34 lines
1.7 KiB
Haskell
34 lines
1.7 KiB
Haskell
module FrequentPatterns (
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frequentPatterns
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) where
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import DataModel
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import qualified Data.Set as Set
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import Debug.Trace (traceShow)
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import qualified Data.List as List
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semiUnion :: ItemSet -> ItemSet -> ItemSet
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semiUnion (ItemSet set1) (ItemSet set2) = ItemSet (if max1 <= max2 && Set.delete max1 set1 == Set.delete max2 set2 then set1 `Set.union` set2 else Set.empty) where
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max1 = Set.findMax set1
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max2 = Set.findMax set2
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-- generate all possible combinations from a set of singletons
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-- generateLevels :: [Item] -> [[ItemSet]]
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-- generateLevels singles = until (\x -> head x == lastLevel) (\x -> generateNextLevel (head x) : x) [firstLevel] where
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-- firstLevel = map (\x -> ItemSet $ Set.fromList [x]) singles
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-- lastLevel = [ItemSet $ Set.fromList singles]
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-- generate the next level in a bottom-up route
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generateNextLevel :: [ItemSet] -> [ItemSet]
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generateNextLevel level = traceShow ("Computing level " ++ show (isSize (head level))) $ foldr (\value old -> generate value ++ old) [] level where
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generate value = takeWhile (/= empty) (foldr (\x old -> semiUnion value x : old) [] (tail $ List.dropWhile (/= value) level))
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empty = ItemSet $ Set.fromList []
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isSize (ItemSet set) = Set.size set
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singletons :: [ItemSet] -> [Item]
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singletons table = Set.toList $ foldr (\(ItemSet row) old -> old `Set.union` row) (Set.fromList []) table where
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frequentPatterns :: Frequency -> [ItemSet] -> [[ItemSet]]
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frequentPatterns thresh table = until (\x -> [] == head x) (\x -> filterByFrequency (generateNextLevel (head x)) : x) [firstLevel] where
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firstLevel = filterByFrequency $ map (\x -> ItemSet $ Set.fromList [x]) (singletons table)
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filterByFrequency = filter (\x -> frequency table x >= thresh)
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