LEMON

LEMON Graph Format (LGF, file extension .lgf) is the native, human-readable serialization used by the LEMON C++ graph library. LEMON — "Library for Efficient Modeling and Optimization in Networks" — is an open-source, template-based C++ library developed by the MTA-ELTE Egerváry Research Group on Combinatorial Optimization (EGRES) at Eötvös Loránd University in Budapest, and distributed as part of the COIN-OR project under the permissive Boost Software License. The library targets combinatorial optimization on graphs and networks (shortest paths, maximum and minimum-cost flow, minimum cut, matching, spanning trees, plus LP/MIP solver interfaces), and LGF is the format its DigraphReader/DigraphWriter and GraphReader/GraphWriter classes read and write.

Structurally, LGF is a plain-text, column-oriented format organized into named sections whose header lines begin with an "@" character. The standard section types are @nodes, @arcs, @edges, and @attributes; each may carry an optional name so that several sections of the same type can coexist in one file. Within a section, each line is a row of whitespace-separated tokens, and each column corresponds to a graph "map" (an attribute keyed by node, arc, or edge). A token is either plain (a run of non-whitespace characters) or quoted with double quotes, in which case it may contain spaces and escape sequences. In the @nodes section a special "label" map holds unique node identifiers; @arcs (directed) and @edges (undirected) rows begin with a source and target token referring to those labels, followed by the arc/edge map values. The @attributes section stores standalone key/value pairs, and lines beginning with "#" are comments.

Because columns map directly onto typed C++ maps, LGF round-trips a graph together with arbitrary node/arc weights, costs, capacities, and coordinates in a single file, which makes it convenient for feeding optimization instances into LEMON algorithms. Conceptually it sits alongside other lightweight, application-tied edge/attribute formats such as LEDA, Pajek, DIMACS, and TGF, and contrasts with verbose, tool-agnostic XML formats like GraphML or GEXF.

Its main limitation is ecosystem scope: LGF is defined by, and primarily consumed by, the LEMON library (LEMON), with little native support in general-purpose graph tools such as NetworkX, igraph, or Gephi, so interchange usually requires converting to a more widely supported format. The specification is also comparatively informal, documented mainly through LEMON's tutorial and API reference rather than an independent standard.