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1994-06-23
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257 lines
------------------------------------------------------------------------------
-- Here is a version of the graph algorithms described in:
--
-- Lazy Depth-First Search and Linear Graph Algorithms in Haskell
-- David King and John Launchbury
--
-- Together with some additional code for printing tree structures ...
--
-- This program requires array.gs, iomonad.gs, and ioarray.gs to run.
-- For example, in the demos/IO directory, try:
--
-- :load ../../array.gs ../../iomonad.gs ../../ioarray.gs ldfs.gs
--
-- Of course, it would be sensible to put these things in a project file!
--
------------------------------------------------------------------------------
type Vertex = Char
-- Representing graphs:
type Table a = Array Vertex a
type Graph = Table [Vertex]
vertices :: Graph -> [Vertex]
vertices = indices
type Edge = Assoc Vertex Vertex
edges :: Graph -> [Edge]
edges g = [ v := w | v <- vertices g, w <- g!v ]
mapT :: (Vertex -> a -> b) -> Table a -> Table b
mapT f t = array (bounds t) [ v := f v (t!v) | v <- indices t ]
type Bounds = (Vertex, Vertex)
outdegree :: Graph -> Table Int
outdegree = mapT numEdges
where numEdges v ws = length ws
buildG :: Bounds -> [Edge] -> Graph
buildG = accumArray (flip (:)) []
graph = buildG ('a','j')
(reverse
[ 'a' := 'b', 'a' := 'f', 'b' := 'c',
'b' := 'e', 'c' := 'a', 'c' := 'd',
'e' := 'd', 'g' := 'h', 'g' := 'j',
'h' := 'f', 'h' := 'i', 'h' := 'j' ]
)
transposeG :: Graph -> Graph
transposeG g = buildG (bounds g) (reverseE g)
reverseE :: Graph -> [Edge]
reverseE g = [ w := v | (v := w) <- edges g ]
indegree :: Graph -> Table Int
indegree = outdegree . transposeG
-- Depth-first search
-- Specification and implementation of depth-first search:
data Tree a = Node a (Forest a)
type Forest a = [Tree a]
dff :: Graph -> Forest Vertex
dff g = dfs g (vertices g)
dfs :: Graph -> [Vertex] -> Forest Vertex
dfs g vs = prune (bounds g) (map (generate g) vs)
generate :: Graph -> Vertex -> Tree Vertex
generate g v = Node v (map (generate g) (g!v))
type Set s = MutArr s Vertex Bool
mkEmpty :: Bounds -> ST s (Set s)
mkEmpty bnds = newArr bnds False
contains :: Set s -> Vertex -> ST s Bool
contains m v = readArr m v
include :: Set s -> Vertex -> ST s ()
include m v = writeArr m v True
prune :: Bounds -> Forest Vertex -> Forest Vertex
prune bnds ts = runST (mkEmpty bnds `thenST` \m ->
chop m ts)
chop :: Set s -> Forest Vertex -> ST s (Forest Vertex)
chop m [] = returnST []
chop m (Node v ts : us)
= contains m v `thenST` \visited ->
if visited then
chop m us
else
include m v `thenST` \_ ->
chop m ts `thenST` \as ->
chop m us `thenST` \bs ->
returnST (Node v as : bs)
-- Depth-first search algorithms
-- Algorithm 1: depth first search numbering
preorder :: Tree a -> [a]
preorder (Node a ts) = [a] ++ preorderF ts
preorderF :: Forest a -> [a]
preorderF ts = concat (map preorder ts)
preOrd :: Graph -> [Vertex]
preOrd = preorderF . dff
tabulate :: Bounds -> [Vertex] -> Table Int
tabulate bnds vs = array bnds (zipWith (:=) vs [1..])
preArr :: Bounds -> Forest Vertex -> Table Int
preArr bnds = tabulate bnds . preorderF
-- Algorithm 2: topological sorting
postorder :: Tree a -> [a]
postorder (Node a ts) = postorderF ts ++ [a]
postorderF :: Forest a -> [a]
postorderF ts = concat (map postorder ts)
postOrd :: Graph -> [Vertex]
postOrd = postorderF . dff
topSort :: Graph -> [Vertex]
topSort = reverse . postOrd
-- Algorithm 3: connected components
components :: Graph -> Forest Vertex
components = dff . undirected
undirected :: Graph -> Graph
undirected g = buildG (bounds g) (edges g ++ reverseE g)
-- Algorithm 4: strongly connected components
scc :: Graph -> Forest Vertex
scc g = dfs (transposeG g) (reverse (postOrd g))
scc' :: Graph -> Forest Vertex
scc' g = dfs g (reverse (postOrd (transposeG g)))
-- Algorithm 5: Classifying edges
tree :: Bounds -> Forest Vertex -> Graph
tree bnds ts = buildG bnds (concat (map flat ts))
where flat (Node v rs) = [ v := w | Node w us <- ts ] ++
concat (map flat ts)
back :: Graph -> Table Int -> Graph
back g post = mapT select g
where select v ws = [ w | w <- ws, post!v < post!w ]
cross :: Graph -> Table Int -> Table Int -> Graph
cross g pre post = mapT select g
where select v ws = [ w | w <- ws, post!v > post!w, pre!v > pre!w ]
forward :: Graph -> Graph -> Table Int -> Graph
forward g tree pre = mapT select g
where select v ws = [ w | w <- ws, pre!v < pre!w ] \\ tree!v
-- Algorithm 6: Finding reachable vertices
reachable :: Graph -> Vertex -> [Vertex]
reachable g v = preorderF (dfs g [v])
path :: Graph -> Vertex -> Vertex -> Bool
path g v w = w `elem` (reachable g v)
-- Algorithm 7: Biconnected components
bcc :: Graph -> Forest [Vertex]
bcc g = (concat . map bicomps . map (label g dnum)) forest
where forest = dff g
dnum = preArr (bounds g) forest
label :: Graph -> Table Int -> Tree Vertex -> Tree (Vertex,Int,Int)
label g dnum (Node v ts) = Node (v,dnum!v,lv) us
where us = map (label g dnum) ts
lv = minimum ([dnum!v] ++ [dnum!w | w <- g!v]
++ [lu | Node (u,du,lu) xs <- us])
bicomps :: Tree (Vertex,Int,Int) -> Forest [Vertex]
bicomps (Node (v,dv,lv) ts)
= [ Node (v:vs) us | (l,Node vs us) <- map collect ts]
collect :: Tree (Vertex,Int,Int) -> (Int, Tree [Vertex])
collect (Node (v,dv,lv) ts) = (lv, Node (v:vs) cs)
where collected = map collect ts
vs = concat [ ws | (lw, Node ws us) <- collected, lw<dv]
cs = concat [ if lw<dv then us else [Node (v:ws) us]
| (lw, Node ws us) <- collected ]
figure4 = buildG ('a','i') (vs ++ reverse [ v:=w | (w:=v) <- vs ])
where vs = [ 'b' := 'a', 'e' := 'a', 'c' := 'b',
'd' := 'c', 'b' := 'd', 'f' := 'e',
'h' := 'e', 'g' := 'f', 'e' := 'g',
'i' := 'h', 'a' := 'i', 'h' := 'a' ]
figure5 = showForest (map (label figure4 dnum) f)
where f = dff figure4
dnum = preArr (bounds figure4) f
figure7 = showForest (bcc figure4)
-- Utility functions for drawing trees and forests:
showTree :: Text a => Tree a -> String
showTree = drawTree . mapTree show
showForest :: Text a => Forest a -> String
showForest = unlines . map showTree
mapTree :: (a -> b) -> (Tree a -> Tree b)
mapTree f (Node x ts) = Node (f x) (map (mapTree f) ts)
drawTree :: Tree String -> String
drawTree = unlines . draw
draw (Node x ts) = grp this (space (length this)) (stLoop ts)
where this = s1 ++ x ++ " "
stLoop [] = [""]
stLoop [t] = grp s2 " " (draw t)
stLoop (t:ts) = grp s3 s4 (draw t) ++ [s4] ++ rsLoop ts
rsLoop [t] = grp s5 " " (draw t)
rsLoop (t:ts) = grp s6 s4 (draw t) ++ [s4] ++ rsLoop ts
grp fst rst = zipWith (++) (fst:repeat rst)
[s1,s2,s3,s4,s5,s6] = ["- ", "--", "-+", " |", " `", " +"]
-- Instances of Eq and Text that are not included in the Gofer preludes:
instance (Eq a, Eq b, Eq c) => Eq (a,b,c) where
(a,b,c)==(p,q,r) = a==p && b==q && c==r
instance (Text a, Text b, Text c) => Text (a,b,c) where
showsPrec d (x,y,z) = showChar '(' . shows x . showChar ',' .
shows y . showChar ',' .
shows z . showChar ')'
------------------------------------------------------------------------------