Finding all the elementary circuits of a directed graph
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Algorithm by R. Tarjan


    Enumeration of the elementary circuits of a directed graph
    R. Tarjan, SIAM Journal on Computing, 2 (1973), pp. 211-216
    http://dx.doi.org/10.1137/0202017

Usage
-----

    echo "0 1\n0 2\n1 0\n1 3\n2 0\n3 0\n3 1\n3 2" | python cycles.py

First argument is the number of vertices. Ordered pairs of space separated
vertices are given via standard input and make up the directed edges of the
graph.

DOT file input
--------------

For simplicity, there is no DOT file parser included but the following allows
to create a suitable argument string and standard input for simple DOT graphs.

Given a DOT file of a simple (no labels, colors, styles, only pairs of
vertices...) directed graph, the following lines generate the number of
vertices as well as the edge list expected on standard input.

        sed -n -e '/^\s*[0-9]\+;$/p' graph.dot | wc -l
        sed -n -e 's/^\s*\([0-9]\) -> \([0-9]\);$/\1 \2/p' graph.dot

The above lines work on DOT files like the following:

    digraph G {
      0;
      1;
      2;
      0 -> 1;
      0 -> 2;
      1 -> 0;
      2 -> 0;
      2 -> 1;
      }

They would produce the following output:

    3
    0 1
    0 2
    1 0
    2 0
    2 1