lib/poset: avoid useless call of `to_a` in `add_edge`

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/lib/poset.nit b/lib/poset.nit
index 93afcf1..3288e5c 100644 (file)
--- a/lib/poset.nit
+++ b/lib/poset.nit
@@ -149,20 +149,37 @@ class POSet[E]
                # Update the transitive reduction
                if te.tos.has(f) then return # Skip the reduction if there is a loop
 
-               for x in te.dfroms.to_a do
+               # Remove transitive edges.
+               # Because the sets of direct is iterated, the list of edges to remove
+               # is stored and is applied after the iteration.
+               # The usual case is that no direct edges need to be removed,
+               # so start with a `null` list of edges.
+               var to_remove: nullable Array[E] = null
+               for x in te.dfroms do
                        var xe = self.elements[x]
                        if xe.tos.has(f) then
-                               te.dfroms.remove(x)
+                               if to_remove == null then to_remove = new Array[E]
+                               to_remove.add x
                                xe.dtos.remove(t)
                        end
                end
-               for x in fe.dtos.to_a do
+               if to_remove != null then
+                       for x in to_remove do te.dfroms.remove(x)
+                       to_remove.clear
+               end
+
+               for x in fe.dtos do
                        var xe = self.elements[x]
                        if xe.froms.has(t) then
                                xe.dfroms.remove(f)
-                               fe.dtos.remove(x)
+                               if to_remove == null then to_remove = new Array[E]
+                               to_remove.add x
                        end
                end
+               if to_remove != null then
+                       for x in to_remove do fe.dtos.remove(x)
+               end
+
                fe.dtos.add t
                te.dfroms.add f
        end