dist1.c

#include <math.h>
#include <sys/time.h>
#include <stdio.h>

double clock() {
  struct timeval tv;
  gettimeofday(&tv,NULL);
  return tv.tv_sec + 1e-6*tv.tv_usec;
}

static inline double sqr(double x) {return x*x;}
static double radians(double deg) {return deg * M_PI / 180;}
static double distance(double latA,double lngA,double latB,double lngB) {
  double
    radius = 6371,
    latAr = radians(latA),
    lngAr = radians(lngA),
    latBr = radians(latB),
    lngBr = radians(lngB),
    deltaLat = latBr - latAr,
    deltaLng = lngBr - lngAr;
  return radius * 2 * asin(sqrt(sqr(sin(deltaLat/2))
                                + cos(latAr) * cos(latBr) * sqr(sin(deltaLng/2))));
}

int main(void) {
  int i,j,k,l;
  double ii,jj,kk,ll,start,total = 0;
  start = clock();
  for (i=0,ii=-90; i<73; i++,ii+=2.5)
    for (j=0,jj=-180; j<145; j++,jj+=2.5)
      for (k=0,kk=-90; k<73; k++,kk+=2.5)
        for (l=0,ll=-180; l<145; l++,ll+=2.5)
          total += distance(ii,jj,kk,ll);
  printf("total = %g, time = %g\n",total,clock()-start);
  return 0;
}