codeforces 600D Area of Two Circles' Intersection

#include<bits/stdc++.h>
using namespace std;

typedef long double ld;
typedef long long ll;

ld x[2],y[2],r[2];

inline ld sqr(ld x){ return x*x; }
inline ll sqrl(ll x) { return x*x; }
inline ld fcos(ld a, ld b, ld c)
{
    return acosl((a*a+b*b-c*c)/(2*a*b));
}

inline ld cut(ld ang, ld r)
{
    ld s1 = ang*r*r;
    ld s2 = sinl(ang)*cosl(ang)*r*r;
    return s1 - s2;
}

const ld pi = acosl(-1);

double solve()
{
    if(r[0] > r[1]){
        swap(r[0],r[1]);
        swap(x[0],x[1]);
        swap(y[0],y[1]);
    }
    ll dx = x[0]-x[1], dy = y[0]-y[1];
    ll ds = sqrl(dx)+sqrl(dy);
    if(ds >= sqrl(r[0]+r[1])) return 0;
    ld d = sqrtl(ds);
    if(d+r[0] <= r[1]) return sqr(r[0])*pi;

    ld ang[2];
    ang[0] = fcos(d,r[0],r[1]);
    ang[1] = fcos(d,r[1],r[0]);
    /*
    WA 28
    ld area = ang[1]*sqr(r[1]) + ang[0]*sqr(r[0]);
    ld s = (d+r[0]+r[1])/2;
    area -= sqrtl(s*(s-d)*(s-r[0])*(s-r[1]))*2; // O(n^4) 拆分会有符号问题 对数也许可行
    return area;
    */
    return cut(ang[0],r[0]) + cut(ang[1],r[1]);
}

//#define LOCAL
int main()
{
#ifdef LOCAL
    freopen("in.txt","r",stdin);
#endif
    cin>>x[0]>>y[0]>>r[0]>>x[1]>>y[1]>>r[1];
    printf("%.10f\n", solve());
    return 0;
}