1 module UnitMinCostFlow (umcf, flowDiff) where
3 import Data.Graph.Inductive.Graph
4 import Data.Graph.Inductive.Internal.RootPath
7 maybeDelete :: Eq a => a -> [a] -> Maybe [a]
8 maybeDelete _ [] = Nothing
9 maybeDelete e (h:t) = if e == h
11 else do t1 <- maybeDelete e t; return (h:t1)
13 -- If the edge occurs in the graph, return Just the graph with one occurrence
14 -- deleted; otherwise return Nothing. (delLEdge deletes all occurrences.)
15 maybeDelOneLEdge :: (DynGraph gr, Eq b) => LEdge b -> gr a b -> Maybe (gr a b)
16 maybeDelOneLEdge (src, dest, lbl) theGraph =
17 let (mc, moreGraph) = match src theGraph in do
19 s2 <- maybeDelete (lbl, dest) s
20 return ((p, v, l, s2) & moreGraph)
22 flipEdge (src, dest, lbl) = (dest, src, -lbl)
24 flipEdgeIn :: (DynGraph gr, Real b) => LEdge b -> gr a b -> gr a b
25 flipEdgeIn edge theGraph =
26 let Just graph1 = maybeDelOneLEdge edge theGraph in
27 insEdge (flipEdge edge) graph1
29 augment :: (DynGraph gr, Real b) => [LNode b] -> gr a b -> gr a b
30 augment augPath@((v1, d1) : t1) theGraph =
33 (v2, d2) : t2 -> augment (tail augPath)
34 (flipEdgeIn (v1, v2, d2 - d1) theGraph)
36 -- Find a min-cost flow from s to t in theGraph.
37 -- Each edge of the input graph has unit capacity and cost given by its label.
38 -- Returns: flow value, residual graph.
39 umcf :: (DynGraph gr, Real b) => Node -> Node -> gr a b -> (b, gr a b)
41 -- Use Bellman-Ford to find an augmenting path from s to t, if one exists.
42 -- NOTE: getLPath reverses it into s-to-t order!
43 let LP augPath = getLPath t (spTree s theGraph) in
48 -- Augment, and continue flowing in the resulting graph.
49 let graph2 = augment augPath theGraph in
50 let (fval1, resid) = umcf s t graph2 in (fval1 + 1, resid)
52 -- Diffs an original graph and a residual graph, producing the flow graph.
53 flowDiff :: (DynGraph gr, Real b) => gr a b -> gr a b -> gr a b
54 flowDiff ograph rgraph = case labEdges ograph of
55 [] -> mkGraph (labNodes ograph) []
56 oedge:_ -> let Just ograph2 = maybeDelOneLEdge oedge ograph in
57 case maybeDelOneLEdge oedge rgraph of
58 Just rgraph2 -> flowDiff ograph2 rgraph2
59 Nothing -> let Just rgraph2 = maybeDelOneLEdge (flipEdge oedge) rgraph in
60 insEdge oedge (flowDiff ograph2 rgraph2)