Draw Circle

<!--
  示例：画圆
  核心逻辑：点击取圆心，按球面公式算出半径对应的 64 个边界点，拼成多边形源
  数据：圆环坐标全部在页面内计算，不依赖几何计算库
  依赖：MapLibre GL JS 4 · jsDelivr CDN · demotiles 样式
-->
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta name="robots" content="noindex,follow">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>画圆</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/maplibre-gl@4/dist/maplibre-gl.css">
<style>
  html,body{height:100%;margin:0}
  body{display:flex;flex-direction:column;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif;color:#16181d}
  .bar{flex:0 0 auto;display:flex;gap:14px;align-items:center;padding:8px 12px;
       background:#f6f7f9;border-bottom:1px solid #e3e6ea;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar b{color:#16181d;font-weight:600}
  .bar input[type=range]{width:140px}
  #map{flex:1 1 auto;min-height:0}
</style>
</head>
<body>
<div class="bar">
  <span>圆心 <b id="c">116.390, 39.900</b></span>
  <label>半径 <input id="r" type="range" min="10" max="300" step="10" value="80"> <b id="rv">80 km</b></label>
  <span>点击地图任意位置重新定圆心</span>
</div>
<div id="map"></div>
<script src="https://cdn.jsdelivr.net/npm/maplibre-gl@4/dist/maplibre-gl.js"></script>
<script>
(function () {
  'use strict';

  if (typeof maplibregl === 'undefined') {
    document.body.innerHTML =
      '<div style="padding:24px;color:#b42318">MapLibre 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    return;
  }

  var R = 6371.0088;
  var RAD = Math.PI / 180;

  /* 从圆心沿某方位角走 dist 公里，返回落点经纬度 */
  function destination(lng, lat, bearing, dist) {
    var d = dist / R;
    var b = bearing * RAD;
    var p1 = lat * RAD;
    var l1 = lng * RAD;

    var p2 = Math.asin(
      Math.sin(p1) * Math.cos(d) + Math.cos(p1) * Math.sin(d) * Math.cos(b)
    );
    var l2 = l1 + Math.atan2(
      Math.sin(b) * Math.sin(d) * Math.cos(p1),
      Math.cos(d) - Math.sin(p1) * Math.sin(p2)
    );

    return [((l2 / RAD + 540) % 360) - 180, p2 / RAD];
  }

  function circlePolygon(lng, lat, radiusKm) {
    var pts = [];
    for (var i = 0; i <= 64; i++) {
      pts.push(destination(lng, lat, i / 64 * 360, radiusKm));
    }
    return pts;
  }

  var center = [116.39, 39.90];
  var radius = 80;

  var map = new maplibregl.Map({
    container: 'map',
    style: 'https://demotiles.maplibre.org/style.json',
    center: center,
    zoom: 6,
    attributionControl: { compact: true, customAttribution: 'MapLibre demo tiles' }
  });

  map.addControl(new maplibregl.NavigationControl(), 'top-right');
  map.addControl(new maplibregl.ScaleControl({ maxWidth: 110 }), 'bottom-left');

  var cEl = document.getElementById('c');
  var rvEl = document.getElementById('rv');

  map.on('load', function () {
    map.addSource('circle', {
      type: 'geojson',
      data: {
        type: 'FeatureCollection',
        features: [{
          type: 'Feature',
          properties: {},
          geometry: { type: 'Polygon', coordinates: [circlePolygon(center[0], center[1], radius)] }
        }]
      }
    });

    map.addLayer({
      id: 'circle-fill',
      type: 'fill',
      source: 'circle',
      paint: { 'fill-color': '#2e86de', 'fill-opacity': 0.18 }
    });

    map.addLayer({
      id: 'circle-line',
      type: 'line',
      source: 'circle',
      paint: { 'line-color': '#2e86de', 'line-width': 2, 'line-dasharray': [3, 2] }
    });

    map.addSource('center', {
      type: 'geojson',
      data: {
        type: 'FeatureCollection',
        features: [{
          type: 'Feature',
          properties: {},
          geometry: { type: 'Point', coordinates: center }
        }]
      }
    });

    map.addLayer({
      id: 'center-dot',
      type: 'circle',
      source: 'center',
      paint: {
        'circle-radius': 5,
        'circle-color': '#f03e3e',
        'circle-stroke-width': 2,
        'circle-stroke-color': '#ffffff'
      }
    });
  });

  function update() {
    if (!map.getSource('circle')) return;
    map.getSource('circle').setData({
      type: 'FeatureCollection',
      features: [{
        type: 'Feature',
        properties: {},
        geometry: { type: 'Polygon', coordinates: [circlePolygon(center[0], center[1], radius)] }
      }]
    });
    map.getSource('center').setData({
      type: 'FeatureCollection',
      features: [{
        type: 'Feature',
        properties: {},
        geometry: { type: 'Point', coordinates: center }
      }]
    });
    cEl.textContent = center[0].toFixed(3) + ', ' + center[1].toFixed(3);
  }

  map.on('click', function (e) {
    center = [e.lngLat.lng, e.lngLat.lat];
    update();
  });

  document.getElementById('r').addEventListener('input', function () {
    radius = parseInt(this.value, 10);
    rvEl.textContent = radius + ' km';
    update();
  });

  map.on('error', function (e) {
    if (e.error) console.warn('地图错误：' + e.error.message);
  });
})();
</script>
</body>
</html>