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Welcome to the Minecraft Save Seed Reader wiki!
The MC Save Seed Reader is a Java program that reads the seed value from all your save files, so you can regenerate your worlds any time you'd like. Being a Java program, it works on all platforms that Minecraft itself does, and it should automatically find your saves no matter what platform you're using. (If you're using some form of Minecraft Portable, though, you will need to manually direct the program to your MC save directory. Just click on the Help menu, then hit "Select MC Save Folder".)
Thread on the Minecraft forums: http://www.minecraftforum.net/topic/612625-pc-mac-linux15-minecraft-save-seed-reader-171-now-includes-adventure-mode/
Download the program's JAR file here: https://github.com/downloads/thedarkfreak/Minecraft-Save-Seed-Reader/MinecraftSeed.zip
To use the Save Reader program, simply run the .JAR file as you would any other .JAR file. For most platforms, double-clicking the file will open it.
When the screen opens up, select the name of your world from the dropdown box to display the random seed for that world.
Please note: All seeds are stored internally as a numeric value. If you use a phrase as a seed, it will get converted internally to a number. My program cannot get the original string back; it will only display the number that's in the save file.
To switch a 1.8+ world between Creative and Survival, once you select the world in the dropdown box, check/uncheck the 'Creative Mode' checkbox, and then hit 'Save'.
The same thing goes for toggling Hardcore mode.
If the world you load is a pre-1.8 world, the 'Creative Mode' checkbox will be disabled. Simply open the save file in Minecraft 1.8 and save it again to be able to use this feature on it. This also goes for Hardcore mode toggling, only you need to save it in Minecraft Beta 1.9+.
If you have problems getting the JAR file to run, and it's opening in another program(like WinRAR), then you'll need to set the association of JAR files to open with Java. First, try right-clicking on the JAR file, go to "Open With", and see if Java is on the list of programs. If it is, open it with Java. (Optionally, you can set it to open by default in Java, which is preferred).
If that doesn't work, or you see something like "ClassNotFound", this means that the association is set to Java, but it's set improperly. To get it to work properly, use this program:
Download and run that, and it will set the JAR files association to work properly in these.
If you get a message stating it can't find any saves, it will open up a folder selection dialog, and ask you to chose the folder in which your world folders reside. For example, if your Minecraft saves are stored in .minecraft/saves, then you must select THAT folder, not any of the sub-folders containing world data. The program checks to make sure you didn't choose a world folder instead of the saves folder, but it's best to make sure yourself. You can also open the Folder Selection dialog manually by going to Help -> Select MC Save Folder.
If you get an error message asking you to check the error log, please find the error log(it should be named MinecraftSeed.error.log, and it will be in the same folder as MinecraftSeed.jar), and send it to me, with a description of what happened. Post it either in the forum topic at GameFAQs or MinecraftForum, e-mail it to me(my e-mail is cj.no.one gmail.com), or on this GitHub site as an issue, through the issue tracker here or PM it to me here on GitHub(username thedarkfreak). My username on MinecraftForum is thedarkfreak, and on GameFAQs it is cjnoone. You can PM it to either of those accounts, as well.
How it works(technical crap)
First, it determines what the default location of the .minecraft/saves folder would be, depending on what OS is found.
If it finds Windows, it uses the path %APPDATA%\.minecraft\saves\.
If it finds Mac OS X, it uses the path /users/<name>/Library/Application Support/.minecraft/saves/.
If it finds Linux, it uses ~/.minecraft/saves/.
Next, it looks into each sub-folder it finds in the .minecraft/saves/ folder, and looks for a level.dat file. If it finds one, it assumes that this folder is a valid Minecraft save folder, and gets the LevelName tag from the level.dat file to use as the world's name. If the level.dat file doesn't have a LevelName tag(older ones may not), it instead uses the folder's name as the world name.
When a world is selected from the dropdown list, it looks up the RandomSeed tag from that folder's level.dat file.
If you choose a save folder manually, it first checks within the selected folder for the existence of 'level.dat', 'session.lock', and the 'region' folder. If it finds all three, it decides that you accidentally selected a world folder instead of the 'saves' folder, and the program will use the parent directory instead.
When enabling/disabling Creative Mode, the program first checks for the existence of the 'GameType' Tag in the save file. If it doesn't exist, the program won't allow the checkbox or button to be pressed. NEW INFO FOR Beta 1.9 PR5+: Four new tags were added to the Minecraft save file, under the tag "abilities". These seem to control the separate Creative mode abilities, and can actually allow some Creative mode abilities to be used in Survival mode. For now, this program simply enables/disables all four tags in accordance with the user's "Creative Mode" setting.
This goes for Hardcore mode, only it checks for the 'hardcore' Tag instead.