Think back to my 118.2 answer, where I had crazy recursion problems that made it go two days without appreciable progress.

My first move against the Cornucopia of Infinite Loops (and I should make a shirt of that as well) was to avoid reusing squares. The second, obvious but forgotten thing is that, if your path is length n, every other path thats n or longer is definitionally a no-go, so adding one line — `return if length \$trail >= length \$shortest` — is enough to keep the code from going off on unproductive tangents. For any path, there may be paths that are of the same length that won’t show, but we’re looking for the shortest path, not all paths of the same length.

I ran it

``````...

00 21 02 23 04 25 06 27 46 65 44 63 42 61 40 32 24 43 51 72 60 52 71
a b c d e f g h
8 . * . * . * . * 8
7 * * * * * * * * 7
6 * . * . X . * . 6
5 * * . * * * * * 5
4 . * X . . * . * 4
3 * X . * * * * * 3
2 X X * . * . * * 2
1 * X . * * * * * 1
a b c d e f g h

68
00 21 02 23 04 25 06 27 46 65 44 63 42 61 40 32 24 43 51 72 60 52 71

00 21 02 23 04 25 06 27 46 65 44 32 24 43 51 72 60 52 71 50 42 61
a b c d e f g h
8 . * . * . * . * 8
7 * * * * * * * * 7
6 * . * . X . * . 6
5 * * . * * * * * 5
4 * * X . . * . * 4
3 . X . * * * * * 3
2 X X * * * . * * 2
1 * X . * * * * * 1
a b c d e f g h

65
00 21 02 23 04 25 06 27 46 65 44 32 24 43 51 72 60 52 71 50 42 61

00 21 02 23 04 25 06 27 46 65 53 61 42 63 71 52 60 72 51 32 24
a b c d e f g h
8 . * . * . * . * 8
7 * * * * * * * * 7
6 * . * . X . * . 6
5 * * . * * * * * 5
4 * * X * * * . * 4
3 * X . . * * * * 3
2 X X * . * . * * 2
1 * X . * * * * * 1
a b c d e f g h

62
00 21 02 23 04 25 06 27 46 65 53 61 42 63 71 52 60 72 51 32 24

00 21 02 23 04 25 06 27 15 36 24 43 51 72 60 52 71 50 42 61
a b c d e f g h
8 . * . * . * . * 8
7 * * * * * . * * 7
6 * . * . X . * . 6
5 * * * * * * . * 5
4 * * X . * * * * 4
3 . X . * * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

59
00 21 02 23 04 25 06 27 15 36 24 43 51 72 60 52 71 50 42 61

00 21 02 23 04 25 44 63 42 61 40 32 24 43 51 72 60 52 71
a b c d e f g h
8 . * . * . * * * 8
7 * * * * * * * * 7
6 * . * . X . * * 6
5 * * . * * * * * 5
4 . * X . . * * * 4
3 * X . * * * * * 3
2 X X * . * * * * 2
1 * X . * * * * * 1
a b c d e f g h

56
00 21 02 23 04 25 44 63 42 61 40 32 24 43 51 72 60 52 71

00 21 02 23 04 25 44 32 24 43 51 72 60 52 71 50 42 61
a b c d e f g h
8 . * . * . * * * 8
7 * * * * * * * * 7
6 * . * . X . * * 6
5 * * . * * * * * 5
4 * * X . . * * * 4
3 . X . * * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

53
00 21 02 23 04 25 44 32 24 43 51 72 60 52 71 50 42 61

00 21 02 23 04 12 24 43 51 30 42 61 53 72 60 52 71
a b c d e f g h
8 . * . * . * * * 8
7 * * . * * * * * 7
6 * . * . X * * * 6
5 . * * * * * * * 5
4 * * X . * * * * 4
3 * X . . * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

50
00 21 02 23 04 12 24 43 51 30 42 61 53 72 60 52 71

00 21 02 23 04 12 24 43 51 72 60 52 71 50 42 61
a b c d e f g h
8 . * . * . * * * 8
7 * * . * * * * * 7
6 * . * . X * * * 6
5 * * * * * * * * 5
4 * * X . * * * * 4
3 . X . * * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

47
00 21 02 23 04 12 24 43 51 72 60 52 71 50 42 61

00 21 02 23 42 61 40 32 24 43 51 72 60 52 71
a b c d e f g h
8 . * . * * * * * 8
7 * * * * * * * * 7
6 * . * . X * * * 6
5 * * . * * * * * 5
4 . * X . * * * * 4
3 * X . * * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

44
00 21 02 23 42 61 40 32 24 43 51 72 60 52 71

00 21 40 61 42 63 71 52 60 72 51 32 24
a b c d e f g h
8 . * * * * * * * 8
7 * * * * * * * * 7
6 * . * * X * * * 6
5 * * . * * * * * 5
4 . * X * * * * * 4
3 * X . * * * * * 3
2 X X * . * * * * 2
1 * X . * * * * * 1
a b c d e f g h

38
00 21 40 61 42 63 71 52 60 72 51 32 24

00 12 24 43 51 72 60 52 71 50 42 61
a b c d e f g h
8 . * * * * * * * 8
7 * * . * * * * * 7
6 * * * * X * * * 6
5 * * * * * * * * 5
4 * * X . * * * * 4
3 . X . * * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

35
00 12 24 43 51 72 60 52 71 50 42 61

00 12 24 43 51 72 60 52 71 50 42 61
a b c d e f g h
8 . * * * * * * * 8
7 * * . * * * * * 7
6 * * * * X * * * 6
5 * * * * * * * * 5
4 * * X . * * * * 4
3 . X . * * * * * 3
2 X X * * * * * * 2
1 * X . * * * * * 1
a b c d e f g h

real    63m23.304s
user    63m0.984s
sys     0m1.641s
``````

So, instead of incomlete in 48 hours, we get a 12-jump answer in one. That’s winnish to me.