# code: drawterm

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

// extended euclid
//
// For a and b it solves, d = gcd(a,b) and finds x and y s.t.
// ax + by = d
//
// Handbook of Applied Cryptography, Menezes et al, 1997, pg 67

void
mpeuclid(mpint *a, mpint *b, mpint *d, mpint *x, mpint *y)
{
mpint *tmp, *x0, *x1, *x2, *y0, *y1, *y2, *q, *r;

if(mpcmp(a, b) < 0){
tmp = a;
a = b;
b = tmp;
tmp = x;
x = y;
y = tmp;
}

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

a = mpcopy(a);
b = mpcopy(b);
x0 = mpnew(0);
x1 = mpcopy(mpzero);
x2 = mpcopy(mpone);
y0 = mpnew(0);
y1 = mpcopy(mpone);
y2 = mpcopy(mpzero);
q = mpnew(0);
r = mpnew(0);

while(b->top != 0 && b->sign > 0){
// q = a/b
// r = a mod b
mpdiv(a, b, q, r);
// x0 = x2 - qx1
mpmul(q, x1, x0);
mpsub(x2, x0, x0);
// y0 = y2 - qy1
mpmul(q, y1, y0);
mpsub(y2, y0, y0);
// rotate values
tmp = a;
a = b;
b = r;
r = tmp;
tmp = x2;
x2 = x1;
x1 = x0;
x0 = tmp;
tmp = y2;
y2 = y1;
y1 = y0;
y0 = tmp;
}

mpassign(a, d);
mpassign(x2, x);
mpassign(y2, y);

mpfree(x0);
mpfree(x1);
mpfree(x2);
mpfree(y0);
mpfree(y1);
mpfree(y2);
mpfree(q);
mpfree(r);
mpfree(a);
mpfree(b);
}
```