+implementation module scc
+
+import StdEnv, StdMaybe
+import Data.Map => qualified updateAt
+
+:: St a = {nextindex :: Int, stack :: [a], map :: Map a Annot, sccs :: [[a]]}
+:: Annot = {index :: Int, lowlink :: Int, onstack :: Bool}
+
+scc :: [(a, [a])] -> [[a]] | Eq, Ord a
+scc nodes = (foldr strongconnect {nextindex=0,stack=[],map=newMap,sccs=[]} nodes).sccs
+where
+// strongconnect :: (a, [a]) (St a) -> St a | Eq, Ord a
+ strongconnect (v, suc) s
+ | isJust (get v s.map) = s
+ # s = foldr processSucc
+ { s & map = put v {index=s.nextindex, lowlink=s.nextindex, onstack=True} s.map
+ , stack = [v:s.stack]
+ , nextindex = s.nextindex + 1
+ } suc
+ # (Just a) = get v s.map
+ | a.index == a.lowlink
+ # (scc, [sl:stack]) = span ((<>) v) s.stack
+ # scc = scc ++ [sl]
+ = { s & sccs = [scc:s.sccs]
+ , stack= stack
+ , map = foldr (alter \(Just s)->Just {s & onstack=False}) s.map scc
+ }
+ = s
+ where
+// processSucc :: a (St a) -> St a | Eq, Ord a
+ processSucc w s = case get w s.map of
+ Nothing
+ # s = strongconnect (hd [l\\l=:(n, _)<-nodes | n == w]) s
+ # (Just aw) = get w s.map
+ # (Just av) = get v s.map
+ = {s & map=put v {av & lowlink=min av.lowlink aw.lowlink} s.map}
+ Just aw=:{onstack=True}
+ # (Just av) = get v s.map
+ = {s & map=put v {av & lowlink=min aw.index av.lowlink} s.map}
+ Just _ = s