Add missing parallel=:threads implementations

These implementations are extremely basic, but they try to follow the
patterns in the other parts of the Parallel module. My real motivation
is to be able to move the Distributed implementations into an extension,
so that Graphs.jl does not depend on Distributed.

Author: topolarity

src/Parallel/distance.jl

@@ -1,20 +1,55 @@
 # used in shortest path calculations
 
 function eccentricity(
+    g::AbstractGraph,
+    vs=vertices(g),
+    distmx::AbstractMatrix{T}=weights(g);
+    parallel::Symbol=:distributed,
+) where {T<:Number}
+    return if parallel === :threads
+        threaded_eccentricity(g, vs, distmx)
+    elseif parallel === :distributed
+        distr_eccentricity(g, vs, distmx)
+    else
+        throw(
+            ArgumentError(
+                "Unsupported parallel argument '$(repr(parallel))' (supported: ':threads' or ':distributed')",
+            ),
+        )
+    end
+end
+
+function distr_eccentricity(
     g::AbstractGraph, vs=vertices(g), distmx::AbstractMatrix{T}=weights(g)
 ) where {T<:Number}
     vlen = length(vs)
     eccs = SharedVector{T}(vlen)
     @sync @distributed for i in 1:vlen
-        eccs[i] = maximum(Graphs.dijkstra_shortest_paths(g, vs[i], distmx).dists)
+        local d = Graphs.dijkstra_shortest_paths(g, vs[i], distmx)
+        eccs[i] = maximum(d.dists)
     end
     d = sdata(eccs)
     maximum(d) == typemax(T) && @warn("Infinite path length detected")
     return d
 end
 
-function eccentricity(g::AbstractGraph, distmx::AbstractMatrix)
-    return eccentricity(g, vertices(g), distmx)
+function threaded_eccentricity(
+    g::AbstractGraph, vs=vertices(g), distmx::AbstractMatrix{T}=weights(g)
+) where {T<:Number}
+    vlen = length(vs)
+    eccs = Vector{T}(undef, vlen)
+    Base.Threads.@threads for i in 1:vlen
+        d = Graphs.dijkstra_shortest_paths(g, vs[i], distmx)
+        eccs[i] = maximum(d.dists)
+    end
+    maximum(eccs) == typemax(T) && @warn("Infinite path length detected")
+    return eccs
+end
+
+function eccentricity(
+    g::AbstractGraph, distmx::AbstractMatrix; parallel::Symbol=:distributed
+)
+    return eccentricity(g, vertices(g), distmx; parallel)
 end
 
 function diameter(g::AbstractGraph, distmx::AbstractMatrix=weights(g))

src/Parallel/shortestpaths/dijkstra.jl

@@ -9,14 +9,54 @@ struct MultipleDijkstraState{T<:Number,U<:Integer} <: AbstractPathState
 end
 
 """
-    Parallel.dijkstra_shortest_paths(g, sources=vertices(g), distmx=weights(g))
+    Parallel.dijkstra_shortest_paths(g, sources=vertices(g), distmx=weights(g), parallel=:distributed)
 
 Compute the shortest paths between all pairs of vertices in graph `g` by running
 [`dijkstra_shortest_paths`] for every vertex and using an optional list of source vertex `sources` and
 an optional distance matrix `distmx`. Return a [`Parallel.MultipleDijkstraState`](@ref) with relevant
-traversal information.
+traversal information. The `parallel` argument can be set to `:threads` or `:distributed` for multi-
+threaded or multi-process parallelism, respectively.
 """
 function dijkstra_shortest_paths(
+    g::AbstractGraph{U},
+    sources=vertices(g),
+    distmx::AbstractMatrix{T}=weights(g);
+    parallel::Symbol=:distributed,
+) where {T<:Number} where {U}
+    return if parallel === :threads
+        threaded_dijkstra_shortest_paths(g, sources, distmx)
+    elseif parallel === :distributed
+        distr_dijkstra_shortest_paths(g, sources, distmx)
+    else
+        throw(
+            ArgumentError(
+                "Unsupported parallel argument '$(repr(parallel))' (supported: ':threads' or ':distributed')",
+            ),
+        )
+    end
+end
+
+function threaded_dijkstra_shortest_paths(
+    g::AbstractGraph{U}, sources=vertices(g), distmx::AbstractMatrix{T}=weights(g)
+) where {T<:Number} where {U}
+    n_v = nv(g)
+    r_v = length(sources)
+
+    # TODO: remove `Int` once julialang/#23029 / #23032 are resolved
+    dists = Matrix{T}(undef, Int(r_v), Int(n_v))
+    parents = Matrix{U}(undef, Int(r_v), Int(n_v))
+
+    Base.Threads.@threads for i in 1:r_v
+        state = Graphs.dijkstra_shortest_paths(g, sources[i], distmx)
+        dists[i, :] = state.dists
+        parents[i, :] = state.parents
+    end
+
+    result = MultipleDijkstraState(dists, parents)
+    return result
+end
+
+function distr_dijkstra_shortest_paths(
     g::AbstractGraph{U}, sources=vertices(g), distmx::AbstractMatrix{T}=weights(g)
 ) where {T<:Number} where {U}
     n_v = nv(g)

src/Parallel/traversals/greedy_color.jl

@@ -1,4 +1,31 @@
-function random_greedy_color(g::AbstractGraph{T}, reps::Integer) where {T<:Integer}
+function random_greedy_color(
+    g::AbstractGraph{T}, reps::Integer; parallel::Symbol=:distributed
+) where {T<:Integer}
+    return if parallel === :threads
+        threaded_random_greedy_color(g, reps)
+    elseif parallel === :distributed
+        distr_random_greedy_color(g, reps)
+    else
+        throw(
+            ArgumentError(
+                "Unsupported parallel argument '$(repr(parallel))' (supported: ':threads' or ':distributed')",
+            ),
+        )
+    end
+end
+
+function threaded_random_greedy_color(g::AbstractGraph{T}, reps::Integer) where {T<:Integer}
+    local_best = Vector{Graphs.Coloring{T}}(undef, reps)
+    Base.Threads.@threads for i in 1:reps
+        seq = shuffle(vertices(g))
+        local_best[i] = Graphs.perm_greedy_color(g, seq)
+    end
+    best = reduce(Graphs.best_color, local_best)
+
+    return convert(Graphs.Coloring{T}, best)
+end
+
+function distr_random_greedy_color(g::AbstractGraph{T}, reps::Integer) where {T<:Integer}
     best = @distributed (Graphs.best_color) for i in 1:reps
         seq = shuffle(vertices(g))
         Graphs.perm_greedy_color(g, seq)
@@ -8,11 +35,14 @@ function random_greedy_color(g::AbstractGraph{T}, reps::Integer) where {T<:Integ
 end
 
 function greedy_color(
-    g::AbstractGraph{U}; sort_degree::Bool=false, reps::Integer=1
+    g::AbstractGraph{U};
+    sort_degree::Bool=false,
+    reps::Integer=1,
+    parallel::Symbol=:distributed,
 ) where {U<:Integer}
     return if sort_degree
         Graphs.degree_greedy_color(g)
     else
-        Parallel.random_greedy_color(g, reps)
+        Parallel.random_greedy_color(g, reps; parallel)
     end
 end

test/parallel/distance.jl

@@ -1,4 +1,4 @@
-@testset "Parallel.Distance" begin
+@testset "Parallel.Distance" for parallel in [:threads, :distributed]
     g4 = path_digraph(5)
     adjmx1 = [0 1 0; 1 0 1; 0 1 0] # graph
     adjmx2 = [0 1 0; 1 0 1; 1 1 0] # digraph
@@ -9,7 +9,7 @@
 
     for g in testgraphs(a1)
         z = @inferred(Graphs.eccentricity(g, distmx1))
-        y = @inferred(Parallel.eccentricity(g, distmx1))
+        y = @inferred(Parallel.eccentricity(g, distmx1; parallel))
         @test isapprox(y, z)
         @test @inferred(Graphs.diameter(y)) ==
             @inferred(Parallel.diameter(g, distmx1)) ==
@@ -21,9 +21,15 @@
         @test @inferred(Graphs.center(y)) == @inferred(Parallel.center(g, distmx1)) == [2]
     end
 
+    let g = testgraphs(a1)[1]
+        # An error should be reported if the parallel mode could not be understood
+        @test_throws ArgumentError Parallel.eccentricity(g, distmx1; parallel=:thread)
+        @test_throws ArgumentError Parallel.eccentricity(g, distmx1; parallel=:distriibuted)
+    end
+
     for g in testdigraphs(a2)
         z = @inferred(Graphs.eccentricity(g, distmx2))
-        y = @inferred(Parallel.eccentricity(g, distmx2))
+        y = @inferred(Parallel.eccentricity(g, distmx2; parallel))
         @test isapprox(y, z)
         @test @inferred(Graphs.diameter(y)) ==
             @inferred(Parallel.diameter(g, distmx2)) ==

test/parallel/shortestpaths/dijkstra.jl

@@ -1,4 +1,4 @@
-@testset "Parallel.Dijkstra" begin
+@testset "Parallel.Dijkstra" for parallel in [:threads, :distributed]
     g4 = path_digraph(5)
     d1 = float([0 1 2 3 4; 5 0 6 7 8; 9 10 0 11 12; 13 14 15 0 16; 17 18 19 20 0])
     d2 = sparse(float([0 1 2 3 4; 5 0 6 7 8; 9 10 0 11 12; 13 14 15 0 16; 17 18 19 20 0]))
@@ -8,7 +8,7 @@
 
     for g in testgraphs(g3)
         z = floyd_warshall_shortest_paths(g, d)
-        zp = @inferred(Parallel.dijkstra_shortest_paths(g, collect(1:5), d))
+        zp = @inferred(Parallel.dijkstra_shortest_paths(g, collect(1:5), d; parallel))
         @test all(isapprox(z.dists, zp.dists))
 
         for i in 1:5
@@ -21,7 +21,7 @@
         end
 
         z = floyd_warshall_shortest_paths(g)
-        zp = @inferred(Parallel.dijkstra_shortest_paths(g))
+        zp = @inferred(Parallel.dijkstra_shortest_paths(g; parallel))
         @test all(isapprox(z.dists, zp.dists))
 
         for i in 1:5
@@ -34,7 +34,7 @@
         end
 
         z = floyd_warshall_shortest_paths(g)
-        zp = @inferred(Parallel.dijkstra_shortest_paths(g, [1, 2]))
+        zp = @inferred(Parallel.dijkstra_shortest_paths(g, [1, 2]; parallel))
         @test all(isapprox(z.dists[1:2, :], zp.dists))
 
         for i in 1:2
@@ -51,9 +51,17 @@
     g3 = path_digraph(5)
     d = float([0 1 2 3 4; 5 0 6 7 8; 9 10 0 11 12; 13 14 15 0 16; 17 18 19 20 0])
 
+    # An error should be reported if the parallel mode could not be understood
+    @test_throws ArgumentError Parallel.dijkstra_shortest_paths(
+        testdigraphs(g3)[1], collect(1:5), d; parallel=:thread
+    )
+    @test_throws ArgumentError Parallel.dijkstra_shortest_paths(
+        testdigraphs(g3)[1], collect(1:5), d; parallel=:distriibuted
+    )
+
     for g in testdigraphs(g3)
         z = floyd_warshall_shortest_paths(g, d)
-        zp = @inferred(Parallel.dijkstra_shortest_paths(g, collect(1:5), d))
+        zp = @inferred(Parallel.dijkstra_shortest_paths(g, collect(1:5), d; parallel))
         @test all(isapprox(z.dists, zp.dists))
 
         for i in 1:5
@@ -66,7 +74,7 @@
         end
 
         z = floyd_warshall_shortest_paths(g)
-        zp = @inferred(Parallel.dijkstra_shortest_paths(g))
+        zp = @inferred(Parallel.dijkstra_shortest_paths(g; parallel))
         @test all(isapprox(z.dists, zp.dists))
 
         for i in 1:5
@@ -79,7 +87,7 @@
         end
 
         z = floyd_warshall_shortest_paths(g)
-        zp = @inferred(Parallel.dijkstra_shortest_paths(g, [1, 2]))
+        zp = @inferred(Parallel.dijkstra_shortest_paths(g, [1, 2]; parallel))
         @test all(isapprox(z.dists[1:2, :], zp.dists))
 
         for i in 1:2

test/parallel/traversals/greedy_color.jl

@@ -1,19 +1,31 @@
-@testset "Parallel.Greedy Coloring" begin
+@testset "Parallel.Greedy Coloring" for parallel in [:threads, :distributed]
     g3 = star_graph(10)
     for g in testgraphs(g3)
         for op_sort in (true, false)
-            C = @inferred(Parallel.greedy_color(g, reps=5, sort_degree=op_sort))
+            C = @inferred(Parallel.greedy_color(g; reps=5, sort_degree=op_sort, parallel))
             @test C.num_colors == 2
         end
     end
 
     g4 = path_graph(20)
     g5 = complete_graph(20)
 
+    let g = testgraphs(g4)[1]
+        # An error should be reported if the parallel mode could not be understood
+        @test_throws ArgumentError Parallel.greedy_color(
+            g; reps=5, sort_degree=false, parallel=:thread
+        )
+        @test_throws ArgumentError Parallel.greedy_color(
+            g; reps=5, sort_degree=false, parallel=:distriibuted
+        )
+    end
+
     for graph in [g4, g5]
         for g in testgraphs(graph)
             for op_sort in (true, false)
-                C = @inferred(Parallel.greedy_color(g, reps=5, sort_degree=op_sort))
+                C = @inferred(
+                    Parallel.greedy_color(g; reps=5, sort_degree=op_sort, parallel)
+                )
 
                 @test C.num_colors <= maximum(degree(g)) + 1
                 correct = true