# enzo-3.0 / src / enzo / utilities / MersenneTwister.C

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 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76``` ```/* This is an implementation of the Mersenne Twister which was written by Michael Brundage. The original source code, which was copied and pasted into this file, can be found at http://qbrundage.com/michaelb/pubs/essays/random_number_generation, and a description of the Mersenne Twister random number generator can be found here: http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html It has a claimed periodicity of 2^{19937}-1, which is pretty incredible. The algorithm as-is is mainly intended for Monte Carlo realizations, and is not intended to be used for cryptography. See the website for more information. This code was placed into the public domain by Michael Brundage, and was put into Enzo by Brian W. O'Shea on 11 December 2007. It uses the system random number generator as an initial seed, and the user must specify a seed for the system random number generator. */ #include "preincludes.h" #include "macros_and_parameters.h" #define MT_LEN 624 int mt_index; unsigned_long_int mt_buffer[MT_LEN]; void mt_init(unsigned_int seed) { int i; srand(seed); for (i = 0; i < MT_LEN; i++) mt_buffer[i] = ((unsigned_long_int) rand() ); mt_index = 0; } #define MT_IA 397 #define MT_IB (MT_LEN - MT_IA) #define UPPER_MASK 0x80000000 #define LOWER_MASK 0x7FFFFFFF #define MATRIX_A 0x9908B0DF #define TWIST(b,i,j) ((b)[i] & UPPER_MASK) | ((b)[j] & LOWER_MASK) #define MAGIC(s) (((s)&1)*MATRIX_A) unsigned_long_int mt_random() { unsigned_long_int * b = mt_buffer; int idx = mt_index; unsigned_long_int s; int i; if (idx == MT_LEN*sizeof(unsigned_long_int)) { idx = 0; i = 0; for (; i < MT_IB; i++) { s = TWIST(b, i, i+1); b[i] = b[i + MT_IA] ^ (s >> 1) ^ MAGIC(s); } for (; i < MT_LEN-1; i++) { s = TWIST(b, i, i+1); b[i] = b[i - MT_IB] ^ (s >> 1) ^ MAGIC(s); } s = TWIST(b, MT_LEN-1, 0); b[MT_LEN-1] = b[MT_IA-1] ^ (s >> 1) ^ MAGIC(s); } mt_index = idx + sizeof(unsigned_long_int); return *(unsigned_long_int *)((unsigned char *)b + idx); /* Matsumoto and Nishimura additionally confound the bits returned to the caller but this doesn't increase the randomness, and slows down the generator by as much as 25%. So I omit these operations here. r ^= (r >> 11); r ^= (r << 7) & 0x9D2C5680; r ^= (r << 15) & 0xEFC60000; r ^= (r >> 18); */ } ```
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