Reduce the process time..??!! complex algorithm

Question

HI,
I have a complex algorithm which takes almost 900 milliseconds per line to process this . I have to calculate the probability for hundreds of thousand lines. Just imagine if have to calculate this for some 500000 lines which is not feasilble at al.. Can anyone help me in simplifing this algorithm & improve the performance..
This is very urgent & your help will desperately needed. Pls...pls ..help.
thanks in advacnce...
probabilty(int a, int b, int c, int d)     {
     int a1 = 0;
     int b1 = 0;
     int c1 = 0;
     int d1 = 0;
     double p = 0.0;
     double q = 0.0;
     double sum = 0.0;
     if(a*b*c*d == 0) {
          if (d == 0) {
               a1 = a;
               a = b;
               b = a1;
               a1 = c;
               c = d;
               d = a1;
     } else if (((a + 0.0)/(b + 0.0)) < ((c + 0.0)/(d + 0.0))) {
          a1 = a;
          a = b;
          b = a1;
          a1 = c;
          c = d;
          d = a1;
     p = logFact(a+b) +
          logFact(a+c) +
          logFact(b+d) +
          logFact(c+d) -
          logFact(a+b+c+d);
     for (int i=b; i>=0; i--) {
          b1 = i;
          a1 = a + b - b1;
          c1 = a + c - a1;
          d1 = b + d - b1;
          q = logFact(a1) +
               logFact(b1) +
               logFact(c1) +
               logFact(d1);
          sum = Math.exp(p-q) + sum;
          if(a*b*c*d == 0) {
               break;
     }

Answer 1

Replace
if (a*b*c*d == 0) ==> if (a | b | c | d == 0)
Try to avoid arithmetic operation, if it cannot be avoided, try to use integer than double. For example :
the line:
if (((a + 0.0)/(b + 0.0)) < ((c + 0.0)/(d + 0.0))) can be rewritten as
if (a * d < b * c)
If the above has not reduce much processing time...I guess you need to take a closer look at logFact() function. Go thru' its mathematic algo, see if there is anyway to rewrite it, or u can try to post the body of logFact() in this forum.
CT