Add stubs

README.md

@@ -90,3 +90,18 @@ _LightGraphs.jl_ and _Graphs.jl_ are functionally identical, still there are som
 - There was also an older package also called _Graphs.jl_ (git tags `v0.2.5` through `v0.10.3`), but the current code base here is a fork of _LightGraphs.jl_ v1.3.5.
 - All older _LightGraphs.jl_ versions are tagged using the naming scheme `lg-vX.Y.Z` rather than plain `vX.Y.Z`, which is used for old _Graphs.jl_ versions (≤ v0.10) and newer versions derived from _LightGraphs.jl_ but released with the _Graphs.jl_ name (≥ v1.4).
 - If you are using a version of Julia prior to 1.x, then you should use _LightGraphs.jl_ at `lg-v.12.*` or _Graphs.jl_ at `v0.10.3`
+
+## Graphs.jl + igraph support
+
+Graphs.jl now works with the igraph C library through the IGraphs.jl package.
+Just load the compatibility layer:
+
+```julia
+using Graphs, IGraphs
+using IGraphs.GraphsCompat  # enable Graphs.jl interface for igraph
+
+g = Graphs.grid([5,5])                # Graphs.jl graph
+ig = GraphsCompat.IGSimpleGraph(g)    # igraph-backed graph
+
+Graphs.nv(ig)                         # number of vertices
+Graphs.connected_components(ig)      # runs igraph’s fast implementation

src/Graphs.jl

@@ -463,7 +463,7 @@ include("interface.jl")
 include("utils.jl")
 include("deprecations.jl")
 include("core.jl")
-
+include("igraph_stubs.jl")
 include("SimpleGraphs/SimpleGraphs.jl")
 using .SimpleGraphs
 """

src/igraph_stubs.jl

@@ -0,0 +1,332 @@
+#### Optional APIs backed by external packages (e.g., IGraphs.jl)
+#### These functions are PUBLIC names that Graphs.jl exposes, but by default
+#### they throw a helpful error until a backend (IGraphs.jl) provides methods.
+
+# Friendly message builder
+_igraph_backend_msg(fname) = """
+`$(fname)` is not implemented in base Graphs.jl.
+It is available via the **IGraphs.jl** backend.
+
+Enable it and re-run:
+
+    using IGraphs, IGraphs.GraphsCompat
+    $(fname)(g, IGraphAlgorithm())
+
+"""
+
+# -----------------------
+# Assortativity
+# -----------------------
+@static if !isdefined(@__MODULE__, :assortativity)
+"""
+    assortativity(g::AbstractGraph, attr; normalized::Bool=true)
+
+Degree- or attribute-based assortativity coefficient.
+
+!!! note "Availability"
+    Backend-only: use **IGraphs.jl** (see message if not loaded).
+"""
+function assortativity end
+assortativity(g::AbstractGraph, attr; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:assortativity)))
+end
+
+@static if !isdefined(@__MODULE__, :assortativity_degree)
+"""
+    assortativity_degree(g::AbstractGraph; normalized::Bool=true)
+
+Degree assortativity (scalar coefficient).
+"""
+function assortativity_degree end
+assortativity_degree(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:assortativity_degree)))
+end
+
+@static if !isdefined(@__MODULE__, :assortativity_nominal)
+"""
+    assortativity_nominal(g::AbstractGraph, attr)
+
+Nominal/categorical assortativity based on a vertex attribute.
+"""
+function assortativity_nominal end
+assortativity_nominal(g::AbstractGraph, attr; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:assortativity_nominal)))
+end
+
+# -----------------------
+# Coreness / k-core number
+# -----------------------
+@static if !isdefined(@__MODULE__, :coreness)
+"""
+    coreness(g::AbstractGraph)
+
+Return the k-core number (coreness) per vertex.
+"""
+function coreness end
+coreness(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:coreness)))
+end
+
+# -----------------------
+# Connectivity & disjoint paths
+# -----------------------
+@static if !isdefined(@__MODULE__, :edge_connectivity)
+"""
+    edge_connectivity(g::AbstractGraph)
+
+Global edge connectivity (minimum edge cut size).
+"""
+function edge_connectivity end
+edge_connectivity(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:edge_connectivity)))
+end
+
+@static if !isdefined(@__MODULE__, :vertex_connectivity)
+"""
+    vertex_connectivity(g::AbstractGraph)
+
+Global vertex connectivity (minimum vertex cut size).
+"""
+function vertex_connectivity end
+vertex_connectivity(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:vertex_connectivity)))
+end
+
+@static if !isdefined(@__MODULE__, :edge_disjoint_paths)
+"""
+    edge_disjoint_paths(g::AbstractGraph, s::Integer, t::Integer)
+
+Maximum number of edge-disjoint s–t paths.
+"""
+function edge_disjoint_paths end
+edge_disjoint_paths(g::AbstractGraph, s::Integer, t::Integer; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:edge_disjoint_paths)))
+end
+
+@static if !isdefined(@__MODULE__, :vertex_disjoint_paths)
+"""
+    vertex_disjoint_paths(g::AbstractGraph, s::Integer, t::Integer)
+
+Maximum number of vertex-disjoint s–t paths.
+"""
+function vertex_disjoint_paths end
+vertex_disjoint_paths(g::AbstractGraph, s::Integer, t::Integer; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:vertex_disjoint_paths)))
+end
+
+# -----------------------
+# Cuts & cut trees
+# -----------------------
+@static if !isdefined(@__MODULE__, :gomory_hu_tree)
+"""
+    gomory_hu_tree(g::AbstractGraph; capacity=:weight)
+
+Compute the Gomory–Hu cut tree (all-pairs min-cuts).
+"""
+function gomory_hu_tree end
+gomory_hu_tree(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:gomory_hu_tree)))
+end
+
+@static if !isdefined(@__MODULE__, :minimum_size_separators)
+"""
+    minimum_size_separators(g::AbstractGraph)
+
+Enumerate minimum cardinality vertex separators.
+"""
+function minimum_size_separators end
+minimum_size_separators(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:minimum_size_separators)))
+end
+
+# -----------------------
+# Isomorphism, VF2, automorphisms
+# -----------------------
+@static if !isdefined(@__MODULE__, :isomorphic)
+"""
+    isomorphic(g1::AbstractGraph, g2::AbstractGraph)
+
+Graph isomorphism test (e.g., VF2 backend).
+"""
+function isomorphic end
+isomorphic(g1::AbstractGraph, g2::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:isomorphic)))
+end
+
+@static if !isdefined(@__MODULE__, :subgraph_isomorphisms_vf2)
+"""
+    subgraph_isomorphisms_vf2(pat::AbstractGraph, g::AbstractGraph)
+
+Find (all) VF2 subgraph isomorphisms of `pat` in `g`.
+"""
+function subgraph_isomorphisms_vf2 end
+subgraph_isomorphisms_vf2(pat::AbstractGraph, g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:subgraph_isomorphisms_vf2)))
+end
+
+@static if !isdefined(@__MODULE__, :count_automorphisms)
+"""
+    count_automorphisms(g::AbstractGraph)
+
+Count graph automorphisms (backend dependent).
+"""
+function count_automorphisms end
+count_automorphisms(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:count_automorphisms)))
+end
+
+# -----------------------
+# Graphlets & motifs
+# -----------------------
+@static if !isdefined(@__MODULE__, :graphlets)
+"""
+    graphlets(g::AbstractGraph; k=5)
+
+Graphlet counts / signatures up to size `k`.
+"""
+function graphlets end
+graphlets(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:graphlets)))
+end
+
+@static if !isdefined(@__MODULE__, :motifs)
+"""
+    motifs(g::AbstractGraph; size)
+
+Motif (small subgraph) counts for a given size.
+"""
+function motifs end
+motifs(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:motifs)))
+end
+
+# -----------------------
+# Line graph, spectral embeddings, modularity
+# -----------------------
+@static if !isdefined(@__MODULE__, :linegraph)
+"""
+    linegraph(g::AbstractGraph)
+
+Construct the line graph of `g`.
+"""
+function linegraph end
+linegraph(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:linegraph)))
+end
+
+@static if !isdefined(@__MODULE__, :adjacency_spectral_embedding)
+"""
+    adjacency_spectral_embedding(g::AbstractGraph; nev, which=:LM)
+
+Compute an adjacency spectral embedding (ASE).
+"""
+function adjacency_spectral_embedding end
+adjacency_spectral_embedding(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:adjacency_spectral_embedding)))
+end
+
+@static if !isdefined(@__MODULE__, :modularity_matrix)
+"""
+    modularity_matrix(g::AbstractGraph; weights=:weight)
+
+Return the modularity matrix (backend dependent).
+"""
+function modularity_matrix end
+modularity_matrix(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:modularity_matrix)))
+end
+
+# -----------------------
+# Clustering (transitivity) & strength
+# -----------------------
+@static if !isdefined(@__MODULE__, :transitivity_avglocal_undirected)
+"""
+    transitivity_avglocal_undirected(g::AbstractGraph)
+
+Average local clustering coefficient (undirected).
+"""
+function transitivity_avglocal_undirected end
+transitivity_avglocal_undirected(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:transitivity_avglocal_undirected)))
+end
+
+@static if !isdefined(@__MODULE__, :transitivity_local_undirected)
+"""
+    transitivity_local_undirected(g::AbstractGraph)
+
+Local clustering coefficients per vertex (undirected).
+"""
+function transitivity_local_undirected end
+transitivity_local_undirected(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:transitivity_local_undirected)))
+end
+
+@static if !isdefined(@__MODULE__, :strength)
+"""
+    strength(g::AbstractGraph; mode=:out, weights=:weight)
+
+Weighted degree ("strength") per vertex.
+"""
+function strength end
+strength(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:strength)))
+end
+
+# -----------------------
+# Independent sets, neighborhood graphs
+# -----------------------
+@static if !isdefined(@__MODULE__, :independence_number)
+"""
+    independence_number(g::AbstractGraph)
+
+Size of a maximum independent set.
+"""
+function independence_number end
+independence_number(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:independence_number)))
+end
+
+@static if !isdefined(@__MODULE__, :independent_vertex_sets)
+"""
+    independent_vertex_sets(g::AbstractGraph)
+
+Enumerate maximal (or all) independent sets (backend dependent).
+"""
+function independent_vertex_sets end
+independent_vertex_sets(g::AbstractGraph; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:independent_vertex_sets)))
+end
+
+@static if !isdefined(@__MODULE__, :neighborhood_graphs)
+"""
+    neighborhood_graphs(g::AbstractGraph, vset; order=1, mode=:all)
+
+Induced neighborhood graphs around `vset`.
+"""
+function neighborhood_graphs end
+neighborhood_graphs(g::AbstractGraph, vset; kwargs...) =
+    throw(ArgumentError(_igraph_backend_msg(:neighborhood_graphs)))
+end
+
+# -----------------------
+# Layouts (visualization helpers)
+# (Backends often provide: circle, kk, fr, drl, graphopt, grid, lgl, mds, random, reingold_tilford, sugiyama)
+# -----------------------
+for fname in (
+    :layout_circle, :layout_kamada_kawai, :layout_fruchterman_reingold, :layout_drl,
+    :layout_graphopt, :layout_grid, :layout_lgl, :layout_mds,
+    :layout_random, :layout_reingold_tilford, :layout_sugiyama
+)
+    @eval begin
+        @static if !isdefined(@__MODULE__, $fname)
+            """
+                $(Symbol($fname))(g::AbstractGraph; kwargs...)
+
+            Compute a 2D/3D layout embedding (backend dependent).
+            """
+            function $(Symbol(fname)) end
+            $(Symbol(fname))(g::AbstractGraph; kwargs...) =
+                throw(ArgumentError(_igraph_backend_msg($fname)))
+        end
+    end
+end