O(logn) for calculating fibonacci number

O(logn) code for computing fib(n)

#include long int fib(long int n) {       long int a=1, b=0, p=0, q=1, prev_a, prev_p = 0;     while(n>0) {         if (n%2 == 0) {             prev_p = p;             p = p*p + q*q;             q = 2*prev_p*q + q*q;             n /= 2;         } else {             prev_a = a;             a = b*q + a*q + a*p;             b = b*p + prev_a*q;             n -= 1;         }     }     return b; } int main(int argc, char **argv) {       long int n;       scanf("%ld", &n);     printf("%ld", fib(n));         return 0;   }

Reference: http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-11.html#%_thm_1.19