Add johnson.

graph.rkt

@@ -40,7 +40,7 @@
            min-st-kruskal max-st-kruskal min-st-prim max-st-prim
 
            bellman-ford dijkstra dag-shortest-paths
-           floyd-warshall transitive-closure
+           floyd-warshall transitive-closure johnson
 
            graphviz)
 
@@ -210,6 +210,8 @@
     (g:floyd-warshall (gg g)))
   (define (transitive-closure g)
     (g:transitive-closure (gg g)))
+  (define (johnson g)
+    (g:johnson (gg g)))
 
   ;; 10 Graphviz
   (define (graphviz g #:output [output #f] #:colors [colors #f])
@@ -313,6 +315,7 @@
   ;; 7 All-pairs Shortest Paths
   [floyd-warshall (-> Graph (Mutable-HashTable (List Any Any) Number))]
   [transitive-closure (-> Graph (Mutable-HashTable (List Any Any) Boolean))]
+  [johnson (-> Graph (Mutable-HashTable (List Any Any) Number))]
 
   ;; 10 Graphviz
   [graphviz (->* (Graph)
@@ -495,7 +498,25 @@
                     ((b d) . #t)
                     ((b b) . #t)
                     ((d b) . #f)
-                    ((a b) . #t))))
+                    ((a b) . #t)))
+
+    (check-equal? (hash->ordered-list (johnson g0))
+                  '(((a d) . 4)
+                    ((c c) . 0)
+                    ((b a) . +inf.0)
+                    ((a a) . 0)
+                    ((d c) . +inf.0)
+                    ((a c) . 2)
+                    ((c b) . +inf.0)
+                    ((d d) . 0)
+                    ((c a) . +inf.0)
+                    ((b c) . +inf.0)
+                    ((d a) . +inf.0)
+                    ((c d) . 3)
+                    ((b d) . 3)
+                    ((b b) . 0)
+                    ((d b) . +inf.0)
+                    ((a b) . 1))))
 
   (test-case "10 Graphviz"
     (define g (directed-graph '((a b) (b c))))