POJ 2115-c looooops (basic unary linear with congruence)

Test instructions

For C's for (i=a; i!=b; I +=c) loop, ask how many loops in the K-bit storage system will end.

If the end is within a finite number of times, the output cycle times.

Otherwise output forever

Ideas:

Easy to list the congruence equation:

X*c+y*2^k = B-a can be solved by expanding GCD

148k0msc++808b#include<cstdio> #include <iostream> #include <algorithm> #include <cstring> Using namespace Std;typedef long long ll;ll a,b,c;ll x,y;int k;ll e_gcd (ll a,ll b,ll &x,ll &y) {    ll ans;    if (b==0)    {        ans=a;        x=1,y=0;    }    else    {        ans=e_gcd (b,a%b,y,x);        y-= (A/b) *x;    }    return ans;} int main () {    while (scanf ("%i64d%i64d%i64d%d", &a,&b,&c,&k), (a+b+c+k))    {        ll m= (LL) 1 <<k;        ll r= (b-a+m)%m;        ll D=E_GCD (c,m,x,y);        if (r%d) puts ("FOREVER");        else        {            ll ans=x* (r/d);            ll t=m/d;            Ans= (ans%t+t)%t;            printf ("%i64d\n", ans);        }    }    return 0;}

POJ 2115-c looooops (basic unary linear with congruence)