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++: #define _USE_MATH_DEFINES
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<climits>
#include<string>
#include<vector>
#include<list>
#include<map>
#include<set>
#include<cmath>
#include<queue>
#include<cstring>
#include<stack>
using namespace std;
#define EPS 1e-8
#define INF 1000000

struct Point{
    double x,y;
    Point(){}
    Point(double _x,double _y){
        x=_x; y=_y;
    }
    Point operator +(const Point p)const{
        return Point(x+p.x,y+p.y);
    }
    Point operator -(const Point p)const{
        return Point(x-p.x,y-p.y);
    }
    Point operator *(const double d)const{
        return Point(x*d,y*d);
    }
    bool operator <(const Point &p)const{
        if(x==p.x) return y<p.y;
        return x<p.x;
    }
    double norm(){
        return x*x+y*y;
    }
	bool input(){
		if(cin>>x>>y) return true;
		return false;
	}
};

struct Line{
    Point a,b;
    Line(){}
    Line(Point _a,Point _b){
        a=_a; b=_b;
    }
	bool input(){
		if(a.input() && b.input()) return true;
		return false;
	}
};

struct Circle{
    Point c;
    double r;
    Circle(){}
    Circle(Point _c,double _r){
        c=_c; r=_r;
    }
};

typedef vector<Point> Polygon;

double dot(Point p,Point q){
    return p.x*q.x+p.y*q.y;
}

double cross(Point p,Point q){
    return p.x*q.y-q.x*p.y;
}

int ccw(Point a,Point b,Point c){
	Point v1 = b-a;
	Point v2 = c-a;
    if(cross(v1,v2)>EPS) return +1;
    if(cross(v1,v2)<-EPS) return -1;
	if(dot(v1,v2)<-EPS) return +2;
	if(v1.norm()<v2.norm()) return -2;
    return 0;
}

bool intersect_ss(Line l,Line m){
	return ccw(l.a,l.b,m.a)*ccw(l.a,l.b,m.b)<=0 && ccw(m.a,m.b,l.a)*ccw(m.a,m.b,l.b)<=0;
}

bool crosspoint_ll(Line l,Line m,Point &p){
    Point a1,a2,b1,b2;
    a1 = l.b-l.a; a2 = m.b-m.a;
    b1 = m.a-l.a; b2 = l.a-m.b;
    double s1,s2;
    s1 = cross(a1,b1)/2; s2 = cross(a1,b2)/2;
    if(s1+s2<EPS) return false;
    p = Point(m.a.x+a2.x*s1/(s1+s2),m.a.y+a2.y*s1/(s1+s2));
    return true;
}

int crosspoint_ss(Line l,Line m,Point &p){
    if(intersect_ss(l,m)==false) return 0;
    if(crosspoint_ll(l,m,p)==true) return 1;
	return -1;
}

int crosspoint_cc(Circle c1,Circle c2,Point &p1,Point &p2){
    double d,a,t;
    d = sqrt((c2.c-c1.c).norm());
	if(abs(c1.c.x-c2.c.x)<EPS && abs(c1.c.y-c2.c.y)<EPS && abs(c1.r-c2.r)<EPS)
		return -1;
    if(d<abs(c1.r-c2.r) || c1.r+c2.r<d) return 0;
    a = acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    t = atan2(c2.c.y-c1.c.y,c2.c.x-c1.c.x);
    p1 = Point(c1.c.x+c1.r*cos(t+a),c1.c.y+c1.r*sin(t+a));
    p2 = Point(c1.c.x+c1.r*cos(t-a),c1.c.y+c1.r*sin(t-a));
    if(abs(p1.x-p2.x)<EPS && abs(p1.y-p2.y)<EPS) return 1;
    return 2;
}

int contains(Polygon g,Point p){
	Line l = Line(p,Point(INF,p.y));
	int cnt = 0, n = g.size();
	for(int i=0;i<n;i++){
		Point a = g[i]-p;
		Point b = g[(i+1)%n]-p;
		if(ccw(g[i],g[(i+1)%n],p)==0) return 1;
		if(a.y>b.y) swap(a,b);
		if(a.y<=EPS && EPS<b.y && cross(a,b)>EPS) cnt++;
	}
	if((cnt&1)==1) return 2;
	return 0;
}

Polygon andrewScan(Polygon s){
	if(s.size()<=2) return s;
	sort(s.begin(),s.end());
	Polygon g;
	for(int i=0;i<s.size();i++){
		for(int n=g.size(); n>=2 && ccw(g[n-2],g[n-1],s[i])!=-1; n--){
			g.pop_back();
		}
		g.push_back(s[i]);
	}
	int upper_n = g.size();
	for(int i=s.size()-2;i>=0;i--){
		for(int n=g.size(); n>upper_n && ccw(g[n-2],g[n-1],s[i])!=-1; n--){
			g.pop_back();
		}
		g.push_back(s[i]);
	}
	reverse(g.begin(),g.end());
	g.pop_back();
	return g;
}

int cntInsidePoint(Circle c,Polygon s){
	int cnt=0;
	for(int i=0;i<s.size();i++){
		if((s[i]-c.c).norm()<=1.0) cnt++;
	}
	return cnt;
}

int main(){
	int N;
	while(cin>>N,N){
		Polygon S;
		S.resize(N);
		for(int i=0;i<N;i++) S[i].input();
		int ans = 0;
		for(int i=0;i<N;i++){
			for(int j=i+1;j<N;j++){
				Point p1,p2;
				if(crosspoint_cc(Circle(S[i],1),Circle(S[j],1),p1,p2)>0){
					ans = max(ans,max(cntInsidePoint(Circle(p1,1),S),cntInsidePoint(Circle(p2,1),S)));
				}
			}
		}
		printf(\"%d\\n\",ans);
	}
}

C++: #include<bits/stdc++.h>
#define eps 1e-9
using namespace std;
typedef complex<double> point;

pair<point, point> norm(point p){
  double unit = 1 / sqrt(p.real()*p.real() + p.imag()*p.imag());
  return pair<point, point>(p*point(0, 1)/unit,
                            p*point(0, -1)/unit);
}

vector<point> calcC(point a, point b){
  point p = (b - a)/2.0;
  pair<point, point> n = norm(p);
  double unit = sqrt(1 - abs(p)*abs(p));
  n.first  *= unit;
  n.second *= unit;
  vector<point> res(2);
  res[0] = a + p + n.first;
  res[1] = a + p + n.second;
  return res;
}
int main(int argc, char *argv[]){
  int n;
  while(cin >> n, n){;
    point p[n];
    for (int i = 0; i < n; i++) {
      double real, imag;
      cin >> real >> imag;
      p[i] = complex<double>(real, imag);
    }
    int ans = 0, cnt = 0;
    for (int i = 0; i < n; i++) {
      for (int j = i + 1; j < n; j++) {
        point a = p[i], b = p[j];
        if(abs(b - a) > 2 - eps)continue;
        vector<point> center = calcC(a, b);
        for (int c = 0; c < 2; c++) {
          cnt = 0;
          // std::cout << center[c].real() << " " <<
          //   center[c].imag()<< std::endl;
          for (int k = 0; k < n; k++) {
            if(abs(center[c] - p[k]) <= 1 - eps)cnt++;
          }
          ans = max(ans, cnt);
        }
      }
    }
    std::cout << ans << std::endl;
  }
  return 0;
}