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Author Topic: Games with fixed RNGs  (Read 618 times)

McKnight

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Games with fixed RNGs
« on: October 14, 2017, 07:25:25 pm »
I've just started playing a translated copy of Mother (GBA version) today, and noticed that everything that's supposed to be generated randomly (barring stat raises when leveling up) follows the same sequence of events: What enemies appear in relation to one another and how far apart, what they're gonna do, the outcomes of physical attacks, whether or not Hippies' bullhorns or running away will work.

I've noticed this happen with certain other games before, but chalked it up to resetting on the emulator I played each on compatibility with their RNGs.  However, I most certainly remember this NOT happening with Earthbound when I recorded an aborted playthrough months ago.  I'm not even playing this on an emulator; I'm playing on a Nintendo DS, which does not have a reset switch (and neither does the Game Boy Advance).

If I had noticed this while playing Earthbound Zero on FCE Ultra five years ago, I would in all likelihood have exploited it by selecting Defend whenever someone was due to take a major hit.

How do some games' RNGs just stay where they are when the games themselves get reset or turned off, while others' reset back to where they were from where you last saved?  How did this port of something originally for the Famicom get changed to reset its RNG instead of just letting it roll (or if that was also the case with the Famicom port, how did that change with the NES prototype)?

Jorpho

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Re: Games with fixed RNGs
« Reply #1 on: October 14, 2017, 07:55:45 pm »
Earthbound has a fixed RNG, if I'm not mistaken.  It is exploited heavily in TAS runs to generate critical hits.  You may not have noticed as the generator gets "ticked" just by moving through menus.
https://forum.starmen.net/forum/Games/Mother2/Can-you-manipulate-the-game-s-Random-Number-Generator-to-improve-drops

There's a lovely video here on the Super Mario World RNG.  It is also technically "fixed".
https://www.youtube.com/watch?v=q15yNrJHOak

And there's DOOM.
http://doom.wikia.com/wiki/Pseudorandom_number_generator
« Last Edit: October 15, 2017, 11:14:21 am by Jorpho »
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Chronosplit

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Re: Games with fixed RNGs
« Reply #2 on: October 15, 2017, 08:46:50 am »
FF1 I believe has a fixed RNG.  Final Fantasy Restored has an optional patch with a new RNG that's not fixed.

KingMike

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Re: Games with fixed RNGs
« Reply #3 on: October 15, 2017, 12:40:52 pm »
Yes, FF1 was the first example I could think of.
I remember the GDQ speedrun heavily exploiting that (they've also done a couple RNG-exploitation runs of Chrono Trigger as well). Though it sounded like CT RNG abuse required resetting the console and loading on certain frames, which would be more impressive for a real-time speedrun.
I know there was a Pokemon HeartGold speedrun that was supposed to be RNG-abuse based (of course, Pokemon would be a pretty bad game to try to speedrun live without predicting RNG) though the runner had quite a struggle setting up the RNG live, and even his second DS with a pre-loaded RNG manipulation failed to run as expected.
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SleepyFist

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Re: Games with fixed RNGs
« Reply #4 on: October 15, 2017, 01:49:54 pm »
I could be wrong but did the first Persona have some of this?

I'm also pretty sure that the Chao Garden RNG can be manipulated in SA2 Battle as well, I always did it to get a particular sharp-toothed white Chao from the Black Market.
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Bregalad

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Re: Games with fixed RNGs
« Reply #5 on: October 15, 2017, 04:17:40 pm »
All RNGs are "fixed". There's nothing really availalbe else. But many games will trigger their RNG regularly (for example every frame) to make it more random when it's actually used. When not doing this, then the RNG is very easily exploitable. Otherwise it's still exploitable but harder.

Fire Emblem GBA games comes to mind immediately. The RNG abuse is almost required to beat Sword of Seals.

FAST6191

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Re: Games with fixed RNGs
« Reply #6 on: October 15, 2017, 06:18:53 pm »
While I have seen many a game use a PRNG, and often so poorly that you can game it, there are just as many that might use uninitialised memory or noise from the audio capture to seed something else. Others may still use a weak PRNG but still more than any human, or especially any human without the ability to peer into memory, can handle.

Anyway Puzzle Quest on the DS. Many the boards which form the game are anything but random. No so bad for some things but pattern recognition is a trait those that like puzzle games... have been known to have. A pity as the rest of the game is really quite good. I did look into fixing it once (and possibly also breaking the AI in the process) but it turns out it is some kind of lua bytecode equivalent (the PC version notably used lua, much to the delight of the modding community) so I would first have to learn that and that rapidly turned into a more effort than it is worth situation.

KingMike

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Re: Games with fixed RNGs
« Reply #7 on: October 15, 2017, 10:48:06 pm »
Although Deep Dungeon III was broken in many ways, the fact that (as I recall) the RNG seed advances only on frames when no buttons are touched is probably amazingly breakable.
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Azkadellia

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Re: Games with fixed RNGs
« Reply #8 on: October 16, 2017, 01:49:55 am »
There's a real time speedrun that abuses the RNG to beat Dragon Warrior 1 at a low level. Not as fast as the tool assisted speedrun, though.
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Chronosplit

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Re: Games with fixed RNGs
« Reply #9 on: October 16, 2017, 11:18:55 am »
I know there was a Pokemon HeartGold speedrun that was supposed to be RNG-abuse based (of course, Pokemon would be a pretty bad game to try to speedrun live without predicting RNG) though the runner had quite a struggle setting up the RNG live, and even his second DS with a pre-loaded RNG manipulation failed to run as expected.
In Emerald you're able to exploit the RNG and produce certain results.  This is because due to a bug the PRNG code is a tiny bit incomplete compared to Ruby/Sapphire.  This is typically fixed by comparing it to R/S's code, but kinda funny when you think about it because the later game uses mostly the same engine.

Golden Sun 1 and 2 can also be abused like this, in particular people cheat artifacts dropped by monsters and Tolbi Springs to get all the loot (both of which are frankly a bit annoying otherwise).

Bregalad

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Re: Games with fixed RNGs
« Reply #10 on: October 17, 2017, 02:10:18 am »
While I have seen many a game use a PRNG, and often so poorly that you can game it, there are just as many that might use uninitialised memory or noise from the audio capture to seed something else.
FF1 uses uninitialized memory, but it is in fact "initialized" to $ff on most consoles, sometimes a bit would randomly be clear, but then it would always be clear on the same console. Or not. http://wiki.nesdev.com/w/index.php/CPU_power_up_state In all cases even though this is technically random it's a very poor source of actual "randomness".

Using "noise from the audio capture", is not going to be available on most platform who does not have audio capture. Even the famicom who have a mic on the 2nd controller, it can be turned off with a potentiometer and then there's no noise.