Orbit Playground

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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
<title>Orbit Playground — sloprun.dev</title>
<style>
/* ── sloprun design tokens (verbatim from demos/tokens.css) ───────────────── */
:root {
  --bg: #F6F6F3;          /* neutral paper, not cream */
  --surface: #FFFFFF;
  --ink: #1A1D21;
  --muted: #5A6068;
  --line: #E3E2DC;
  --accent: #C05600;      /* phosphor amber, darkened for light bg */
  --accent-soft: #FFF3E6;
  --run: #1A7F37;         /* earned green */
  --run-soft: #E7F4EA;
  --danger: #C93C37;
  --radius: 10px;
  --font-sans: system-ui, -apple-system, "Segoe UI", sans-serif;
  --font-mono: ui-monospace, "SF Mono", "Cascadia Code", Menlo, Consolas, monospace;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #14161A; --surface: #1C1F26; --ink: #E8E6E1; --muted: #9AA0A8;
    --line: #2A2E36; --accent: #FFB454; --accent-soft: #2A2118;
    --run: #3FB950; --run-soft: #16281B; --danger: #F47067;
  }
}
:root[data-theme="dark"] {
  --bg: #14161A; --surface: #1C1F26; --ink: #E8E6E1; --muted: #9AA0A8;
  --line: #2A2E36; --accent: #FFB454; --accent-soft: #2A2118;
  --run: #3FB950; --run-soft: #16281B; --danger: #F47067;
}
body { background: var(--bg); color: var(--ink); font-family: var(--font-sans); }
/* ── end tokens ───────────────────────────────────────────────────────────── */

/* demo-local additions (scrims need alpha; satellite hues need more than 1 accent) */
:root {
  --scrim: rgba(246,246,243,.9);
  --shadow: 0 1px 2px rgba(26,29,33,.05);
  --s1: #C05600; --s2: #0E7490; --s3: #6D28D9; --s4: #B4126A; --s5: #1D4ED8;
  --planet: #4A515D; --planet2: #20242B;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --scrim: rgba(20,22,26,.9);
    --shadow: 0 1px 2px rgba(0,0,0,.35);
    --s1: #FFB454; --s2: #45C8DE; --s3: #B79CFF; --s4: #FF86BC; --s5: #7FA9FF;
    --planet: #454C57; --planet2: #21252C;
  }
}
:root[data-theme="dark"] {
  --scrim: rgba(20,22,26,.9);
  --shadow: 0 1px 2px rgba(0,0,0,.35);
  --s1: #FFB454; --s2: #45C8DE; --s3: #B79CFF; --s4: #FF86BC; --s5: #7FA9FF;
  --planet: #454C57; --planet2: #21252C;
}

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
html { -webkit-text-size-adjust: 100%; }
body { min-height: 100%; overflow-x: hidden; }

.wrap {
  max-width: 1140px; margin: 0 auto;
  padding: 22px 16px 26px;
  display: flex; flex-direction: column; gap: 16px;
}

h1 { margin: 0 0 6px; font-size: clamp(21px, 3.6vw, 29px); letter-spacing: -.021em; line-height: 1.15; }
.lede { margin: 0; max-width: 68ch; color: var(--muted); font-size: 14.5px; line-height: 1.55; }
.lede b { color: var(--ink); font-weight: 600; }
.lede code {
  font-family: var(--font-mono); font-size: .87em; color: var(--accent);
  background: var(--accent-soft); border-radius: 4px; padding: 1px 5px;
}

.stage { display: grid; grid-template-columns: minmax(0,1fr) 300px; gap: 16px; align-items: stretch; }
@media (max-width: 900px) { .stage { grid-template-columns: minmax(0,1fr); align-items: start; } }

.field {
  position: relative; overflow: hidden;
  border: 1px solid var(--line); border-radius: var(--radius);
  background: var(--surface);
  min-height: 440px; box-shadow: var(--shadow);
}
@media (max-width: 900px) { .field { aspect-ratio: 4 / 3; min-height: 0; } }
@media (max-width: 560px) { .field { aspect-ratio: 1 / 1; } }
canvas { display: block; width: 100%; height: 100%; touch-action: none; cursor: crosshair; }
canvas:focus { outline: none; }
canvas:focus-visible { outline: 2px solid var(--accent); outline-offset: -3px; }

.hint {
  position: absolute; left: 10px; bottom: 10px; pointer-events: none;
  font-family: var(--font-mono); font-size: 11px; letter-spacing: .01em;
  color: var(--muted); background: var(--scrim);
  border: 1px solid var(--line); border-radius: 999px; padding: 5px 10px;
  transition: opacity .45s ease; max-width: calc(100% - 20px);
}
.hint.gone { opacity: 0; }

/* Paused affordance. Deliberately NOT a full-field scrim: pausing is how you
   stop and read an orbit, so the field has to stay completely legible. */
.overlay {
  position: absolute; left: 50%; top: 10px; transform: translateX(-50%);
  max-width: calc(100% - 20px); pointer-events: none;
}
.overlay[hidden] { display: none; }
.overlay .box {
  display: flex; align-items: center; gap: 9px;
  background: var(--scrim); border: 1px solid var(--line);
  border-radius: 999px; padding: 5px 6px 5px 13px; box-shadow: var(--shadow);
}
.overlay p {
  margin: 0; font-family: var(--font-mono); font-size: 11px; line-height: 1.3;
  color: var(--muted); letter-spacing: .01em;
}
.overlay button { pointer-events: auto; flex: none; padding: 5px 12px; font-size: 12.5px; }
@media (max-width: 460px) {
  /* wrap only when the note is too long to sit beside the button */
  .overlay .box { border-radius: 14px; flex-wrap: wrap; justify-content: center; padding: 5px 7px; gap: 7px; }
  .overlay p { text-align: center; }
}

.panel { display: flex; flex-direction: column; gap: 12px; }
.card {
  background: var(--surface); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 12px 13px; box-shadow: var(--shadow);
}
.card h2 {
  margin: 0 0 10px; font-family: var(--font-mono); font-size: 10.5px;
  letter-spacing: .1em; text-transform: uppercase; color: var(--muted); font-weight: 600;
  display: flex; align-items: center; justify-content: space-between; gap: 8px;
}
.card h2 .tag { letter-spacing: .04em; text-transform: none; color: var(--muted); font-weight: 400; }

/* classification pill */
.cls {
  display: inline-flex; align-items: center; gap: 7px; width: 100%;
  font-family: var(--font-mono); font-size: 13px; letter-spacing: .06em;
  text-transform: uppercase; padding: 7px 11px; border-radius: 8px;
  border: 1px solid var(--line); color: var(--muted); background: var(--bg);
  margin-bottom: 11px;
}
.cls .dot { width: 8px; height: 8px; border-radius: 50%; background: currentColor; flex: none; }
.cls.ok   { color: var(--run);    background: var(--run-soft);    border-color: transparent; }
.cls.warn { color: var(--accent); background: var(--accent-soft); border-color: transparent; }
.cls.bad  { color: var(--danger); background: var(--bg);          border-color: var(--danger); }
.cls.live { color: var(--ink); background: var(--bg); border-color: var(--line); }

.stats { display: grid; grid-template-columns: repeat(3, minmax(0,1fr)); gap: 11px 10px; }
.stat { display: flex; flex-direction: column; gap: 2px; min-width: 0; }
.stat .v {
  font-family: var(--font-mono); font-size: 16px; line-height: 1.05;
  font-variant-numeric: tabular-nums; letter-spacing: -.02em;
  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}
.stat .k {
  font-family: var(--font-mono); font-size: 9px; letter-spacing: .09em;
  text-transform: uppercase; color: var(--muted);
  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}
.stat .v.warn { color: var(--accent); }
.stat .v.bad  { color: var(--danger); }

.strip {
  display: flex; flex-wrap: wrap; gap: 4px 14px; margin-top: 12px;
  padding-top: 10px; border-top: 1px solid var(--line);
  font-family: var(--font-mono); font-size: 11px; color: var(--muted);
  font-variant-numeric: tabular-nums;
}
.strip b { color: var(--ink); font-weight: 500; }

/* satellite list */
.satlist { display: flex; flex-direction: column; gap: 4px; margin-top: 11px; }
.satlist:empty { display: none; }
.satrow {
  display: flex; align-items: center; gap: 8px; width: 100%;
  font-family: var(--font-mono); font-size: 11px; text-align: left;
  padding: 5px 8px; border-radius: 7px; border: 1px solid transparent;
  background: transparent; color: var(--muted); cursor: pointer; line-height: 1.2;
  font-variant-numeric: tabular-nums;
}
.satrow:hover { background: var(--bg); }
.satrow[aria-pressed="true"] { border-color: var(--line); background: var(--bg); color: var(--ink); }
.satrow .chip { width: 9px; height: 9px; border-radius: 50%; flex: none; }
.satrow .nm { flex: none; }
.satrow .cl { flex: 1 1 auto; text-align: right; color: var(--muted); overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
.satrow:focus-visible { outline: 2px solid var(--accent); outline-offset: 1px; }

/* controls */
.ctl { display: block; margin-bottom: 13px; }
.ctl:last-child { margin-bottom: 0; }
.ctl-top { display: flex; justify-content: space-between; align-items: baseline; gap: 8px; margin-bottom: 3px; }
.ctl-top label { font-size: 13px; }
.ctl-top .val {
  font-family: var(--font-mono); font-size: 12px; color: var(--muted);
  font-variant-numeric: tabular-nums; white-space: nowrap;
}
.ctl .desc { display: block; font-size: 11.5px; color: var(--muted); line-height: 1.35; margin-top: 2px; }

input[type=range] {
  -webkit-appearance: none; appearance: none; display: block;
  width: 100%; height: 20px; margin: 0; background: transparent; cursor: pointer;
}
input[type=range]::-webkit-slider-runnable-track { height: 4px; border-radius: 2px; background: var(--line); }
input[type=range]::-webkit-slider-thumb {
  -webkit-appearance: none; width: 15px; height: 15px; margin-top: -5.5px;
  border-radius: 50%; background: var(--accent); border: 2px solid var(--surface);
  box-shadow: 0 0 0 1px var(--line);
}
input[type=range]::-moz-range-track { height: 4px; border-radius: 2px; background: var(--line); }
input[type=range]::-moz-range-thumb {
  width: 13px; height: 13px; border-radius: 50%; background: var(--accent);
  border: 2px solid var(--surface); box-shadow: 0 0 0 1px var(--line);
}
input[type=range]:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 6px; }

button {
  font: inherit; font-size: 13px; font-family: var(--font-sans);
  color: var(--ink); background: var(--surface);
  border: 1px solid var(--line); border-radius: 8px;
  padding: 7px 11px; cursor: pointer; line-height: 1.2;
  transition: border-color .12s ease, color .12s ease, background .12s ease;
}
button:hover:not(:disabled) { border-color: var(--accent); color: var(--accent); }
button:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
button:disabled { opacity: .42; cursor: not-allowed; }
button.primary { background: var(--accent); color: var(--surface); border-color: var(--accent); font-weight: 600; }
button.primary:hover:not(:disabled) { color: var(--surface); filter: brightness(1.07); }
button.big { font-size: 14px; padding: 9px 18px; }
button.wide { width: 100%; }

.row { display: flex; flex-wrap: wrap; gap: 7px; }
.row > button { flex: 1 1 auto; }

.preview {
  margin: 9px 0 0; font-family: var(--font-mono); font-size: 11px;
  color: var(--muted); line-height: 1.45; font-variant-numeric: tabular-nums;
  display: flex; align-items: baseline; gap: 6px;
}
.preview .arrow { color: var(--line); flex: none; }
.preview .body { min-width: 0; }
.preview b { color: var(--ink); font-weight: 500; }
.preview.warn b { color: var(--accent); }
.preview.bad b { color: var(--danger); }

.switch { display: flex; align-items: center; gap: 9px; cursor: pointer; font-size: 13px; margin-top: 12px; }
.switch input { position: absolute; opacity: 0; width: 0; height: 0; }
.track {
  width: 34px; height: 19px; border-radius: 999px; background: var(--line);
  border: 1px solid var(--line); position: relative; flex: none; transition: background .15s ease;
}
.track::after {
  content: ""; position: absolute; top: 1.5px; left: 2px; width: 14px; height: 14px;
  border-radius: 50%; background: var(--surface); transition: transform .15s ease;
  box-shadow: 0 1px 2px rgba(0,0,0,.25);
}
.switch input:checked + .track { background: var(--accent); border-color: var(--accent); }
.switch input:checked + .track::after { transform: translateX(15px); }
.switch input:focus-visible + .track { outline: 2px solid var(--accent); outline-offset: 2px; }

footer {
  font-family: var(--font-mono); font-size: 11px; color: var(--muted);
  letter-spacing: .05em; padding-top: 2px;
}

.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition-duration: .001ms !important; animation-duration: .001ms !important; }
}
</style>
</head>
<body>
<div class="wrap">

  <header>
    <h1>Orbit Playground</h1>
    <p class="lede">
      One planet, one rule: <b>everything falls toward mass</b>. Press anywhere on the field and
      <b>drag out a velocity arrow</b> — the dashed curve shows the orbit you'd get before you let go.
      Release to launch. Too slow and it falls back into the planet; too fast and it never comes back.
      A short tap gives you a perfect circle.
    </p>
  </header>

  <main class="stage">

    <div class="field" id="field">
      <canvas id="c" tabindex="0" aria-label="Orbit field. A planet sits at the centre with satellites orbiting it. Drag with the pointer to launch a satellite. With the keyboard: left and right arrows move the launch point around the planet, up and down arrows change its altitude, plus and minus change launch speed, and Enter launches.">
        An interactive canvas showing satellites orbiting a planet. Your browser does not support canvas.
      </canvas>
      <div class="hint" id="hint">drag anywhere to launch &middot; tap for a circular orbit</div>
      <div class="overlay" id="overlay" hidden>
        <div class="box">
          <p id="overlayNote">paused</p>
          <button class="primary" id="overlayPlay" type="button">&#9654;&nbsp;Play</button>
        </div>
      </div>
    </div>

    <div class="panel">

      <section class="card" aria-label="Orbit readout">
        <h2>Readout <span class="tag" id="selTag">satellite 1</span></h2>

        <span class="cls" id="clsPill"><span class="dot"></span><span id="clsText">elliptical</span></span>

        <div class="stats">
          <div class="stat"><span class="v" id="stSpeed">&mdash;</span><span class="k">speed km/s</span></div>
          <div class="stat"><span class="v" id="stAlt">&mdash;</span><span class="k">altitude km</span></div>
          <div class="stat"><span class="v" id="stEcc">&mdash;</span><span class="k">eccentricity</span></div>
          <div class="stat"><span class="v" id="stPeri">&mdash;</span><span class="k">periapsis km</span></div>
          <div class="stat"><span class="v" id="stApo">&mdash;</span><span class="k">apoapsis km</span></div>
          <div class="stat"><span class="v" id="stPer">&mdash;</span><span class="k">period</span></div>
        </div>

        <div class="strip">
          <span>clock <b id="stClock">0m 00s</b></span>
          <span>laps <b id="stLaps">0</b></span>
          <span>in orbit <b id="stCount">1</b></span>
        </div>

        <div class="satlist" id="satList" role="group" aria-label="Satellites in the field"></div>
      </section>

      <section class="card" aria-label="Launch a satellite">
        <h2>Launch</h2>

        <div class="ctl">
          <div class="ctl-top">
            <label for="alt">Altitude</label><span class="val" id="altV" aria-hidden="true">12,000 km</span>
          </div>
          <input type="range" id="alt" min="400" max="30000" step="200" value="12000">
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="spd">Speed</label><span class="val" id="spdV" aria-hidden="true">100% of circular</span>
          </div>
          <input type="range" id="spd" min="40" max="170" step="1" value="100">
          <span class="desc">100% holds a circle &middot; 141% is escape speed</span>
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="ang">Start angle</label><span class="val" id="angV" aria-hidden="true">200&deg;</span>
          </div>
          <input type="range" id="ang" min="0" max="355" step="5" value="200">
        </div>

        <p class="preview" id="preview"><span class="arrow">&rarr;</span><span class="body" id="previewBody">&mdash;</span></p>

        <div class="row" style="margin-top:10px">
          <button type="button" class="primary wide" id="launchBtn">Launch satellite</button>
        </div>
      </section>

      <section class="card" aria-label="Simulation controls">
        <h2>Controls</h2>

        <div class="ctl">
          <div class="ctl-top">
            <label for="warp">Time warp</label><span class="val" id="warpV" aria-hidden="true">&times;2000</span>
          </div>
          <input type="range" id="warp" min="100" max="6000" step="100" value="2000">
          <span class="desc" id="warpDesc">1 second here &asymp; 33 minutes up there</span>
        </div>

        <div class="row" style="margin-bottom:7px">
          <button type="button" class="primary" id="playBtn">Pause</button>
          <button type="button" id="resetBtn">Reset</button>
        </div>
        <div class="row">
          <button type="button" id="clearBtn">Clear field</button>
        </div>

        <label class="switch" for="paths">
          <input type="checkbox" id="paths" checked>
          <span class="track" aria-hidden="true"></span>
          <span>Orbit paths</span>
        </label>
        <label class="switch" for="trails">
          <input type="checkbox" id="trails" checked>
          <span class="track" aria-hidden="true"></span>
          <span>Fading trails</span>
        </label>
      </section>

    </div>
  </main>

  <footer>demo &middot; sloprun.dev</footer>
</div>

<div class="sr-only" role="status" aria-live="polite" id="live"></div>

<script>
(function () {
  "use strict";

  /* ── theme plumbing ─────────────────────────────────────────────────────── */
  var root = document.documentElement;
  var mqDark = window.matchMedia ? matchMedia("(prefers-color-scheme: dark)") : null;
  var mqRM   = window.matchMedia ? matchMedia("(prefers-reduced-motion: reduce)") : null;

  window.addEventListener("message", function (e) {
    var d = e.data;
    if (d && d.type === "sloprun:theme" && (d.theme === "light" || d.theme === "dark")) {
      root.setAttribute("data-theme", d.theme);
      readTheme();
      if (!running) draw();
    }
  });
  if (mqDark) {
    var onScheme = function () { readTheme(); if (!running) draw(); };
    if (mqDark.addEventListener) mqDark.addEventListener("change", onScheme);
    else if (mqDark.addListener) mqDark.addListener(onScheme);
  }

  /* ── colour reading ─────────────────────────────────────────────────────── */
  var COL = {
    surface: [255,255,255], bg: [246,246,243], ink: [26,29,33], muted: [90,96,104],
    line: [227,226,220], accent: [192,86,0], run: [26,127,55], danger: [201,60,55],
    planet: [58,64,72], planet2: [23,26,31]
  };
  var SAT = [[192,86,0],[14,116,144],[109,40,217],[180,18,106],[29,78,216]];

  function hexToRgb(h) {
    h = String(h).trim();
    if (h.charAt(0) === "#") h = h.slice(1);
    if (h.length === 3) h = h.charAt(0)+h.charAt(0)+h.charAt(1)+h.charAt(1)+h.charAt(2)+h.charAt(2);
    var n = parseInt(h, 16);
    if (h.length !== 6 || isNaN(n)) return null;
    return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
  }
  function tok(name, fallback) {
    var v = getComputedStyle(root).getPropertyValue(name);
    return hexToRgb(v) || fallback;
  }
  function rgba(c, a) { return "rgba(" + c[0] + "," + c[1] + "," + c[2] + "," + a + ")"; }

  function readTheme() {
    COL.surface = tok("--surface", COL.surface);
    COL.bg      = tok("--bg",      COL.bg);
    COL.ink     = tok("--ink",     COL.ink);
    COL.muted   = tok("--muted",   COL.muted);
    COL.line    = tok("--line",    COL.line);
    COL.accent  = tok("--accent",  COL.accent);
    COL.run     = tok("--run",     COL.run);
    COL.danger  = tok("--danger",  COL.danger);
    COL.planet  = tok("--planet",  COL.planet);
    COL.planet2 = tok("--planet2", COL.planet2);
    SAT[0] = tok("--s1", SAT[0]);
    SAT[1] = tok("--s2", SAT[1]);
    SAT[2] = tok("--s3", SAT[2]);
    SAT[3] = tok("--s4", SAT[3]);
    SAT[4] = tok("--s5", SAT[4]);
  }

  /* ── dom ────────────────────────────────────────────────────────────────── */
  var $ = function (id) { return document.getElementById(id); };
  var field = $("field"), canvas = $("c"), ctx = canvas.getContext("2d");
  var hint = $("hint"), overlay = $("overlay"), overlayPlay = $("overlayPlay"), overlayNote = $("overlayNote");
  var clsPill = $("clsPill"), clsText = $("clsText"), selTag = $("selTag");
  var stSpeed = $("stSpeed"), stAlt = $("stAlt"), stEcc = $("stEcc");
  var stPeri = $("stPeri"), stApo = $("stApo"), stPer = $("stPer");
  var stClock = $("stClock"), stLaps = $("stLaps"), stCount = $("stCount");
  var satList = $("satList");
  var elAlt = $("alt"), elSpd = $("spd"), elAng = $("ang"), elWarp = $("warp");
  var altV = $("altV"), spdV = $("spdV"), angV = $("angV"), warpV = $("warpV"), warpDesc = $("warpDesc");
  var preview = $("preview"), previewBody = $("previewBody");
  var launchBtn = $("launchBtn"), playBtn = $("playBtn"), resetBtn = $("resetBtn"), clearBtn = $("clearBtn");
  var elPaths = $("paths"), elTrails = $("trails"), live = $("live");

  /* ── constants (classroom Newtonian gravity, round numbers) ─────────────── */
  var MU = 398600;          /* gravitational parameter, km^3/s^2 */
  var RP = 6000;            /* planet radius, km */
  var WORLD_H = 72000;      /* km of world mapped to the canvas height */
  var TAU_DRAG = 1500;      /* drag arrow reads as "distance covered in 1500 s" */
  var MAXSAT = 5;          /* one distinct hue each — see SAT[] */
  var KSTEP = 0.0010;       /* sub-step h = KSTEP * (periapsis radius) / (periapsis speed) */
  var MAXSUB = 1200;
  var TRAIL_MAX = 190;      /* committed trail points kept per satellite */
  var TRAIL_MIN = WORLD_H * 0.0035;   /* commit one point every ~250 km of path */
  var TRAIL_MIN2 = TRAIL_MIN * TRAIL_MIN;
  var VMAX = 30;            /* km/s clamp on a hand-drawn launch */
  var MONO = 'ui-monospace, "SF Mono", Menlo, Consolas, monospace';

  /* ── state ──────────────────────────────────────────────────────────────── */
  var W = 0, H = 0, scale = 1, cx = 0, cy = 0;
  var sats = [], ghosts = [], nextId = 1, selId = 0;
  var clock = 0, running = true, warp = 2000;
  var showPaths = true, showTrails = true;
  var aim = { alt: 12000, pct: 100, ang: 200 };
  var aimHot = false;
  var drag = null;                     /* {kind:'new'|'grab', sat, x, y, vx, vy, moved} */
  var rafId = 0, lastT = 0, uiT = 0;
  var reduced = !!(mqRM && mqRM.matches);

  /* ── formatting ─────────────────────────────────────────────────────────── */
  function fmtInt(v) {
    if (!isFinite(v)) return "∞";
    var n = Math.round(v), s = n < 0 ? "-" : "", a = String(Math.abs(n)), out = "", k = 0;
    for (var i = a.length - 1; i >= 0; i--) { out = a.charAt(i) + out; if (++k % 3 === 0 && i > 0) out = "," + out; }
    return s + out;
  }
  function pad2(n) { return (n < 10 ? "0" : "") + n; }
  function fmtDur(s) {
    if (!isFinite(s) || s <= 0) return "—";
    s = Math.round(s);
    var d = Math.floor(s / 86400); s -= d * 86400;
    var h = Math.floor(s / 3600);  s -= h * 3600;
    var m = Math.floor(s / 60);    s -= m * 60;
    if (d) return d + "d " + h + "h";
    if (h) return h + "h " + pad2(m) + "m";
    return m + "m " + pad2(s) + "s";
  }
  function fmtClock(s) {
    s = Math.max(0, Math.round(s));
    var d = Math.floor(s / 86400); s -= d * 86400;
    var h = Math.floor(s / 3600);  s -= h * 3600;
    var m = Math.floor(s / 60);    s -= m * 60;
    if (d) return d + "d " + h + "h";
    if (h) return h + "h " + pad2(m) + "m";
    return m + "m " + pad2(s) + "s";
  }
  function say(t) { live.textContent = t; }

  /* ── orbital elements from state vector ─────────────────────────────────── */
  function elems(x, y, vx, vy) {
    var r = Math.sqrt(x * x + y * y) || 1e-9;
    var v2 = vx * vx + vy * vy;
    var h = x * vy - y * vx;
    var c = v2 - MU / r, rv = x * vx + y * vy;
    var ex = (c * x - rv * vx) / MU, ey = (c * y - rv * vy) / MU;
    var e = Math.sqrt(ex * ex + ey * ey);
    var p = h * h / MU;
    var eps = v2 / 2 - MU / r;
    var o = {
      r: r, v: Math.sqrt(v2), h: h, e: e, p: p, eps: eps, rv: rv,
      w: (e > 1e-7 ? Math.atan2(ey, ex) : Math.atan2(y, x)),
      a: Infinity, peri: 0, apo: Infinity, T: Infinity, kind: "escape"
    };
    o.peri = p / (1 + e);
    /* e is exactly 1 for a purely radial throw, so admit that degenerate case
       too: it is a flat ellipse with apoapsis 2a and periapsis 0, and the
       energy sign is what actually decides whether it comes back. */
    if (eps < -1e-9 && e < 1 + 1e-9) {
      o.a = -MU / (2 * eps);
      o.apo = o.a * (1 + e);
      o.peri = o.a * (1 - e);
      o.T = 2 * Math.PI * Math.sqrt(o.a * o.a * o.a / MU);
      o.kind = (e < 0.02) ? "circular" : "elliptical";
    }
    /* A closed orbit with its low point inside the planet always hits, whichever
       way it is currently moving — it comes back round. An escape path only hits
       if it is still falling inward (rv < 0); once past the low point it is gone. */
    o.impact = (o.peri < RP) && (o.kind !== "escape" || rv < 0);
    return o;
  }
  function classOf(o) {
    if (!(o.p > 1e-6)) {
      /* a purely radial throw: only leaves for good if it has escape energy
         AND is still climbing; anything else is on its way back down */
      return (o.eps >= 0 && o.rv >= 0)
        ? { text: "escape", cls: "warn" }
        : { text: "straight down", cls: "bad" };
    }
    if (o.impact) return { text: "impact course", cls: "bad" };
    if (o.kind === "escape") return { text: "escape", cls: "warn" };
    if (o.kind === "circular") return { text: "circular", cls: "ok" };
    return { text: "elliptical", cls: "live" };
  }

  /* ── conic sampling (same polar formula for ellipse and hyperbola) ──────── */
  function conic(o, out) {
    out.length = 0;
    if (!(o.p > 1e-6) || !isFinite(o.p)) return out;
    var cw = Math.cos(o.w), sw = Math.sin(o.w);
    var RMAX = WORLD_H * 3.2;
    var lo, hi, n;
    if (o.e < 1) { lo = 0; hi = Math.PI * 2; n = 260; }
    else {
      var nmax = Math.acos(Math.max(-1, Math.min(1, -1 / o.e))) * 0.988;
      lo = -nmax; hi = nmax; n = 200;
    }
    var gap = true;
    for (var i = 0; i <= n; i++) {
      var nu = lo + (hi - lo) * i / n;
      var den = 1 + o.e * Math.cos(nu);
      var r = (den < 1e-6) ? Infinity : o.p / den;
      if (!(r <= RMAX)) { gap = true; continue; }
      /* NaN marks a break: without it a huge ellipse whose apoapsis falls off
         the map would be "closed" with a straight chord across the gap */
      if (gap && out.length) out.push(NaN, NaN);
      gap = false;
      var px = r * Math.cos(nu), py = r * Math.sin(nu);
      out.push(px * cw - py * sw, px * sw + py * cw);
    }
    return out;
  }
  var conicBuf = [];

  /* ── satellites ─────────────────────────────────────────────────────────── */
  function makeSat(x, y, vx, vy) {
    var o = elems(x, y, vx, vy);
    return {
      id: nextId++, x: x, y: y, vx: vx, vy: vy,
      trail: [], laps: 0, sweep: 0, ang: Math.atan2(y, x),
      born: clock, o: o, h: chooseStep(o)
    };
  }

  /* Symplectic Euler only conserves energy if the step size never changes, so
     each satellite gets ONE step size, fixed for its whole life: fine enough to
     resolve its fastest moment (periapsis) and then never touched again. */
  function chooseStep(o) {
    var rp = Math.max(o.peri, RP * 0.85);
    var inv = isFinite(o.a) ? 1 / o.a : 0;
    var vp = Math.sqrt(Math.max(MU * (2 / rp - inv), 1e-6));
    var h = KSTEP * rp / vp;
    if (h < 0.05) h = 0.05;
    if (h > 30) h = 30;
    return h;
  }
  function satColor(s) { return SAT[(s.id - 1) % SAT.length]; }
  function selected() {
    for (var i = 0; i < sats.length; i++) if (sats[i].id === selId) return sats[i];
    return null;
  }
  function addSat(s, quiet) {
    if (sats.length >= MAXSAT) return null;
    sats.push(s); selId = s.id;
    fillTrail(s);
    renderList(); syncButtons();
    if (!quiet) {
      var c = classOf(s.o);
      say("Satellite " + s.id + " launched. " + c.text +
          (s.o.impact ? ", it will hit the surface."
           : s.o.kind === "escape" ? ", it will not come back."
           : ". Period " + fmtDur(s.o.T) + "."));
    }
    return s;
  }
  function removeSat(s, why) {
    var i = sats.indexOf(s);
    if (i < 0) return;
    sats.splice(i, 1);
    if (s.trail.length > 3) ghosts.push({ trail: s.trail, col: satColor(s), fade: 1 });
    if (selId === s.id) selId = sats.length ? sats[sats.length - 1].id : 0;
    renderList(); syncButtons();
    if (why) say("Satellite " + s.id + " " + why + ".");
  }

  /* pre-fill a trail along the orbit behind the satellite, so a fresh field
     already reads as motion (and a paused first frame still looks alive) */
  function fillTrail(s) {
    var o = s.o;
    s.trail.length = 0;
    if (o.kind === "escape" || !(o.p > 0)) return;
    var nu0 = Math.atan2(s.y, s.x) - o.w;
    var dir = (o.h >= 0) ? -1 : 1;              /* walk backwards along motion */
    var step = 0.004, minD2 = TRAIL_MIN2;
    var lx = s.x, ly = s.y, pts = [];
    for (var k = 1; k * step < Math.PI * 2 && pts.length < TRAIL_MAX * 2; k++) {
      var nu = nu0 + dir * k * step;
      var den = 1 + o.e * Math.cos(nu);
      if (den < 1e-6) break;
      var r = o.p / den;
      if (r < RP) break;
      var px = r * Math.cos(nu + o.w), py = r * Math.sin(nu + o.w);
      var dx = px - lx, dy = py - ly;
      if (dx * dx + dy * dy >= minD2) { pts.push(px, py); lx = px; ly = py; }
    }
    for (var i = pts.length - 2; i >= 0; i -= 2) s.trail.push(pts[i], pts[i + 1]);
  }

  /* Commit a point only once the satellite has travelled far enough for the
     spacing to matter. The live position is NOT stored — it is drawn as the
     head segment every frame, so the trail always reaches the satellite
     without the committed points being dragged along behind it. */
  function pushTrail(s) {
    var t = s.trail, n = t.length;
    if (n >= 2) {
      var dx = s.x - t[n - 2], dy = s.y - t[n - 1];
      if (dx * dx + dy * dy < TRAIL_MIN2) return;
    }
    t.push(s.x, s.y);
    if (t.length > TRAIL_MAX * 2) t.splice(0, t.length - TRAIL_MAX * 2);
  }

  /* ── physics: symplectic Euler with adaptive sub-steps ──────────────────── */
  function stepSat(s, dt) {
    var h = s.h, left = dt, guard = 0;
    if (dt / h > MAXSUB) h = dt / MAXSUB;     /* safety valve, extreme warp only */
    while (left > 1e-9 && guard++ < MAXSUB + 2) {
      var step = (left > h) ? h : left;
      var r2 = s.x * s.x + s.y * s.y;
      var r = Math.sqrt(r2) || 1e-9;
      var g = -MU / (r2 * r);
      s.vx += g * s.x * step;                 /* velocity first ... */
      s.vy += g * s.y * step;
      s.x  += s.vx * step;                    /* ... then position: symplectic */
      s.y  += s.vy * step;
      left -= step;
      if (s.x * s.x + s.y * s.y < RP * RP) return "reached the surface";
    }
    var rr = Math.sqrt(s.x * s.x + s.y * s.y);
    if (rr > WORLD_H * 3) return "left the neighbourhood";
    return null;
  }

  function tick(dt) {
    clock += dt;
    for (var i = sats.length - 1; i >= 0; i--) {
      var s = sats[i];
      var end = stepSat(s, dt);
      var a = Math.atan2(s.y, s.x), d = a - s.ang;
      while (d > Math.PI) d -= Math.PI * 2;
      while (d < -Math.PI) d += Math.PI * 2;
      s.sweep += Math.abs(d); s.ang = a;
      s.laps = Math.floor(s.sweep / (Math.PI * 2));
      s.o = elems(s.x, s.y, s.vx, s.vy);
      pushTrail(s);
      if (end) removeSat(s, end);
    }
    for (var g = ghosts.length - 1; g >= 0; g--) {
      ghosts[g].fade -= dt / (warp * 1.6);
      if (ghosts[g].fade <= 0) ghosts.splice(g, 1);
    }
  }

  /* ── geometry helpers ───────────────────────────────────────────────────── */
  function sxOf(x) { return cx + x * scale; }
  function syOf(y) { return cy - y * scale; }
  function toWorld(px, py) { return { x: (px - cx) / scale, y: (cy - py) / scale }; }

  function aimState() {
    var r = RP + aim.alt;
    var t = aim.ang * Math.PI / 180;
    var x = r * Math.cos(t), y = r * Math.sin(t);
    var vc = Math.sqrt(MU / r) * (aim.pct / 100);
    return { x: x, y: y, vx: -Math.sin(t) * vc, vy: Math.cos(t) * vc };
  }

  /* ── drawing ────────────────────────────────────────────────────────────── */
  function polyPath(pts) {
    var pen = false, drew = false;
    ctx.beginPath();
    for (var i = 0; i < pts.length; i += 2) {
      var x = pts[i];
      if (x !== x) { pen = false; continue; }        /* NaN break marker */
      var sx = sxOf(x), sy = syOf(pts[i + 1]);
      if (pen) { ctx.lineTo(sx, sy); drew = true; }
      else { ctx.moveTo(sx, sy); pen = true; }
    }
    return drew;
  }

  /* hx,hy = the satellite's live position, appended for the duration of the
     draw so the freshest segment always joins the trail to the dot */
  function drawTrail(t, col, mul, hx, hy) {
    var head = (hx !== undefined);
    if (head) t.push(hx, hy);
    var n = t.length / 2;
    if (n >= 2) {
      var B = 14;
      for (var b = 0; b < B; b++) {
        var i0 = Math.floor(b * (n - 1) / B), i1 = Math.floor((b + 1) * (n - 1) / B);
        if (i1 <= i0) continue;
        var f = (b + 1) / B;
        ctx.beginPath();
        ctx.moveTo(sxOf(t[i0 * 2]), syOf(t[i0 * 2 + 1]));
        for (var i = i0 + 1; i <= i1; i++) ctx.lineTo(sxOf(t[i * 2]), syOf(t[i * 2 + 1]));
        ctx.strokeStyle = rgba(col, (0.05 + 0.55 * f * f) * mul);
        ctx.lineWidth = 1 + 1.6 * f;
        ctx.stroke();
      }
    }
    if (head) t.length -= 2;
  }

  function drawPlanet() {
    var R = RP * scale;
    var glow = ctx.createRadialGradient(cx, cy, R * 0.9, cx, cy, R * 5.5);
    glow.addColorStop(0, rgba(COL.accent, 0.09));
    glow.addColorStop(1, rgba(COL.accent, 0));
    ctx.fillStyle = glow;
    ctx.fillRect(0, 0, W, H);

    var g = ctx.createRadialGradient(cx - R * 0.35, cy - R * 0.4, R * 0.1, cx, cy, R * 1.05);
    g.addColorStop(0, rgba(COL.planet, 1));
    g.addColorStop(1, rgba(COL.planet2, 1));
    ctx.beginPath(); ctx.arc(cx, cy, R, 0, Math.PI * 2);
    ctx.fillStyle = g; ctx.fill();

    ctx.beginPath(); ctx.arc(cx, cy, R + 0.6, 0, Math.PI * 2);
    ctx.strokeStyle = rgba(COL.accent, 0.42); ctx.lineWidth = 1.2; ctx.stroke();
  }

  function drawRings() {
    ctx.save();
    ctx.setLineDash([2, 5]);
    ctx.lineWidth = 1;
    ctx.font = "9px " + MONO;
    ctx.textAlign = "left"; ctx.textBaseline = "middle";
    for (var k = 1; k <= 3; k++) {
      var r = k * 15000;
      var pr = r * scale;
      if (pr > Math.max(W, H) * 0.85) break;
      ctx.beginPath(); ctx.arc(cx, cy, pr, 0, Math.PI * 2);
      ctx.strokeStyle = rgba(COL.muted, 0.22); ctx.stroke();
      ctx.setLineDash([]);
      ctx.fillStyle = rgba(COL.muted, 0.55);
      var lbl = fmtInt(r) + " km";
      var lw = ctx.measureText(lbl).width;
      var lx = cx + pr * 0.72 + 4, ly = cy - pr * 0.72;
      if (lx + lw > W - 4) lx = W - 4 - lw;
      if (lx < 4) lx = 4;
      if (ly < 8) ly = 8;
      ctx.fillText(lbl, lx, ly);
      ctx.setLineDash([2, 5]);
    }
    ctx.restore();
  }

  function drawApsis(o, col, sel) {
    if (o.kind === "escape" || o.kind === "circular" || !(o.p > 0)) return;
    var pts = [
      { r: o.peri, a: o.w, lbl: "peri" },
      { r: o.apo,  a: o.w + Math.PI, lbl: "apo" }
    ];
    ctx.font = "9px " + MONO;
    ctx.textAlign = "center"; ctx.textBaseline = "middle";
    for (var i = 0; i < pts.length; i++) {
      var p = pts[i];
      if (!isFinite(p.r)) continue;
      var ux = Math.cos(p.a), uy = Math.sin(p.a);
      var x = sxOf(p.r * ux), y = syOf(p.r * uy);
      ctx.beginPath(); ctx.arc(x, y, 3.4, 0, Math.PI * 2);
      ctx.strokeStyle = rgba(col, 0.85); ctx.lineWidth = 1.3; ctx.stroke();
      var lx = x + ux * 13, ly = y - uy * 13;
      var sx0 = sxOf(sel ? sel.x : 1e9), sy0 = syOf(sel ? sel.y : 1e9);
      if ((lx - sx0) * (lx - sx0) + (ly - sy0) * (ly - sy0) > 15 * 15) {
        ctx.fillStyle = rgba(COL.muted, 0.9);
        ctx.fillText(p.lbl, lx, ly);
      }
    }
  }

  function drawArrow(x0, y0, x1, y1, col, wdt) {
    var dx = x1 - x0, dy = y1 - y0, len = Math.sqrt(dx * dx + dy * dy);
    if (len < 2) return;
    var ux = dx / len, uy = dy / len;
    var hx = x1 - ux * 9, hy = y1 - uy * 9;
    ctx.beginPath(); ctx.moveTo(x0, y0); ctx.lineTo(hx, hy);
    ctx.strokeStyle = col; ctx.lineWidth = wdt; ctx.stroke();
    ctx.beginPath();
    ctx.moveTo(x1, y1);
    ctx.lineTo(hx - uy * 4.6, hy + ux * 4.6);
    ctx.lineTo(hx + uy * 4.6, hy - ux * 4.6);
    ctx.closePath();
    ctx.fillStyle = col; ctx.fill();
  }

  function drawGhost() {
    /* the pending launch: dashed conic + start marker, brighter when focused */
    var st, alpha, showArrow = false, label = null;
    if (drag) {
      st = { x: drag.x, y: drag.y, vx: drag.vx, vy: drag.vy };
      alpha = 1; showArrow = true;
    } else {
      if (sats.length >= MAXSAT) return;
      st = aimState();
      alpha = aimHot ? 0.85 : 0.42;
    }
    var o = elems(st.x, st.y, st.vx, st.vy);
    var c = classOf(o);
    /* the pending launch reads in accent so it never gets lost among the
       muted range rings; danger when the curve ends in the surface */
    var col = (c.cls === "bad") ? COL.danger : COL.accent;

    ctx.save();
    ctx.setLineDash([5, 5]);
    ctx.lineWidth = 1.4;
    ctx.strokeStyle = rgba(col, 0.75 * alpha);
    if (polyPath(conic(o, conicBuf))) ctx.stroke();
    ctx.restore();

    var px = sxOf(st.x), py = syOf(st.y);

    if (showArrow) {
      ctx.save();
      ctx.setLineDash([3, 4]);
      ctx.beginPath(); ctx.moveTo(cx, cy); ctx.lineTo(px, py);
      ctx.strokeStyle = rgba(COL.muted, 0.4); ctx.lineWidth = 1; ctx.stroke();
      ctx.restore();
      drawArrow(px, py, sxOf(st.x + st.vx * TAU_DRAG), syOf(st.y + st.vy * TAU_DRAG), rgba(col, 1), 2);
      label = o.v.toFixed(2) + " km/s · " + c.text;
    }

    ctx.beginPath(); ctx.arc(px, py, 4.5, 0, Math.PI * 2);
    ctx.fillStyle = rgba(COL.surface, alpha);
    ctx.fill();
    ctx.strokeStyle = rgba(col, alpha); ctx.lineWidth = 1.6; ctx.stroke();

    if (label) {
      ctx.font = "11px " + MONO;
      ctx.textAlign = "left"; ctx.textBaseline = "middle";
      var tw = ctx.measureText(label).width;
      var lx = px + 12, ly = py - 16;
      if (lx + tw + 10 > W) lx = px - tw - 12;
      if (lx < 6) lx = 6;
      if (ly < 12) ly = py + 18;
      ctx.fillStyle = rgba(COL.surface, 0.88);
      ctx.fillRect(lx - 5, ly - 9, tw + 10, 18);
      ctx.strokeStyle = rgba(COL.line, 1); ctx.lineWidth = 1;
      ctx.strokeRect(lx - 5.5, ly - 9.5, tw + 11, 19);
      ctx.fillStyle = rgba(col, 1);
      ctx.fillText(label, lx, ly);
    }
  }

  function draw() {
    if (!W || !H) return;
    ctx.fillStyle = rgba(COL.surface, 1);
    ctx.fillRect(0, 0, W, H);
    ctx.lineJoin = "round"; ctx.lineCap = "round";

    drawRings();
    drawPlanet();

    var i, s, col;

    for (i = 0; i < ghosts.length; i++) {
      if (showTrails) drawTrail(ghosts[i].trail, ghosts[i].col, ghosts[i].fade * 0.6);
    }

    if (showPaths) {
      ctx.save();
      for (i = 0; i < sats.length; i++) {
        s = sats[i]; col = satColor(s);
        ctx.setLineDash(s.o.kind === "escape" ? [6, 5] : []);
        ctx.lineWidth = 1;
        ctx.strokeStyle = rgba(col, s.id === selId ? 0.42 : 0.2);
        if (polyPath(conic(s.o, conicBuf))) ctx.stroke();
      }
      ctx.restore();
    }

    for (i = 0; i < sats.length; i++) {
      s = sats[i]; col = satColor(s);
      if (showTrails) drawTrail(s.trail, col, 1, s.x, s.y);
    }

    var sel = selected();
    if (sel && showPaths) drawApsis(sel.o, satColor(sel), sel);

    for (i = 0; i < sats.length; i++) {
      s = sats[i]; col = satColor(s);
      var x = sxOf(s.x), y = syOf(s.y);
      if (s.id === selId) {
        ctx.beginPath(); ctx.arc(x, y, 9, 0, Math.PI * 2);
        ctx.strokeStyle = rgba(col, 0.5); ctx.lineWidth = 1.2; ctx.stroke();
      }
      ctx.beginPath(); ctx.arc(x, y, 4.6, 0, Math.PI * 2);
      ctx.fillStyle = rgba(col, 1); ctx.fill();
      ctx.strokeStyle = rgba(COL.surface, 0.95); ctx.lineWidth = 1.4; ctx.stroke();
    }

    drawGhost();

    if (!sats.length && !drag) {
      ctx.font = "12px " + MONO;
      ctx.textAlign = "center"; ctx.textBaseline = "middle";
      ctx.fillStyle = rgba(COL.muted, 0.8);
      ctx.fillText("empty field — drag to launch, or press Reset", cx, H - 26);
    }
  }

  /* ── readout ────────────────────────────────────────────────────────────── */
  function setCls(el, textEl, c) {
    el.className = "cls " + c.cls;
    textEl.textContent = c.text;
  }

  function syncPressed() {
    var rows = satList.children;
    for (var i = 0; i < rows.length; i++) {
      rows[i].setAttribute("aria-pressed",
        (+rows[i].getAttribute("data-id") === selId) ? "true" : "false");
    }
  }

  function renderList() {
    satList.textContent = "";
    if (sats.length < 2) return;
    for (var i = 0; i < sats.length; i++) {
      var s = sats[i], col = satColor(s);
      var b = document.createElement("button");
      b.type = "button"; b.className = "satrow";
      b.setAttribute("aria-pressed", s.id === selId ? "true" : "false");
      b.setAttribute("data-id", String(s.id));
      var chip = document.createElement("span");
      chip.className = "chip";
      chip.style.background = rgba(col, 1);
      var nm = document.createElement("span");
      nm.className = "nm"; nm.textContent = "sat " + s.id;
      var cl = document.createElement("span");
      cl.className = "cl"; cl.textContent = classOf(s.o).text;
      b.appendChild(chip); b.appendChild(nm); b.appendChild(cl);
      satList.appendChild(b);
    }
  }

  function updateReadout() {
    var s = selected(), o = null, tag = "", laps = 0;
    /* a grabbed satellite is lifted out of `sats` for the duration of the drag,
       so add it back for the count */
    var held = (drag && drag.kind === "grab") ? 1 : 0;
    stClock.textContent = fmtClock(clock);
    stCount.textContent = String(sats.length + held);

    if (drag) {
      /* while aiming, the readout follows the pending orbit — that is the
         thing the visitor is actually steering, and going blank here would
         hide the numbers exactly when they matter most */
      o = elems(drag.x, drag.y, drag.vx, drag.vy);
      tag = (drag.kind === "grab" && drag.sat)
              ? "satellite " + drag.sat.id + " · aiming"
              : "new satellite · aiming";
      laps = (drag.kind === "grab" && drag.sat) ? drag.sat.laps : 0;
    } else if (s) {
      o = s.o; tag = "satellite " + s.id; laps = s.laps;
    }

    if (!o) {
      selTag.textContent = "no satellite";
      setCls(clsPill, clsText, { text: "empty field", cls: "live" });
      stSpeed.textContent = stAlt.textContent = stEcc.textContent = "—";
      stPeri.textContent = stApo.textContent = stPer.textContent = "—";
      stPeri.className = stApo.className = stPer.className = "v";
      stLaps.textContent = "0";
      return;
    }
    var c = classOf(o);
    selTag.textContent = tag;
    setCls(clsPill, clsText, c);
    stSpeed.textContent = o.v.toFixed(2);
    stAlt.textContent = fmtInt(o.r - RP);
    stEcc.textContent = o.e.toFixed(3);
    stPeri.textContent = fmtInt(o.peri - RP);
    stPeri.className = "v" + (o.impact ? " bad" : "");
    stApo.textContent = (o.kind === "escape") ? "—" : fmtInt(o.apo - RP);
    stApo.className = "v" + (o.kind === "escape" ? " warn" : "");
    stPer.textContent = (o.kind === "escape") ? "never" : fmtDur(o.T);
    stPer.className = "v" + (o.kind === "escape" ? " warn" : "");
    stLaps.textContent = String(laps);

    var rows = satList.children;
    for (var i = 0; i < rows.length; i++) {
      var id = +rows[i].getAttribute("data-id");
      rows[i].setAttribute("aria-pressed", id === selId ? "true" : "false");
      for (var k = 0; k < sats.length; k++) {
        if (sats[k].id === id) rows[i].lastChild.textContent = classOf(sats[k].o).text;
      }
    }
  }

  function updatePreview() {
    var st = aimState();
    var o = elems(st.x, st.y, st.vx, st.vy);
    var c = classOf(o);
    var txt;
    if (o.impact) txt = "<b>impact course</b> — periapsis is " + fmtInt(RP - o.peri) + " km below the surface";
    else if (o.kind === "escape") txt = "<b>escape</b> — it never comes back";
    else if (o.kind === "circular") txt = "<b>circular</b> · " + fmtInt(o.r - RP) + " km up · one lap every " + fmtDur(o.T);
    else txt = "<b>elliptical</b> · peri " + fmtInt(o.peri - RP) + " · apo " + fmtInt(o.apo - RP) + " km · " + fmtDur(o.T);
    previewBody.innerHTML = txt;
    preview.className = "preview " + c.cls;
  }

  function syncButtons() {
    var full = sats.length >= MAXSAT;
    launchBtn.disabled = full;
    launchBtn.textContent = full ? "Field is full (" + MAXSAT + ")" : "Launch satellite";
    clearBtn.disabled = sats.length === 0;
  }

  /* ── sizing ─────────────────────────────────────────────────────────────── */
  function resize() {
    /* clientWidth/Height is the content box — exactly the canvas's CSS size,
       whereas getBoundingClientRect() would include the field's 1px border */
    var cw = Math.max(160, field.clientWidth);
    var ch = Math.max(160, field.clientHeight);
    var dpr = Math.min(2, window.devicePixelRatio || 1);
    if (cw === W && ch === H && canvas.width) return;
    canvas.width = Math.round(cw * dpr);
    canvas.height = Math.round(ch * dpr);
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    W = cw; H = ch; cx = W / 2; cy = H / 2;
    scale = Math.min(W, H) / WORLD_H;
    draw();
  }

  /* ── set-up / reset ─────────────────────────────────────────────────────── */
  function preload() {
    drag = null;                      /* drop anything held mid-drag */
    sats.length = 0; ghosts.length = 0; nextId = 1; selId = 0; clock = 0;
    /* a pleasing ellipse: 3,000 km up at its lowest, 24,000 km at its highest.
       Placed a little past periapsis rather than exactly on it, so the very
       first frame already shows a satellite mid-climb with its trail behind. */
    var rp = 9000, ra = 30000;
    var a = (rp + ra) / 2;
    var e = (ra - rp) / (ra + rp);
    var pl = a * (1 - e * e);
    var w = -0.55;                       /* which way the low point faces */
    var nu = 1.6;                        /* ~92 degrees past the low point */
    var r = pl / (1 + e * Math.cos(nu));
    var th = nu + w;
    var hh = Math.sqrt(MU * pl);
    var vr = MU / hh * e * Math.sin(nu); /* radial and transverse components */
    var vt = hh / r;
    var s = makeSat(
      r * Math.cos(th), r * Math.sin(th),
      vr * Math.cos(th) - vt * Math.sin(th),
      vr * Math.sin(th) + vt * Math.cos(th)
    );
    addSat(s, true);
  }

  function reset() {
    preload();
    aim.alt = 12000; aim.pct = 100; aim.ang = 200;
    elAlt.value = "12000"; elSpd.value = "100"; elAng.value = "200";
    syncAim();
    updateReadout(); syncButtons(); draw();
    say("Field reset to one elliptical orbit.");
  }

  /* ── controls ───────────────────────────────────────────────────────────── */
  function syncAim() {
    altV.textContent = fmtInt(aim.alt) + " km";
    spdV.textContent = aim.pct + "% of circular";
    angV.textContent = aim.ang + "°";
    elAlt.setAttribute("aria-valuetext", fmtInt(aim.alt) + " kilometres");
    elSpd.setAttribute("aria-valuetext", aim.pct + " percent of circular speed");
    elAng.setAttribute("aria-valuetext", aim.ang + " degrees");
    updatePreview();
  }

  elAlt.addEventListener("input", function () { aim.alt = +elAlt.value; syncAim(); if (!running) draw(); });
  elSpd.addEventListener("input", function () { aim.pct = +elSpd.value; syncAim(); if (!running) draw(); });
  elAng.addEventListener("input", function () { aim.ang = +elAng.value; syncAim(); if (!running) draw(); });

  function hot(v) { return function () { aimHot = v; if (!running) draw(); }; }
  [elAlt, elSpd, elAng, launchBtn].forEach(function (el) {
    el.addEventListener("focus", hot(true));
    el.addEventListener("blur", hot(false));
    el.addEventListener("pointerenter", hot(true));
    el.addEventListener("pointerleave", function () { if (document.activeElement !== elAlt && document.activeElement !== elSpd && document.activeElement !== elAng && document.activeElement !== canvas) { aimHot = false; if (!running) draw(); } });
  });

  elWarp.addEventListener("input", function () {
    warp = +elWarp.value;
    warpV.textContent = "×" + fmtInt(warp);
    elWarp.setAttribute("aria-valuetext", warp + " times real time");
    var mins = warp / 60;
    warpDesc.textContent = "1 second here ≈ " +
      (mins < 60 ? Math.round(mins) + " minutes" : (mins / 60).toFixed(1) + " hours") + " up there";
  });

  function doLaunch() {
    if (sats.length >= MAXSAT) return;
    var st = aimState();
    addSat(makeSat(st.x, st.y, st.vx, st.vy));
    dismissHint();
    updateReadout(); draw();
  }
  launchBtn.addEventListener("click", doLaunch);

  playBtn.addEventListener("click", function () { setRunning(!running, true); });
  overlayPlay.addEventListener("click", function () { setRunning(true, true); focusField(); });
  resetBtn.addEventListener("click", function () { reset(); });
  clearBtn.addEventListener("click", function () {
    drag = null;                      /* drop anything held mid-drag */
    if (!sats.length) return;
    for (var i = sats.length - 1; i >= 0; i--) removeSat(sats[i]);
    ghosts.length = 0;
    say("Field cleared.");
    updateReadout(); draw();
  });

  elPaths.addEventListener("change", function () { showPaths = elPaths.checked; if (!running) draw(); });
  elTrails.addEventListener("change", function () { showTrails = elTrails.checked; if (!running) draw(); });

  satList.addEventListener("click", function (e) {
    var b = e.target.closest ? e.target.closest(".satrow") : null;
    if (!b) return;
    selId = +b.getAttribute("data-id");
    syncPressed(); updateReadout(); draw();
  });

  function setRunning(v, announce) {
    running = v;
    playBtn.textContent = running ? "Pause" : "Play";
    overlay.hidden = running;
    if (announce) {
      if (!running) overlayNote.textContent = "paused";
      say(running ? "Simulation running." : "Simulation paused.");
    }
    if (running) { lastT = 0; if (!rafId) rafId = requestAnimationFrame(loop); }
    else draw();
  }

  function dismissHint() {
    if (hint.classList.contains("gone")) return;
    hint.classList.add("gone");
    setTimeout(function () { hint.style.display = "none"; }, 500);
  }

  /* ── pointer: drag to launch, or grab an existing satellite ─────────────── */
  function evPos(e) {
    var r = canvas.getBoundingClientRect();
    return { px: e.clientX - r.left, py: e.clientY - r.top };
  }
  /* never scroll the page out from under a drag in progress */
  function focusField() {
    try { canvas.focus({ preventScroll: true }); } catch (err) { canvas.focus(); }
  }

  canvas.addEventListener("pointerdown", function (e) {
    if (e.button !== undefined && e.button !== 0) return;
    var p = evPos(e), i, s;
    focusField();

    for (i = 0; i < sats.length; i++) {
      s = sats[i];
      var dx = sxOf(s.x) - p.px, dy = syOf(s.y) - p.py;
      if (dx * dx + dy * dy < 18 * 18) {
        selId = s.id; syncPressed();
        drag = { kind: "grab", sat: s, x: s.x, y: s.y, vx: s.vx, vy: s.vy, moved: false };
        sats.splice(i, 1);
        break;
      }
    }
    if (!drag) {
      if (sats.length >= MAXSAT) { say("Field is full. Clear it before launching another."); return; }
      var w = toWorld(p.px, p.py);
      var r = Math.sqrt(w.x * w.x + w.y * w.y);
      if (r < RP * 1.06) {
        var t = Math.atan2(w.y, w.x) || 0;
        w.x = RP * 1.15 * Math.cos(t); w.y = RP * 1.15 * Math.sin(t);
        r = RP * 1.15;
      }
      var vc = Math.sqrt(MU / r);
      drag = { kind: "new", sat: null, x: w.x, y: w.y,
               vx: -w.y / r * vc, vy: w.x / r * vc, moved: false };
    }
    if (canvas.setPointerCapture) { try { canvas.setPointerCapture(e.pointerId); } catch (err) {} }
    dismissHint();
    e.preventDefault();
    updateReadout();
    draw();
  });

  canvas.addEventListener("pointermove", function (e) {
    if (!drag) return;
    var p = evPos(e);
    var w = toWorld(p.px, p.py);
    var dx = w.x - drag.x, dy = w.y - drag.y;
    var dpx = (dx * scale), dpy = (dy * scale);
    if (Math.sqrt(dpx * dpx + dpy * dpy) > 5) drag.moved = true;
    if (drag.moved) {
      var vx = dx / TAU_DRAG, vy = dy / TAU_DRAG;
      var v = Math.sqrt(vx * vx + vy * vy);
      if (v > VMAX) { vx = vx / v * VMAX; vy = vy / v * VMAX; }
      drag.vx = vx; drag.vy = vy;
    }
    e.preventDefault();
    updateReadout();
    if (!running) draw();
  });

  function endDrag(cancel) {
    if (!drag) return;
    var d = drag; drag = null;
    if (cancel && d.kind === "grab" && d.sat) {
      sats.push(d.sat); renderList(); syncButtons();
    } else if (!cancel) {
      if (d.kind === "grab" && d.sat) {
        d.sat.x = d.x; d.sat.y = d.y; d.sat.vx = d.vx; d.sat.vy = d.vy;
        d.sat.o = elems(d.x, d.y, d.vx, d.vy);
        d.sat.h = chooseStep(d.sat.o);
        d.sat.trail.length = 0; d.sat.sweep = 0; d.sat.laps = 0;
        d.sat.ang = Math.atan2(d.y, d.x);
        sats.push(d.sat); selId = d.sat.id;
        fillTrail(d.sat); renderList(); syncButtons();
        var c0 = classOf(d.sat.o);
        say("Satellite " + d.sat.id + " re-launched. " + c0.text + ".");
      } else {
        addSat(makeSat(d.x, d.y, d.vx, d.vy));
      }
    }
    updateReadout(); draw();
  }

  canvas.addEventListener("pointerup", function (e) { endDrag(false); e.preventDefault(); });
  canvas.addEventListener("pointercancel", function () { endDrag(true); });
  /* safety net: these no-op when the canvas handlers above already ran */
  window.addEventListener("pointerup", function () { if (drag) endDrag(false); });
  window.addEventListener("pointercancel", function () { if (drag) endDrag(true); });

  canvas.addEventListener("keydown", function (e) {
    var k = e.key, fine = e.shiftKey, handled = true;
    if (drag && k === "Escape") { endDrag(true); return; }
    if (k === "ArrowLeft")       aim.ang = (aim.ang - (fine ? 1 : 5) + 360) % 360;
    else if (k === "ArrowRight") aim.ang = (aim.ang + (fine ? 1 : 5)) % 360;
    else if (k === "ArrowUp")    aim.alt = Math.min(30000, aim.alt + (fine ? 200 : 1000));
    else if (k === "ArrowDown")  aim.alt = Math.max(400, aim.alt - (fine ? 200 : 1000));
    else if (k === "+" || k === "=") aim.pct = Math.min(170, aim.pct + (fine ? 1 : 3));
    else if (k === "-" || k === "_") aim.pct = Math.max(40, aim.pct - (fine ? 1 : 3));
    else if (k === "Enter" || k === " " || k === "Spacebar") { doLaunch(); return void e.preventDefault(); }
    else handled = false;
    if (!handled) return;
    e.preventDefault();
    elAlt.value = String(aim.alt); elSpd.value = String(aim.pct); elAng.value = String(aim.ang);
    syncAim();
    dismissHint();
    if (!running) draw();
  });
  canvas.addEventListener("focus", function () { aimHot = true; if (!running) draw(); });
  canvas.addEventListener("blur", function () { aimHot = false; if (drag) endDrag(true); if (!running) draw(); });

  /* ── main loop ──────────────────────────────────────────────────────────── */
  function loop(t) {
    rafId = 0;
    if (!running) return;
    if (!lastT) lastT = t;
    var dtReal = (t - lastT) / 1000;
    lastT = t;
    if (dtReal > 0.05) dtReal = 0.05;
    if (dtReal > 0) tick(dtReal * warp);
    draw();
    if (t - uiT > 110) { uiT = t; updateReadout(); }
    rafId = requestAnimationFrame(loop);
  }

  /* ── boot ───────────────────────────────────────────────────────────────── */
  readTheme();
  if (window.ResizeObserver) new ResizeObserver(resize).observe(field);
  window.addEventListener("resize", resize);
  resize();
  preload();
  syncAim();
  warpV.textContent = "×2,000";
  elWarp.setAttribute("aria-valuetext", "2000 times real time");
  updateReadout();
  syncButtons();
  renderList();

  if (mqRM) {
    var onRM = function () {
      reduced = mqRM.matches;
      if (reduced) { setRunning(false, false); overlayNote.textContent = "paused \u00b7 reduced motion"; }
    };
    if (mqRM.addEventListener) mqRM.addEventListener("change", onRM);
    else if (mqRM.addListener) mqRM.addListener(onRM);
  }

  if (reduced) {
    overlayNote.textContent = "paused \u00b7 reduced motion";
    setRunning(false, false);
  } else {
    setRunning(true, false);
  }
  draw();

  document.addEventListener("visibilitychange", function () { if (!document.hidden) lastT = 0; });
})();
</script>
</body>
</html>