src/common/random.c

#ifndef lint
static const char       RCSid[] = "$Id$";
#endif
#ifndef lint
static  char sccsid[] = "@(#)random.c 1.1 87/12/17 SMI"; /* from UCB 4.2 83/01/02 */
#endif

#include        <stdio.h>

/*
 * random.c:
 * 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() routine 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 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
 * the separation between the two lower order coefficients of the trinomial.
 */

#define         TYPE_0          0               /* linear congruential */
#define         BREAK_0         8
#define         DEG_0           0
#define         SEP_0           0

#define         TYPE_1          1               /* x**7 + x**3 + 1 */
#define         BREAK_1         32
#define         DEG_1           7
#define         SEP_1           3

#define         TYPE_2          2               /* x**15 + x + 1 */
#define         BREAK_2         64
#define         DEG_2           15
#define         SEP_2           1

#define         TYPE_3          3               /* x**31 + x**3 + 1 */
#define         BREAK_3         128
#define         DEG_3           31
#define         SEP_3           3

#define         TYPE_4          4               /* x**63 + x + 1 */
#define         BREAK_4         256
#define         DEG_4           63
#define         SEP_4           1

extern long     random();

/*
 * 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            randtbl[ DEG_3 + 1 ]    = { TYPE_3,
                            0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342, 
                            0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb, 
                            0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd, 
                            0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86, 
                            0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7, 
                            0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc, 
                            0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b, 
                                        0xf5ad9d0e, 0x8999220b, 0x27fb47b9 };

/*
 * 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
 * cyclically 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            *fptr                   = &randtbl[ SEP_3 + 1 ];
static  long            *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            *state                  = &randtbl[ 1 ];

static  int             rand_type               = TYPE_3;
static  int             rand_deg                = DEG_3;
static  int             rand_sep                = SEP_3;

static  long            *end_ptr                = &randtbl[ DEG_3 + 1 ];


/*
 * srandom:
 * 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.
 */

srandom( x )

    unsigned            x;
{
        register  int           i, j;

        if(  rand_type  ==  TYPE_0  )  {
            state[ 0 ] = x;
        }
        else  {
            j = 1;
            state[ 0 ] = x;
            for( i = 1; i < rand_deg; i++ )  {
                state[i] = 1103515245*state[i - 1] + 12345;
            }
            fptr = &state[ rand_sep ];
            rptr = &state[ 0 ];
            for( i = 0; i < 10*rand_deg; i++ )  random();
        }
}


/*
 * initstate:
 * 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.
 */

char  *
initstate( seed, arg_state, n )

    unsigned            seed;                   /* seed for R. N. G. */
    char                *arg_state;             /* pointer to state array */
    int                 n;                      /* # bytes of state info */
{
        register  char          *ostate         = (char *)( &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  )  {
                fprintf( stderr, "initstate: not enough state (%d bytes) with which to do jack; ignored.\n" );
                return;
            }
            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 *)arg_state )[1]  );        /* first location */
        end_ptr = &state[ rand_deg ];   /* must set end_ptr before srandom */
        srandom( seed );
        if(  rand_type  ==  TYPE_0  )  state[ -1 ] = rand_type;
        else  state[ -1 ] = MAX_TYPES*(rptr - state) + rand_type;
        return( ostate );
}


/*
 * setstate:
 * 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.
 */

char  *
setstate( arg_state )

    char                *arg_state;
{
        register  long          *new_state      = (long *)arg_state;
        register  int           type            = new_state[0]%MAX_TYPES;
        register  int           rear            = new_state[0]/MAX_TYPES;
        char                    *ostate         = (char *)( &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:
                fprintf( stderr, "setstate: state info has been munged; not changed.\n" );
        }
        state = &new_state[ 1 ];
        if(  rand_type  !=  TYPE_0  )  {
            rptr = &state[ rear ];
            fptr = &state[ (rear + rand_sep)%rand_deg ];
        }
        end_ptr = &state[ rand_deg ];           /* set end_ptr too */
        return( ostate );
}


/*
 * random:
 * 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
random()
{
        long            i;
        
        if(  rand_type  ==  TYPE_0  )  {
            i = state[0] = ( state[0]*1103515245 + 12345 )&0x7fffffff;
        }
        else  {
            *fptr += *rptr;
            i = (*fptr >> 1)&0x7fffffff;        /* chucking least random bit */
            if(  ++fptr  >=  end_ptr  )  {
                fptr = state;
                ++rptr;
            }
            else  {
                if(  ++rptr  >=  end_ptr  )  rptr = state;
            }
        }
        return( i );
}

Revision:	3.1
Committed:	Sat Feb 22 02:07:22 2003 UTC (21 years, 2 months ago) by greg
Content type:	text/plain
Branch:	MAIN
CVS Tags:	rad3R6P1, rad3R5, rad3R6
Log Message:	Changes and check-in for 3.5 release Includes new source files and modifications not recorded for many years See ray/doc/notes/ReleaseNotes for notes between 3.1 and 3.5 release
#	Content
1	#ifndef lint
2	static const char RCSid[] = "$Id$";
3	#endif
4	#ifndef lint
5	static char sccsid[] = "@(#)random.c 1.1 87/12/17 SMI"; /* from UCB 4.2 83/01/02 */
6	#endif
7
8	#include <stdio.h>
9
10	/*
11	* random.c:
12	* An improved random number generation package. In addition to the standard
13	* rand()/srand() like interface, this package also has a special state info
14	* interface. The initstate() routine is called with a seed, an array of
15	* bytes, and a count of how many bytes are being passed in; this array is then
16	* initialized to contain information for random number generation with that
17	* much state information. Good sizes for the amount of state information are
18	* 32, 64, 128, and 256 bytes. The state can be switched by calling the
19	* setstate() routine with the same array as was initiallized with initstate().
20	* By default, the package runs with 128 bytes of state information and
21	* generates far better random numbers than a linear congruential generator.
22	* If the amount of state information is less than 32 bytes, a simple linear
23	* congruential R.N.G. is used.
24	* Internally, the state information is treated as an array of longs; the
25	* zeroeth element of the array is the type of R.N.G. being used (small
26	* integer); the remainder of the array is the state information for the
27	* R.N.G. Thus, 32 bytes of state information will give 7 longs worth of
28	* state information, which will allow a degree seven polynomial. (Note: the
29	* zeroeth word of state information also has some other information stored
30	* in it -- see setstate() for details).
31	* The random number generation technique is a linear feedback shift register
32	* approach, employing trinomials (since there are fewer terms to sum up that
33	* way). In this approach, the least significant bit of all the numbers in
34	* the state table will act as a linear feedback shift register, and will have
35	* period 2^deg - 1 (where deg is the degree of the polynomial being used,
36	* assuming that the polynomial is irreducible and primitive). The higher
37	* order bits will have longer periods, since their values are also influenced
38	* by pseudo-random carries out of the lower bits. The total period of the
39	* generator is approximately deg(2*deg - 1); thus doubling the amount of
40	* state information has a vast influence on the period of the generator.
41	* Note: the deg(2*deg - 1) is an approximation only good for large deg,
42	* when the period of the shift register is the dominant factor. With deg
43	* equal to seven, the period is actually much longer than the 7(2*7 - 1)
44	* predicted by this formula.
45	*/
46
47
48
49	/*
50	* For each of the currently supported random number generators, we have a
51	* break value on the amount of state information (you need at least this
52	* many bytes of state info to support this random number generator), a degree
53	* for the polynomial (actually a trinomial) that the R.N.G. is based on, and
54	* the separation between the two lower order coefficients of the trinomial.
55	*/
56
57	#define TYPE_0 0 /* linear congruential */
58	#define BREAK_0 8
59	#define DEG_0 0
60	#define SEP_0 0
61
62	#define TYPE_1 1 /* x7 + x3 + 1 */
63	#define BREAK_1 32
64	#define DEG_1 7
65	#define SEP_1 3
66
67	#define TYPE_2 2 /* x*15 + x + 1 /
68	#define BREAK_2 64
69	#define DEG_2 15
70	#define SEP_2 1
71
72	#define TYPE_3 3 /* x31 + x3 + 1 */
73	#define BREAK_3 128
74	#define DEG_3 31
75	#define SEP_3 3
76
77	#define TYPE_4 4 /* x*63 + x + 1 /
78	#define BREAK_4 256
79	#define DEG_4 63
80	#define SEP_4 1
81
82	extern long random();
83
84	/*
85	* Array versions of the above information to make code run faster -- relies
86	* on fact that TYPE_i == i.
87	*/
88
89	#define MAX_TYPES 5 /* max number of types above */
90
91	static int degrees[ MAX_TYPES ] = { DEG_0, DEG_1, DEG_2,
92	DEG_3, DEG_4 };
93
94	static int seps[ MAX_TYPES ] = { SEP_0, SEP_1, SEP_2,
95	SEP_3, SEP_4 };
96
97
98
99	/*
100	* Initially, everything is set up as if from :
101	* initstate( 1, &randtbl, 128 );
102	* Note that this initialization takes advantage of the fact that srandom()
103	* advances the front and rear pointers 10*rand_deg times, and hence the
104	* rear pointer which starts at 0 will also end up at zero; thus the zeroeth
105	* element of the state information, which contains info about the current
106	* position of the rear pointer is just
107	* MAX_TYPES*(rptr - state) + TYPE_3 == TYPE_3.
108	*/
109
110	static long randtbl[ DEG_3 + 1 ] = { TYPE_3,
111	0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342,
112	0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb,
113	0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
114	0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86,
115	0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7,
116	0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc,
117	0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b,
118	0xf5ad9d0e, 0x8999220b, 0x27fb47b9 };
119
120	/*
121	* fptr and rptr are two pointers into the state info, a front and a rear
122	* pointer. These two pointers are always rand_sep places aparts, as they cycle
123	* cyclically through the state information. (Yes, this does mean we could get
124	* away with just one pointer, but the code for random() is more efficient this
125	* way). The pointers are left positioned as they would be from the call
126	* initstate( 1, randtbl, 128 )
127	* (The position of the rear pointer, rptr, is really 0 (as explained above
128	* in the initialization of randtbl) because the state table pointer is set
129	* to point to randtbl[1] (as explained below).
130	*/
131
132	static long *fptr = &randtbl[ SEP_3 + 1 ];
133	static long *rptr = &randtbl[ 1 ];
134
135
136
137	/*
138	* The following things are the pointer to the state information table,
139	* the type of the current generator, the degree of the current polynomial
140	* being used, and the separation between the two pointers.
141	* Note that for efficiency of random(), we remember the first location of
142	* the state information, not the zeroeth. Hence it is valid to access
143	* state[-1], which is used to store the type of the R.N.G.
144	* Also, we remember the last location, since this is more efficient than
145	* indexing every time to find the address of the last element to see if
146	* the front and rear pointers have wrapped.
147	*/
148
149	static long *state = &randtbl[ 1 ];
150
151	static int rand_type = TYPE_3;
152	static int rand_deg = DEG_3;
153	static int rand_sep = SEP_3;
154
155	static long *end_ptr = &randtbl[ DEG_3 + 1 ];
156
157
158
159	/*
160	* srandom:
161	* Initialize the random number generator based on the given seed. If the
162	* type is the trivial no-state-information type, just remember the seed.
163	* Otherwise, initializes state[] based on the given "seed" via a linear
164	* congruential generator. Then, the pointers are set to known locations
165	* that are exactly rand_sep places apart. Lastly, it cycles the state
166	* information a given number of times to get rid of any initial dependencies
167	* introduced by the L.C.R.N.G.
168	* Note that the initialization of randtbl[] for default usage relies on
169	* values produced by this routine.
170	*/
171
172	srandom( x )
173
174	unsigned x;
175	{
176	register int i, j;
177
178	if( rand_type == TYPE_0 ) {
179	state[ 0 ] = x;
180	}
181	else {
182	j = 1;
183	state[ 0 ] = x;
184	for( i = 1; i < rand_deg; i++ ) {
185	state[i] = 1103515245*state[i - 1] + 12345;
186	}
187	fptr = &state[ rand_sep ];
188	rptr = &state[ 0 ];
189	for( i = 0; i < 10*rand_deg; i++ ) random();
190	}
191	}
192
193
194
195	/*
196	* initstate:
197	* Initialize the state information in the given array of n bytes for
198	* future random number generation. Based on the number of bytes we
199	* are given, and the break values for the different R.N.G.'s, we choose
200	* the best (largest) one we can and set things up for it. srandom() is
201	* then called to initialize the state information.
202	* Note that on return from srandom(), we set state[-1] to be the type
203	* multiplexed with the current value of the rear pointer; this is so
204	* successive calls to initstate() won't lose this information and will
205	* be able to restart with setstate().
206	* Note: the first thing we do is save the current state, if any, just like
207	* setstate() so that it doesn't matter when initstate is called.
208	* Returns a pointer to the old state.
209	*/
210
211	char *
212	initstate( seed, arg_state, n )
213
214	unsigned seed; /* seed for R. N. G. */
215	char arg_state; / pointer to state array */
216	int n; /* # bytes of state info */
217	{
218	register char ostate = (char )( &state[ -1 ] );
219
220	if( rand_type == TYPE_0 ) state[ -1 ] = rand_type;
221	else state[ -1 ] = MAX_TYPES*(rptr - state) + rand_type;
222	if( n < BREAK_1 ) {
223	if( n < BREAK_0 ) {
224	fprintf( stderr, "initstate: not enough state (%d bytes) with which to do jack; ignored.\n" );
225	return;
226	}
227	rand_type = TYPE_0;
228	rand_deg = DEG_0;
229	rand_sep = SEP_0;
230	}
231	else {
232	if( n < BREAK_2 ) {
233	rand_type = TYPE_1;
234	rand_deg = DEG_1;
235	rand_sep = SEP_1;
236	}
237	else {
238	if( n < BREAK_3 ) {
239	rand_type = TYPE_2;
240	rand_deg = DEG_2;
241	rand_sep = SEP_2;
242	}
243	else {
244	if( n < BREAK_4 ) {
245	rand_type = TYPE_3;
246	rand_deg = DEG_3;
247	rand_sep = SEP_3;
248	}
249	else {
250	rand_type = TYPE_4;
251	rand_deg = DEG_4;
252	rand_sep = SEP_4;
253	}
254	}
255	}
256	}
257	state = &( ( (long )arg_state )[1] ); / first location */
258	end_ptr = &state[ rand_deg ]; /* must set end_ptr before srandom */
259	srandom( seed );
260	if( rand_type == TYPE_0 ) state[ -1 ] = rand_type;
261	else state[ -1 ] = MAX_TYPES*(rptr - state) + rand_type;
262	return( ostate );
263	}
264
265
266
267	/*
268	* setstate:
269	* Restore the state from the given state array.
270	* Note: it is important that we also remember the locations of the pointers
271	* in the current state information, and restore the locations of the pointers
272	* from the old state information. This is done by multiplexing the pointer
273	* location into the zeroeth word of the state information.
274	* Note that due to the order in which things are done, it is OK to call
275	* setstate() with the same state as the current state.
276	* Returns a pointer to the old state information.
277	*/
278
279	char *
280	setstate( arg_state )
281
282	char *arg_state;
283	{
284	register long new_state = (long )arg_state;
285	register int type = new_state[0]%MAX_TYPES;
286	register int rear = new_state[0]/MAX_TYPES;
287	char ostate = (char )( &state[ -1 ] );
288
289	if( rand_type == TYPE_0 ) state[ -1 ] = rand_type;
290	else state[ -1 ] = MAX_TYPES*(rptr - state) + rand_type;
291	switch( type ) {
292	case TYPE_0:
293	case TYPE_1:
294	case TYPE_2:
295	case TYPE_3:
296	case TYPE_4:
297	rand_type = type;
298	rand_deg = degrees[ type ];
299	rand_sep = seps[ type ];
300	break;
301
302	default:
303	fprintf( stderr, "setstate: state info has been munged; not changed.\n" );
304	}
305	state = &new_state[ 1 ];
306	if( rand_type != TYPE_0 ) {
307	rptr = &state[ rear ];
308	fptr = &state[ (rear + rand_sep)%rand_deg ];
309	}
310	end_ptr = &state[ rand_deg ]; /* set end_ptr too */
311	return( ostate );
312	}
313
314
315
316	/*
317	* random:
318	* If we are using the trivial TYPE_0 R.N.G., just do the old linear
319	* congruential bit. Otherwise, we do our fancy trinomial stuff, which is the
320	* same in all ther other cases due to all the global variables that have been
321	* set up. The basic operation is to add the number at the rear pointer into
322	* the one at the front pointer. Then both pointers are advanced to the next
323	* location cyclically in the table. The value returned is the sum generated,
324	* reduced to 31 bits by throwing away the "least random" low bit.
325	* Note: the code takes advantage of the fact that both the front and
326	* rear pointers can't wrap on the same call by not testing the rear
327	* pointer if the front one has wrapped.
328	* Returns a 31-bit random number.
329	*/
330
331	long
332	random()
333	{
334	long i;
335
336	if( rand_type == TYPE_0 ) {
337	i = state[0] = ( state[0]*1103515245 + 12345 )&0x7fffffff;
338	}
339	else {
340	fptr += rptr;
341	i = (fptr >> 1)&0x7fffffff; / chucking least random bit */
342	if( ++fptr >= end_ptr ) {
343	fptr = state;
344	++rptr;
345	}
346	else {
347	if( ++rptr >= end_ptr ) rptr = state;
348	}
349	}
350	return( i );
351	}
352