[Haskell-cafe] Optimization problem
ross at soi.city.ac.uk
Fri Sep 15 22:03:02 EDT 2006
On Fri, Sep 15, 2006 at 05:13:29AM +0200, Bertram Felgenhauer wrote:
> Just to prove the point, here's the same code with balancing:
How embarrassing. Still, your code is quite subtle. As penance, here's
my explanation, separating the main idea from the knot-tying.
The ingredients are a map type with an insertion function
data Key k a = ...
instance Functor (Map k)
insert :: Ord k => k -> a -> Map k a -> Map k a
with a partial inverse
uninsert :: Ord k => k -> Map k () -> Map k a -> (a, Map k a)
uninsert k (fmap (const ()) m) (insert k v m) = (v, m)
for any map m not containing k. We also need an update function
update :: Ord k => k -> (a -> a) -> Map k x -> Map k a -> Map k a
where the two map arguments have the same shape. Then splitSeq becomes:
splitSeq :: Ord a => [(a,b)] -> [(a,[b])]
splitSeq = fst . splitSeq' Leaf
splitSeq' :: Ord a => Map a () -> [(a,b)] -> ([(a,[b])], Map a [b])
splitSeq' bp  = (, map (const ) bp)
splitSeq' bp ((a,b):xs)
| member a bp =
let (l, m) = splitSeq' bp xs
in (l, update a (b:) bp m)
| otherwise =
let bp' = insert a () bp
(l, m') = splitSeq' bp' xs
(bs, m) = uninsert a bp m'
in ((a, b : bs) : l, m)
Applying a tupling transformation to insert+uninsert gives your version.
It's interesting that these composed transformations don't seem to cost
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