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>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define repr(i, n) for (int i = (n); i >= 0; --i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define FORR(i, m, n) for (int i = (m); i >= (n); --i)
#define equals(a, b) (fabs((a) - (b)) < EPS)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
const ll mod = 1000000007;
//const ll mod = 998244353;
const int inf = 1e9 + 10;
const ll INF = 1e18;
const ld EPS = 1e-10;
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

struct Point {
  ld x, y;

  Point(ld x = 0, ld 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 * (ld k) {
    return Point(x * k, y * k);
  }

  Point operator / (ld k) {
    return Point(x / k, y / k);
  }

  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;
  }
};

ld norm(Point p) {
  return p.x * p.x + p.y * p.y;
}

ld dist(Point p) {
  return sqrt(norm(p));
}

ld dot(Point p1, Point p2) {
  return p1.x * p2.x + p1.y * p2.y;
}

ld cross(Point p1, Point p2) {
  return p1.x * p2.y - p1.y * p2.x;
}

Point rotate(Point p, ld theta) {
  return Point(p.x * cos(theta) - p.y * sin(theta), p.x * sin(theta) + p.y * cos(theta));
}

//1:反時計回り
//-1:時計回り
//2:p2, p0, p1の順で同一直線上
//-2:p0, p1, p2の順
//0:p2が線分p0, p1上
int ccw(Point p0, Point p1, Point p2) {
  Point v1 = p1 - p0, v2 = p2 - p0;
  if (cross(v1, v2) > 0) return 1;
  else if (cross(v1, v2) < 0) return -1;
  else if (dot(v1, v2) < 0) return 2;
  else if (norm(v1) < norm(v2)) return -2;
  else return 0;
}

//線分p1, p2がp3, p4と交差するか
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);
}

//直線p2-p1と点p3の距離
ld getDistanceLP(Point p1, Point p2, Point p3) {
  return fabs(cross(p2-p1, p3-p1) / dist(p2-p1));
}

//線分s(p2-p1)と点p3の距離
ld getDistanceSP(Point p1, Point p2, Point p3) {
  if (dot(p2-p1, p3-p1) < 0) return dist(p3-p1);
  else if (dot(p1-p2, p3-p2) < 0) return dist(p3-p2);
  else return getDistanceLP(p1, p2, p3);
}

//線分s1(p2-p1)と線分s2(p4-p3)の距離
ld getDistanceTS(Point p1, Point p2, Point p3, Point p4) {
  if (intersect(p1, p2, p3, p4)) return 0;
  ld ans = INF;
  ans = min(ans, getDistanceSP(p1, p2, p3));
  ans = min(ans, getDistanceSP(p1, p2, p4));
  ans = min(ans, getDistanceSP(p3, p4, p1));
  ans = min(ans, getDistanceSP(p3, p4, p2));
  return ans;
}

vector<Point> convex_hull(vector<Point>& ps, int n) {
  sort(ps.begin(), ps.end());
  int k = 0;
  vector<Point> qs(2 * n);
  rep(i, n) {
    while (k > 1 && cross(qs[k - 1] - qs[k - 2], ps[i] - qs[k - 1]) <= 0) k--;
    qs[k++] = ps[i];
  }
  for (int i = n - 2, t = k; i >= 0; i--) {
    while (k > t && (cross(qs[k - 1] - qs[k - 2], ps[i] - qs[k - 1]) <= 0)) k--;
    qs[k++] = ps[i];
  }
  qs.resize(k - 1);
  return qs;
}

//円
struct Circle {
  Point p;
  ld r;

  Circle() {}
  Circle(Point p, ld r): p(p), r(r) {}
};

//２円の交点
vector<Point> getIntersections(Circle c1, Circle c2) {
  ld d = dist(c2.p - c1.p);
  ld rc = (c1.r * c1.r + d * d - c2.r * c2.r) / (2 * d);
  ld rs = sqrt(c1.r * c1.r - rc * rc);
  Point e1 = (c2.p - c1.p) / d;
  Point e2 = rotate(e1, M_PI / 2) * rs;
  Point e3 = rotate(e1, -M_PI / 2) * rs;
  e1 = e1 * rc;
  Point a = c1.p + e1 + e2;
  Point b = c1.p + e1 + e3;
  return {a, b};
}

int n;
Point p[305];
Circle c[305];

void solve() {
  rep(i, n) c[i] = {p[i], 1};
  int ans = 1;
  rep(i, n) {
    int res = 0;
    rep(j, n) {
      if (dist(p[i] - p[j]) < 1 + EPS) res++;
    }
    chmax(ans, res);
  }
  rep(i, n) {
    rep(j, i) {
      if (dist(p[i] - p[j]) > 2 + EPS) continue;
      vector<Point> vs = getIntersections(c[i], c[j]);
      for (Point v: vs) {
        int res = 0;
        rep(k, n) {
          if (dist(v - p[k]) < 1 + EPS) res++;
        }
        chmax(ans, res);
      }
    }
  }
  cout << ans << '\n';
}

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(25);

  while (cin >> n, n) {
    rep(i, n) cin >> p[i].x >> p[i].y;

    solve();
  }

  






  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);
    }
}