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Random seeds in classic BD games
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Dustin
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Joined: 23 Sep 2007
Posts: 504
Location: Erlangen, Germany

PostPosted: Mon Dec 14, 2020 9:50 pm    Post subject: Reply with quote

BD2-E and the RNG

In BD2-E, there's an interesting effect concerning the slime: If Rockford gets crushed in a way that the explosion destroys 3 slimes, then some boulders/diamonds will never fall through the slime afterwards! Most probably everyone failed this way at least one in their BD career, and perhaps you also wondered why this is happening! However, if Rockford gets crushed at the left border, so that only 2 slimes explode, such an effect does not occur. Now, based on Arno's RNG analysis, I think I've found the reason!

Arno, perhaps we can make a puzzle out of it? I'm absolutely sure you'll love the solution, especially if you figure it out for yourself! Just check the map carefully, perhaps counting something is not a bad idea, and then I'm sure you'll be as stunned as I was today! Very Happy
_________________
Boulder Dash X Rock, Paper, Scissors:
ROCKFORD collects DIAMOND, digs DIRT
DIAMOND outvalues DIRT & BOULDER
DIRT carries BOULDER, blocks FIREFLY
BOULDER kills FIREFLY & ROCKFORD
FIREFLY kills ROCKFORD, guards DIAMOND
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Dustin
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Joined: 23 Sep 2007
Posts: 504
Location: Erlangen, Germany

PostPosted: Fri Dec 18, 2020 10:08 am    Post subject: Reply with quote

At Krissz's site, we've already discussed the solution, so here it is, and it could be an idea for very interesting slime caves as well:

In BD2-E, there are 4*19=76 slimes at the start. If 3 get destroyed, then 73 remain, which exactly matches Arno's 73-cycle of the RNG discussed above! Funny coincidence! Very Happy

Every time the cavescanner scans a piece of slime, it calculates a new random number (more precisely: a set of 2 numbers) to decide whether the slime is active for this frame. This is done even if the slime can't swallow anything currently! This means that with exactly 73 slimes in the cave, each slime always gets the same seed2 value (in the long run). Also the seed1 value (which decides about the slime being active or not) then isn't completely random anymore (as deeply analyzed by Arno above). So each slime only gets a few different ssed1 values, and if none of them make the slime active, then it will never be!

As the exact cycle length of the C64 RNG (for the whole seed1/seed2 pair) is 949=73*13 (source: Nesdori), similar effects should also be possible with exactly 13 slimes in the cave.

Do caves using this effect already exist? I don't remember any, but I also don't know every fangame by far. If someone knows/ has made such a cave, it would be very interesting to see it! Very Happy
_________________
Boulder Dash X Rock, Paper, Scissors:
ROCKFORD collects DIAMOND, digs DIRT
DIAMOND outvalues DIRT & BOULDER
DIRT carries BOULDER, blocks FIREFLY
BOULDER kills FIREFLY & ROCKFORD
FIREFLY kills ROCKFORD, guards DIAMOND
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Arno
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Joined: 17 Mar 2007
Posts: 2450
Location: netherlands

PostPosted: Fri Dec 18, 2020 8:02 pm    Post subject: Reply with quote

Dustin wrote:
... So each slime only gets a few different ssed1 values, and if none of them make the slime active, then it will never be!

Excellent research again! Very Happy
I remember I have seen such effect before, but not in BD2 cave E, but just when I designed a cave in the CKit that some pieces of slime refused to let a boulder/diamond through, while all other slimes did that very fast.
Very cool that we now know the explanation, and indeed a nice source for new cave ideas! Very Happy
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