/*
 * Copyright (c) 1983 Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted
 * provided that: (1) source distributions retain this entire copyright
 * notice and comment, and (2) distributions including binaries display
 * the following acknowledgement:  ``This product includes software
 * developed by the University of California, Berkeley and its contributors''
 * in the documentation or other materials provided with the distribution
 * and in all advertising materials mentioning features or use of this
 * software. Neither the name of the University nor the names of its
 * contributors may 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 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)random.c	5.7 (Berkeley) 6/1/90";
#endif /* LIBC_SCCS and not lint */

#include "tweak.h"
#include <stdio.h>

#ifdef NEED_RANDOM
/*
 * 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

/*
 * 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.
 */

void srandom PROTO1(unsigned int, x)
{
  register  int		i, j;
  long random();
  
  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 PROTO3(unsigned int, seed, char *, arg_state, int, n )
{
  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); ignored.\n", n);
      return 0;
    }
    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 PROTO1(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 PROTO0((void))
{
  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);
}
#endif /* NEED_RANDOM */
