Illegal starting positions (already supported by OGS!)

Suppose we have a position on the board where some chains don’t have liberties:

Different implementations handle this differently. A literal reading of most rulesets would mean that after any white move on the board, the black stone would be removed. And if suicide is illegal, all black moves may be illegal, since they leave a black chain without liberties on the board!

But of course, it would be wasteful for the computer to check every single chain after every single move: it makes more sense to check chains only adjacent to the last stone (this is what humans do anyways).

OGS actually does something inbetween: it seems that all opposing chains next to the just placed stone’s chain are checked. This means that in the above picture, a white move at E2 will capture the black stone, but a move at E3 will not.

You can try this out on a demo board (make sure to use image to set up the position), and I guess it’s even possible to challenge another player from such a position! But more importantly, we can make problems :smiley:

I made 4 easy ones to show the idea, click the image below to go to the collection:

Can you come up with some more interesting puzzles? Or maybe just construct some funny positions?




These are good ones.






Absolutely genius @le_4TC


OMG I love this! Problem 4 blew my smol brain


Problem 4 was very clever. I mean they all are.


That is totally legend. I wonder if you can do something like a ko or even Eternal Life from it?

So it’s different to send two return one because even in Japanese rules white isn’t gaining points from Black playing the repetition to avoid dieing?

In rules with superko though, I guess it’s alive (pending ko threats maybe)? Assuming white started with the throwin on the first line, Black plays the sequence, and then white can’t play the same throw in at the end, so would have to choose the seki line or play elsewhere a threat like a ko.

I don’t follow at all, could you make a diagram?

Illegal starting positions by themselves won’t lead to any new cyclic behavior I think, since the “ghost chains” are non-renewable. But they could probably make other interesting positions, non-removable ko threats for instance, which in turn can make it easier to set up some standard superko stuff. (so just doing things we can already do with normal positions, but using less area)

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I was just wondering if eternal life is only eternal life in Japanese rules.

It’s just dead with the other superko rules I think?

It does seem like a silly name of eternal life if its dead in a lot of the rule sets. I don’t understand the position really.

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Ah, then I understand!

Hm yes, from a quick glance it seems that it is dead unless black has infinite ko threats. But perhaps there’s weirdness where black could change where the cycle “starts” to get a more favorable result? (pure conjecture, let me play around with it a bit…)

So actually it behaves very similar to a regular ko! For instance, assume the sequence starts with this white throw-in:
Black can exchange 1-2, but then needs to find a ko threat 3-4:
Then black can continue 5-6-7, and now white has to find the next ko threat 8-9:
It continues back and forth like this. So the side with more ko threats wins, just like a regular ko fight :slight_smile:


Superko is very unintuitive to me. I did get a feeling that it might behave a bit like a ko, but I didn’t realise it was Black that couldn’t recapture (from the way I set it up) first.

It’s pretty interesting all the same, and I guess like the page says, it probably doesn’t come up very often, probably meaning there isn’t some standard sequence where it would arise.

The unintuitiveness I suppose is the main argument for ending games with long cycles as soon as they come up (ending in no-result, tie, or both players lose), rather than playing it out under superko. There are some difficult positions possible in theory for sure, but they are even more astronomically rare than eternal life or triple ko (both of which have actually happened a few times).

Eternal life in particular doesn’t seem very hard… is it really so bad to play a ko where you make three moves in between each threat instead of only one move?

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Curiously, of the three linked “Eternal life” games on the sensei’s page, two of them are not the standard shape, but rather “Eternal ko”, which is slightly easier to understand in my opinion (the same types of moves are played, but as ko threats - you don’t have to consider the eye-shape of the group anymore).

Here are waltheri links since go4go requires an account to view:

Eternal ko from 1993:

Eternal ko from 2009:

Eternal life from 2013:

I think reading out the position from a static image/board and remembering at which point the moves were forbidden by superko, that was hard yes, and it still is even after looking at your explanations :stuck_out_tongue:

Sure, but even regular ko is very difficult if we think about it this way! Once you’ve understood how regular ko works, you just think in terms of “take the ko, respond to threat, opponent takes back ko, make threat, take back ko”. And once you’ve analyzed eternal life once (like we did here) you could do the same if it comes up, it’s just that “taking the ko” is suddenly a three-move sequence, but it’s straightforward to play.

Now triple ko and the like is a bit more complicated, because there may be multiple possible ways to “take the ko”, and it indeed does get a bit hard for a human mind to track. My personal opinion is that it’s rare enough that it’s not worth making the rules uglier to avoid, but I can at least understand the argument for it.

Although in that case my preference would be for something like “either side may claim a draw after the same position has occurred 3 times”, similar to chess.

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The SGF file format supports setting up illegal positions (via the add color properties) and it even allows illegal moves that overwrite existing stones! However, the capture mechanics are a little bit different than how OGS implements them.

See SGF FF[5] - Move vs. Adding Stones

My SGF editor BesoGo supports these interesting quirks of the SGF standard. However, one needs to use a PC to perform a “ctrl+click” in order to play a move that overwrites an existing stone (or immediately retakes a ko or suicides).

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