Avoid materializing intermediate adjacency list (#372)

* Avoid materializing intermediate adjacency list

* Do the same for undirected graphs

* Fix bug

Author: simsurace

src/SimpleGraphs/simpleedgeiter.jl

@@ -32,24 +32,23 @@ eltype(::Type{SimpleEdgeIter{SimpleDiGraph{T}}}) where {T} = SimpleDiGraphEdge{T
     eit::SimpleEdgeIter{G}, state=(one(eltype(eit.g)), 1)
 ) where {G <: AbstractSimpleGraph; !IsDirected{G}}
     g = eit.g
-    fadjlist = fadj(g)
     T = eltype(g)
     n = T(nv(g))
     u, i = state
 
     @inbounds while u < n
-        list_u = fadjlist[u]
+        list_u = fadj(g, u)
         if i > length(list_u)
             u += one(u)
-            i = searchsortedfirst(fadjlist[u], u)
+            i = searchsortedfirst(fadj(g, u), u)
             continue
         end
         e = SimpleEdge(u, list_u[i])
         state = (u, i + 1)
         return e, state
     end
 
-    @inbounds (n == 0 || i > length(fadjlist[n])) && return nothing
+    @inbounds (n == 0 || i > length(fadj(g, n))) && return nothing
 
     e = SimpleEdge(n, n)
     state = (u, i + 1)
@@ -60,20 +59,19 @@ end
     eit::SimpleEdgeIter{G}, state=(one(eltype(eit.g)), 1)
 ) where {G <: AbstractSimpleGraph; IsDirected{G}}
     g = eit.g
-    fadjlist = fadj(g)
     T = eltype(g)
     n = T(nv(g))
     u, i = state
 
     n == 0 && return nothing
 
     @inbounds while true
-        list_u = fadjlist[u]
+        list_u = fadj(g, u)
         if i > length(list_u)
             u == n && return nothing
 
             u += one(u)
-            list_u = fadjlist[u]
+            list_u = fadj(g, u)
             i = 1
             continue
         end