Go: package main

import (
	"fmt"
	"math"
	"math/cmplx"
)

func main() {
	for {
		var n int
		fmt.Scan(&n)

		if n == 0 {
			return
		}

		arr := make([]complex128, n)

		for i := 0; i < n; i++ {
			var x, y float64

			fmt.Scanf("%f%f", &x, &y)

			arr[i] = complex(x, y)
		}

		max := 1
		for i := 0; i < n; i++ {
			for j := i + 1; j < n; j++ {
				sub := arr[j] - arr[i]

				if cmplx.Abs(sub) >= 2 {
					continue
				}
				sum := arr[j] + arr[i]

				v := 1
				{
					halfd := cmplx.Abs(sub / 2)

					center := sum/2 + sub*cmplx.Rect(math.Sqrt(1-halfd*halfd)/cmplx.Abs(sub), float64(v)*math.Pi/2)

					counter := 0
					for k := 0; k < n; k++ {
						if i == k || j == k || cmplx.Abs(arr[k]-center) <= 1 {
							counter++
						}
					}
					if max < counter {
						max = counter
					}
				}

				v = -1
				{
					halfd := cmplx.Abs(sub / 2)

					center := sum/2 + sub*cmplx.Rect(math.Sqrt(1-halfd*halfd)/cmplx.Abs(sub), float64(v)*math.Pi/2)

					counter := 0
					for k := 0; k < n; k++ {
						if i == k || j == k || cmplx.Abs(arr[k]-center) <= 1 {
							counter++
						}
					}
					if max < counter {
						max = counter
					}
				}

			}
		}

		fmt.Println(max)
	}
}

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

#define rep(i,x,y) for(int i=(x);i<(y);++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define dump(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define dump(x)
#endif

typedef long long int ll;
typedef pair<int,int> pii;
//template<typename T> using vec=std::vector<T>;

const int inf=1<<30;
const long long int infll=1LL<<58;
const double eps=1e-3;
const int dx[]={1,0,-1,0},dy[]={0,1,0,-1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){
    os << "[";
    for (const auto &v : vec) {
    	os << v << ",";
    }
    os << "]";
    return os;
}

void solve(){
    while(true){
        int n;
        cin >> n;
        if(n==0) break;
        vector<long double> xs(n),ys(n);
        rep(i,0,n) cin >> xs[i] >> ys[i];

        auto count=[&](const long double x,const long double y){
            int res=0;
            rep(i,0,n) if((xs[i]-x)*(xs[i]-x)+(ys[i]-y)*(ys[i]-y)<=1+eps) ++res;
            return res;
        };

        int ans=0;
        rep(i,0,401){
            const long double y=10.*i/400;
            rep(j,0,n){
                const long double tmp=1-(ys[j]-y)*(ys[j]-y);
                if(tmp<0) continue;
                const long double x1=xs[j]-sqrtl(tmp),x2=xs[j]+sqrt(tmp);
                ans=max({ans,count(x1,y),count(x2,y)});
            }
        }
        cout << ans << endl;
    }
}

int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    cout << fixed << setprecision(8);
    solve();
    return 0;
}

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

#define rep(i,N) for(int i=0;i<(int)(N);i++)
#define REP(i,k,N) for(int i=k;i<(int)(N);i++)

#define vi vector<int>
#define pb push_back

#define pdd pair<double,double>
#define fi first
#define se second
const double  EPS = 1e-9;

double dist(double x,double y){
    return x*x+y*y;
};

int main(){
    ifstream in("d_in.txt");
    cin.rdbuf(in.rdbuf());
    int n;
    while(cin >> n,n){
        int ma = 0;
        pdd p[330];
        rep(i,n) cin >> p[i].fi >> p[i].se;
        rep(i,n-1){
            REP(j,i+1,n){

                double x1,y1,x2,y2,x3,y3;
                tie(x1,y1) = p[i];
                tie(x2,y2) = p[j];
                double cx1,cy1,cx2,cy2;
                if(dist(x1-x2,y1-y2)  > 4+EPS  ) continue;

                //はりまコーナー
//                double ax,ay,bx1,by1,bx2,by2,mx,my,dx,dy;
//                ax=x1-x2;ay=y1-y2;
//                mx=(x1+x2)/2;my=(y1+y2)/2;
//                by1 = (-0.5)*pow(ax,4)+pow(ax,2)+(-0.5)*pow(ax,2)*pow(ay,2)
//                        /dist(ax,ay);
//                by1 = sqrt(abs(by1));
//                by2 = -1*by1;
//                cy1= my+by1;
//                cy2= my+by2;
//                bx1= -1*(ay*by1)/ax;
//                bx2= -1*(ay*by2)/ax;
//                cx1=bx1+mx;
//                cx2=bx2+mx;


                //あじのりコーナー
                double mx=(x1+x2)/2,my=(y1+y2)/2;

                double ld = sqrt(1-dist((x1-x2),(y1-y2))/4);
                double theta = atan((y2-y1)/(x2-x1));
                cx1 = mx - sin(theta)*ld;
                cy1 = my + cos(theta)*ld;
                cx2 = mx + sin(theta)*ld;
                cy2 = my - cos(theta)*ld;

                int cnt1 = 0,cnt2=0;
                for(int k = 0;k<n;k++) {
                    tie(x3, y3) = p[k];
                    if (EPS + 1> dist((cx1 - x3), (cy1 - y3))) cnt1++;
                    if (EPS + 1> dist((cx2 - x3), (cy2 - y3))) cnt2++;
                }
                ma = max({cnt1,cnt2,ma});
            }
        }
        cout<< ma << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <utility>
#include <cmath>

using namespace std;

typedef pair<double,double> pos;
const double r = 1;
const double eps = 0.0001;
int count(vector<pos> ps,double x,double y,int ban1,int ban2){
	int c=0;//2;
	for(int i=0;i<ps.size();i++){
		if(//i!=ban1&&i!=ban2&&
				pow(ps[i].first-x,2)+pow(ps[i].second-y,2)<=pow(1.0+eps,2)){
			c++;
		}
	}
	return c;
}

bool range(pos p1,pos p2,double r){
	return pow(p1.first-p2.first,2)+pow(p1.second-p2.second,2) <= r*r;
}

double len(pos p){
	return sqrt(pow(p.first,2)+pow(p.second,2));
}

double len2(pos p){
	return (pow(p.first,2)+pow(p.second,2));
}

vector<pos> point2(pos p1,pos p2){
	pos m = make_pair((p1.first+p2.first)/2,(p1.second+p2.second)/2);
	pos vec = make_pair(p1.first-p2.first,p1.second-p2.second);
	pos q = m;//make_pair(p2.first+vec.first/2,p2.second+vec.second/2);
	double l = sqrt(1-pow(len(vec)/2.0,2));
	double ll = len(vec);
	pos vec2 = make_pair(-vec.second,vec.first);
	vec2.first=vec2.first/ll*l;
	vec2.second = vec2.second/ll*l;
	vector<pos> ret;
	ret.push_back(make_pair(q.first+vec2.first,q.second+vec2.second));
	ret.push_back(make_pair(q.first-vec2.first,q.second-vec2.second));
	return ret;
}

int solve(vector<pos> ps){
	int mx=0;
	for(int i=0;i<ps.size()-1;i++)
		for(int k=i+1;k<ps.size();k++){
			if(range(ps[i],ps[k],2)){
				//cerr << "*****" << i << " " << k << endl;
				vector<pos> vp = point2(ps[i],ps[k]);
				mx = max(mx,count(ps,vp[0].first,vp[0].second,i,k));
				mx = max(mx,count(ps,vp[1].first,vp[1].second,i,k));
			}
		}
	return mx;
}

int main(){
	//cerr << point2(make_pair(0,0),make_pair(1,1))[1].first << endl;
	while(true){
		int n;
		vector<pos> v;
		cin >> n;
		if(n==0)return 0;
		for(int i=0;i<n;i++){
			double x,y;
			cin >> x >> y;
			v.push_back(make_pair(x,y));
		}
		cout << solve(v) << endl;
	}
}