`belt_balancer` fails with `--fast` (with custom + partial)

[Yodo9000]

When I run
belt_balancer networks/balancer/6x6 10 12 --custom --underground-length 8 --partial ./blueprints/6-6_base.txt --fast | blueprint encode
I get no output (only EOF error), but if I run it without --fast:
belt_balancer networks/balancer/6x6 10 12 --custom --underground-length 8 --partial ./blueprints/6-6_base.txt | blueprint encode
it prints

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

together with the usual EOF error message (see issue 21).

The partial solution I provide is

0eNqllmtvgyAUhv/L+YyNgKj4V5plsS3bSBSN0G1N438f7bK7Z0L55u08vK/nAmfYdUc1Tto4aM6g94Ox0GzPYPWjabvLM9P2Chp4aK3L3NQaOw6Ty3aqczAT0OagXqGh8x0BZZx2Wr0Drjene3Psd2ryH5AP0MswHJTJ9k/KOiAwDtYHDeaylAdlRbkRBE7+iuZMbMQ8kz8wdhOsWIZx8p/FBWaxzixSmIhpEcsU6zrLFCais4plsnVmHZxwvm5a3gRDlNE81i4NgIZ3Sx5AY5ESeb3+EylPgWJKY/uGywClIgWKKS1DM8SrAFqV0occgdYpQwiDyhSlDBnneUotIUoZTUk7Bo1tpe8jhCIFynjCxEShv1rJjp12zr9Z0vgJw/ZbkTDkUIXRm08eAK3CbdNV23VCCaEKZUKxY1CeB9vm8qdtf27TTvU+8uskSOBZTfYaJEomCymFKASTnM7zG/YYQ3I=

And the network file was generated with network create ./networks/balancer/6x6 6

Partial solution and result (without --fast) next to each other:

[ignaloidas]

--fast essentially speeds up solving by disallowing some configurations that supposed to be "unoptimal" (not all of them are strictly that, I have gotten some balancers smaller without fast). One of those are straight belts without any undergrounds going in parallel. The provided partial solution doesn't comply with that rule that --fast adds, so there's no solutions possible.

I'd either remove the longer stubs of belts, or just don't use the --fast option.

[Yodo9000]

Ah, I tried putting wooden chests in place of the straight belts, but I didn't get a solution at all. I thought this was because the ends need to be on the edge of the area, even if they are custom, is that right?

[Yodo9000]

And if try to trick it by using underground belts:

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

Then it doesn't work with or without --fast.

[ignaloidas]

I'm pretty sure undergrounds without any gap are disallowed by the internal SAT encoding in general, they need a gap of at least one tile.

The ends should be at the edge of the area, even if they are custom. If you want to use --fast, I'd have only the ends on the edge of the area, without the extension into the area at all. Though I personally feel like --fast isn't that good for custom layouts in general, there are some rules in there that assume that the splitter is laid out like a standard one, and they might break with weirder input/output configurations.

[Yodo9000]

I tried a gap of one tile with wooden chests filling it, but it didn't work, I think I'll just stick with not using --fast if you think it's not good for custom layouts.

Could the help info for --fast be improved: something like Enables all speed improving options, can fail to generate a solution? I can make a PR.