faster erdos_renyi

src/SimpleGraphs/generators/randgraphs.jl

@@ -1,4 +1,4 @@
-using Random: randperm, shuffle!
+using Random: randperm, shuffle!, randsubseq
 using Statistics: mean
 using Graphs: sample!
 
@@ -126,6 +126,18 @@ function randbn(
     return x
 end
 
+"maps 1:binomial(n,2) into an upper triangle of [1,n]×[1,n]"
+function trianglemap(r, n)
+    j, i = fldmod1(r, n - 1)
+    return i ≥ j ? Edge(i + 1, j) : Edge(n - i + 1, n - j + 1)
+end
+
+"maps 1:n*(n-1) into non-diagonal elements of [1,n]×[1,n]"
+function nondiagmap(r, n)
+    i, j = fldmod1(r, n - 1)
+    return Edge(i + (i ≥ j), j)
+end
+
 """
     erdos_renyi(n, p)
 
@@ -149,7 +161,7 @@ julia> erdos_renyi(10, 0.5)
 julia> using Graphs
 
 julia> erdos_renyi(10, 0.5, is_directed=true, seed=123)
-{10, 49} directed simple Int64 graph
+{10, 41} directed simple Int64 graph
 ```
 """
 function erdos_renyi(
@@ -160,13 +172,16 @@ function erdos_renyi(
     seed::Union{Nothing,Integer}=nothing,
 )
     p >= 1 && return is_directed ? complete_digraph(n) : complete_graph(n)
-    m = is_directed ? n * (n - 1) : div(n * (n - 1), 2)
-    ne = randbn(m, p; rng=rng, seed=seed)
-    return if is_directed
-        SimpleDiGraph(n, ne; rng=rng, seed=seed)
+    m = is_directed ? n * (n - 1) : binomial(n, 2)
+    seq = randsubseq(rng_from_rng_or_seed(rng, seed), 1:m, p)
+    g = if is_directed
+        SimpleDiGraphFromIterator(nondiagmap(r, n) for r in seq)
     else
-        SimpleGraph(n, ne; rng=rng, seed=seed)
+        SimpleGraphFromIterator(trianglemap(r, n) for r in seq)
     end
+    # complete to exactly n vertices
+    add_vertices!(g, n - nv(g))
+    return g
 end
 
 """