expl.c

#define EXP_TAYLOR_TERMS 18

static uint64_t fact[]=
{
  1,
  1,
  2,
  6,
  24,
  120,
  720,
  5040,
  40320,
  362880,
  3628800,
  39916800,
  479001600,
  6227020800,
  87178291200,
  1307674368000,
  20922789888000,
  355687428096000,
  6402373705728000
};

long double expl(long double x)
{
  long double res; /* Result of the operation */
  long double xpow; /* At each iteration, will be multiplied by x to simulate pow(x, i) */
  uint64_t i; /* Iteration variable */
  uint64_t k; /* How many times we have to divide x by 2 to get x<1 */
  
  res=0.0;
  xpow=1.0;
  
  if (x<0.0)
    return (1.0/expl(-x));
  for(k=0; x>1.0; ++k)
    x /= 2.0;
  for (i=0; i<EXP_TAYLOR_TERMS; ++i)
  {
    res += xpow/fact[i]
    xpow *= x;
  }
  for (i=0; i<k; ++i)
    res *= res;
  return (res);
}