Scotch

Scotch is the native source-graph file format of the Scotch software package for static mapping, graph and mesh partitioning, and sparse-matrix reordering. Scotch is developed by François Pellegrini and collaborators within the TADaaM team at Inria Bordeaux and LaBRI (Laboratoire Bordelais de Recherche en Informatique), and is distributed under the CeCILL-C license. The format is used to feed graphs into Scotch's command-line tools (such as gpart, gmap and gord) and, through its libScotch API, into scientific-computing codes that need to distribute unstructured meshes and matrices across processors or to reduce fill-in during sparse factorization. Files conventionally carry the ".grf" (or ".src") extension for the graph itself, with companion geometry stored separately in ".xyz" files.

A source graph file is plain ASCII text describing an undirected graph by adjacency. A version-number line is followed by a line giving the number of vertices (vertnbr) and the number of arcs (edgenbr), where the arc count is twice the number of edges because each edge is listed from both endpoints. A third line holds the base index (baseval, typically 0 for graphs written by C code and 1 for Fortran) and a three-digit numeric flag: a non-zero units digit signals that per-vertex weights (loads) are present, the tens digit signals per-edge weights, and the hundreds digit signals explicit vertex labels. The body then contains one record per vertex: an optional load, an optional label, the vertex degree, and the list of neighbor indices, each optionally preceded by its edge load. Because the graph is symmetric and stored twice, the format targets partitioning and ordering rather than directed-graph interchange.

Within the format-conversion landscape Scotch occupies the same niche as other numeric partitioning formats and is frequently discussed alongside METIS, Chaco and DIMACS; the METIS and Scotch ecosystems each provide converters that translate between the two, since many finite-element and linear-algebra codes support both. Scotch also exposes a subset of the METIS API for drop-in replacement. It is largely disjoint from the graph-visualization and semantic-web world, so it shares little with formats such as GraphML, GML or DOT beyond being a graph description.

The format's strengths are compactness, streaming-friendly line orientation, and a direct mapping to compressed-sparse adjacency structures used internally, making very large graphs (billions of vertices via the MPI-based PT-Scotch variant) practical. Its limitations are corresponding: it carries no arbitrary attributes, no styling or coordinates (geometry is offloaded to a separate file), no directed-edge or hyperedge semantics beyond the mesh extension, and its terse positional layout is easy to misread without the flag conventions, so it is best treated as a computational input format rather than a general graph interchange or archival format.