# Collaborative filtering

This page was added to discuss different versions of the code for collaborative filtering at Bryan's blog.

## Chris' version

I renamed the variables and then reorganized the code a bit.

The(update,SlopeOne,predict) definitions can be compared with the newer (update2,SlopeOne',predict') definitions.

```
module WeightedSlopeOne (Rating, SlopeOne, empty, predict, update) where
import Data.List (foldl',foldl1')
import qualified Data.Map as M
-- The item type is a polymorphic parameter. Since it goes into a Map
-- it must be able to be compared, so item must be an instance of Ord.
type Count = Int
type RatingValue = Double
-- The Rating is the known (item,Rating) information for a particular "user"
type Rating item = M.Map item RatingValue
-- The SlopeOne matrix is indexed by pairs of items and is implemented
-- as a sparse map of maps.
newtype SlopeOne item = SlopeOne (M.Map item (M.Map item (Count,RatingValue)))
deriving (Show)
-- The SlopeOne' matrix is an un-normalized version of SlopeOne
newtype SlopeOne' item = SlopeOne' (M.Map item (M.Map item (Count,RatingValue)))
deriving (Show)
empty = SlopeOne M.empty
empty' = SlopeOne' M.empty
-- This performs a strict addition on pairs made of two nuumeric types
addT (a,b) (c,d) = let (l,r) = (a+c, b+d) in l `seq` r `seq` (l, r)
-- There is never an entry for the "diagonal" elements with equal
-- items in the pair: (foo,foo) is never in the SlopeOne.
update :: Ord item => SlopeOne item -> [Rating item] -> SlopeOne item
update (SlopeOne matrixInNormed) usersRatings =
SlopeOne . M.map (M.map norm) . foldl' update' matrixIn $ usersRatings
where update' oldMatrix userRatings =
foldl' (\oldMatrix (itemPair, rating) -> insert oldMatrix itemPair rating)
oldMatrix itemCombos
where itemCombos = [ ((item1, item2), (1, rating1 - rating2))
| (item1, rating1) <- ratings
, (item2, rating2) <- ratings
, item1 /= item2]
ratings = M.toList userRatings
insert outerMap (item1, item2) newRating = M.insertWith' outer item1 newOuterEntry outerMap
where newOuterEntry = M.singleton item2 newRating
outer _ innerMap = M.insertWith' addT item2 newRating innerMap
norm (count,total_rating) = (count, total_rating / fromIntegral count)
un_norm (count,rating) = (count, rating * fromIntegral count)
matrixIn = M.map (M.map un_norm) matrixInNormed
-- This version of update2 makes an unnormalize slopeOne' from each
-- Rating and combines them using Map.union* operations and addT.
update2 :: Ord item => SlopeOne' item -> [Rating item] -> SlopeOne' item
update2 s@(SlopeOne' matrixIn) usersRatingsIn | null usersRatings = s
| otherwise =
SlopeOne' . M.unionsWith (M.unionWith addT) . (matrixIn:) . map fromRating $ usersRatings
where usersRatings = filter ((1<) . M.size) usersRatingsIn
fromRating userRating = M.mapWithKey expand1 userRating
where expand1 item1 rating1 = M.mapMaybeWithKey expand2 userRating
where expand2 item2 rating2 | item1 == item2 = Nothing
| otherwise = Just (1,rating1 - rating2)
predict :: Ord a => SlopeOne a -> Rating a -> Rating a
predict (SlopeOne matrixIn) userRatings =
let freqM = foldl' insert M.empty
[ (item1,found_rating,user_rating)
| (item1,innerMap) <- M.assocs matrixIn
, M.notMember item1 userRatings
, (user_item, user_rating) <- M.toList userRatings
, item1 /= user_item
, found_rating <- M.lookup user_item innerMap
]
insert oldM (item1,found_rating,user_rating) =
let (count,norm_rating) = found_rating
total_rating = fromIntegral count * (norm_rating + user_rating)
in M.insertWith' addT item1 (count,total_rating) oldM
normM = M.map (\(count, total_rating) -> total_rating / fromIntegral count) freqM
in M.filter (\norm_rating -> norm_rating > 0) normM
-- This is a modified version of predict. It also expect the
-- unnormalized SlopeOne' but this is a small detail
predict' :: Ord a => SlopeOne' a -> Rating a -> Rating a
predict' (SlopeOne' matrixIn) userRatings =
M.mapMaybe calcItem (M.difference matrixIn userRatings)
where calcItem innerMap | M.null combined = Nothing
| norm_rating <= 0 = Nothing
| otherwise = Just norm_rating
where combined = M.intersectionWith weight innerMap userRatings
(total_count,total_rating) = foldl1' addT (M.elems combined)
norm_rating = total_rating / fromIntegral total_count
weight (count,rating) user_rating =
(count,rating + fromIntegral count * user_rating)
userData :: [Rating String]
userData = map M.fromList [
[("squid", 1.0), ("cuttlefish", 0.5), ("octopus", 0.2)],
[("squid", 1.0), ("octopus", 0.5), ("nautilus", 0.2)],
[("squid", 0.2), ("octopus", 1.0), ("cuttlefish", 0.4), ("nautilus", 0.4)],
[("cuttlefish", 0.9), ("octopus", 0.4), ("nautilus", 0.5)]
]
userInfo = M.fromList [("squid", 0.4),("cuttlefish",0.9),("dolphin",1.0)]
predictions = predict (update empty userData) userInfo
predictions' = predict' (update2 empty' userData) userInfo
```

## More optimized storage

The changes to SlopeOne/update/predict below use a different internal data structure for storing the sparse matrix of SlopeOne. Instead of a Map of Map design it uses a Map of List design and keeps the List in distinct ascending form. The list values are a strict (data Tup) type which should help save space compared to the previous inner Map design and yet efficiently provide all the operations needed by update and predict.

Much of the logic of prediction is in the computeRating helper function.

```
-- The SlopeOne matrix is indexed by pairs of items and is implemented
-- as a sparse map of distinct ascending lists. The 'update' and
-- 'predict' functions do not need the inner type to actually be a
-- map, so the list saves space and complexity.
newtype SlopeOne item = SlopeOne (M.Map item [Tup item])
deriving (Show)
-- Strict triple tuple type for SlopeOne internals
data Tup item = Tup { itemT :: !item, countT :: !Count, ratingT :: !RatingValue }
deriving (Show)
empty :: SlopeOne item
empty = SlopeOne M.empty
update :: Ord item => SlopeOne item -> [Rating item] -> SlopeOne item
update s@(SlopeOne matrixIn) usersRatingsIn | null usersRatings = s
| otherwise =
SlopeOne . M.unionsWith mergeAdd . (matrixIn:) . map fromRating $ usersRatings
where usersRatings = filter ((1<) . M.size) usersRatingsIn
-- fromRating converts a Rating into a Map of Lists, a singleton SlopeOne.
fromRating userRatings = M.mapWithKey expand userRatings
where expand item1 rating1 = map makeTup . M.toAscList . M.delete item1 $ userRatings
where makeTup (item2,rating2) = Tup item2 1 (rating1-rating2)
-- 'mergeAdd' is a helper for 'update'.
-- Optimized traversal of distinct ascending lists to perform additive merge.
mergeAdd :: Ord item => [Tup item] -> [Tup item] -> [Tup item]
mergeAdd !xa@(x:xs) !ya@(y:ys) =
case compare (itemT x) (itemT y) of
LT -> x : mergeAdd xs ya
GT -> y : mergeAdd xa ys
EQ -> Tup (itemT x) (countT x + countT y) (ratingT x + ratingT y) : mergeAdd xs ys
mergeAdd xs [] = xs
mergeAdd [] ys = ys
-- The output Rating has no items in common with the input Rating and
-- only includes positively weighted ratings.
predict :: Ord item => SlopeOne item -> Rating item -> Rating item
predict (SlopeOne matrixIn) userRatings =
M.mapMaybe (computeRating ratingList) (M.difference matrixIn userRatings)
where ratingList = M.toAscList userRatings
-- 'computeRating' is a helper for 'predict'.
-- Optimized traversal of distinct ascending lists to compute positive weighted rating.
computeRating :: (Ord item) => [(item,RatingValue)] -> [Tup item] -> Maybe RatingValue
computeRating !xa@(x:xs) !ya@(y:ys) =
case compare (fst x) (itemT y) of
LT -> computeRating xs ya
GT -> computeRating xa ys
EQ -> helper (countT y) (ratingT y + fromIntegral (countT y) * snd x) xs ys
where
helper :: (Ord item) => Count -> RatingValue -> [(item,RatingValue)] -> [Tup item] -> Maybe RatingValue
helper !count !rating !xa@(x:xs) !ya@(y:ys) =
case compare (fst x) (itemT y) of
LT -> helper count rating xs ya
GT -> helper count rating xa ys
EQ -> helper (count + countT y) (rating + ratingT y + fromIntegral (countT y) * (snd x)) xs ys
helper !count !rating _ _ | rating > 0 = Just (rating / fromIntegral count)
| otherwise = Nothing
computeRating _ _ = Nothing
```