;;; lib/obj/graph.lisp --- Graphs

 ;; Graph objects and algorithms

 ;;; Commentary:

 ;; Mostly modeled off of eschulte's GRAPH library - see also DAT/DOT

 ;; ref: https://eschulte.github.io/graph/

 ;; Our goals are slightly different than the original library - we prioritize

 ;; flexibility over speed or code size. To this end the base GRAPH class

 ;; accepts NODE vectors in addition to hash-tables. We also make minimal

 ;; assumptions about the underling node types - it is a blank class

 ;; definition. Edges are a bit more complicated - they subclass ID which can be

 ;;; Code:

 (in-package :obj/graph)

 (in-readtable :std)

 (defclass node () ())

 ;;; Vertex

 (defclass vertex (id node)

   ()

   (:documentation "generic vertex mixin. The difference between this class and NODE is

 that a vertex always carries an ID slot."))

 ;;; Edge

 (defclass edge (node)

   (:documentation "generic edge mixin. Compatible with the NODE and ID protocols."))

 (defclass edgex (edge id)

   ()

   (:documentation "Edge compatible with the NODE and ID protocols."))

 (defclass directed-edge (edge)

   ()

   (:documentation "An edge with an implicit direction from node A to B."))

 (defclass weighted-edge (edge)

   ((weight :initform 1d0 :initarg :weight :accessor weight-of)))

 ;;; Hashing

 (defun copy-hash (hash &optional test comb)

   "Return a copy of HASH.

 Optional argument TEST specifies a new equality test to use for the

 copy.  Second optional argument COMB specifies a function to use to

 combine the values of elements of HASH which collide in the copy due

 to a new equality test specified with TEST."

   (let ((comb (when comb (fdefinition comb)))

         (copy (make-hash-table :test (or test (hash-table-test hash)))))

     (maphash (lambda (k v) (setf (gethash k copy)

                             (if (and (gethash k copy) comb)

                                 (funcall comb (gethash k copy) v)

                                 v)))

              hash)

     copy))

 (defun node-hash-equal (hash1 hash2)

   "Test node hashes HASH1 and HASH2 for equality."

   (set-equal (hash-table-alist hash1)

              (hash-table-alist hash2)

              :test (lambda (a b)

                      (and (equalp (car a) (car b))

                           (set-equal (cdr a) (cdr b) :test 'tree-equal)))))

 (defun edge-hash-equal (hash1 hash2)

   "Test edge hashes HASH1 and HASH2 for equality."

   (set-equal (hash-table-alist hash1)

              (hash-table-alist hash2)

              :test 'equalp))

 (defun edge-equalp (edge1 edge2)

   (set-equal edge1 edge2 :test 'equal))

 (defun directed-edge-equalp (edge1 edge2)

   (tree-equal edge1 edge2))

 (defun sxhash-edge (edge)

   (sxhash (sort (copy-tree edge)

                 (cond

                   ((and (numberp (car edge)) (numberp (cdr edge)))

                    (lambda (a b)

                      (or (< (imagpart a) (imagpart b))

                          (and (= (imagpart a) (imagpart b))

                               (< (realpart a) (realpart b))))))

                   ((or (numberp (car edge)) (numberp (second edge)))

                    (lambda (a b) (declare (ignore a b)) t))

                   (t #'string<)))))

 (sb-ext:define-hash-table-test edge-equalp sxhash-edge)

 (sb-ext:define-hash-table-test directed-edge-equalp sxhash)

 ;;; Proto

 (defgeneric nodes (graph))

 (defgeneric (setf nodes) (graph nodes))

 (defgeneric edges (graph))

 (defgeneric (setf edges) (graph edges))

 (defgeneric graph-equal (graph1 graph2))

 (defgeneric subgraph (graph nodes)

   (:documentation "Return the subgraph of GRAPH restricted to NODES."))

 (defgeneric delete-node (graph node)

   (:documentation "Delete NODE from GRAPH.

 Delete and return the old edges of NODE in GRAPH."))

 (defgeneric has-node-p (graph node)

   (:documentation "Return non-nil if GRAPH has node NODE."))

 (defgeneric has-edge-p (graph edge)

   (:documentation "Return `true' if GRAPH has edge EDGE."))

 (defgeneric edge-weight (edge &key &allow-other-keys)

   (:method ((edge t) &key &allow-other-keys) (values 1.0)))

 (defgeneric edge-value (graph edge)

   (:method ((graph t) (edge t)) (values nil)))

 (defgeneric (setf edge-value) (new graph edge))

 (defgeneric delete-edge (graph edge)

   (:documentation "Delete EDGE from GRAPH.

 Return the old value of EDGE."))

 (defgeneric node-edges (graph node)

   (:documentation "Return the edges of NODE in GRAPH."))

 (defgeneric (setf node-edges) (new graph node)

   (:documentation "Set the edges of NODE in GRAPH to NEW.

 Delete and return the old edges of NODE in GRAPH."))

 (defgeneric add-node (graph node))

 (defgeneric add-edge (graph edge &optional value))

 ;;; Graph

 (defclass graph (node)

   ((nodes :initform (make-hash-table :test 'equal)

           :type (or (vector node) hash-table)

           :accessor nodes

           :initarg :nodes)

    (edges :initform (make-hash-table :test 'edge-equalp)

           :type hash-table

           :accessor edges

           :initarg :edges))

   (:documentation "generic graph object."))

 (defmethod copy-graph ((graph graph))

   (make-instance (type-of graph) :nodes (copy-hash (nodes graph)) :edges (copy-hash (edges graph))))

 (defmethod subgraph ((graph graph) nodes)

   (make-instance (type-of graph) :nodes nodes :edges (copy-hash (edges graph))))

 (defmethod has-edge-p ((graph graph) edge)

   (multiple-value-bind (value included) (gethash edge (edges graph))

     (declare (ignorable value)) included))

 (defmethod has-node-p ((graph graph) node)

   (multiple-value-bind (value included) (gethash node (nodes graph))

     (declare (ignorable value)) included))

 (defmethod delete-node ((graph graph) node)

   (prog1 (mapcar (lambda (edge) (cons edge (delete-edge graph edge)))

                  (node-edges graph node))

     (remhash node (nodes graph))))

 (defmethod delete-edge ((graph graph) edge)

   (prog1 (edge-value graph edge)

     (mapc (lambda (node) (setf (gethash node (nodes graph))

                           (remove edge (gethash node (nodes graph))

                                   :test 'edge-equalp)))

           edge)

     (remhash edge (edges graph))))

 (defmethod node-edges ((graph graph) node)

   (multiple-value-bind (edges included) (gethash node (nodes graph))

     (assert included (node graph) "~S doesn't include ~S" graph node)

     edges))

 (defmethod (setf node-edges) (new (graph graph) node)

   (prog1 (mapc {delete-edge graph} (gethash node (nodes graph)))

     (mapc {add-edge graph} new)))

 (defmethod add-edge ((graph graph) edge &optional value)

   (mapc (lambda (node)

           (add-node graph node)

           (pushnew (case (type-of graph)

                      (graph (remove-duplicates edge))

                      (directed-graph edge))

                    (gethash node (nodes graph))

                    :test 'edge-equalp))

         edge)

   (setf (gethash edge (edges graph)) value)

   edge)

 (defmethod edge-value ((graph graph) edge)

   (multiple-value-bind (value included) (gethash edge (edges graph))

     (assert included (edge graph) "~S doesn't include ~S" graph edge)

     value))

 (defmethod (setf edge-value) (new (graph graph) edge)

   (setf (gethash edge (edges graph)) new))

 (defgeneric merge-nodes (graph node1 node2 &key new)

   (:documentation "Combine NODE1 and NODE2 in GRAPH into the node NEW.

 All edges of NODE1 and NODE2 in GRAPH will be combined into a new node

 of value NEW.  Edges between only NODE1 and NODE2 will be removed."))

 (defmethod merge-nodes ((graph graph) node1 node2 &key (new node1))

   ;; replace all removed edges with NEW instead of NODE1 or NODE2

   (mapcar

    (lambda (l)

      (destructuring-bind(edge . value) l

        (let ((e (mapcar (lambda (n) (if (member n (list node1 node2)) new n)) edge)))

          (if (has-edge-p graph e)

              (when (and (edge-value graph e) value)

                (setf (edge-value graph e) (+ (edge-value graph e) value)))

              (add-edge graph e value)))))

    ;; drop edges between only node1 and node2

    (remove-if-not [{set-difference _ (list node1 node2)} #'car]

                   ;; delete both nodes keeping their edges and values

                   (prog1 (append (delete-node graph node1)

                                  (delete-node graph node2))

                     ;; add the new node

                     (add-node graph new))))

   graph)

 (defgeneric merge-edges (graph edge1 edge2 &key value)

   (:documentation "Combine EDGE1 and EDGE2 in GRAPH into a new EDGE.

 Optionally provide a value for the new edge, the values of EDGE1 and

 EDGE2 will be combined."))

 (defmethod merge-edges ((graph graph) edge1 edge2 &key value)

   (add-edge graph (remove-duplicates (append edge1 edge2))

             (or value

                 (when (and (edge-value graph edge1) (edge-value graph edge2))

                   (+ (edge-value graph edge1) (edge-value graph edge2)))))

   (append (delete-edge graph edge1)

           (delete-edge graph edge2)))

 (defgeneric degree (graph node)

   (:documentation "Return the degree of NODE in GRAPH."))

 (defmethod degree ((graph graph) node)

   (length (node-edges graph node)))

 (defmethod add-node ((graph graph) node)

   ;; NOTE: This is where our implementation breaks character from Eschulte's

   ;; implementation. We currently accept strings in addition to numbers and symbols.

   (assert (or (numberp node) (symbolp node) (stringp node)) (node)

           "Nodes must be numbers, symbols or keywords, not ~S.~%Invalid node:~S"

           (type-of node) node)

   (unless (has-node-p graph node)

     (setf (gethash node (nodes graph)) nil)

     node))

 ;;; Directed Graph

 (defclass directed-graph (graph)

   ((edges :initform (make-hash-table :test 'directed-edge-equalp)

           :type (or (vector directed-edge) hash-table)

           :accessor edges

           :initarg :edges))

   (:documentation "graph with only directed edges."))

 (defgeneric indegree (digraph node)

   (:documentation "The number of edges directed to NODE in GRAPH."))

 (defmethod indegree ((digraph directed-graph) node)

   (length (remove-if-not [{member node} #'cdr] (node-edges digraph node))))

 (defgeneric outdegree (digraph node)

   (:documentation "The number of edges directed from NODE in DIGRAPH."))

 (defmethod outdegree ((digraph directed-graph) node)

   (length (remove-if-not [{equal node} #'car] (node-edges digraph node))))

 ;;; Shortest Path

 (defgeneric shortest-path (graph a b &optional heuristic)

   (:documentation "Return the shortest path in GRAPH from A to B.

 Implemented using A* search.  Optional argument HEURISTIC may be a

 function which returns an estimated heuristic cost from an node to the

 target B.  The default value for HEURISTIC is the constant function of

 0, reducing this implementation to Dijkstra's algorithm.  The

 HEURISTIC function must satisfy HEURITIC(x)≤d(x,y)+HEURITIC(y) ∀ x,y

 in GRAPH allowing the more efficient monotonic or \"consistent\"

 implementation of A*.")

   (:method ((graph graph) a b

             &optional

               (heuristic (constantly 0))

             &aux

               (from (make-hash-table))

               (fringe (sb-concurrency:make-queue))

               (open (make-hash-table))

               (closed (make-hash-table))

               (g (make-hash-table))

               (f (make-hash-table)))

     (when (equal a b) (return-from shortest-path nil))

     (labels ((reconstruct-path (current)

                (destructuring-bind (node . edge) (gethash current from)

                  (cons edge (unless (member a edge) (reconstruct-path node))))))

       (setf (gethash a g) 0

             (gethash a f) (funcall heuristic a)

             (gethash a open) t)

       (sb-concurrency:enqueue fringe (gethash a f))

       (do ((current (sb-concurrency:dequeue fringe) (sb-concurrency:dequeue fringe)))

           ((zerop (hash-table-count open))

            (multiple-value-bind (value present-p) (gethash b f)

              (when present-p

                (values (nreverse (reconstruct-path b)) value))))

         (when (eql current b)

           (return-from shortest-path

             (values (nreverse (reconstruct-path current))

                     (gethash current f))))

         (remhash current open)

         (setf (gethash current closed) t)

         (mapc (lambda (edge)

                 (let ((weight (or (edge-value graph edge) 1)))

                   (mapc (lambda (next)

                           (unless (gethash next closed)

                             (setf (gethash next open) t)

                             (let ((tentative (+ (gethash current g) weight)))

                               (multiple-value-bind (value present-p)

                                   (gethash next g)

                                 (when (or (not present-p)

                                           (< tentative value))

                                   (setf (gethash next from) (cons current edge)

                                         (gethash next g) tentative

                                         (gethash next f)

                                         (+ tentative (funcall heuristic next)))

                                   (sb-concurrency:enqueue fringe (gethash next f)))))))

                         (etypecase graph

                           (directed-graph (cdr (member current edge)))

                           (graph (remove current edge))))))

               (node-edges graph current))))))

 ;;; Min Cut