Add VNGraphs.jl stubs for chromatic_number and edge_chromatic_number

Declares chromatic_number and edge_chromatic_number in Graphs.jl with
helpful error hints directing users to install VNGraphs.jl for the
fast C-backed very_nauty implementations.

Includes unit tests asserting the stubs throw the expected errors.

Author: mahmudsudo

src/Graphs.jl

@@ -454,7 +454,9 @@ export
 
     # planarity
     is_planar,
-    planar_maximally_filtered_graph
+    planar_maximally_filtered_graph,
+    chromatic_number,
+    edge_chromatic_number
 """
     Graphs
 
@@ -575,6 +577,7 @@ include("independentset/degree_ind_set.jl")
 include("independentset/maximal_ind_set.jl")
 include("vertexcover/degree_vertex_cover.jl")
 include("vertexcover/random_vertex_cover.jl")
+include("vngraphs_stubs.jl")
 include("Experimental/Experimental.jl")
 include("Parallel/Parallel.jl")
 include("Test/Test.jl")

src/vngraphs_stubs.jl

@@ -0,0 +1,39 @@
+# Algorithm stubs for VNGraphs.jl integration
+
+"""
+    chromatic_number(g, timeout=0)
+
+Return the chromatic number of the graph `g`.
+Note: This is a stub provided by `Graphs.jl`. For a concrete implementation,
+please use the `VNGraphs.jl` package which provides a fast C implementation
+via `very_nauty`.
+
+```julia
+using VNGraphs
+chromatic_number(g)
+```
+"""
+function chromatic_number(g::AbstractGraph, args...; kwargs...)
+    error("chromatic_number is not implemented in Graphs.jl. " *
+          "Please load VNGraphs.jl to use the very_nauty implementation.")
+end
+
+"""
+    edge_chromatic_number(g, timeout=0)
+
+Return the edge chromatic number of the graph `g`.
+Note: This is a stub provided by `Graphs.jl`. For a concrete implementation,
+please use the `VNGraphs.jl` package which provides a fast C implementation
+via `very_nauty`.
+
+```julia
+using VNGraphs
+edge_chromatic_number(g)
+```
+"""
+function edge_chromatic_number(g::AbstractGraph, args...; kwargs...)
+    error("edge_chromatic_number is not implemented in Graphs.jl. " *
+          "Please load VNGraphs.jl to use the very_nauty implementation.")
+end
+
+export chromatic_number, edge_chromatic_number

test/runtests.jl

@@ -155,6 +155,7 @@ tests = [
     "trees/prufer",
     "experimental/experimental",
     "planarity",
+    "vngraphs_stubs",
 ]
 
 @testset verbose = true "Graphs" begin

test/vngraphs_stubs.jl

@@ -0,0 +1,22 @@
+using Graphs
+using Test
+
+@testset "VNGraphs Stubs" begin
+    g = path_graph(5)
+    
+    @test_throws ErrorException chromatic_number(g)
+    @test_throws ErrorException edge_chromatic_number(g)
+    
+    # Verify the error message contains the suggestion to load VNGraphs.jl
+    try
+        chromatic_number(g)
+    catch e
+        @test contains(e.msg, "VNGraphs.jl")
+    end
+    
+    try
+        edge_chromatic_number(g)
+    catch e
+        @test contains(e.msg, "VNGraphs.jl")
+    end
+end