passed to the graph creation function) form a set of /consecutive/ values (for example : [4,5,6,7] or [5,6,4,7]), we can have an O(1) lookup by substracting the value of the smallest key, and checking for bounds.
I'm not sure I follow this part; are you ignoring order in these lists (you're referring to sets but using list notation)?
Hence, graph creation complexity is improved, and user algorithms using (key -> Maybe Vertex) lookups will see their complexity reduced by a factor of up-to O(log V).
The PR introduces this lookup and uses it for functions graphFromEdgesWithConsecutiveKeys and graphFromEdgesWithConsecutiveAscKeys.
Here is a summary of complexities for (graph creation, lookup function) as they stand in the current state of the PR:
- graphFromEdges (the currently existing function): O( (V+E) * log V ), O(log V) - graphFromEdgesWithConsecutiveKeys (new function): O( E + (V*log V) ), O(1) - graphFromEdgesWithConsecutiveAscKeys (new function) : O( V+E ), O(1)
- Proposal II : Deprecate `graphFromEdges` taking [(node, key, [key])] in favor of `graphFromMap` taking (Map key (node,[key]))
If we pass the same key twice in the list we pass to 'graphFromEdges' it is undefined which node for that key will actually be used. Introducing 'graphFromMap', taking a (Map key (node,[key]) would alleviate this issue, through the type used.
Off the top of my head, I'm not a big fan of this. If we're going to improve this, then I'd prefer to do so in such a way that allowed for usage with IntMap (is there an existing type-class that covers association list-style data structures?). Ideally you could also use HashMap from unordered-containers as well, but since we ultimately want `type Vertex = Int` I'm not sure if that's worth it; IntMap, however, is.
Also, using a Map makes the implementation a bit more "natural" : there is no need for sorting by key, as Map.toAscList gives exactly the sorted list we want.
We could also deprecate graphFromEdgesWithConsecutiveKeys and graphFromEdgesWithConsecutiveAscKeys (introduced in proposal I) in favor of graphFromConsecutiveMap.
About the naming, I propose two different schemes:
Either: - graphFromEdges (takes a List, deprecated, existing function) - graphFromEdgesInMap (takes a Map) - graphFromEdgesInConsecutiveMap (takes a Map with consecutive keys) Or: - graphFromEdges (takes a List, deprecated, existing function) - graphFromMap - graphFromConsecutiveMap with these, to reflect the Map / List duality in the naming scheme: - graphFromList (takes a List, deprecated, redirects to graphFromEdges) - graphFromConsecutiveList (takes a List, deprecated, redirects to graphFromEdgesWithConsecutiveKeys) - graphFromConsecutiveAscList (takes a List, deprecated, redirects to graphFromEdgesWithConsecutiveAscKeys)
Cheers, Olivier Sohn
[1] https://github.com/haskell/containers/pull/549
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