Add LEMON graph structures and tests

[MAVRICK-1]

Summary

This PR introduces the foundational structure for the LEMON wrapper module in Graphs.jl, addressing issue #447. This is the first of several small PRs that will incrementally build the complete LEMON wrapper functionality.

What's included

• Created src/LEMON/LEMON.jl module with basic graph types:
  • AbstractLEMONGraph abstract type
  • LEMONGraph for undirected graphs
  • LEMONDiGraph for directed graphs
• Implemented minimal required interface methods:
  • nv() - number of vertices
  • ne() - number of edges
  • vertices() - vertex iterator
  • is_directed() - graph directionality check
  • show() - display methods
• Added basic test suite in test/LEMON/lemon.jl
• Integrated LEMON module into main Graphs.jl exports

Testing

All tests pass:

julia> include("test/LEMON/lemon.jl")
Test Summary: | Pass  Total  Time
LEMON         |   16     16  0.1s

Example usage

using Graphs, Graphs.LEMON

# Create undirected LEMON graph with 5 vertices
g = LEMONGraph(5)
# Output: {5, 0} undirected LEMON graph

# Create directed LEMON graph
dg = LEMONDiGraph(10)
# Output: {10, 0} directed LEMON graph

nv(g)  # 5
ne(g)  # 0
is_directed(g)  # false
is_directed(dg) # true

Next steps

Future PRs will add:

1. Graph conversion utilities between SimpleGraph and LEMON formats
2. Complete Graph API implementation (add_vertex!, add_edge!, etc.)
3. Algorithm dispatch to LEMON implementations
4. GraphsInterfaceChecker.jl integration

Related to

• Issue A reliable idiomatic wrapper for the C++ library LEMON [$400] #447: A reliable idiomatic wrapper for the C++ library LEMON
• This work is funded by the National Science Foundation and NSF Center for Quantum Networks

Checklist

• Tests pass locally
• Code follows Julia formatting conventions
• Module is properly exported
• Basic documentation included

[MAVRICK-1]

@Krastanov can you please review this pr

[Krastanov]

Thanks for looking into this. I do not believe this is a an appropriate way to start this work. This does not have any reference to the actual c++ library "LEMON". And even if it was related to that library, it should probably be done in LEMONGraphs.jl, not in Graphs.jl.

I would suggest investigating LEMONGraphs.jl first and doing as much as possible there, as suggested in the description of the bounty.