# shithub: drawterm

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```#include "os.h"
#include <mp.h>

#define iseven(a)	(((a)->p[0] & 1) == 0)

// extended binary gcd
//
// For a and b it solves, v = gcd(a,b) and finds x and y s.t.
// ax + by = v
//
// Handbook of Applied Cryptography, Menezes et al, 1997, pg 608.
void
mpextendedgcd(mpint *a, mpint *b, mpint *v, mpint *x, mpint *y)
{
mpint *u, *A, *B, *C, *D;
int g;

if(v == nil){
v = mpnew(0);
mpextendedgcd(a, b, v, x, y);
mpfree(v);
return;
}
assert(x == nil || (x->flags & MPtimesafe) == 0);
assert(y == nil || (y->flags & MPtimesafe) == 0);
assert((a->flags&b->flags) & MPnorm);
assert(((a->flags|b->flags|v->flags) & MPtimesafe) == 0);

if(a->sign < 0 || b->sign < 0){
mpassign(mpzero, v);
mpassign(mpzero, y);
mpassign(mpzero, x);
return;
}

if(a->top == 0){
mpassign(b, v);
mpassign(mpone, y);
mpassign(mpzero, x);
return;
}
if(b->top == 0){
mpassign(a, v);
mpassign(mpone, x);
mpassign(mpzero, y);
return;
}

g = 0;
a = mpcopy(a);
b = mpcopy(b);

while(iseven(a) && iseven(b)){
mpright(a, 1, a);
mpright(b, 1, b);
g++;
}

u = mpcopy(a);
mpassign(b, v);
A = mpcopy(mpone);
B = mpcopy(mpzero);
C = mpcopy(mpzero);
D = mpcopy(mpone);

for(;;) {
//		print("%B %B %B %B %B %B\n", u, v, A, B, C, D);
while(iseven(u)){
mpright(u, 1, u);
if(!iseven(A) || !iseven(B)) {
mpadd(A, b, A);
mpsub(B, a, B);
}
mpright(A, 1, A);
mpright(B, 1, B);
}

//		print("%B %B %B %B %B %B\n", u, v, A, B, C, D);
while(iseven(v)){
mpright(v, 1, v);
if(!iseven(C) || !iseven(D)) {
mpadd(C, b, C);
mpsub(D, a, D);
}
mpright(C, 1, C);
mpright(D, 1, D);
}

//		print("%B %B %B %B %B %B\n", u, v, A, B, C, D);
if(mpcmp(u, v) >= 0){
mpsub(u, v, u);
mpsub(A, C, A);
mpsub(B, D, B);
} else {
mpsub(v, u, v);
mpsub(C, A, C);
mpsub(D, B, D);
}

if(u->top == 0)
break;

}
mpassign(C, x);
mpassign(D, y);
mpleft(v, g, v);

mpfree(A);
mpfree(B);
mpfree(C);
mpfree(D);
mpfree(u);
mpfree(a);
mpfree(b);
}
```