ccan/junkcode/tterribe@email.unc.edu-nmbrthry/egcd.c

#include "egcd.h"

/*Computes the coefficients of the smallest positive linear combination of two
   integers _a and _b.
  These are a solution (u,v) to the equation _a*u+_b*v==gcd(_a,_b).
  _a: The first integer of which to compute the extended gcd.
  _b: The second integer of which to compute the extended gcd.
  _u: Returns the coefficient of _a in the smallest positive linear
       combination.
  _v: Returns the coefficient _of b in the smallest positive linear
       combination.
  Return: The non-negative gcd of _a and _b.
          If _a and _b are both 0, then 0 is returned, though in reality the
           gcd is undefined, as any integer, no matter how large, will divide 0
           evenly.
          _a*u+_b*v will not be positive in this case.
          Note that the solution (u,v) of _a*u+_b*v==gcd(_a,_b) returned is not
           unique.
          (u+(_b/gcd(_a,_b))*k,v-(_a/gcd(_a,_b))*k) is also a solution for all
           k.
          The coefficients (u,v) might not be positive.*/
int egcd(int _a,int _b,int *_u,int *_v){
  int a;
  int b;
  int s;
  int t;
  int u;
  int v;
  /*Make both arguments non-negative.
    This forces the return value to be non-negative.*/
  a=_a<0?-_a:_a;
  b=_b<0?-_b:_b;
  /*Simply use the extended Euclidean algorithm.*/
  s=v=0;
  t=u=1;
  while(b){
    int q;
    int r;
    int w;
    q=a/b;
    r=a%b;
    a=b;
    b=r;
    w=s;
    s=u-q*s;
    u=w;
    w=t;
    t=v-q*t;
    v=w;
  }
  /*u and v were computed for non-negative a and b.
    If the arguments passed in were negative, flip the sign.*/
  *_u=_a<0?-u:u;
  *_v=_b<0?-v:v;
  return a;
}