source: trunk/sys/libupc/upc_c_randdp.c @ 66

#if defined(USE_POW)
#define r23 pow(0.5, 23.0)
#define r46 (r23*r23)
#define t23 pow(2.0, 23.0)
#define t46 (t23*t23)
#else
#define r23 (0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5*0.5)
#define r46 (r23*r23)
#define t23 (2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0*2.0)
#define t46 (t23*t23)
#endif

double randlc (double *x, double a) {

/*  This routine returns a uniform pseudorandom double precision number in the
   range (0, 1) by using the linear congruential generator

   x_{k+1} = a x_k  (mod 2^46)

   where 0 < x_k < 2^46 and 0 < a < 2^46.  This scheme generates 2^44 numbers
   before repeating.  The argument A is the same as 'a' in the above formula,
   and X is the same as x_0.  A and X must be odd double precision integers
   in the range (1, 2^46).  The returned value RANDLC is normalized to be
   between 0 and 1, i.e. RANDLC = 2^(-46) * x_1.  X is updated to contain
   the new seed x_1, so that subsequent calls to RANDLC using the same
   arguments will generate a continuous sequence.

   This routine should produce the same results on any computer with at least
   48 mantissa bits in double precision floating point data.  On 64 bit
   systems, double precision should be disabled.

   David H. Bailey     October 26, 1990
*/
    double t1,t2,t3,t4,a1,a2,x1,x2,z;

/*  Break A into two parts such that A = 2^23 * A1 + A2. */
    t1 = r23 * a;
    a1 = (int)t1;
    a2 = a - t23 * a1;

/*  Break X into two parts such that X = 2^23 * X1 + X2, compute
    Z = A1 * X2 + A2 * X1  (mod 2^23), and then
    X = 2^23 * Z + A2 * X2  (mod 2^46).  */
    t1 = r23 * (*x);
    x1 = (int)t1;
    x2 = (*x) - t23 * x1;
    t1 = a1 * x2 + a2 * x1;
    t2 = (int)(r23 * t1);
    z = t1 - t23 * t2;
    t3 = t23 * z + a2 * x2;
    t4 = (int)(r46 * t3);
    (*x) = t3 - t46 * t4;

    return (r46 * (*x));
}

void vranlc (int n, double *x_seed, double a, double y[]) {
/* This routine generates N uniform pseudorandom double precision numbers in
   the range (0, 1) by using the linear congruential generator

   x_{k+1} = a x_k  (mod 2^46)

   where 0 < x_k < 2^46 and 0 < a < 2^46.  This scheme generates 2^44 numbers
   before repeating.  The argument A is the same as 'a' in the above formula,
   and X is the same as x_0.  A and X must be odd double precision integers
   in the range (1, 2^46).  The N results are placed in Y and are normalized
   to be between 0 and 1.  X is updated to contain the new seed, so that
   subsequent calls to VRANLC using the same arguments will generate a
   continuous sequence.  If N is zero, only initialization is performed, and
   the variables X, A and Y are ignored.

   This routine is the standard version designed for scalar or RISC systems.
   However, it should produce the same results on any single processor
   computer with at least 48 mantissa bits in double precision floating point
   data.  On 64 bit systems, double precision should be disabled.  */

    int i;
    double x,t1,t2,t3,t4,a1,a2,x1,x2,z;

/* Break A into two parts such that A = 2^23 * A1 + A2.  */
    t1 = r23 * a;
    a1 = (int)t1;
    a2 = a - t23 * a1;
    x = *x_seed;

/* Generate N results.   This loop is not vectorizable. */
    for (i = 1; i <= n; i++) {

/* Break X into two parts such that X = 2^23 * X1 + X2, compute
   Z = A1 * X2 + A2 * X1  (mod 2^23), and then
   X = 2^23 * Z + A2 * X2  (mod 2^46).  */
        t1 = r23 * x;
        x1 = (int)t1;
        x2 = x - t23 * x1;
        t1 = a1 * x2 + a2 * x1;
        t2 = (int)(r23 * t1);
        z = t1 - t23 * t2;
        t3 = t23 * z + a2 * x2;
        t4 = (int)(r46 * t3);
        x = t3 - t46 * t4;
        y[i] = r46 * x;
    }
    *x_seed = x;
}