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 unsigned long long ULL;
typedef pair<int, int> P;
const LL MOD = 1000000007LL;
const double EPS = 1e-10;
struct Point {
	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);
	}
	double abs() {
		return sqrt(norm());
	}
	double norm() {
		return x*x + y*y;
	}
	bool operator<(const Point &p) const {
		return x != p.x ? x < p.x : y < p.y;
	}
	bool operator==(const Point &p) const {
		return fabs(x - p.x) < EPS && fabs(y - p.y) < EPS;
	}
};
typedef Point Vector;
struct Segment {
	Point p1, p2;
};
typedef Segment Line;
struct Circle {
	Point c;
	double r;
	Circle(Point c = Point(), double r = 0.0) :c(c), r(r) {}
};
typedef vector<Point> Polygon;
double norm(Vector a) {
	return a.x*a.x + a.y*a.y;
}
double abs(Vector a) {
	return sqrt(norm(a));
}
double dot(Vector a, Vector b) {
	return a.x*b.x + a.y*b.y;
}
double cross(Vector a, Vector b) {
	return a.x*b.y - a.y*b.x;
}
bool equals(double a, double b) {
	return fabs(a - b) < EPS;
}
bool isOrthogonal(Vector a, Vector b) {
	return equals(dot(a, b), 0.0);
}
bool isOrthogonal(Point a1, Point a2, Point b1, Point b2) {
	return isOrthogonal(a1 - a2, b1 - b2);
}
bool isOrthogonal(Segment s1, Segment s2) {
	return equals(dot(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}
bool isParallel(Vector a, Vector b) {
	return equals(cross(a, b), 0.0);
}
bool isParallel(Point a1, Point a2, Point b1, Point b2) {
	return isParallel(a1 - a2, b1 - b2);
}
bool isParallel(Segment s1, Segment s2) {
	return equals(cross(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}
Point project(Segment s, Point p) {
	Vector base = s.p2 - s.p1;
	double r = dot(p - s.p1, base) / norm(base);
	return s.p1 + base*r;
}
Point reflect(Segment s, Point p) {
	return p + (project(s, p) - p)*2.0;
}
static const int COUNTER_CLOCKWISE = 1;
static const int CLOCKWISE = -1;
static const int ONLINE_BACK = 2;
static const int ONLINE_FRONT = -2;
static const int ON_SEGMENT = 0;
int ccw(Point p0, Point p1, Point p2) {
	Vector a = p1 - p0;
	Vector b = p2 - p0;
	if (cross(a, b) > EPS) return COUNTER_CLOCKWISE;
	if (cross(a, b) < -EPS) return CLOCKWISE;
	if (dot(a, b) < -EPS) return ONLINE_BACK;
	if (a.norm() < b.norm()) return ONLINE_FRONT;
	return ON_SEGMENT;
}
double getDistance(Point a, Point b) {
	return abs(a - b);
}
double getDistanceLP(Line l, Point p) {
	return abs(cross(l.p2 - l.p1, p - l.p1) / abs(l.p2 - l.p1));
}
double getDistanceSP(Segment s, Point p) {
	if (dot(s.p2 - s.p1, p - s.p1) < 0.0) return abs(p - s.p1);
	if (dot(s.p1 - s.p2, p - s.p2) < 0.0) return abs(p - s.p2);
	return getDistanceLP(s, p);
}
bool intersect(Point p1, Point p2, Point p3, Point p4) {
	return (ccw(p1, p2, p3)*ccw(p1, p2, p4) <= 0 && ccw(p3, p4, p1)*ccw(p3, p4, p2) <= 0);
}
bool intersect(Segment s1, Segment s2) {
	return intersect(s1.p1, s1.p2, s2.p1, s2.p2);
}
double getDistance(Segment s1, Segment s2) {
	if (intersect(s1, s2)) return 0.0;
	return min(min(getDistanceSP(s1, s2.p1), getDistanceSP(s1, s2.p2)), min(getDistanceSP(s2, s1.p1), getDistanceSP(s2, s1.p2)));
}
bool intersect(Circle c, Line l) {
	return getDistanceLP(l, c.c) <= c.r;
}
bool intersect(Circle c1, Circle c2) {
	return getDistance(c1.c, c2.c) <= c1.r + c2.r;
}
Point getCrossPoint(Segment s1, Segment s2) {
	Vector base = s2.p2 - s2.p1;
	double d1 = abs(cross(base, s1.p1 - s2.p1));
	double d2 = abs(cross(base, s1.p2 - s2.p1));
	double t = d1 / (d1 + d2);
	return s1.p1 + (s1.p2 - s1.p1)*t;
}
pair<Point, Point> getCrossPoints(Circle c, Line l) {
	assert(intersect(c, l));
	Vector pr = project(l, c.c);
	Vector e = (l.p2 - l.p1) / abs(l.p2 - l.p1);
	double base = sqrt(c.r*c.r - norm(pr - c.c));
	return make_pair(pr + e*base, pr - e*base);
}
double arg(Vector p) {
	return atan2(p.y, p.x);
}
Vector polar(double a, double r) {
	return Vector(cos(r)*a, sin(r)*a);
}
pair<Point, Point> getCrossPoints(Circle c1, Circle c2) {
	assert(intersect(c1, c2));
	double d = abs(c1.c - c2.c);
	double a = acos((c1.r*c1.r + d*d - c2.r*c2.r) / (2 * c1.r*d));
	double t = arg(c2.c - c1.c);
	return make_pair(c1.c + polar(c1.r, t + a), c1.c + polar(c1.r, t - a));
}
int main() {
	int N;
	while (cin >> N, N) {
		Point p[300];
		for (int i = 0; i < N; i++) {
			double x, y;
			cin >> x >> y;
			p[i] = Point(x, y);
		}
		int ans = 0;
		for (int i = 0; i < N; i++) {
			for (int j = i + 1; j < N; j++) {
				Vector d = p[j] - p[i];
				if (d.abs() > 2.0) continue;
				Point m = p[i] + d / 2.0;
				Vector r = Vector(-d.y, d.x);
				r = r / r.abs()*sqrt(1 - pow(d.abs() / 2.0, 2));
				Point p1 = m + r;
				Point p2 = m - r;
				int cnt1 = 0;
				int cnt2 = 0;
				for (int k = 0; k < N; k++) {
					if ((p[k] - p1).abs() <= 1) cnt1++;
					if ((p[k] - p2).abs() <= 1) cnt2++;
				}
				ans = max(ans, max(cnt1, cnt2));
			}
		}
		cout << ans << endl;
	}
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <complex>
#include <cstring>
#include <cstdlib>
using namespace std;

#define REP(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()

typedef complex<double> P;
const double EPS = 1e-8;

int main() {
  int n;
  while(cin>>n,n) {
    vector<P> v;
    REP(i,n) {
      double x,y;
      cin >> x >> y;
      v.push_back(P(x,y));
    }
    int ans = 1;
    REP(i,n-1) {
      for (int j=i+1;j<n;j++) {
        P m = (v[i]+v[j])*P(0.5,0);
        //cout << "m "<< m << endl;
        P a = v[i]-v[j];
        double dis = abs(a);
        if (1-dis*dis/4<0) continue;
        double d = sqrt(1-dis*dis/4);
        int hoge[]={1,-1};
        REP(k,2) {
          P un = (a * P(0, hoge[k])) / dis;
          P o1 = m + un*d;
          //cout << o1 << endl;
          int sum = 0;
          REP(l,n) {
            if (norm(v[l]-o1)<1+EPS) sum++;
          }
          ans = max(ans,sum);
        }
      }
    }
    cout << ans << endl;
    
  }
}