半平面求交——放置守卫

#include <bits/stdc++.h>
//using namespcace std;
const int maxn = 1510;
const double EPS = 1E-12;
struct Point
{
	double x, y;
};
Point convex[maxn];
struct Line
{
	Point a, b;
	double ang;
};
Line l[maxn], st[maxn];
int n, ccnt;
double operator *(const Point &x, const Point &y)
{
	return x.x * y.y - x.y * y.x;
}
Point operator - (Point x, const Point &y)
{
	x.x -= y.x, x.y -= y.y;
	return x;
}
Point operator * (const Line &x, const Line &y)
{
	double a1 = (y.b - x.a) * (y.a - x.a) , a2 = (y.a - x.b) * (y.b - x.b);
	Point r;
	r.x =  (x.a.x * a2 + x.b.x * a1) / (a2 + a1);
	r.y = (x.a.y * a2 + x.b.y * a1) / (a2 + a1);
	return r;
}
bool operator == (const Point &a, const Point &b)
{
	return fabs(a.x - b.x) < EPS && fabs(a.y - b.y) < EPS;
}
bool operator <(const Line &x, const Line &y)
{
	if (fabs(x.ang - y.ang) < EPS)
		return (y.b - x.a) * (x.b - y.a) > EPS;
	return x.ang < y.ang;
}
bool JudgeOut(const Line &x, const Point &p)
{
	return (p - x.a) * (x.b - x.a) > EPS;
}
bool Parellel(const Line &x, const Line &y)
{
	return fabs((x.b - x.a) * (y.b - y.a)) < -EPS;
}
void InputData()
{
	scanf("%d", &n);
	scanf("%lf%lf", &l[0].b.x, &l[0].b.y);
	l[n - 1].a.x = l[0].b.x, l[n - 1].a.y = l[0].b.y;
	for (int i = 1; i < n; i++)
	{
		scanf("%lf%lf", &l[i].b.x, &l[i].b.y);
		l[i - 1].a.x = l[i].b.x; l[i - 1].a.y = l[i].b.y;
	}
	for (int i = 0; i < n; i++)
		l[i].ang = atan2(l[i].b.y - l[i].a.y, l[i].b.x - l[i].a.x);
}
double HplaneIntersection()
{
	int top = 1, bot  = 0;
	std::sort(l, l + n);
	int tmp = 1;
	for (int i = 1; i < n; i++)
		if (l[i].ang - l[i - 1].ang > EPS) l[tmp++] = l[i];
	n = tmp;
	st[0] = l[0], st[1] = l[1];
	for (int i = 2; i < n; i++)
	{
		if (Parellel(st[top], st[top - 1]) || Parellel(st[bot], st[bot - 1])) return 0;
		while (bot < top && JudgeOut(l[i], st[top]*st[top - 1])) top--;
		while (bot < top && JudgeOut(l[i], st[bot]*st[bot + 1])) bot++;
		st[++top] = l[i];
	}
	while (bot < top && JudgeOut(st[bot], st[top]*st[top - 1])) top--;
	while (bot < top && JudgeOut(st[top], st[bot]*st[bot + 1])) bot++;
	if (top <= bot + 1) return 0.00;
	st[++top] = st[bot];
	ccnt = 0;
	for (int i = bot; i < top; i++)
		convex[ccnt++] = st[i] * st[i + 1];
	double ans = 0;
	convex[ccnt] = convex[0];
	for (int i = 0; i < ccnt; i++)
		ans += convex[i] * convex[i + 1];
	return ans / 2;
}
double CheckDirection()
{
	double ans = 0;
	for (int i = 0; i < n; i++)
		ans += l[i].a * l[i].b;
	return ans;
}
void ChangeDirection()
{
	for (int i = 0; i < n; i++)
		std::swap(l[i].a, l[i].b);
}
int main()
{
	int t;
	scanf("%d", &t);
	while (t--)
	{
		InputData();
		if (CheckDirection() < 0) ChangeDirection();
		printf("%.2f\n", HplaneIntersection());
	}
	return 0;
}