Net.jl

module NET

using Graphs: Graphs
using Graphs
using Graphs: AbstractGraphFormat

import Graphs: loadgraph, loadgraphs, savegraph

export NETFormat

struct NETFormat <: AbstractGraphFormat end
"""
    savenet(io, g, gname="g")

Write a graph `g` to an IO stream `io` in the [Pajek NET](https://docs.gephi.org/desktop/User_Manual/Import/Pajek_NET_Format)
format. Return 1 (number of graphs written).
"""
function savenet(io::IO, g::Graphs.AbstractGraph, gname::String="g")
    println(io, "*Vertices $(nv(g))")
    # write edges
    if is_directed(g)
        println(io, "*Arcs")
    else
        println(io, "*Edges")
    end
    for e in Graphs.edges(g)
        println(io, "$(src(e)) $(dst(e))")
    end
    return 1
end

"""
    loadnet(io::IO, gname="graph")

Read a graph from IO stream `io` in the [Pajek NET](https://docs.gephi.org/desktop/User_Manual/Import/Pajek_NET_Format)
format. Return the graph.
"""
function loadnet(io::IO, gname::String="graph")
    line = readline(io)
    # skip comments
    while startswith(line, "%")
        line = readline(io)
    end
    n = parse(Int, match(r"\d+", line).match)
    for ioline in eachline(io)
        line = ioline
        (occursin(r"^\*[Aa]rcs", line) || occursin(r"^\*[Ee]dges", line)) && break
    end
    if occursin(r"^\*[Aa]rcs", line)
        g = Graphs.DiGraph(n)
    else
        g = Graphs.Graph(n)
    end
    while occursin(r"^\*[Aa]rcs", line)
        for ioline in eachline(io)
            line = ioline
            ms = collect(m.match for m in eachmatch(r"\d+", line; overlap=false))
            length(ms) < 2 && break
            add_edge!(g, parse(Int, ms[1]), parse(Int, ms[2]))
        end
    end
    while occursin(r"^\*[Ee]dges", line) # add edges in both directions
        for ioline in eachline(io)
            line = ioline
            ms = collect(m.match for m in eachmatch(r"\d+", line; overlap=false))
            length(ms) < 2 && break
            i1, i2 = parse(Int, ms[1]), parse(Int, ms[2])
            add_edge!(g, i1, i2)
            add_edge!(g, i2, i1)
        end
    end
    return g
end

loadgraph(io::IO, gname::String, ::NETFormat) = loadnet(io, gname)
loadgraphs(io::IO, ::NETFormat) = Dict("graph" => loadnet(io, "graph"))
savegraph(io::IO, g::AbstractGraph, gname::String, ::NETFormat) = savenet(io, g, gname)

end # module