Add Simple(Di)Graph constructors from AbstractGraph

src/SimpleGraphs/simpledigraph.jl

@@ -24,7 +24,6 @@ function SimpleDiGraph(
     return SimpleDiGraph{T}(ne, fadjlist, badjlist)
 end
 
-
 # DiGraph{UInt8}(6), DiGraph{Int16}(7), DiGraph{Int8}()
 """
     SimpleDiGraph{T}(n=0)
@@ -281,6 +280,29 @@ function SimpleDiGraph(edge_list::Vector{SimpleDiGraphEdge{T}}) where {T<:Intege
     return g
 end
 
+"""
+    SimpleDiGraph{T}(g::AbstractGraph)
+    SimpleDiGraph(g::AbstractGraph)
+
+Construct a `SimpleDiGraph` from any `AbstractGraph` by enumerating edges.
+
+If `g` is undirected, both directed edges `(u, v)` and `(v, u)` are added if undirected edge `{u, v}` exists.
+"""
+function SimpleDiGraph{T}(g::AbstractGraph) where {T}
+    eds = edges(g)
+    srcs = src.(eds)
+    dsts = dst.(eds)
+    if !is_directed(g)
+        append!(srcs, dst.(eds))
+        append!(dsts, src.(eds))
+    end
+    newg = SimpleDiGraph(Edge{T}.(srcs, dsts))
+    add_vertices!(newg, nv(g) - nv(newg))
+    return newg
+end
+
+SimpleDiGraph(g::AbstractGraph{T}) where {T} = SimpleDiGraph{T}(g)
+
 @inbounds function add_to_lists!(
     fadjlist::Vector{Vector{T}}, badjlist::Vector{Vector{T}}, s::T, d::T
 ) where {T<:Integer}

src/SimpleGraphs/simplegraph.jl

@@ -265,6 +265,25 @@ function SimpleGraph(edge_list::Vector{SimpleGraphEdge{T}}) where {T<:Integer}
     return g
 end
 
+"""
+    SimpleGraph{T}(g::AbstractGraph)
+    SimpleGraph(g::AbstractGraph)
+
+Construct a `SimpleGraph` from any `AbstractGraph` by enumerating edges.
+
+If `g` is directed, a directed edge `{u, v}` is added if either directed edge `(u, v)` or `(v, u)` exists.
+"""
+function SimpleGraph{T}(g::AbstractGraph) where {T}
+    eds = edges(g)
+    srcs = src.(eds)
+    dsts = dst.(eds)
+    newg = SimpleGraph(Edge{T}.(srcs, dsts))
+    add_vertices!(newg, nv(g) - nv(newg))
+    return newg
+end
+
+SimpleGraph(g::AbstractGraph{T}) where {T} = SimpleGraph{T}(g)
+
 @inbounds function add_to_fadjlist!(
     fadjlist::Vector{Vector{T}}, s::T, d::T
 ) where {T<:Integer}

test/simplegraphs/simplegraphs.jl

@@ -1,4 +1,5 @@
 using Random: Random
+using Graphs.Test
 
 @testset "SimpleGraphs" begin
     rng = StableRNG(1)
@@ -509,4 +510,41 @@ using Random: Random
         SimpleDiGraphFromIterator(Edge.([(1, 2), (3, 2)]))
     @test SimpleDiGraphFromIterator(Edge.([(1, 2), (2, 3)])) !=
         SimpleDiGraphFromIterator(Edge.([(1, 2), (1, 3)]))
+
+    # Tests for constructors from AbstractGraph
+
+    g = path_graph(4)
+    add_vertex!(g)
+    for h in [g, GenericGraph(g)]
+        hu = SimpleGraph(h)
+        hd = SimpleDiGraph(h)
+        @test nv(hu) == 5
+        @test ne(hu) == 3
+        @test nv(hd) == 5
+        @test ne(hd) == 6
+        @test eltype(SimpleGraph{Int32}(h)) == Int32
+        @test eltype(SimpleDiGraph{Int32}(h)) == Int32
+    end
+
+    g = path_digraph(4)
+    add_vertex!(g)
+    for h in [g, GenericDiGraph(g)]
+        hu = SimpleGraph(h)
+        hd = SimpleDiGraph(h)
+        @test nv(hu) == 5
+        @test ne(hu) == 3
+        @test nv(hd) == 5
+        @test ne(hd) == 3
+    end
+
+    g = union(path_digraph(4), reverse(path_digraph(4)))
+    add_vertex!(g)
+    for h in [g, GenericDiGraph(g)]
+        hu = SimpleGraph(h)
+        hd = SimpleDiGraph(h)
+        @test nv(hu) == 5
+        @test ne(hu) == 3
+        @test nv(hd) == 5
+        @test ne(hd) == 6
+    end
 end