c标准库源代码之 随机数产生算法

这是一个较老的不太复杂的算法

/* This algorithm is mentioned in the ISO C standard, here extended for 32 bits. */intrand_r (unsigned int *seed){ unsigned int next = *seed; int result; next *= 1103515245; next += 12345; result = (unsigned int) (next / 65536) % 2048; next *= 1103515245; next += 12345; result <<= 10; result ^= (unsigned int) (next / 65536) % 1024; next *= 1103515245; next += 12345; result <<= 10; result ^= (unsigned int) (next / 65536) % 1024; *seed = next; return result;}

 

 

这一个太复杂了，先看主函数

后面是它所依赖的

long int__random (){ int32_t retval; __libc_lock_lock (lock); (void) __random_r (&unsafe_state, &retval); __libc_lock_unlock (lock); return retval;}/* For each of the currently supported random number generators, we have a break value on the amount of state information (you need at least this many bytes of state info to support this random number generator), a degree for the polynomial (actually a trinomial) that the R.N.G. is based on, and separation between the two lower order coefficients of the trinomial. *//* Linear congruential. */#defineTYPE_00#defineBREAK_08#defineDEG_00#defineSEP_00/* x**7 + x**3 + 1. */#defineTYPE_11#defineBREAK_132#defineDEG_17#defineSEP_13/* x**15 + x + 1. */#defineTYPE_22#defineBREAK_264#defineDEG_215#defineSEP_21/* x**31 + x**3 + 1. */#defineTYPE_33#defineBREAK_3128#defineDEG_331#defineSEP_33/* x**63 + x + 1. */#defineTYPE_44#defineBREAK_4256#defineDEG_463#defineSEP_41/* Array versions of the above information to make code run faster. Relies on fact that TYPE_i == i. */#defineMAX_TYPES5/* Max number of types above. */struct random_poly_info{ int seps[MAX_TYPES]; int degrees[MAX_TYPES];};static const struct random_poly_info random_poly_info ={ { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 }, { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 }};/* Initialize the random number generator based on the given seed. If the type is the trivial no-state-information type, just remember the seed. Otherwise, initializes state[] based on the given "seed" via a linear congruential generator. Then, the pointers are set to known locations that are exactly rand_sep places apart. Lastly, it cycles the state information a given number of times to get rid of any initial dependencies introduced by the L.C.R.N.G. Note that the initialization of randtbl[] for default usage relies on values produced by this routine. */int__srandom_r (seed, buf) unsigned int seed; struct random_data *buf;{ int type; int32_t *state; long int i; int32_t word; int32_t *dst; int kc; if (buf == NULL) goto fail; type = buf->rand_type; if ((unsigned int) type >= MAX_TYPES) goto fail; state = buf->state; /* We must make sure the seed is not 0. Take arbitrarily 1 in this case. */ if (seed == 0) seed = 1; state[0] = seed; if (type == TYPE_0) goto done; dst = state; word = seed; kc = buf->rand_deg; for (i = 1; i < kc; ++i) { /* This does: state[i] = (16807 * state[i - 1]) % 2147483647; but avoids overflowing 31 bits. */ long int hi = word / 127773; long int lo = word % 127773; word = 16807 * lo - 2836 * hi; if (word < 0)word += 2147483647; *++dst = word; } buf->fptr = &state[buf->rand_sep]; buf->rptr = &state[0]; kc *= 10; while (--kc >= 0) { int32_t discard; (void) __random_r (buf, &discard); } done: return 0; fail: return -1;}weak_alias (__srandom_r, srandom_r)/* Initialize the state information in the given array of N bytes for future random number generation. Based on the number of bytes we are given, and the break values for the different R.N.G.'s, we choose the best (largest) one we can and set things up for it. srandom is then called to initialize the state information. Note that on return from srandom, we set state[-1] to be the type multiplexed with the current value of the rear pointer; this is so successive calls to initstate won't lose this information and will be able to restart with setstate. Note: The first thing we do is save the current state, if any, just like setstate so that it doesn't matter when initstate is called. Returns a pointer to the old state. */int__initstate_r (seed, arg_state, n, buf) unsigned int seed; char *arg_state; size_t n; struct random_data *buf;{ if (buf == NULL) goto fail; int32_t *old_state = buf->state; if (old_state != NULL) { int old_type = buf->rand_type; if (old_type == TYPE_0)old_state[-1] = TYPE_0; elseold_state[-1] = (MAX_TYPES * (buf->rptr - old_state)) + old_type; } int type; if (n >= BREAK_3) type = n < BREAK_4 ? TYPE_3 : TYPE_4; else if (n < BREAK_1) { if (n < BREAK_0)goto fail; type = TYPE_0; } else type = n < BREAK_2 ? TYPE_1 : TYPE_2; int degree = random_poly_info.degrees[type]; int separation = random_poly_info.seps[type]; buf->rand_type = type; buf->rand_sep = separation; buf->rand_deg = degree; int32_t *state = &((int32_t *) arg_state)[1];/* First location. */ /* Must set END_PTR before srandom. */ buf->end_ptr = &state[degree]; buf->state = state; __srandom_r (seed, buf); state[-1] = TYPE_0; if (type != TYPE_0) state[-1] = (buf->rptr - state) * MAX_TYPES + type; return 0; fail: __set_errno (EINVAL); return -1;}weak_alias (__initstate_r, initstate_r)/* Restore the state from the given state array. Note: It is important that we also remember the locations of the pointers in the current state information, and restore the locations of the pointers from the old state information. This is done by multiplexing the pointer location into the zeroth word of the state information. Note that due to the order in which things are done, it is OK to call setstate with the same state as the current state Returns a pointer to the old state information. */int__setstate_r (arg_state, buf) char *arg_state; struct random_data *buf;{ int32_t *new_state = 1 + (int32_t *) arg_state; int type; int old_type; int32_t *old_state; int degree; int separation; if (arg_state == NULL || buf == NULL) goto fail; old_type = buf->rand_type; old_state = buf->state; if (old_type == TYPE_0) old_state[-1] = TYPE_0; else old_state[-1] = (MAX_TYPES * (buf->rptr - old_state)) + old_type; type = new_state[-1] % MAX_TYPES; if (type < TYPE_0 || type > TYPE_4) goto fail; buf->rand_deg = degree = random_poly_info.degrees[type]; buf->rand_sep = separation = random_poly_info.seps[type]; buf->rand_type = type; if (type != TYPE_0) { int rear = new_state[-1] / MAX_TYPES; buf->rptr = &new_state[rear]; buf->fptr = &new_state[(rear + separation) % degree]; } buf->state = new_state; /* Set end_ptr too. */ buf->end_ptr = &new_state[degree]; return 0; fail: __set_errno (EINVAL); return -1;}weak_alias (__setstate_r, setstate_r)/* If we are using the trivial TYPE_0 R.N.G., just do the old linear congruential bit. Otherwise, we do our fancy trinomial stuff, which is the same in all the other cases due to all the global variables that have been set up. The basic operation is to add the number at the rear pointer into the one at the front pointer. Then both pointers are advanced to the next location cyclically in the table. The value returned is the sum generated, reduced to 31 bits by throwing away the "least random" low bit. Note: The code takes advantage of the fact that both the front and rear pointers can't wrap on the same call by not testing the rear pointer if the front one has wrapped. Returns a 31-bit random number. */int__random_r (buf, result) struct random_data *buf; int32_t *result;{ int32_t *state; if (buf == NULL || result == NULL) goto fail; state = buf->state; if (buf->rand_type == TYPE_0) { int32_t val = state[0]; val = ((state[0] * 1103515245) + 12345) & 0x7fffffff; state[0] = val; *result = val; } else { int32_t *fptr = buf->fptr; int32_t *rptr = buf->rptr; int32_t *end_ptr = buf->end_ptr; int32_t val; val = *fptr += *rptr; /* Chucking least random bit. */ *result = (val >> 1) & 0x7fffffff; ++fptr; if (fptr >= end_ptr){ fptr = state; ++rptr;} else{ ++rptr; if (rptr >= end_ptr) rptr = state;} buf->fptr = fptr; buf->rptr = rptr; } return 0; fail: __set_errno (EINVAL); return -1;}