feat: Add discovery stubs and documentation for IGraphs.jl integration (#446)

CHANGELOG.md

@@ -4,6 +4,7 @@ We follow SemVer as most of the Julia ecosystem. Below you might see the "breaki
 
 ## unreleased
 - `is_articulation(g, v)` for checking whether a single vertex is an articulation point
+- Added documentation stubs for `community_leiden`, `modularity_matrix`, `sir_model`, `layout_kamada_kawai`, and `layout_fruchterman_reingold` pointing users to `IGraphs.jl`.
 
 
 ## v1.14.0 - 2026-02-26

docs/src/advanced/experimental.md

@@ -36,4 +36,16 @@ Pages = [
     "Experimental/Traversals/Traversals.jl",
 ]
 
+```
+
+## External Algorithm Stubs
+
+The following algorithms are not implemented natively in Graphs.jl but define documentation stubs guiding users to external wrapper packages like `IGraphs.jl`.
+
+```@docs
+Graphs.community_leiden
+Graphs.modularity_matrix
+Graphs.sir_model
+Graphs.layout_kamada_kawai
+Graphs.layout_fruchterman_reingold
 ```

docs/src/ecosystem/graphalgorithms.md

@@ -38,6 +38,21 @@ true
 
 Here, dispatching via `NautyAlg()` implicitly converts `g` to a _nauty_-compatible format and uses _nauty_ for the isomorphism computation.
 
+Similarly, _IGraphs.jl_ provides a backend for algorithms like `pagerank` and `betweenness_centrality`. You can dispatch to the C implementation by passing `IGraphs.IGraphAlgorithm()`:
+
+```julia
+julia> using Graphs, IGraphs
+
+julia> g = star_graph(5)
+{5, 4} undirected simple Int64 graph
+
+julia> pagerank(g, IGraphs.IGraphAlgorithm())
+5-element Vector{Float64}:
+...
+```
+
+To quickly convert a _Graphs.jl_ graph to an _IGraphs.jl_ structure, you can use `IGraphs.igraph(g)`.
+
 ### Functions extended by IGraphs.jl
 
 A list of functions extended by _IGraphs.jl_ can be obtained with
@@ -47,6 +62,11 @@ import IGraphs
 IGraphs.igraphalg_methods()
 ```
 
+### Exclusive algorithms provided by IGraphs.jl
+
+_IGraphs.jl_ also provides algorithms that are not natively available in _Graphs.jl_ (e.g., `community_leiden`, `sir_model`, `modularity_matrix`, and various layout algorithms such as `layout_kamada_kawai`). Users looking for these functionalities should use `IGraphs.jl` directly.
+
+
 ### Functions extended by NautyGraphs.jl
 
 A list of functions extended by _NautyGraphs.jl_ can be obtained with

src/Graphs.jl

@@ -200,6 +200,7 @@ export
     # diffusion
     diffusion,
     diffusion_rate,
+    sir_model,
 
     # eulerian
     eulerian,
@@ -320,6 +321,8 @@ export
 
     # community
     modularity,
+    modularity_matrix,
+    community_leiden,
     core_periphery_deg,
     local_clustering,
     local_clustering_coefficient,
@@ -454,7 +457,11 @@ export
 
     # planarity
     is_planar,
-    planar_maximally_filtered_graph
+    planar_maximally_filtered_graph,
+
+    # Layouts
+    layout_kamada_kawai,
+    layout_fruchterman_reingold
 """
     Graphs
 
@@ -478,6 +485,7 @@ and tutorials are available at the
 """
 Graphs
 include("interface.jl")
+include("igraphs_stubs.jl")
 include("utils.jl")
 include("frozenvector.jl")
 include("deprecations.jl")

src/igraphs_stubs.jl

@@ -0,0 +1,64 @@
+"""
+    community_leiden(g::AbstractGraph; resolution=1.0, beta=0.01)
+
+Find communities in a graph using the Leiden algorithm.
+This algorithm is not implemented natively in Graphs.jl.
+Please load `IGraphs.jl` to use this function.
+"""
+function community_leiden(g::AbstractGraph; kwargs...)
+    error(
+        "community_leiden is not implemented natively in Graphs.jl. Please use IGraphs.jl: `community_leiden(g; kwargs...)`",
+    )
+end
+
+"""
+    modularity_matrix(g::AbstractGraph)
+
+Calculate the modularity matrix of a graph.
+This algorithm is not implemented natively in Graphs.jl.
+Please load `IGraphs.jl` to use this function.
+"""
+function modularity_matrix(g::AbstractGraph; kwargs...)
+    error(
+        "modularity_matrix is not implemented natively in Graphs.jl. Please use IGraphs.jl: `modularity_matrix(g; kwargs...)`",
+    )
+end
+
+"""
+    sir_model(g::AbstractGraph, beta, gamma; no_sim=100)
+
+Simulate a SIR (Susceptible-Infected-Recovered) model on a graph.
+This algorithm is not implemented natively in Graphs.jl.
+Please load `IGraphs.jl` to use this function.
+"""
+function sir_model(g::AbstractGraph, beta, gamma; kwargs...)
+    error(
+        "sir_model is not implemented natively in Graphs.jl. Please use IGraphs.jl: `sir_model(g, beta, gamma; kwargs...)`",
+    )
+end
+
+"""
+    layout_kamada_kawai(g::AbstractGraph)
+
+Calculate a graph layout using the Kamada-Kawai algorithm.
+This algorithm is not implemented natively in Graphs.jl.
+Please load `IGraphs.jl` to use this function.
+"""
+function layout_kamada_kawai(g::AbstractGraph; kwargs...)
+    error(
+        "layout_kamada_kawai is not implemented natively in Graphs.jl. Please use IGraphs.jl: `layout_kamada_kawai(g; kwargs...)`",
+    )
+end
+
+"""
+    layout_fruchterman_reingold(g::AbstractGraph)
+
+Calculate a graph layout using the Fruchterman-Reingold algorithm.
+This algorithm is not implemented natively in Graphs.jl.
+Please load `IGraphs.jl` to use this function.
+"""
+function layout_fruchterman_reingold(g::AbstractGraph; kwargs...)
+    error(
+        "layout_fruchterman_reingold is not implemented natively in Graphs.jl. Please use IGraphs.jl: `layout_fruchterman_reingold(g; kwargs...)`",
+    )
+end