a seed is more than the number
The seed is the full recipe for a world rather than a single random value. It records the size, the difficulty, which evil biome you got, which special seeds were switched on, and then the random number. Copy Seed in the world select screen hands you all five parts joined by periods. Two people entering the same complete string get the same world, down to the chest contents.
reading the five fields
- Size: 1 for small, 2 for medium, 3 for large.
- Difficulty: 1 Classic, 2 Expert, 3 Master, 4 Journey.
- Evil biome: 1 Corruption, 2 Crimson.
- Special seeds: a single packed number, explained below.
- Identifier: the actual random value, anywhere from 0 up to 2147483647.
the special seed field is a sum
The fourth number is not a code to look up. Each special world seed carries a value and the active ones are added together, so a world reading 40 has the seeds worth 32 and 8 turned on and nothing else. That is why the numbers look random. Zenith is the one worth memorizing: it cannot run without every other special seed except Skyblock, so any Zenith world shows 255, or 511 when Skyblock is on as well.
The named secret seeds, the ones you type as a phrase rather than toggle, are stored differently. They appear as plain text in front of the identifier, separated from it by a vertical bar.
why a pasted seed gives you the wrong world
The usual mistake is copying only the number off the end. If you enter just the identifier, the game leaves size, difficulty and evil biome at whatever the menu currently has selected, and all three feed generation, so you get a map that shares nothing with the one you were trying to copy. Paste the whole string.
The parser also fails quietly. If the shape of your input is valid but one value is out of range, that setting is left alone and the others apply. If the shape is wrong anywhere at all, the entire string is treated as the identifier and no setting moves. Either way you get a world with no warning. After pasting, look at the size, difficulty and evil selections and check they actually changed.
text seeds and how the game handles them
The identifier field takes up to 40 characters and accepts anything. A value that is already a number is used as it is. Anything containing letters or symbols is run through CRC-32 and reduced to a number first, so a word and its numeric equivalent build the same world. Nothing about the text influences terrain beyond that: it only chooses a starting point. Recognized secret seed phrases are the exception, since they are checked before this happens.
Pasting a complete four part seed eats six characters of the limit, leaving 34 for the identifier itself.
what seeds are actually good for
Same seed plus same settings gives the same world every time, which makes a few things possible.
- Scout a world quickly, decide the Jungle is on the wrong side of the Dungeon or the shrines are all in bad spots, delete it and roll again, without losing the record of the good ones.
- Regenerate a world you already ruined and get an untouched copy of it back.
- Hand a string to a friend so you both explore the same map, or race the same start.
- Rebuild a world after a save file is lost, which is the only recovery route there is.
Keep them in a text file with a line about what each one contains. The world list shows you the world but not why you kept it.
old worlds have no seed
Worlds created before 1.3.4 cannot be reproduced. A random value was used to build them and then discarded rather than written into the file, so there is nothing to copy. Anything made since always carries its seed, and since 1.4.0.1 you can view, copy and enter seeds with no settings to switch on first.