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| #include <bits/stdc++.h>
using namespace std;
using ld = long double;
static const ld EPS = 1e-12L;
static const ld INF = 1e100L;
static const ld PI = acosl(-1.0L);
struct Pt {
ld x, y;
Pt() : x(0), y(0) {}
Pt(ld x_, ld y_) : x(x_), y(y_) {}
Pt operator + (const Pt& o) const { return Pt(x + o.x, y + o.y); }
Pt operator - (const Pt& o) const { return Pt(x - o.x, y - o.y); }
Pt operator * (ld k) const { return Pt(x * k, y * k); }
};
struct Seg {
Pt a, b;
};
ld dot(const Pt& a, const Pt& b) { return a.x * b.x + a.y * b.y; }
ld cross(const Pt& a, const Pt& b) { return a.x * b.y - a.y * b.x; }
ld norm2(const Pt& a) { return dot(a, a); }
bool isParallel(const Pt& a, const Pt& b) {
ld den = fabsl(cross(a, b));
ld scale = sqrtl(max((ld)1.0, norm2(a) * norm2(b)));
return den <= EPS * scale;
}
bool isCollinear(const Pt& a, const Pt& b) {
ld den = fabsl(cross(a, b));
ld scale = sqrtl(max((ld)1.0, norm2(a) * norm2(b)));
return den <= EPS * scale;
}
Pt reflectPoint(const Pt& p, const Seg& s) {
Pt e = s.b - s.a;
ld t = dot(p - s.a, e) / dot(e, e);
Pt h = s.a + e * t;
return h * 2.0L - p;
}
Pt reflectDir(const Pt& d, const Seg& s) {
Pt e = s.b - s.a;
ld k = 2.0L * dot(d, e) / dot(e, e);
return e * k - d;
}
bool raySegIntersectT(const Pt& p, const Pt& d, const Seg& s, ld& tOut) {
Pt e = s.b - s.a;
if (isParallel(d, e)) return false;
Pt ap = s.a - p;
ld den = cross(d, e);
ld t = cross(ap, e) / den;
ld u = cross(ap, d) / den;
if (t <= EPS) return false;
if (u <= EPS || u >= 1.0L - EPS) return false;
tOut = t;
return true;
}
bool rayHitOriginT(const Pt& p, const Pt& d, ld& tOut) {
Pt v(-p.x, -p.y);
if (!isCollinear(v, d)) return false;
ld t = dot(v, d) / norm2(d);
if (t <= EPS) return false;
tOut = t;
return true;
}
int angleRound(const Pt& d) {
ld deg = atan2l(d.y, d.x) * 180.0L / PI;
if (deg < 0) deg += 360.0L;
int a = (int)floorl(deg + 0.5L);
a %= 360;
return a;
}
bool simulate(const Pt& initDir, const vector<Seg>& mirrors) {
Pt p(0.0L, 0.0L), d = initDir;
int refl = 0;
while (true) {
ld tOrigin = INF, tTmp;
if (rayHitOriginT(p, d, tTmp)) tOrigin = tTmp;
ld bestT = INF;
int bestIdx = -1;
for (int i = 0; i < (int)mirrors.size(); ++i) {
if (raySegIntersectT(p, d, mirrors[i], tTmp) && tTmp < bestT) {
bestT = tTmp;
bestIdx = i;
}
}
if (tOrigin + EPS < bestT) {
return (refl >= 1 && refl <= 7);
}
if (refl == 7) return false;
if (bestIdx == -1) return false;
p = p + d * bestT;
d = reflectDir(d, mirrors[bestIdx]);
++refl;
}
}
void dfs(int depth, int limit, Pt imagePoint,
const vector<Seg>& mirrors, set<int>& ans) {
if (depth == limit) {
if (fabsl(imagePoint.x) <= EPS && fabsl(imagePoint.y) <= EPS) return;
if (simulate(imagePoint, mirrors)) {
ans.insert(angleRound(imagePoint));
}
return;
}
for (int i = 0; i < (int)mirrors.size(); ++i) {
Pt nxt = reflectPoint(imagePoint, mirrors[i]);
dfs(depth + 1, limit, nxt, mirrors, ans);
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
while (cin >> n) {
if (n == 0) break;
vector<Seg> mirrors(n);
for (int i = 0; i < n; ++i) {
cin >> mirrors[i].a.x >> mirrors[i].a.y
>> mirrors[i].b.x >> mirrors[i].b.y;
}
set<int> ans;
Pt O(0.0L, 0.0L);
for (int len = 1; len <= 7; ++len) {
dfs(0, len, O, mirrors, ans);
}
if (ans.empty()) {
cout << "no danger\n";
} else {
bool first = true;
for (int a : ans) {
if (!first) cout << ' ';
cout << a;
first = false;
}
cout << '\n';
}
}
return 0;
}
|
