Documentation Index
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cubiomes implements two RNG systems that match Minecraft’s own: the Java Random linear congruential generator (used in Minecraft ≤ 1.17 and for most structure positioning), and Xoroshiro 128 (used in Minecraft 1.18+ world generation). Both are defined as inline functions in rng.h.
Compile with -fwrapv when using this library. This enforces two’s complement arithmetic for signed integer overflow, which is required to correctly emulate Java’s integer behavior.
Type aliases
typedef int8_t i8; typedef uint8_t u8;
typedef int16_t i16; typedef uint16_t u16;
typedef int32_t i32; typedef uint32_t u32;
typedef int64_t i64; typedef uint64_t u64;
typedef float f32; typedef double f64;
Java Random (LCG)
Implements the 48-bit linear congruential generator from java.util.Random.
setSeed
void setSeed(uint64_t *seed, uint64_t value);
*seed = (value ^ 0x5deece66d) & ((1ULL << 48) - 1).
next
int next(uint64_t *seed, const int bits);
bits bits.
nextInt
int nextInt(uint64_t *seed, const int n);
[0, n). Handles powers-of-two as a special case matching Java’s behavior.
nextLong
uint64_t nextLong(uint64_t *seed);
nextFloat / nextDouble
float nextFloat(uint64_t *seed);
double nextDouble(uint64_t *seed);
[0, 1) using 24 or 53 bits respectively.
skipNextN
void skipNextN(uint64_t *seed, uint64_t n);
n steps in O(log n) time using the fast-forward formula for linear recurrences.
JAVA_NEXT_INT24
#define JAVA_NEXT_INT24(S, X) /* ... */
nextInt(S, 24) with optimal assembly output. Used in hot paths like isQuadBaseFeature24.
Xoroshiro 128
Used by Minecraft 1.18+ world generation. Operates on a 128-bit state stored in a Xoroshiro struct.
STRUCT(Xoroshiro) { uint64_t lo, hi; };
xSetSeed
void xSetSeed(Xoroshiro *xr, uint64_t value);
xNextLong / xNextInt / xNextDouble / xNextFloat
uint64_t xNextLong(Xoroshiro *xr);
int xNextInt(Xoroshiro *xr, uint32_t n);
double xNextDouble(Xoroshiro *xr);
float xNextFloat(Xoroshiro *xr);
xSkipN
void xSkipN(Xoroshiro *xr, int count);
count steps.
Java-compatible variants
uint64_t xNextLongJ(Xoroshiro *xr);
int xNextIntJ(Xoroshiro *xr, uint32_t n);
RandomSource interface by combining two 32-bit halves, used when cubiomes needs to replicate Minecraft’s exact behavior for Xoroshiro-based worlds.
MC seed helpers
These functions implement the layer-seed pipeline that determines biome generation per chunk.
getLayerSalt(n) → layerSalt (ls)
(ls, worldSeed) → startSalt (st), startSeed (ss)
(ss, x, z) → chunkSeed (cs)
mcFirstInt(cs, mod) → first PRNG output
mcStepSeed
uint64_t mcStepSeed(uint64_t s, uint64_t salt);
mcFirstInt / mcFirstIsZero
int mcFirstInt(uint64_t s, int mod);
int mcFirstIsZero(uint64_t s, int mod);
mod. mcFirstIsZero is an optimized check for whether the result is zero.
getChunkSeed
uint64_t getChunkSeed(uint64_t ss, int x, int z);
getLayerSalt / getStartSalt / getStartSeed
uint64_t getLayerSalt(uint64_t salt);
uint64_t getStartSalt(uint64_t ws, uint64_t ls);
uint64_t getStartSeed(uint64_t ws, uint64_t ls);
Math utilities
double lerp(double part, double from, double to);
double lerp2(double dx, double dy, double v00, double v10, double v01, double v11);
double lerp3(double dx, double dy, double dz, /* 8 corner values */);
double clampedLerp(double part, double from, double to);
uint64_t mulInv(uint64_t x, uint64_t m); // modular inverse
uint64_t rotl64(uint64_t x, uint8_t b);
uint32_t rotr32(uint32_t a, uint8_t b);
int32_t floordiv(int32_t a, int32_t b);