Infinite loop in TwoOptHeuristicTSP.

 * JGraphT version: 1.3.1
 * Java version (java -version)/platform: 1.8 but should be any version 

In some the cases TwoOptHeuristicTSP fails to produce a tour but enters an infinite loop and consumes 100% CPU

Unable to reproduce reliably

Have a loop guard (max iterations) or detect duplicate attempted solutions

My math knowledge is insufficient to fully grasp this code. I solved it for my purposes by placing an iteration guard around the loop but there exists probably a better solution.

private int[] improve(int[] tour) {
    int[] newTour = new int[n + 1];
    double minChange;
    // WERS: limiter
    int iterations = MAX_ITERATIONS;
    do {
        - - -
        if (--iterations < 0) {
            /* 
            WERS it seems that this algoritm can encounter a set of edges
            such that same edges are swapped over and over again in an
            infite loop.
             */
            break;
        }
    } while (minChange < 0d);

[HannesWell]

You could build your random route with a explicit seed value for the Random instance you use and then iterate over the seed values until you run into the endless loop. With that you then know a seed value that creates a critical case that is now reproducible.

[Walter-Stroebel]

Perfect! Thanks for the effort and good to know I did find a real issue. As it seems that all angles are now covered, closing this issue as “already fixed in next release”.