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 <iostream>
#include <cmath>
using namespace std;

#define rep(i,x) for(int i = 0 ; i < x ; i ++)
#define rep1(i,x) for(int i = 1 ; i <= x ; i ++)

typedef long double ld;
const ld PI = 3.141592653589793238462643383279;

struct po{
	ld x;
	ld y;
	po(ld a){ x = a; y = 0; }
	po(ld a,ld b){ x = a; y = b; }
	po(){}
	inline ld operator()(const po& rhs) const {
		return x*rhs.x+y*rhs.y;
	}
	inline ld norm() {
		return sqrt((*this)(*this));
	}
	inline po& operator+=(const po& rhs){
		x += rhs.x;
		y += rhs.y;
		return *this;
	}
	inline po& operator-=(const po& rhs){
		x -= rhs.x;
		y -= rhs.y;
		return *this;
	}
	inline po& operator*=(const po& rhs){
		ld x_ = x*rhs.x-y*rhs.y;
		y = x*rhs.y+y*rhs.x;
		x = x_;
		return *this;
	}
	inline po& operator/=(const po& rhs){
		ld x_ = x*rhs.x+y*rhs.y;
		y = -x*rhs.y+y*rhs.x;
		x = x_;
		ld nor = rhs(rhs);
		x /= nor;
		y /= nor;
		return *this;
	}
	inline void in(){
		scanf("%Lf%Lf",&x,&y);
	}
};
inline po operator+(const po& a, const po& b){ return po(a) += b; }
inline po operator-(const po& a, const po& b){ return po(a) -= b; }
inline po operator*(const po& a, const po& b){ return po(a) *= b; }
inline po operator/(const po& a, const po& b){ return po(a) /= b; }

inline po similar(const po& a,const po& b,const po& c,const po& p,const po& q){
	return p+(q-p)*(c-a)/(b-a);
}

typedef pair<po,po> se;
#define fr first
#define sc second

struct li{
	ld a;
	ld b;
	ld c;
	li(po p,po q){
		a = q.y-p.y;
		b = p.x-q.x;
		c = p.y*q.x-p.x*q.y;
	}
	/*explicit li(se s){
		a = s.sc.y-s.fr.y;
		b = s.fr.x-s.sc.x;
		c = s.fr.y*s.sc.x-s.fr.x*s.sc.y;
		li(s.fr,s.sc);
	}*/
	li(){}
	inline ld operator()(const po& p) const {
		return a*p.x+b*p.y+c;
	}
};

void intersect(const li &l,const li &m,po* &ret){
	if(abs(l.a*m.b-m.a*l.b) <= 1e-18){
		ret = nullptr;
		return;
	}
	ret = new po;
	ret->x = -(l.c*m.b-m.c*l.b)/(l.a*m.b-m.a*l.b);
	ret->y = -(l.a*m.c-m.a*l.c)/(l.a*m.b-m.a*l.b);
}
inline bool on(const po &p,const se &s){
	return (s.fr-p)(s.sc-p) <= 1e-9;
}
void intersect(const se &s,const se &t,po* &ret){
	li l = li(s.fr,s.sc);
	li m = li(t.fr,t.sc);
	po *p;
	intersect(l,m,p);
	if(p == nullptr || (on(*p,s)&&on(*p,t)))ret = p;
	else{
		delete p;
		ret = nullptr;
	}
}
inline ld dist(const po &p,const po &q){
	return sqrt((p-q)(p-q));
}
inline li perpen(const po &p,const li &l){
	li ret;
	ret.a = l.b;
	ret.b = -l.a;
	ret.c = l.a*p.y-l.b*p.x;
	return ret;
}
inline po perpen_leg(const po &p,const li &l){
	po ret;
	li m = perpen(p,l);
	ret.x = -(l.c*m.b-m.c*l.b)/(l.a*m.b-m.a*l.b);
	ret.y = -(l.a*m.c-m.a*l.c)/(l.a*m.b-m.a*l.b);
	return ret;
}
inline ld dist(const po &p,const se &s){
	po h = perpen_leg(p,li(s.fr,s.sc));
	if(on(h,s))return dist(p,h);
	else return min( dist(p,s.fr) , dist(p,s.sc) );
}
inline ld dist(const se &s,const se &t){
	po *p;
	intersect(s,t,p);
	if(p != nullptr){
		delete p;
		return 0.0;
	}
	ld ret = min( dist(s.fr,t) , dist(s.sc,t) );
	ret = min( ret , min( dist(t.fr,s) , dist(t.sc,s) ) );
	return ret;
}

struct ci{
	po c;
	ld r;
	inline bool internal(const po &p){
		return dist(p,c) <= r+1e-9;
	}
};

ci circum(const po &p,const po &q,const po &r){
	ci ret;
	li l = perpen((p+q)/2.0,li(p,q));
	li m = perpen((q+r)/2.0,li(q,r));
	po* _c;
	intersect(l,m,_c);
	if(_c == nullptr){
		ret.c = po(0.0,0.0);
		ret.r = -1.0;
		return ret;
	}
	else {
		ret.c = *_c;
		ret.r = dist(ret.c,p);
		delete _c;
		return ret;
	}
}

int main(){
	int n;
	while(1){
		scanf("%d",&n);
		if(n == 0)break;
		po p[302];
		rep1(i,n){
			p[i].in();
		}
		int ret = 1;
		for(int i = 1 ; i <= n ; i ++){
			for(int j = i+1 ; j <= n ; j ++){
				for(int k = j+1 ; k <= n ; k ++){
					ci c = circum(p[i],p[j],p[k]);
					if(c.r < -1e-9 || c.r >= 1.0+1e-9)continue;
					int cnt = 0;
					rep1(l,n){
						if(c.internal(p[l]))cnt ++;
					}
					ret = max( ret , cnt );
				}
			}
		}
		if(ret == 1){
			for(int i = 1 ; i <= n ; i ++){
				for(int j = i+1 ; j <= n ; j ++){
					if(dist(p[i],p[j]) >= 2.0+1e-9)continue;
					int cnt = 0;
					rep1(k,n){
						if(on(p[k],se(p[i],p[j])))cnt ++;
					}
					ret = max( ret , cnt );
				}
			}
		}
		printf("%d\n",ret);
	}
}

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){
                int cnt1 = 0,cnt2=0;
                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 = 1-dist((x1-x2)/2,(y1-y2)/2);
                ld = sqrt(ld);
                cx1 = mx - sin(atan((y2-y1)/(x2-x1)))*ld;
                cy1 = my + cos(atan((y2-y1)/(x2-x1)))*ld;
                cx2 = mx + sin(atan((y2-y1)/(x2-x1)))*ld;
                cy2 = my - cos(atan((y2-y1)/(x2-x1)))*ld;

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