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;

const double r = 1.0;
const double PI = acos(-1.0);

struct Point {
    double x, y;
    Point() {}
    Point(double x, double y) : x(x), y(y) {}
};
inline double Distance2(const Point &a, const Point &b) {
    return (a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y);
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n;
    std::random_device rnd;
    std::mt19937 mt(rnd());
    std::uniform_real_distribution<> rand100(0.0, 10.0);

    while (cin >> n, n) {
        // Input
        vector<Point> p(n);
        for (int i = 0; i < n; ++i)
            cin >> p[i].x >> p[i].y;

        // Solve
        int ans = 1;
        auto start = std::chrono::high_resolution_clock::now();
        auto end = std::chrono::high_resolution_clock::now();
        auto take_time = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);

        while (true) {
            end = std::chrono::high_resolution_clock::now();
            take_time = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
            if (300 < take_time.count())
                break;

            for (int i = 0; i < 10; ++i) {
                Point c(rand100(mt), rand100(mt));
                int num = 0;
                for (int j = 0; j < n; ++j) {
                    double d = Distance2(p[j], c);
                    if (d <= 1.0)
                        ++num;
                }
                ans = max(ans, num);
            }
        }

        // Output
        cout << ans << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

class Point {
public:
  double x, y;
  Point(double x = 0, double y = 0) : x(x), y(y) {}
  Point operator +(Point p) {
    return Point(x+p.x, y+p.y);
  }
  Point operator -(Point p) {
    return Point(x-p.x, y-p.y);
  }
  Point operator *(double a) {
    return Point(x*a, y*a);
  }
  Point operator /(double a) {
    return Point(x/a, y/a);
  }
  bool operator ==(const Point &p) const {
    return x == p.x && y == p.y;
  }
};

typedef Point Vector;

double norm(Point p1, Point p2) {
  return (p1.x-p2.x)*(p1.x-p2.x) + (p1.y-p2.y)*(p1.y-p2.y);
}

int n;
Point p[300];

int calc(Point p1, Point p2) {
  Point m = Point((p1.x+p2.x)/2.0, (p1.y+p2.y)/2.0);
  Point o;
  Vector moto = Vector(p2.y-p1.y, p1.x-p2.x);
  double l = sqrt(4.0/norm(p1,p2)-1)/2.0;
  int cnt = 0;
  int newCnt;
  for(int t = 0; t < 2; t++) {
    newCnt = 0;
    if(t == 1) {
      moto = moto * (-1);
    }
    o = m + moto * l;

    for(int i = 0; i < n; i++) {
      if(p[i] == p1 || p[i] == p2) continue;
      if(newCnt + n-i <= cnt) break;
      if((p[i].x-o.x)*(p[i].x-o.x) + (p[i].y-o.y)*(p[i].y-o.y) <= 1.0) {
	newCnt++;
      }
    }
    if(newCnt > cnt) cnt = newCnt;
  }
  return cnt + 2;
}

main() {
  while(1) {
    int ans;
    cin >> n;
    if(n == 0) break;
    for(int i = 0; i < n; i++) {
      cin >> p[i].x >> p[i].y;
    }
    ans = -1;
    for(int i = 0; i < n; i++) {
      for(int j = i+1; j < n; j++) {
	ans = max(ans, calc(p[i],p[j]));
      }
    }
    cout << ans << endl;
  }
}

C++: #include <bits/stdc++.h>
#include<stdio.h>
using namespace std;
#define FOR(i,k,n) for(int i = (k); i < (n); i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) begin(a),end(a)
#define MS(m,v) memset(m,v,sizeof(m))
#define D10  fixed<<setprecision(10)
typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int, int> P;
typedef long long ll;
const ll INF = 114514810;
const ll MOD = 1000000007;
const double EPS = 1e-8;
const double PI = acos(-1.0);
struct edge
{
	int to, cost;
	bool operator < (const edge& e) const { return cost < e.cost; }
	bool operator >(const edge& e) const { return cost > e.cost; }
};
int dx[] = { -1, 0, 0, 1 }; int dy[] = { 0, -1, 1, 0 };
template<class T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<class T> T &chmax(T &a, const T &b) { return a = max(a, b); }
bool valid(int x, int y, int h, int w) { return (x >= 0 && y >= 0 && x < h&&y < w); }
///*************************************************************************************///
///*************************************************************************************///
///*************************************************************************************///

bool eq(double a, double b) { return fabs(a - b) < EPS; }
typedef complex<double> Point;
typedef vector<Point> Polygon;

namespace std
{
	bool operator < (const Point& a, const Point& b)
	{
		return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
	}
}

struct Line
{
	Point a, b;
	Line(Point p, Point q) :a(p), b(q) {};
};

struct Circle
{
	Point p; double r;
	Circle(Point a, double b) :p(a), r(b) {};
};

double dot(Point a, Point b)
{
	return real(conj(a)*b);
}

double cross(Point a, Point b)
{
	return imag(conj(a)*b);
}

int ccw(Point a, Point b, Point c)
{
	b -= a; c -= a;
	if (cross(b, c) > EPS) return 1;	//counter cloclwise
	if (cross(b, c) < -EPS) return -1;  //cloclwise
	if (dot(b, c) < 0) return 2;		//c--a--b on line 
	if (norm(b) < norm(c)) return -2;   //a--b--c on line
	return 0;							//a--c--b on line
}

vector<Point> is_cc(Circle c1, Circle c2)
{
	vector<Point> res;
	double d = abs(c1.p - c2.p);
	double rc = (d*d + pow(c1.r, 2) - pow(c2.r, 2)) / (2 * d);
	double dfr = pow(c1.r, 2) - rc*rc;
	if (abs(dfr) < EPS) dfr = 0;
	if (dfr < 0.0) return res;
	double rs = sqrt(dfr);
	Point diff = (c2.p - c1.p) / d;
	res.push_back(c1.p + diff*Point(rc, rs));
	if (dfr != 0.0) res.push_back(c1.p + diff*Point(rc, -rs));
	return res;
}




int main()
{
	int n;
	while (cin >> n, n)
	{
		int ans = 1;
		vector<Point> v;
		REP(i, n)
		{
			double x, y;
			cin >> x >> y;
			v.push_back(Point(x, y));
		}
		REP(i, n - 1)
		{
			FOR(j, 1 + 1, n)
			{
				Circle c1(v[i], 1), c2(v[j], 1);
				auto vp = is_cc(c1, c2);
				int a = vp.size();
				chmax(ans, a);
				REP(k, vp.size())
				{
					int cnt = 0;
					Point tmp = vp[k];
					REP(l, n)
					{
						if (abs(tmp - v[l])<= 1) cnt++;
					}
					chmax(ans, cnt);
				}
			}
		}
		cout << ans << endl;
	}
	return 0;
}