SGU 106 The Equation 扩展欧几里得应用

#include <algorithm>
#include <cstdio>
#include <iostream>

using namespace std;

long long extend_gcd(long long a,long long b,long long &x,long long &y)
{
if(a==0&&b==0) return -1;
if(b==0){x=1;y=0;return a;}
long long d=extend_gcd(b,a%b,y,x);
y-=a/b*x;
return d;
}

long long a,b,c,x1,x2,y1,y2,ans;

int main()
{
cin>>a>>b>>c>>x1>>x2>>y1>>y2;
c=-c;
if(a==0&&b==0)
{
if(c==0)  ans=(x2-x1+1)*(y2-y1+1);
}
else if(a==0)
{
if(c%b==0&&c/b>=y1&&c/b<=y2)
ans=x2-x1+1;
}
else if(b==0)
{
if(c%a==0&&c/a>=x1&&c/a<=x2)
ans=y2-y1+1;
}
else
{
long long x=0,y=0;
long long d=extend_gcd(a,b,x,y);
if(c%d==0)
{
long long X=x*(c/d);
long long Y=y*(c/d);
long long k1,k2,k3,k4;
if(x1<=X||(x1-X)*d%b==0)
k1=(x1-X)*d/b;
else
k1=(x1-X)*d/b+1;
if(x2>=X||(X-x2)*d%b==0)
k2=(x2-X)*d/b;
else
k2=(x2-X)*d/b-1;
if(y1<=Y||(y1-Y)*d%a==0)
k3=(Y-y1)*d/a;
else
k3=(Y-y1)*d/a-1;
if(y2>=Y||(Y-y2)*d%a==0)
k4=(Y-y2)*d/a;
else
k4=(Y-y2)*d/a+1;
if(k1>k2) swap(k1,k2);
if(k3>k4) swap(k3,k4);
ans=min(k2,k4)-max(k1,k3)+1;
}
}
printf("%I64d\n",ans);
return 0;
}