iterator interface for CCGraph.scc

src/data/CCGraph.ml

@@ -41,6 +41,7 @@ module Seq = struct
     let acc = ref acc in
     a (fun x -> acc := f !acc x);
     !acc
+  let to_list seq = fold (fun acc x->x::acc) [] seq |> List.rev
 end
 
 let (|>) x f = f x
@@ -368,14 +369,15 @@ module SCC = struct
     ) else pop_down_to ~id (cell.vertex::acc) stack
 
   let explore ~tbl ~graph seq =
+    let first = ref true in
+    fun k ->
+      if !first then first := false else raise Sequence_once;
       (* stack of nodes being explored, for the DFS *)
       let to_explore = Stack.create() in
       (* stack for Tarjan's algorithm itself *)
       let stack = Stack.create () in
       (* unique ID *)
       let n = ref 0 in
-    (* result *)
-    let res = ref [] in
       (* exploration *)
       Seq.iter
         (fun v ->
@@ -412,12 +414,12 @@ module SCC = struct
                (* pop from stack if SCC found *)
                if cell.id = cell.min_id then (
                  let scc = pop_down_to ~id:cell.id [] stack in
-               res := scc :: !res
+                 k scc
                )
            done
         ) seq;
       assert (Stack.is_empty stack);
-    !res
+      ()
 end
 
 type 'v scc_state = 'v SCC.state
@@ -443,7 +445,7 @@ let scc ?(tbl=mk_table 128) ~graph seq = SCC.explore ~tbl ~graph seq
     ; "h", "d"
     ; "h", "g"
   ] in
-  let res = scc ~graph (Seq.return "a") in
+  let res = scc ~graph (Seq.return "a") |> Seq.to_list in
   assert_bool "scc"
     (set_eq ~eq:(set_eq ?eq:None) res
       [ [ "a"; "b"; "e" ]

src/data/CCGraph.mli

@@ -43,6 +43,7 @@ module Seq : sig
   val filter_map : ('a -> 'b option) -> 'a t -> 'b t
   val iter : ('a -> unit) -> 'a t -> unit
   val fold: ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b
+  val to_list : 'a t -> 'a list
 end
 
 (** {2 Interfaces for graphs} *)
@@ -224,7 +225,7 @@ type 'v scc_state
 val scc : ?tbl:('v, 'v scc_state) table ->
           graph:('v, 'e) t ->
           'v sequence ->
-          'v list list
+          'v list sequence_once
 (** Strongly connected components reachable from the given vertices.
     Each component is a list of vertices that are all mutually reachable
     in the graph.