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	lrand.c - 9base - revived minimalist port of Plan 9 userland to Unix
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	---
	lrand.c (1113B)
	---
	1 #include <u.h>
	2 #include <libc.h>
	3
	4 /*
	5 * algorithm by
	6 * D. P. Mitchell & J. A. Reeds
	7 */
	8
	9 #define LEN 607
	10 #define TAP 273
	11 #define MASK 0x7fffffffL
	12 #define A 48271
	13 #define M 2147483647
	14 #define Q 44488
	15 #define R 3399
	16 #define NORM (1.0/(1.0+MASK))
	17
	18 static ulong rng_vec[LEN];
	19 static ulong* rng_tap = rng_vec;
	20 static ulong* rng_feed = 0;
	21 static Lock lk;
	22
	23 static void
	24 isrand(long seed)
	25 {
	26 long lo, hi, x;
	27 int i;
	28
	29 rng_tap = rng_vec;
	30 rng_feed = rng_vec+LEN-TAP;
	31 seed = seed%M;
	32 if(seed < 0)
	33 seed += M;
	34 if(seed == 0)
	35 seed = 89482311;
	36 x = seed;
	37 /*
	38 * Initialize by x[n+1] = 48271 * x[n] mod (2**31 - 1)
	39 */
	40 for(i = -20; i < LEN; i++) {
	41 hi = x / Q;
	42 lo = x % Q;
	43 x = Alo - Rhi;
	44 if(x < 0)
	45 x += M;
	46 if(i >= 0)
	47 rng_vec[i] = x;
	48 }
	49 }
	50
	51 void
	52 p9srand(long seed)
	53 {
	54 lock(&lk);
	55 isrand(seed);
	56 unlock(&lk);
	57 }
	58
	59 long
	60 p9lrand(void)
	61 {
	62 ulong x;
	63
	64 lock(&lk);
	65
	66 rng_tap--;
	67 if(rng_tap < rng_vec) {
	68 if(rng_feed == 0) {
	69 isrand(1);
	70 rng_tap--;
	71 }
	72 rng_tap += LEN;
	73 }
	74 rng_feed--;
	75 if(rng_feed < rng_vec)
	76 rng_feed += LEN;
	77 x = (rng_feed + rng_tap) & MASK;
	78 *rng_feed = x;
	79
	80 unlock(&lk);
	81
	82 return x;
	83 }