C++ solution for problem E

#include <cstring>
#include <map>
#include <deque>
#include <queue>
#include <stack>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <algorithm>
#include <vector>
#include <set>
#include <complex>
#include <list>
#include <climits>
#include <cctype>

using namespace std;

typedef complex<double> point;
#define cross(a,b) ((conj(a)*(b)).imag())
#define X real()
#define Y imag()

const double eps = 1e-9;

int comp(double a, double b) {
    if (fabs(a - b) < eps)
        return 0;
    return a > b ? 1 : -1;
}

struct pComp {
    bool operator()(const point &A, const point &B) const {
        if (comp(A.X, B.X))
            return A.X < B.X;
        if (comp(A.Y, B.Y))
            return A.Y < B.Y;
        return 0;
    }
};

double len(point A, point B) {
    return hypot(A.X - B.X, A.Y - B.Y);
}

bool pointOnSeg(point A, point B, point R) {
    return comp(len(A, B), len(A, R) + len(B, R)) == 0;
}

bool lineInter(point A, point B, point P, point Q, point &R) {
    double d1 = cross(P-A,B-A);
    double d2 = cross(Q-A,B-A);
    if (comp(d1, d2) == 0)
        return false;
    R = (d1 * Q - d2 * P) / (d1 - d2);
    if (!pointOnSeg(A, B, R) || !pointOnSeg(P, Q, R))
        return false;
    return true;
}

int n;
point st, en;
vector<point> pol;

double mat[34][34];

int main() {
    int x, y;
    cin >> x >> y;
    st = point(x, y);
    cin >> x >> y;
    en = point(x, y);
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> x >> y;
        pol.push_back(point(x, y));
    }
    double ret = len(st, en);
    set<point, pComp> intrs;
    for (int i = 0; i < n; i++) {
        point R;
        if (lineInter(st, en, pol[i], pol[(i + 1) % n], R))
            intrs.insert(R);
    }
    if (intrs.size() == 2) {
        point p1 = *intrs.begin();
        point p2 = *(++intrs.begin());
        double d1 = -1, d2 = 0;
        for (int i = 0; i < n; i++) {
            int j = (i + 1) % n;
            d2 += len(pol[i], pol[j]);
            if (d1 == -1 && pointOnSeg(pol[i], pol[j], p1)) {
                if (pointOnSeg(pol[i], pol[j], p2))
                    goto PRINT;
                d1 = len(p1, pol[j]);
                while (1) {
                    int j2 = (j + 1) % n;
                    if (pointOnSeg(pol[j], pol[j2], p2)) {
                        d1 += len(pol[j], p2);
                        break;
                    }
                    d1 += len(pol[j], pol[j2]);
                    j = j2;
                }
            }
        }
        d2 -= d1;
        double d3 = len(p1, p2);
        ret -= d3;
        d3 *= 2;
        ret += min(d3, min(d1, d2));
    }
    PRINT: ;
    printf("%.9lf\n", ret);
    return 0;
}