How would I find the Seed of a MC world without /seed? [duplicate]

No, there is no such program and likely never will be.

As a programming task, it's identical to the least efficient method of password cracking. We can't reverse-calculate the seed no matter how many landmark coordinates we have, because the type of calculation Minecraft uses for seed generation is one-way. Thus, what such a program would have to do is try every seed until it finds one that matches the landmarks.

In computer security terms this is called a brute-force attack, and it can take a very long time:

1. Minecraft's seeds have 18 quintillion possible values.
2. A GPU-based password cracker on consumer-grade hardware today, very loosely, can try about a billion numbers a second, given a target result (such as our landmarks).
3. 1 quintillion is a billion billion.
4. That gives us about 18 billion seconds needed to try every possible seed.
5. On average we will have to only try half the seeds, so that brings us down to an average of 9 billion seconds to find a match.
6. 9 billion seconds is over 285 years.
7. In reality the program would have to run world-generation (or some kind of faked world generation) on every seed in order to find out if it matches our landmarks, so multiply 285 years by how long it takes Minecraft to start a new world. If we're generous and say a fake-generation takes only 3 seconds, that gives us 856 years to find a match, on average.

And all that is ignoring the question of how the program would handle the fact that our coordinates of landmarks are going to be guesses (which block in a village is the coordinate the world generator got from the seed?) and not exact matches for what the seed actually produced. That adds at least a few processor cycles to every time the program needs to check a particular seed for matches.

So as you can see, the idea is lovely, but in practice finding a seed from landmark coordinates is impractical enough that no-one is going to write such a program – by the time it finishes running, we'd all be dead (and Minecraft will likely have changed its world-generation again…). At least, not practical until we have quantum computers.

It is totally possible to find a Minecraft seed in a reasonable amount of time on CPU. Even though there are about 2^64 possible seeds, there is a lot of bias in how structure and decorator features are placed. By approaching the problem in a smart way, you can get rid of major brute-forcing overhead.

When it comes to structures(villages, temples, monuments, etc) and decorators(dungeons, emerald ore, end gateways, etc), the game creates a java.util.Random object which is seeded with the world seed before sending it through a layer of hashing. Looking closely, one realizes that the java Random class is a 48-bit linear congruential generator. What does that mean? Well, the upper 16 bits of the seed get trimmed off when passed on to the LCG. In fact, ALL features in the game do this mistake: structures, decorators, heightmap, slime chunks, etc. This means that you only need to brute-force 2^48 seeds which is much more reasonable on a GPU(a few hours worth). Those lower 48 bits are referred to as "structure seed".

The odd one out is biomes. Those use the full 64 bit of the seed. Once you have fully reversed the structure seed, you can then proceed to brute-force across the upper 2^16 with a biome search. This was the way to do it prior to 1.9.

In 1.9, Mojang changed the end pillars so they would generate in a circle with random heights. First, the game generates a list of indices, 0 through 9 for the 10 pillars. Then, it shuffles the list through Collections.shuffle() by passing on a custom Random object.

Random rand = new Random(worldSeed); long pillarSeed = rand.nextLong() & 65535L; List pillars = IntStream.range(0, 10).boxed().collect(Collectors.toList()); Collections.shuffle(pillars, new Random(pillarSeed));

For the sake of this post not being massive, you'll have to take my word for it. Given the 10 pillar heights, We can reverse 16 direct bits from the world seed in a matter of milliseconds. Those 16 bits all overlap with the structure seed, tuning down the brute-forcing work from 2^48 to 2^32.

To conclude, YES, it is possible to reverse-engineer a Minecraft seed. The most advanced seed crackers nowadays can reverse a seed in about 2 minutes of CPU once all the data is provided. I first learned about this technique through Neil's channel:

The seed usually runs through Java's Random class, which only uses 48 bits of the seed. There are some complicating factors, but I'm finishing off a tool to do this. At the moment it narrows the options for the lower 48 bits to a few tens of thousands - will need to chain with some other technique to reduce further. With one technique, I'm extracting about 32 bits and have 32 to go ("only" 4 billion possibilities).

Actually there is a way to find a seed using a program. All you need is this program linked below https://github.com/KaptainWutax/SeedCracker