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Diffstat (limited to 'stdlib/random.c')
| -rw-r--r-- | stdlib/random.c | 357 |
1 files changed, 357 insertions, 0 deletions
diff --git a/stdlib/random.c b/stdlib/random.c new file mode 100644 index 0000000..fb32b36 --- /dev/null +++ b/stdlib/random.c @@ -0,0 +1,357 @@ +/* + * Copyright (c) 1983 Regents of the University of California. + * All rights reserved. + * + * Redistribution and use in source and binary forms are permitted + * provided that the above copyright notice and this paragraph are + * duplicated in all such forms and that any documentation, + * advertising materials, and other materials related to such + * distribution and use acknowledge that the software was developed + * by the University of California, Berkeley. The name of the + * University may not be used to endorse or promote products derived + * from this software without specific prior written permission. + * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR + * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED + * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE. + */ + +/* + * This is derived from the Berkeley source: + * @(#)random.c 5.5 (Berkeley) 7/6/88 + * It was reworked for the GNU C Library by Roland McGrath. + */ + +#include <ansidecl.h> +#include <errno.h> +#include <limits.h> +#include <stddef.h> +#include <stdlib.h> + + +/* An improved random number generation package. In addition to the standard + rand()/srand() like interface, this package also has a special state info + interface. The initstate() routine is called with a seed, an array of + bytes, and a count of how many bytes are being passed in; this array is + then initialized to contain information for random number generation with + that much state information. Good sizes for the amount of state + information are 32, 64, 128, and 256 bytes. The state can be switched by + calling the setstate() function with the same array as was initiallized + with initstate(). By default, the package runs with 128 bytes of state + information and generates far better random numbers than a linear + congruential generator. If the amount of state information is less than + 32 bytes, a simple linear congruential R.N.G. is used. Internally, the + state information is treated as an array of longs; the zeroeth element of + the array is the type of R.N.G. being used (small integer); the remainder + of the array is the state information for the R.N.G. Thus, 32 bytes of + state information will give 7 longs worth of state information, which will + allow a degree seven polynomial. (Note: The zeroeth word of state + information also has some other information stored in it; see setstate + for details). The random number generation technique is a linear feedback + shift register approach, employing trinomials (since there are fewer terms + to sum up that way). In this approach, the least significant bit of all + the numbers in the state table will act as a linear feedback shift register, + and will have period 2^deg - 1 (where deg is the degree of the polynomial + being used, assuming that the polynomial is irreducible and primitive). + The higher order bits will have longer periods, since their values are + also influenced by pseudo-random carries out of the lower bits. The + total period of the generator is approximately deg*(2**deg - 1); thus + doubling the amount of state information has a vast influence on the + period of the generator. Note: The deg*(2**deg - 1) is an approximation + only good for large deg, when the period of the shift register is the + dominant factor. With deg equal to seven, the period is actually much + longer than the 7*(2**7 - 1) predicted by this formula. */ + + + +/* For each of the currently supported random number generators, we have a + break value on the amount of state information (you need at least thi + 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. */ +#define TYPE_0 0 +#define BREAK_0 8 +#define DEG_0 0 +#define SEP_0 0 + +/* x**7 + x**3 + 1. */ +#define TYPE_1 1 +#define BREAK_1 32 +#define DEG_1 7 +#define SEP_1 3 + +/* x**15 + x + 1. */ +#define TYPE_2 2 +#define BREAK_2 64 +#define DEG_2 15 +#define SEP_2 1 + +/* x**31 + x**3 + 1. */ +#define TYPE_3 3 +#define BREAK_3 128 +#define DEG_3 31 +#define SEP_3 3 + +/* x**63 + x + 1. */ +#define TYPE_4 4 +#define BREAK_4 256 +#define DEG_4 63 +#define SEP_4 1 + + +/* Array versions of the above information to make code run faster. + Relies on fact that TYPE_i == i. */ + +#define MAX_TYPES 5 /* Max number of types above. */ + +static int degrees[MAX_TYPES] = { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 }; +static int seps[MAX_TYPES] = { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 }; + + + +/* Initially, everything is set up as if from: + initstate(1, randtbl, 128); + Note that this initialization takes advantage of the fact that srandom + advances the front and rear pointers 10*rand_deg times, and hence the + rear pointer which starts at 0 will also end up at zero; thus the zeroeth + element of the state information, which contains info about the current + position of the rear pointer is just + (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3. */ + +static long int randtbl[DEG_3 + 1] = + { + TYPE_3, + -851904987, -43806228, -2029755270, 1390239686, -1912102820, + -485608943, 1969813258, -1590463333, -1944053249, 455935928, 508023712, + -1714531963, 1800685987, -2015299881, 654595283, -1149023258, + -1470005550, -1143256056, -1325577603, -1568001885, 1275120390, + -607508183, -205999574, -1696891592, 1492211999, -1528267240, + -952028296, -189082757, 362343714, 1424981831, 2039449641, + }; + +/* FPTR and RPTR are two pointers into the state info, a front and a rear + pointer. These two pointers are always rand_sep places aparts, as they + cycle through the state information. (Yes, this does mean we could get + away with just one pointer, but the code for random is more efficient + this way). The pointers are left positioned as they would be from the call: + initstate(1, randtbl, 128); + (The position of the rear pointer, rptr, is really 0 (as explained above + in the initialization of randtbl) because the state table pointer is set + to point to randtbl[1] (as explained below).) */ + +static long int *fptr = &randtbl[SEP_3 + 1]; +static long int *rptr = &randtbl[1]; + + + +/* The following things are the pointer to the state information table, + the type of the current generator, the degree of the current polynomial + being used, and the separation between the two pointers. + Note that for efficiency of random, we remember the first location of + the state information, not the zeroeth. Hence it is valid to access + state[-1], which is used to store the type of the R.N.G. + Also, we remember the last location, since this is more efficient than + indexing every time to find the address of the last element to see if + the front and rear pointers have wrapped. */ + +static long int *state = &randtbl[1]; + +static int rand_type = TYPE_3; +static int rand_deg = DEG_3; +static int rand_sep = SEP_3; + +static long int *end_ptr = &randtbl[sizeof(randtbl) / sizeof(randtbl[0])]; + +/* 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. */ +void +DEFUN(__srandom, (x), unsigned int x) +{ + state[0] = x; + if (rand_type != TYPE_0) + { + register long int i; + for (i = 1; i < rand_deg; ++i) + state[i] = (1103515145 * state[i - 1]) + 12345; + fptr = &state[rand_sep]; + rptr = &state[0]; + for (i = 0; i < 10 * rand_deg; ++i) + (void) __random(); + } +} + +weak_alias (__srandom, srandom) +weak_alias (__srandom, srand) + +/* 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. */ +PTR +DEFUN(__initstate, (seed, arg_state, n), + unsigned int seed AND PTR arg_state AND size_t n) +{ + PTR ostate = (PTR) &state[-1]; + + if (rand_type == TYPE_0) + state[-1] = rand_type; + else + state[-1] = (MAX_TYPES * (rptr - state)) + rand_type; + if (n < BREAK_1) + { + if (n < BREAK_0) + { + errno = EINVAL; + return NULL; + } + rand_type = TYPE_0; + rand_deg = DEG_0; + rand_sep = SEP_0; + } + else if (n < BREAK_2) + { + rand_type = TYPE_1; + rand_deg = DEG_1; + rand_sep = SEP_1; + } + else if (n < BREAK_3) + { + rand_type = TYPE_2; + rand_deg = DEG_2; + rand_sep = SEP_2; + } + else if (n < BREAK_4) + { + rand_type = TYPE_3; + rand_deg = DEG_3; + rand_sep = SEP_3; + } + else + { + rand_type = TYPE_4; + rand_deg = DEG_4; + rand_sep = SEP_4; + } + + state = &((long int *) arg_state)[1]; /* First location. */ + /* Must set END_PTR before srandom. */ + end_ptr = &state[rand_deg]; + __srandom(seed); + if (rand_type == TYPE_0) + state[-1] = rand_type; + else + state[-1] = (MAX_TYPES * (rptr - state)) + rand_type; + + return ostate; +} + +weak_alias (__initstate, initstate) + +/* 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 zeroeth 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. */ +PTR +DEFUN(__setstate, (arg_state), PTR arg_state) +{ + register long int *new_state = (long int *) arg_state; + register int type = new_state[0] % MAX_TYPES; + register int rear = new_state[0] / MAX_TYPES; + PTR ostate = (PTR) &state[-1]; + + if (rand_type == TYPE_0) + state[-1] = rand_type; + else + state[-1] = (MAX_TYPES * (rptr - state)) + rand_type; + + switch (type) + { + case TYPE_0: + case TYPE_1: + case TYPE_2: + case TYPE_3: + case TYPE_4: + rand_type = type; + rand_deg = degrees[type]; + rand_sep = seps[type]; + break; + default: + /* State info munged. */ + errno = EINVAL; + return NULL; + } + + state = &new_state[1]; + if (rand_type != TYPE_0) + { + rptr = &state[rear]; + fptr = &state[(rear + rand_sep) % rand_deg]; + } + /* Set end_ptr too. */ + end_ptr = &state[rand_deg]; + + return ostate; +} + +weak_alias (__setstate, setstate) + +/* 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 ther 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. */ + +long int +DEFUN_VOID(__random) +{ + if (rand_type == TYPE_0) + { + state[0] = ((state[0] * 1103515245) + 12345) & LONG_MAX; + return state[0]; + } + else + { + long int i; + *fptr += *rptr; + /* Chucking least random bit. */ + i = (*fptr >> 1) & LONG_MAX; + ++fptr; + if (fptr >= end_ptr) + { + fptr = state; + ++rptr; + } + else + { + ++rptr; + if (rptr >= end_ptr) + rptr = state; + } + return i; + } +} + +weak_alias (__random, random) |