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;
typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,P> P1;
typedef pair<P,P> P2;
#define pu push
#define pb push_back
#define mp make_pair
#define INF 1000000000
#define fi first
#define sc second
#define rep(i,x) for(int i=0;i<x;i++)
#define SORT(x) sort(x.begin(),x.end())
#define ERASE(x) x.erase(unique(x.begin(),x.end()),x.end())
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin())
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin())
typedef complex<double> pt;
typedef pair<pt,pt> L;
typedef vector<P> poly;
const double EPS = 1e-9;
#define x real()
#define y imag()
 
double dot(pt a,pt b){
	return (conj(a)*b).x;
}
double cross(pt a,pt b){
	return (conj(a)*b).y;
}
int ccw(pt a,pt b,pt c){
	b -= a; c -= a;
	if(cross(b,c) > EPS) return 1; // counter clockwise
	if(cross(b,c) < -EPS) return -1; // clockwise
	if(dot(b,c) < -EPS) return 2; //c-a-b
	if(norm(b) < norm(c)) return -2; //a-b-c
	return 0; //a-c-b
}
bool cmp(const pt& a,const pt& b){
	if(-EPS < a.x-b.x && a.x-b.x < EPS) return a.y < b.y;
	else return a.x < b.x;
}
vector<pt>convex_hull(vector<pt>ps)
{
	sort(ps.begin(),ps.end(),cmp);
	int k=0,n = ps.size();
	vector<pt>qs(n*2);
	for(int i=0;i<n;i++)
	{
		while(k>1 && ccw(qs[k-2],qs[k-1],ps[i]) == -1) k--;
		qs[k++] = ps[i];
	}
	for(int i=n-2,t=k;i>=0;i--)
	{
		while(k>t && ccw(qs[k-2],qs[k-1],ps[i]) == -1) k--;
		qs[k++] = ps[i];
	}
	qs.resize(k-1);
	return qs;
}
pt intersect(pt a,pt b,pt c,pt d){
	double dev = (b.y-a.y)*(d.x-c.x)-(b.x-a.x)*(d.y-c.y);
	
	double d1, d2;
 
	d1 = (c.y*d.x-c.x*d.y);
	d2 = (a.y*b.x-a.x*b.y);
 
	pt ap1;
	ap1.x = d1*(b.x-a.x) - d2*(d.x-c.x);
	ap1.x /= dev;
	ap1.y = d1*(b.y-a.y) - d2*(d.y-c.y);
	ap1.y /= dev;
 
	return ap1;
}
bool par(pt a,pt b,pt c,pt d){
	b.x -= a.x; b.y -= a.y;
	d.x -= c.x; d.y -= c.y;
	if(abs(b.y*d.x - b.x*d.y) < EPS){
		return 1;
	}
	return 0;
}
int main(){
	while(1){
		int n; scanf("%d",&n); if(!n) return 0;
		pt p[305];
		for(int i=1;i<=n;i++){
			double a,b; scanf("%lf%lf",&a,&b);
			p[i] = pt(a,b);
		}
		int ans = 0;
		for(int i=1;i<=n;i++){
			for(int j=i+1;j<=n;j++){
				pt q = (p[i]+p[j])/2.0;
				pt r = (p[i]-p[j]);
				pt p1 = q + r / abs(r) * pt(0,sqrt(1.0-abs(r)*abs(r)/4.0));
				pt p2 = q - r / abs(r) * pt(0,sqrt(1.0-abs(r)*abs(r)/4.0));
				int c=0,d=0;
				for(int k=1;k<=n;k++){
					if(abs(p1-p[k]) <= 1.0+EPS) c++;
					if(abs(p2-p[k]) <= 1.0+EPS) d++;
				}
				ans = max(ans,max(c,d));
			}
		}
		printf("%d\n",ans);
	}
}

C++: #include <bits/stdc++.h>

using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
template<class T> inline T sqr(T x) {return x*x;}

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define all(a)  (a).begin(),(a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define each(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define exist(s,e) ((s).find(e)!=(s).end())
#define range(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)  range(i,0,n)
#define clr(a,b) memset((a), (b) ,sizeof(a))
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

const double eps = 1e-10;
const double pi  = acos(-1.0);
const ll INF =1LL << 62;
const int inf =1 << 29;

// ??\??? ????????¨???
template<class T> bool operator==(T a, T b){return abs(a-b)< eps;}
template<class T> bool operator< (T a, T b){return a < b-eps;}
template<class T> bool operator<=(T a, T b){return a < b+eps;}
template<class T> int sig(T r) {return (r==0||r==-0) ? 0 : r > 0 ? 1 : -1;}
#define X real()
#define Y imag()

typedef bool B;
typedef long double D;
typedef complex<D> P;

P p[310];

int main(void){
	int n;
	while(cin >> n,n){
		D ix,iy;
		rep(i,n){
			cin >> ix >> iy;
			p[i].real(ix),p[i].imag(iy);
		}
		int ans=1;
		rep(i,n)rep(j,n){
			if(i==j) continue;
			D d=norm(p[j]-p[i]);
			if(d>4.0) continue;
			P v=(p[j]-p[i])/P(2.0,0.0);
			D s=sqrt((1.0-norm(v))/norm(v));
			P cnt1=p[i]+v+s*P(v.imag(),-v.real());
			int num=2;
			rep(k,n){
				if(k==i||k==j) continue;
				if(norm(p[k]-cnt1)<1.0) num++;
			}
			ans=max(ans,num);
			P cnt2=p[i]+v+s*P(-v.imag(),v.real());
			int num2=2;
			rep(k,n){
				if(k==i||k==j) continue;
				if(norm(p[k]-cnt2)<1.0) num2++;
			}
			ans=max(ans,num2);
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cmath>
#include <iostream>
#include <vector>
#include <algorithm>

#define rep(n) for(int i=0;i<n;i++)
#define repp(j, n) for(int j=0;j<n;j++)
#define reppp(i, m, n) for(int i=m;i<n;i++)
#define all(c) c.begin(), c.end()
#define rall(c) c.rbegin(), c.rend()
#define debug(x) cerr << #x << ": " << x << endl

using namespace std;

typedef long long ll;
typedef pair<ll, ll> Pll;
typedef pair<int, int> Pii;

const long double EPS = 10e-6;

class Point{
    public: 
    long double x,y;
    
    Point(long double x = 0.0, long double y = 0.0): x(x), y(y) {}
    
    Point operator + (Point p) const {
        return Point(x+p.x, y+p.y);
    }
    Point operator - (Point p) const {
        return Point(x-p.x, y-p.y);
    }
    Point operator * (long double a) const {
        return Point(x*a, y*a);
    }
    Point operator / (long double a) const {
        return Point(x/a, y/a);
    }

    long double norm() {
        return x*x + y*y;
    }
    
    long double dot(Point p) {
        return x*p.x + y*p.y;
    }
};

Point calc_center(Point p, Point mid, Point midv){
    long double norm = mid.norm();
    midv = midv * sqrt((1.0 - norm) / norm);
    return p+mid+midv;
}

int count(vector<Point> points, Point center){
    int tmp = 0;
    for(Point p: points){
        if((p-center).norm() <= 1.0 + EPS) tmp++;
    }
    return tmp;
}

int main() {
    int n;
    while(scanf("%d", &n) && n){
        vector<Point> points(n);
        rep(n) scanf("%Lf %Lf", &points[i].x, &points[i].y);

        int ans = 1;
        rep(n){
            repp(j, n){
                if(j == i) continue;
                Point mid = (points[j] - points[i]) * 0.5;
                Point center = calc_center(points[i], mid, Point(mid.y, -mid.x));
                ans = max(count(points, center), ans);

                center = calc_center(points[i], mid, Point(-mid.y, mid.x));
                ans = max(count(points, center), ans);
            }
        }
        printf("%d\n", ans);
    }
}