Isochrone

<!--
  示例：等时圈
  核心逻辑：按步行速度换算半径做三级 buffer，再用 union 融合外圈
  依赖：Turf.js 7 · jsDelivr CDN
-->
<!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>等时圈</title>
<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;background:#fff}
  .bar{flex:0 0 auto;display:flex;gap:12px;align-items:center;padding:8px 12px;
       background:#f6f7f9;border-bottom:1px solid #e3e6ea;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar button{font:inherit;font-size:12px;padding:4px 10px;border-radius:4px;cursor:pointer;
       border:1px solid #ccd1d9;background:#fff;color:#16181d}
  .bar button:hover{background:#eef0f3}
  .bar input[type=range]{width:150px}
  .bar b{color:#1f5fd0;font-weight:600}
  .hint{color:#8a929f}
  svg{flex:1 1 auto;min-height:0;width:100%;display:block;background:#fbfcfd}
  .out{flex:0 0 auto;display:flex;gap:20px;flex-wrap:wrap;padding:8px 12px;
       border-top:1px solid #e3e6ea;
       font-family:ui-monospace,Consolas,monospace;font-size:11.5px;color:#3a4250}
  .out b{font-weight:600;color:#1f5fd0}
</style>
</head>
<body>
<div class="bar"><span>中心点固定 · 步行 4 km/h</span><span class="hint">半径按时间乘速度换算</span></div>
<svg id="stage" viewBox="0 0 640 320" preserveAspectRatio="xMidYMid meet"></svg>
<div class="out">
  <span>5 分钟 <b id="a5">—</b></span>
  <span>10 分钟 <b id="a10">—</b></span>
  <span>15 分钟 <b id="a15">—</b></span>
  <span>融合环数 <span id="rings">—</span></span>
</div>
<script src="https://cdn.jsdelivr.net/npm/@turf/turf@7/turf.min.js"></script>
<script>
(function () {
  'use strict';

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

  function makeProjection(BOX, W, H, PAD) {
    var kx = Math.cos(((BOX[1] + BOX[3]) / 2) * Math.PI / 180);
    var x0 = BOX[0] * kx, x1 = BOX[2] * kx;
    var y0 = BOX[1], y1 = BOX[3];
    var s = Math.min((W - PAD * 2) / (x1 - x0), (H - PAD * 2) / (y1 - y0));
    var ox = (W - (x1 - x0) * s) / 2;
    var oy = (H - (y1 - y0) * s) / 2;
    var to = function (c) {
      return [(c[0] * kx - x0) * s + ox, (y1 - c[1]) * s + oy];
    };
    to.inverse = function (px, py) {
      return [((px - ox) / s + x0) / kx, y1 - (py - oy) / s];
    };
    return to;
  }

  function linePath(pr, coords) {
    var d = '';
    for (var i = 0; i < coords.length; i++) {
      var p = pr(coords[i]);
      d += (i ? 'L' : 'M') + p[0].toFixed(1) + ' ' + p[1].toFixed(1);
    }
    return d;
  }

  function ringPath(pr, ring) { return linePath(pr, ring) + 'Z'; }

  function polyPath(pr, rings) {
    var d = '';
    for (var i = 0; i < rings.length; i++) d += ringPath(pr, rings[i]);
    return d;
  }
  var BOX = [116.32, 39.85, 116.48, 39.97];
  var pr = makeProjection(BOX, 640, 320, 24);
  var stage = document.getElementById('stage');
  var center = turf.point([116.40, 39.91]);

  var levels = [
    { min: 5, fill: 'rgba(31,95,208,0.10)', color: '#7fa8e8' },
    { min: 10, fill: 'rgba(31,95,208,0.14)', color: '#3f78d0' },
    { min: 15, fill: 'rgba(31,95,208,0.18)', color: '#1f5fd0' }
  ];

  var parts = ['<rect x="0" y="0" width="640" height="320" fill="none" stroke="#e3e6ea"/>'];
  var acc = null;

  levels.forEach(function (lv) {
    var km = 4 * lv.min / 60;
    var ring = turf.buffer(center, km, { units: 'kilometers', steps: 96 });
    acc = acc ? turf.union(turf.featureCollection([acc, ring])) : ring;

    parts.push('<path d="' + polyPath(pr, ring.geometry.coordinates) +
               '" fill="' + lv.fill + '" fill-rule="evenodd" stroke="' + lv.color +
               '" stroke-width="1.2"/>');

    var a = turf.area(ring) / 1e6;
    document.getElementById('a' + lv.min).textContent = a.toFixed(1) + ' km²';

    var label = pr([116.40 + km / 85, 39.91]);
    parts.push('<text x="' + (label[0] + 6).toFixed(1) + '" y="' + label[1].toFixed(1) +
               '" font-size="11" fill="' + lv.color + '">' + lv.min + ' min</text>');
  });

  var accRings = acc.geometry.type === 'MultiPolygon'
    ? acc.geometry.coordinates[0]
    : acc.geometry.coordinates;
  parts.push('<path d="' + polyPath(pr, accRings) +
             '" fill="none" stroke="#c2410c" stroke-width="2" stroke-dasharray="7 4"/>');

  var c = pr([116.40, 39.91]);
  parts.push('<circle cx="' + c[0] + '" cy="' + c[1] + '" r="4" fill="#1f5fd0"/>');

  stage.innerHTML = parts.join('');
  document.getElementById('rings').textContent = acc.geometry.type;
})();
</script>
</body>
</html>