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(* scc.sml * * COPYRIGHT (c) 1996 Bell Laboratories. * * Calculate strongly-connected components of directed graph. * The graph can have nodes with self-loops. * * author: Matthias Blume * *) signature SCCNODE = sig type node val eq: node * node -> bool val lt: node * node -> bool end signature SCC = sig structure Node: SCCNODE datatype component = SIMPLE of Node.node (* singleton, no self-loop *) | RECURSIVE of Node.node list (* take root node and follow function and return * list of topologically sorted strongly-connected components; * root component goes first *) val topOrder: { root: Node.node, follow: Node.node -> Node.node list } -> component list end functor SCCUtilFun (structure Node: SCCNODE): SCC = struct exception SccBug structure Node = Node structure NodeOrdSet = struct type elem = Node.node val (op <) = Node.lt end structure Map = MapF (NodeOrdSet) type node = Node.node datatype component = SIMPLE of node | RECURSIVE of node list fun topOrder { root, follow } = let fun getNode (n, nm as (npre, m)) = (nm, (Map.lookup m n)) handle Map.MapF => let val r = { pre = npre, low = ref npre } val m' = Map.add (m, n, r) in ((npre + 1, m'), r) end fun theNode x y = Node.eq (x, y) fun component (x, []) = if List.exists (theNode x) (follow x) then RECURSIVE [x] else SIMPLE x | component (x, xl) = RECURSIVE (x :: xl) (* depth-first search in continuation-passing, state-passing style *) fun dfs args = let (* the nodemap represents the mapping from nodes to * pre-order numbers and low-numbers. The latter are ref-cells. * nodemap also remembers the next available pre-order number. *) val { node, node_pre, node_low, parent_low, nodemap, stack, sccl, cont } = args (* loop over the follow-set of a node *) fun loop [] (nodemap, stack, sccl) = let val nl = !node_low in if nl = node_pre then let fun grab (top :: stack, scc) = if Node.eq (top, node) then cont (nodemap, stack, component (top, scc) :: sccl) else grab (stack, top :: scc) | grab _ = raise SccBug in grab (stack, []) end else let val _ = (* propagate node_low up *) if nl < (!parent_low) then parent_low := nl else () in (* `return' *) cont (nodemap, stack, sccl) end end | loop (tn :: tnl) (nodemap as (npre, theMap), stack, sccl) = let val { pre = tn_pre, low = tn_low } = Map.lookup theMap tn (* the lookup succeeded -> we have seen tn before *) val tl = !tn_low in if tl < (!node_low) andalso List.exists (theNode tn) stack then node_low := tl else (); loop tnl (nodemap, stack, sccl) end handle Map.MapF => let (* lookup failed -> tn is a new node *) val tn_pre = npre val tn_low = ref npre val npre = npre + 1 val theMap = Map.add (theMap, tn, { pre = tn_pre, low = tn_low }) val nodemap = (npre, theMap) in dfs { node = tn, node_pre = tn_pre, node_low = tn_low, parent_low = node_low, nodemap = nodemap, stack = tn :: stack, sccl = sccl, cont = loop tnl } end in loop (follow node) (nodemap, stack, sccl) end val root_low = ref 0 in dfs { node = root, node_pre = 0, node_low = root_low, parent_low = ref 0, (* dummy *) nodemap = (1, Map.singleton (root, { pre = 0, low = root_low })), stack = [root], sccl = [], cont = fn (_, _, sccl) => sccl } end end (* * $Log$ *)

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