Create a gist now

Instantly share code, notes, and snippets.

What would you like to do?
Reverse engineered MCPE "slime-chunk" code; by @protolambda and @jocopa3
public static boolean isSlimeChunk(int cX, int cZ){
// MCPE slime-chunk checker
// From Minecraft: Pocket Edition 0.15.0 (
// Reverse engineered by @protolambda and @jocopa3
// NOTE:
// - The world-seed doesn't seem to be incorporated into the randomness, which is very odd.
// This means that every world has its slime-chunks in the exact same chunks!
// Update: @_tomcc has confirmed in a reddit PM that this is "entirely possible!".
// He also noted that this part of the MCPE code was ported from PC back in the day by Johan.
// - Reverse engineering this code cost a lot of time,
// please add CREDITS when you are copying this.
// Copy the following into your program source:
// MCPE slime-chunk checker; reverse engineered by @protolambda and @jocopa3
// chunkX/Z are the chunk-coordinates, used in the DB keys etc.
// Unsigned int32, using 64 bit longs to work-around the sign issue.
long chunkX_uint = cX & 0xffffffffL;
long chunkZ_uint = cZ & 0xffffffffL;
// Combine X and Z into a 32 bit int (again, long to work around sign issue)
long seed = (chunkX_uint * 0x1f1f1f1fL) ^ chunkZ_uint;
// The random function MCPE uses, not the same as MCPC!
// This is a Mersenne Twister; MT19937 by Takuji Nishimura and Makoto Matsumoto.
// Java adaption source:
MTwister random = new MTwister();
// The output of the random function, first operand of the asm umull instruction
long n = random.genrand_int32();
// The other operand, magic bit number that keeps characteristics
// In binary: 1100 1100 1100 1100 1100 1100 1100 1101
long m = 0xcccccccdL;
// umull (unsigned long multiplication)
// Stores the result of multiplying two int32 integers in two registers (lo and hi).
// Java workaround: store the result in a 64 bit long, instead of two 32 bit registers.
long product = n * m;
// The umull instruction puts the result in a lo and a hi register, the lo one is not used.
long hi = (product >> 32) & 0xffffffffL;
// Make room for 3 bits, preparation for decrease of randomness by a factor 10.
long hi_shift3 = (hi >> 0x3) & 0xffffffffL;
// Multiply with 10 (3 bits)
// ---> effect: the 3 bit randomness decrease expresses a 1 in a 10 chance.
long res = (((hi_shift3 + (hi_shift3 * 0x4)) & 0xffffffffL) * 0x2) & 0xffffffffL;
// Final check: is the input equal to 10 times less random, but comparable, output.
// Every chunk has a 1 in 10 chance to be a slime-chunk.
return n == res;
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment