HNOI2008 bzoj1007 Horizontal Visible Straight line __ Math

Semi-planar intersection simplified version.

#include <cstdio> #include <algorithm> #include <cmath> using namespace std;
const int maxn=500010;
const double eps=1e-8;
    int cmp (double x) {if (x>=eps) return 1;
    if (fabs (x) <=eps) return 0;
return-1;
    struct Vector {double x,y;
    Vector operator + (const vector &v) const {return (Vector) {x+v.x,y+v.y};
    Vector operator-(const vector &v) const {return (Vector) {x-v.x,y-v.y};
    Vector operator * (const double &k) const {return (Vector) {x*k,y*k};
}}P[MAXN];
typedef Vector Point; Double dot (vector v,vector u) {return v.x*u.x+v.y*u.y.} Double Cross (vector v,vector u) {return v.x*u.y-v.y*u.x
;
    } struct line {point P;
    Vector v;
    int id;
    /*double angle () const {return atan2 (v.y,v.x);
    }*/BOOL Operator < (const line &l) Const {return CMP (V.Y-L.V.Y) ==-1;
}}A[MAXN],Q[MAXN];
Point intersection (line l,line m) {    Vector U=L.P-M.P;
    Double T=cross (m.v,u)/cross (L.V,M.V);
return l.p+l.v*t;
int N,ANS[MAXN];
    int main () {//freopen ("e.in", "R", stdin);
    Freopen ("E.out", "w", stdout);
    Freopen ("Bzoj_1007.in", "R", stdin);
    Freopen ("Bzoj_1007.out", "w", stdout);
    int hd,tl;
    Double x,y;
    scanf ("%d", &n);
        for (int i=1;i<=n;i++) {scanf ("%lf%lf", &x,&y);
    A[i]= (line) {(point) {0,y}, (Vector) {1,x},i};
    Sort (a+1,a+n+1);
    Q[HD=TL=1]=A[1];
        for (int i=2;i<=n;i++) {while (HD&LT;TL&AMP;&AMP;CMP (Cross (A[I].V,P[TL-1]-A[I].P)) <=0) tl--;
        while (HD&LT;TL&AMP;&AMP;CMP (Cross (A[I].V,P[HD]-A[I].P)) <=0) hd++;
        if (Q[tl]<a[i]) q[++tl]=a[i];
        else if (CMP (cross (A[I].V,Q[TL].P-A[I].P)) <=0) q[tl]=a[i];
    if (HD&LT;TL) p[tl-1]=intersection (Q[tl-1],q[tl]);
    }//while (hd<tl-1&&cmp (Cross (Q[1].V,P[TL-1]-Q[1].P)) <=0) tl--;
 for (int i=hd;i<=tl;i++) ans[q[i].id]=1;   for (int i=1;i<=n;i++) if (Ans[i]) printf ("%d", I);}