Difference

<!--
  示例：差集运算
  核心逻辑：difference 从外框中减去内框，结果保留缺口与洞
  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>差集运算</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>外框减去内框</span><span class="hint">灰色虚线为原始外框</span></div>
<svg id="stage" viewBox="0 0 640 320" preserveAspectRatio="xMidYMid meet"></svg>
<div class="out">
  <span>外框面积 <b id="a0">—</b></span>
  <span>内框面积 <b id="a1">—</b></span>
  <span>差集面积 <b id="a2">—</b></span>
  <span>结果类型 <span id="type">—</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 = [116.28, 39.82, 116.52, 39.96];
  var pr = makeProjection(BOX, 640, 320, 24);
  var stage = document.getElementById('stage');

  var outer = turf.bboxPolygon([116.30, 39.85, 116.50, 39.94]);
  var inner = turf.polygon([[[
    116.36, 39.87], [116.45, 39.875], [116.47, 39.915],
    [116.38, 39.925], [116.36, 39.87]
  ]]);

  var rest = turf.difference(turf.featureCollection([outer, inner]));

  var parts = ['<rect x="0" y="0" width="640" height="320" fill="none" stroke="#e3e6ea"/>'];
  parts.push('<path d="' + polyPath(pr, outer.geometry.coordinates) +
             '" fill="none" stroke="#8a929f" stroke-width="1.4" stroke-dasharray="6 4"/>');
  parts.push('<path d="' + polyPath(pr, inner.geometry.coordinates) +
             '" fill="rgba(194,65,12,0.10)" stroke="#c2410c" stroke-width="1.2" stroke-dasharray="4 3"/>');

  if (rest) {
    parts.push('<path d="' + polyPath(pr, rest.geometry.coordinates) +
               '" fill="rgba(31,95,208,0.28)" fill-rule="evenodd" stroke="#1f5fd0" stroke-width="2"/>');
  }

  stage.innerHTML = parts.join('');

  document.getElementById('a0').textContent = (turf.area(outer) / 1e6).toFixed(2) + ' km²';
  document.getElementById('a1').textContent = (turf.area(inner) / 1e6).toFixed(2) + ' km²';
  document.getElementById('a2').textContent =
    rest ? (turf.area(rest) / 1e6).toFixed(2) + ' km²' : 'null';
  document.getElementById('type').textContent = rest ? rest.geometry.type : '—';
})();
</script>
</body>
</html>