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(* graph-scc.sml * * COPYRIGHT (c) 1999 Lucent Bell Laboratories. * * Calculate strongly-connected components of directed graph. * The graph can have nodes with self-loops. * * author: Matthias Blume *) functor GraphSCCFn (Nd: ORD_KEY) :> GRAPH_SCC where Nd = Nd = struct structure Nd = Nd type node = Nd.ord_key structure Map = RedBlackMapFn (Nd) datatype component = SIMPLE of node | RECURSIVE of node list fun eq x y = (Nd.compare(x, y) = EQUAL) fun topOrder' { roots, follow } = let fun getNode (n, nm as (npre, m)) = ( case Map.find (m, n) of NONE => let val r = { pre = npre, low = ref npre } val m' = Map.insert (m, n, r) in ((npre + 1, m'), r) end | SOME r => (nm, r) (* end case *)) fun component (x, []) = if List.exists (eq 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. * The current node itself is not given as an argument. * Instead, it is represented by grab_cont -- a function * that "grabs" a component from the current stack and then * continues with the regular continuation. We do it this * way to be able to handle the topmost virtual component -- * the one whose sole element is the virtual root node. *) val { follow_nodes, grab_cont, node_pre, node_low, parent_low, nodemap, stack, sccl, nograb_cont } = args (* loop over the follow-set of a node *) fun loop (tn :: tnl) (nodemap as (npre, theMap), stack, sccl) = let val is_tn = eq tn in case Map.find (theMap, tn) of SOME{ pre = tn_pre, low = tn_low } => let val tl = !tn_low in if tl < (!node_low) andalso List.exists is_tn stack then node_low := tl else (); loop tnl (nodemap, stack, sccl) end | NONE =>let (* lookup failed -> tn is a new node *) val tn_pre = npre val tn_low = ref npre val npre = npre + 1 val theMap = Map.insert (theMap, tn, { pre = tn_pre, low = tn_low }) val nodemap = (npre, theMap) val tn_nograb_cont = loop tnl fun tn_grab_cont (nodemap, sccl) = let fun grab (top :: stack, scc) = if eq tn top then tn_nograb_cont (nodemap, stack, component (top, scc) :: sccl) else grab (stack, top :: scc) | grab _ = raise Fail "scc:grab: empty stack" in grab end in dfs { follow_nodes = follow tn, grab_cont = tn_grab_cont, node_pre = tn_pre, node_low = tn_low, parent_low = node_low, nodemap = nodemap, stack = tn :: stack, sccl = sccl, nograb_cont = tn_nograb_cont } end end | loop [] (nodemap, stack, sccl) = let val nl = !node_low in if nl = node_pre then grab_cont (nodemap, sccl) (stack, []) else ((* propagate node_low up *) if nl < (!parent_low) then parent_low := nl else (); (* `return' *) nograb_cont (nodemap, stack, sccl)) end in loop (rev follow_nodes) (nodemap, stack, sccl) end fun top_grab_cont (nodemap, sccl) ([], []) = sccl | top_grab_cont _ _ = raise Fail "scc:top_grab: stack not empty" in dfs { follow_nodes = roots, grab_cont = top_grab_cont, node_pre = 0, node_low = ref 0, (* low of virtual root *) parent_low = ref 0, (* low of virtual parent of virtual root *) nodemap = (1, Map.empty), stack = [], sccl = [], nograb_cont = fn (_, _, _) => raise Fail "scc:top_nograb_cont" } end fun topOrder { root, follow } = topOrder' { roots = [root], follow = follow } end

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