frand.c

XPDC1是针对静电模型中的等离子体模拟程序！通过INP文件输入参数有较好的通用性

float frand(void);
/************************************************************************/
/* irand(iseed) returns values 1 through 2**31-2.                       */
/* From "Random number generators: good ones are hard to find", S. Park */
/* and K. Miller, Communications of ACM, October 1988, pp 1192-1201.    */
/* This is from page 1195, and is to work on any system for which       */
/* maxint is 2**31-1 or larger. Due earlier to Schrage, as cited by P&M.*/
/*                                                                      */
/* Note: OK values for iseed are 1 through 2**31-2. Give it 0 or 2*31-1 */
/* and it will return the same values thereafter!                       */
/*                                                                      */
/* C version 6/91, Bruce Langdon.                                       */
/*                                                                      */
/* Algorithm replaces seed by mod(a*seed,m). First represent            */
/* seed = q*hi + lo.  Then                                              */
/* a*seed = a*q*hi + lo = (m - r)*hi + a*lo = (a*lo - r*hi) + m*hi,     */
/* and new seed = a*lo - r*hi unless negative; if so, then add m.       */

float frand(void)
{
  long a = 16807, m = 2147483647, q = 127773, r = 2836;
  long hi, lo;
  static long seed=31207321;
  float fnumb;

  hi = seed/q;
  lo = seed - q*hi;
  seed = a*lo - r*hi;
  /* "seed" will always be a legal integer of 32 bits (including sign). */
  if(seed <= 0) seed = seed + m;
  fnumb = seed/2147483646.0;

  return(fnumb);
}