Implemented new version of multiply

src/matrix-multiply.hs

-import Data.List
 import Data.Ord
+import Data.List
+import qualified Data.Map
 
 type CoordMat = [(Int, Int, Float)]
 
 type IndexedRowMat = [(Int, [Float])]
 
+-- UTILS
+groupBy :: (Ord b) => (a -> b) -> [a] -> Data.Map.Map b [a]
+groupBy f l = Data.Map.fromListWith (++) [(f v, [v]) | v <- l]
+
 -- SPARSE MATRIX MULTIPLICATION
 
+
 multiply :: CoordMat -> CoordMat -> CoordMat
 multiply left right = 
-    let join = [(k, i, v, j, w) | (k, i, v) <- left, (j, k1, w) <- right, k == k1]
-    in [(i, j, w * v) | (k, i, v, j, w) <- join]
+    let join = [((i, j), v * w) | (k, i, v) <- right, (j, k1, w) <- left, k == k1]
+        getValue (_, v) = v
+    in 
+    map (\((i, j), v) -> (i, j, v)) $
+    Data.Map.toList $
+    Data.Map.map (sum) $
+    Data.Map.map (map snd) $ 
+    Main.groupBy (\(p, v) -> p) join
+
 
 -- DENSE MATRIX MULTIPLICATION
 
@@ -22,7 +35,7 @@ toRowMatrix =
         -- [(Int, Int, Float)] -> [Float]
         aggregateRow = map (\(i, j, v) -> v)
         isZeroRow = all (== 0) 
-    in zip [1..] . filter (not . isZeroRow) . map aggregateRow . groupBy equivalent . sortBy (comparing rowNum)
+    in zip [1..] . filter (not . isZeroRow) . map aggregateRow . Data.List.groupBy equivalent . sortBy (comparing rowNum)
 
 transpose :: CoordMat -> CoordMat
 transpose = map (\(i, j, v) -> (j, i, v))