ref: 5c448c618a89ab8933838da1b5b97ed7c4a42e08
dir: /sys/src/ape/lib/ap/gen/rand.c/
#include <stdlib.h> /* * algorithm by * D. P. Mitchell & J. A. Reeds */ #define LEN 607 #define TAP 273 #define MASK 0x7fffffffL #define A 48271 #define M 2147483647 #define Q 44488 #define R 3399 typedef unsigned long ulong; static ulong rng_vec[LEN]; static ulong* rng_tap = rng_vec; static ulong* rng_feed = 0; void srand(unsigned int seed) { long lo, hi, x; int i; rng_tap = rng_vec; rng_feed = rng_vec+LEN-TAP; seed = seed%M; if(seed == 0) seed = 89482311; x = seed; /* * Initialize by x[n+1] = 48271 * x[n] mod (2**31 - 1) */ for(i = -20; i < LEN; i++) { hi = x / Q; lo = x % Q; x = A*lo - R*hi; if(x < 0) x += M; if(i >= 0) rng_vec[i] = x; } } static long lrand(void) { ulong x; rng_tap--; if(rng_tap < rng_vec) { if(rng_feed == 0) { srand(1); rng_tap--; } rng_tap += LEN; } rng_feed--; if(rng_feed < rng_vec) rng_feed += LEN; x = (*rng_feed + *rng_tap) & MASK; *rng_feed = x; return x; } int rand(void) { return lrand() & 0x7fff; }