initial graph properties API support

Author: nsajko

CONTRIBUTING.md

@@ -70,6 +70,8 @@ This gives users the option of reusing memory and improving performance.
 
 - If the algorithm was presented in a paper, include a reference to the paper (_e.g._, a proper academic citation along with an eprint link).
 
+- When implementing a new graph invariant, just add a method to `graph_property`, instead of exporting a new function. If necessary, also add the property to GraphProperties.jl.
+
 ## Git(Hub) usage
 
 ### Getting started on a package contribution

Project.toml

@@ -6,6 +6,7 @@ version = "1.13.1"
 ArnoldiMethod = "ec485272-7323-5ecc-a04f-4719b315124d"
 DataStructures = "864edb3b-99cc-5e75-8d2d-829cb0a9cfe8"
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
+GraphProperties = "bbdf290f-6e15-4dc1-adeb-6e1d1446781e"
 Inflate = "d25df0c9-e2be-5dd7-82c8-3ad0b3e990b9"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
@@ -18,6 +19,7 @@ Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 ArnoldiMethod = "0.4"
 Distributed = "1"
 DataStructures = "0.18, 0.19"
+GraphProperties = "1"
 Inflate = "0.1.3"
 LinearAlgebra = "1"
 Random = "1"

src/Graphs.jl

@@ -69,6 +69,7 @@ import Base:
     Pair,
     Tuple,
     zero
+using GraphProperties: GraphProperties, GraphProperty
 
 export
     # Interface
@@ -434,6 +435,9 @@ export
     # vertexcover
     vertex_cover,
 
+    # graphproperties
+    graph_property,
+
     # longestpaths
     dag_longest_path
 
@@ -548,6 +552,7 @@ include("biconnectivity/biconnect.jl")
 include("biconnectivity/bridge.jl")
 include("graphcut/normalized_cut.jl")
 include("graphcut/karger_min_cut.jl")
+include("graphproperties.jl")
 include("dominatingset/degree_dom_set.jl")
 include("dominatingset/minimal_dom_set.jl")
 include("independentset/degree_ind_set.jl")

src/graphproperties.jl

@@ -0,0 +1,71 @@
+"""
+    graph_property(graph::AbstractGraph, property_specification::GraphProperty{T}, [options = nothing])::Union{Nothing,Some{<:T}}
+
+Get the graph property specified by `property_specification` of the graph `graph`.
+
+Only some properties are implemented currently.
+
+A `nothing` return value may be returned in some cases, such as when a time limit specified in `options` was reached.
+
+Third-party packages may add methods. Only add three-argument methods, and only if you own the third argument, `options`.
+"""
+function graph_property end
+
+function graph_property(graph::AbstractGraph, prop_spec::GraphProperty)
+    graph_property(graph, prop_spec, nothing)
+end
+
+function graph_property(graph::AbstractGraph, ::GraphProperties.NumberOfVertices, ::Nothing)
+    Some(nv(graph))
+end
+
+function graph_property(graph::AbstractGraph, ::GraphProperties.DegreeSequence, ::Nothing)
+    if is_directed(graph)
+        throw(ArgumentError("expected undirected graph"))
+    end
+    Some(sort(degree(graph)))
+end
+
+function graph_property(graph::AbstractGraph, ::GraphProperties.NumberOfEdges, ::Nothing)
+    if is_directed(graph)
+        throw(ArgumentError("expected undirected graph"))
+    end
+    Some(ne(graph))
+end
+
+function graph_property(graph::AbstractGraph, ::GraphProperties.NumberOfArcs, ::Nothing)
+    if !is_directed(graph)
+        throw(ArgumentError("expected directed graph"))
+    end
+    Some(ne(graph))
+end
+
+function graph_property(
+    graph::AbstractGraph, ::GraphProperties.NumberOfConnectedComponents, ::Nothing
+)
+    if is_directed(graph)
+        throw(ArgumentError("expected undirected graph"))
+    end
+    # TODO: performance: avoid allocating the components
+    Some(length(connected_components(graph)))
+end
+
+function graph_property(
+    graph::AbstractGraph, ::GraphProperties.NumberOfWeaklyConnectedComponents, ::Nothing
+)
+    if !is_directed(graph)
+        throw(ArgumentError("expected directed graph"))
+    end
+    # TODO: performance: avoid allocating the components
+    Some(length(weakly_connected_components(graph)))
+end
+
+function graph_property(
+    graph::AbstractGraph, ::GraphProperties.NumberOfStronglyConnectedComponents, ::Nothing
+)
+    if !is_directed(graph)
+        throw(ArgumentError("expected directed graph"))
+    end
+    # TODO: performance: avoid allocating the components
+    Some(length(strongly_connected_components(graph)))
+end

test/Project.toml

@@ -6,6 +6,7 @@ DataStructures = "864edb3b-99cc-5e75-8d2d-829cb0a9cfe8"
 DelimitedFiles = "8bb1440f-4735-579b-a4ab-409b98df4dab"
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
+GraphProperties = "bbdf290f-6e15-4dc1-adeb-6e1d1446781e"
 Inflate = "d25df0c9-e2be-5dd7-82c8-3ad0b3e990b9"
 JET = "c3a54625-cd67-489e-a8e7-0a5a0ff4e31b"
 JuliaFormatter = "98e50ef6-434e-11e9-1051-2b60c6c9e899"

test/graphproperties.jl

@@ -0,0 +1,22 @@
+@testset "graph properties" begin
+    graph = ladder_graph(6)
+    @testset "properties without parameter" begin
+        properties = (
+            GraphProperties.NumberOfVertices,
+            GraphProperties.DegreeSequence,
+            GraphProperties.NumberOfEdges,
+            GraphProperties.NumberOfArcs,
+            GraphProperties.NumberOfConnectedComponents,
+            GraphProperties.NumberOfWeaklyConnectedComponents,
+            GraphProperties.NumberOfStronglyConnectedComponents,
+        )
+        for options in ((), (nothing,))
+            for property in properties
+                prop = property()
+                @test ((@inferred graph_property(graph, prop, options...)); true)
+                @test something(graph_property(graph, prop, options...)) isa
+                    graph_property_type(prop)
+            end
+        end
+    end
+end

test/runtests.jl

@@ -16,6 +16,7 @@ using Statistics: mean, std
 using StableRNGs
 using Pkg
 using Unitful
+using GraphProperties: GraphProperties, graph_property_type
 
 const testdir = dirname(@__FILE__)
 const KMf = typeof(u"1.0km")
@@ -80,6 +81,7 @@ tests = [
     "interface",
     "core",
     "operators",
+    "graphproperties",
     "degeneracy",
     "distance",
     "digraph/transitivity",