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Tarjan and Kosaraju

This development contains formalizations and correctness proofs, using Coq and the Mathematical Components library, of algorithms originally due to Kosaraju and Tarjan for finding strongly connected components in finite graphs. It also contains a verified implementation of topological sorting with extended guarantees for acyclic graphs.

Meta

• Author(s):
  • Cyril Cohen (initial)
  • Jean-Jacques Lévy (initial)
  • Karl Palmskog
  • Laurent Théry (initial)
• Coq-community maintainer(s):
  • Cyril Cohen (@CohenCyril)
  • Karl Palmskog (@palmskog)
• License: CeCILL-B
• Compatible Coq versions: 8.16 or later
• Additional dependencies:
  • MathComp ssreflect 2.0 or later
  • MathComp fingroup
  • Hierarchy Builder 1.4.0 or later
• Coq namespace: mathcomp.tarjan
• Related publication(s):
  • Formal Proofs of Tarjan's Strongly Connected Components Algorithm in Why3, Coq and Isabelle doi:10.4230/LIPIcs.ITP.2019.13
  • Formally-Proven Kosaraju's algorithm

Building and installation instructions

The easiest way to install the latest released version of Tarjan and Kosaraju is via OPAM:

opam repo add coq-released https://coq.inria.fr/opam/released
opam install coq-mathcomp-tarjan

To instead build and install manually, do:

git clone https://github.com/coq-community/tarjan.git
cd tarjan
make   # or make -j <number-of-cores-on-your-machine> 
make install

Main files

Extra library files

• bigmin.v: extra library to deal with \min(i in A) F i
• extra.v: naive definitions of strongly connected components and various basic extentions of mathcomp libraries on paths and fintypes
• acyclic.v: definition of acyclic graphs and proof that acyclicity can be determined by knowing strongly connected components

Proof of Kosaraju strongly connected component algorithm

• kosaraju.v: proof of topological sorting and Kosaraju connected component algorithm
• acyclic_tsorted.v: properties of topological sorting in acyclic graphs

Proofs of Tarjan strongly connected component algorithm (independent from each other)

• tarjan_rank.v: proof with rank
• tarjan_rank_bigmin.v: same proof but with a \min_ instead of multiple inequalities on the output rank
• tarjan_num.v: same proof as tarjan_rank_bigmin.v but with serial numbers instead of ranks
• tarjan_nocolor.v: new proof, with ranks and without colors, less fields in environement and less invariants, preconditions and postconditions.
• tarjan_nocolor_optim.v: same proof as tarjan_nocolor.v, but with the serial number field of the environement restored, and passing around stack extensions as sets