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Variation of an older version... Less computing (still roughly the same amount of time needed), but more memory usage.
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module Combinaties where | |
deleteFromHead :: Eq a => a -> [a] -> [a] | |
deleteFromHead x [] = [] | |
deleteFromHead x (y:ys) | |
| x == y = ys | |
| otherwise = (y:ys) | |
generate :: Eq a => Int -> [a] -> [a] -> [[a]] | |
generate n values knowns = generate' n knowns | |
where | |
-- base cases | |
generate' 0 [] = [[]] | |
generate' 0 knowns = [] | |
-- recursive cases | |
generate' n [] = [v : subsolution | subsolution <- generate' (n-1) [], v <- values] | |
generate' n knowns@(k:ks) | |
| n > m = [k : subsolution | subsolution <- generate' (n-1) ks] | |
++ | |
[v : subsolution | subsolution <- generate' (n-1) knowns, v <- values, v /= k] | |
| otherwise = [knowns] | |
where | |
m = length knowns | |
digits = ['0'..'9'] | |
test = generate 9 digits "783659" | |
-- length test == 64234 -- (0.05 secs, 13067840 bytes) | |
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