Compression Tree

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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Compression Tree</title>
<style>
/* sloprun design tokens — inline this block into every demo (self-contained rule).
   Identity: instrument-panel. Machine facts in mono; human words in sans.
   Green is EARNED: only for "it ran / it worked" states, never decoration. */
: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); }

/* --- demo-local supplements (derived; same three theme states) --- */
:root { --danger-soft: #FCEBEA; --canvas: #FBFBF8; --shadow: 0 1px 2px rgba(20,22,26,.05); --tint: rgba(26,29,33,.085); }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) { --danger-soft: #2B1A18; --canvas: #171A20; --shadow: 0 1px 2px rgba(0,0,0,.3); --tint: rgba(255,255,255,.11); }
}
:root[data-theme="dark"] { --danger-soft: #2B1A18; --canvas: #171A20; --shadow: 0 1px 2px rgba(0,0,0,.3); --tint: rgba(255,255,255,.11); }

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
body { -webkit-text-size-adjust: 100%; line-height: 1.45; overflow-x: hidden; }

.wrap { max-width: 1040px; margin: 0 auto; padding: 22px 16px 30px; }

header h1 { font-size: 21px; margin: 0 0 4px; letter-spacing: -.01em; }
header p.lede { margin: 0; color: var(--muted); font-size: 14.5px; max-width: 66ch; }
.chip {
  display: inline-block; font-family: var(--font-mono); font-size: 10.5px; letter-spacing: .06em;
  text-transform: uppercase; color: var(--accent); background: var(--accent-soft);
  border: 1px solid var(--line); border-radius: 999px; padding: 3px 9px; margin-bottom: 10px;
}

.panel {
  margin-top: 14px; background: var(--surface); border: 1px solid var(--line);
  border-radius: var(--radius); box-shadow: var(--shadow); overflow: hidden;
}
.phead {
  display: flex; align-items: center; gap: 10px; flex-wrap: wrap;
  padding: 8px 12px; border-bottom: 1px solid var(--line); background: var(--bg);
}
.phead h2 {
  margin: 0; font-family: var(--font-mono); font-size: 10px; letter-spacing: .08em;
  text-transform: uppercase; color: var(--muted); font-weight: 500;
}
.phead .sub { font-family: var(--font-mono); font-size: 11px; color: var(--muted); }

/* ---- input ---- */
.inbox { padding: 12px; }
textarea {
  display: block; width: 100%; resize: vertical; min-height: 62px;
  font-family: var(--font-sans); font-size: 15px; line-height: 1.5; color: var(--ink);
  background: var(--bg); border: 1px solid var(--line); border-radius: 8px; padding: 9px 10px;
}
.inmeta { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; margin-top: 8px; }
.inmeta .lab { font-family: var(--font-mono); font-size: 10px; letter-spacing: .07em; text-transform: uppercase; color: var(--muted); }
.count { font-family: var(--font-mono); font-size: 11.5px; color: var(--muted); font-variant-numeric: tabular-nums; margin-left: auto; }
.count.warn { color: var(--danger); }

button {
  font-family: var(--font-sans); font-size: 13.5px; color: var(--ink); background: var(--surface);
  border: 1px solid var(--line); border-radius: 8px; padding: 6px 11px; min-height: 34px;
  cursor: pointer; white-space: nowrap;
}
button:hover:not(:disabled) { border-color: var(--muted); }
button:disabled { opacity: .45; cursor: not-allowed; }
button.primary { background: var(--accent); border-color: var(--accent); color: var(--surface); font-weight: 600; }
button.primary:hover:not(:disabled) { filter: brightness(1.08); }
button.tiny { font-family: var(--font-mono); font-size: 11.5px; padding: 4px 9px; min-height: 30px; }
button.tiny[aria-pressed="true"] { background: var(--accent-soft); border-color: var(--accent); color: var(--accent); font-weight: 600; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

/* ---- stats ---- */
.stats { display: grid; grid-template-columns: repeat(6, 1fr); gap: 1px; background: var(--line); }
.stat { background: var(--surface); padding: 9px 11px; }
.stat .lab { font-family: var(--font-mono); font-size: 9.5px; letter-spacing: .07em; text-transform: uppercase; color: var(--muted); white-space: nowrap; }
.stat .val { font-family: var(--font-mono); font-size: 17px; font-variant-numeric: tabular-nums; line-height: 1.3; }
.stat .val small { font-size: 10.5px; color: var(--muted); font-weight: 400; }

/* ---- verdict + meters ---- */
.verdict { border-top: 1px solid var(--line); padding: 11px 12px; }
.vline {
  display: flex; align-items: baseline; gap: 8px; flex-wrap: wrap;
  font-family: var(--font-mono); font-size: 13.5px;
}
.vbadge {
  font-family: var(--font-mono); font-size: 11px; letter-spacing: .06em; text-transform: uppercase;
  border: 1px solid var(--line); border-radius: 999px; padding: 2px 9px; color: var(--muted); background: var(--bg);
}
.verdict.good .vbadge { color: var(--run); background: var(--run-soft); border-color: var(--run); }
.verdict.bad .vbadge { color: var(--danger); background: var(--danger-soft); border-color: var(--danger); }
.verdict .vnum { font-variant-numeric: tabular-nums; }
.verdict.good .vnum.hero { color: var(--run); font-weight: 600; }
.verdict.bad .vnum.hero { color: var(--danger); font-weight: 600; }
.meters { margin-top: 9px; display: grid; gap: 6px; }
.meter { display: grid; grid-template-columns: 74px 1fr auto; align-items: center; gap: 9px; }
.meter .mlab { font-family: var(--font-mono); font-size: 10px; letter-spacing: .06em; text-transform: uppercase; color: var(--muted); }
.meter .track { height: 12px; border-radius: 4px; background: var(--bg); border: 1px solid var(--line); overflow: hidden; }
.meter .fill { height: 100%; width: 0; background: var(--muted); opacity: .45; transition: width .35s ease; }
.meter .mval { font-family: var(--font-mono); font-size: 12px; font-variant-numeric: tabular-nums; color: var(--muted); min-width: 8.5ch; text-align: right; }
.meter.enc .fill { opacity: 1; background: var(--muted); }
.verdict.good .meter.enc .fill { background: var(--run); }
.verdict.bad .meter.enc .fill { background: var(--danger); }
.verdict.good .meter.enc .mval { color: var(--run); }
.verdict.bad .meter.enc .mval { color: var(--danger); }
.tblrow { display: flex; align-items: center; gap: 7px; margin-top: 10px; font-size: 12.5px; color: var(--muted); flex-wrap: wrap; }
.tblrow label { display: inline-flex; align-items: center; gap: 7px; cursor: pointer; }
.tblrow input[type=checkbox] { width: 15px; height: 15px; accent-color: var(--accent); cursor: pointer; }
.tblrow code { font-family: var(--font-mono); font-size: 11.5px; color: var(--ink); }

/* ---- tree ---- */
.controls { display: flex; flex-wrap: wrap; gap: 7px; align-items: center; padding: 9px 12px; border-bottom: 1px solid var(--line); }
.controls .spacer { flex: 1 1 auto; }
select {
  font-family: var(--font-mono); font-size: 12.5px; color: var(--ink); background: var(--bg);
  border: 1px solid var(--line); border-radius: 8px; padding: 6px 7px; min-height: 34px;
}
.field { display: flex; align-items: center; gap: 6px; }
.field label { font-family: var(--font-mono); font-size: 10px; color: var(--muted); letter-spacing: .05em; text-transform: uppercase; }

.note {
  display: flex; align-items: flex-start; gap: 9px; padding: 9px 12px;
  font-family: var(--font-mono); font-size: 12.5px; line-height: 1.5;
  border-bottom: 1px solid var(--line); border-left: 3px solid var(--line);
  background: var(--bg); min-height: 40px; word-break: break-word;
}
.note .dot { width: 8px; height: 8px; border-radius: 50%; background: var(--muted); flex: 0 0 auto; margin-top: 6px; }
.note.info { border-left-color: var(--accent); background: var(--accent-soft); }
.note.info .dot { background: var(--accent); }
.note.ok { border-left-color: var(--run); background: var(--run-soft); }
.note.ok .dot { background: var(--run); }

.canvas-shell { position: relative; background: var(--canvas); }
.canvas-wrap {
  overflow-x: auto; overflow-y: hidden; padding: 4px 0 2px;
  display: flex; align-items: flex-start; justify-content: center; justify-content: safe center;
}
svg.tree { display: block; flex: 0 0 auto; }
.fade { position: absolute; top: 0; bottom: 0; width: 26px; pointer-events: none; opacity: 0; transition: opacity .18s linear; }
.fade-l { left: 0; background: linear-gradient(to right, var(--canvas), transparent); }
.fade-r { right: 0; background: linear-gradient(to left, var(--canvas), transparent); }
.canvas-shell.can-l .fade-l, .canvas-shell.can-r .fade-r { opacity: 1; }

.nd .box { fill: var(--surface); stroke: var(--line); stroke-width: 1.25; }
.nd .dotn { fill: var(--bg); stroke: var(--line); stroke-width: 1.25; }
.nd text.g { fill: var(--ink); font-family: var(--font-mono); font-size: 13px; text-anchor: middle; }
.nd text.w { fill: var(--muted); font-family: var(--font-mono); font-size: 9.5px; text-anchor: middle; font-variant-numeric: tabular-nums; }
.nd text.iw { fill: var(--muted); font-family: var(--font-mono); font-size: 10px; text-anchor: middle; font-variant-numeric: tabular-nums; }
.nd .hit { fill: transparent; cursor: pointer; }
.nd.pick .box { stroke: var(--accent); stroke-width: 2.2; fill: var(--accent-soft); }
.nd.pick .dotn { stroke: var(--accent); stroke-width: 2.2; fill: var(--accent-soft); }
.nd.pick text.w, .nd.pick text.iw { fill: var(--accent); }
.nd.hot .box { stroke: var(--accent); stroke-width: 2.2; fill: var(--accent-soft); }
.nd.hot text.g { fill: var(--accent); font-weight: 600; }
.nd.born .dotn { stroke: var(--run); stroke-width: 2.2; fill: var(--run-soft); }
.nd.born text.iw { fill: var(--run); }
.edge { stroke: var(--line); stroke-width: 1.5; fill: none; }
.edge.hot { stroke: var(--accent); stroke-width: 2.4; }
.edge.fresh { stroke: var(--run); stroke-width: 2.2; }
text.bit { font-family: var(--font-mono); font-size: 9px; fill: var(--muted); text-anchor: middle; }
text.bit.hot { fill: var(--accent); font-weight: 700; }
.rootcap { font-family: var(--font-mono); font-size: 13px; fill: var(--muted); text-anchor: middle; letter-spacing: .08em; }

/* queue strip */
.queue { display: flex; gap: 6px; align-items: center; padding: 8px 12px; border-top: 1px solid var(--line); overflow-x: auto; }
.queue:empty { display: none; }
.queue .qlab { font-family: var(--font-mono); font-size: 9.5px; letter-spacing: .07em; text-transform: uppercase; color: var(--muted); flex: 0 0 auto; margin-right: 2px; }
.qchip {
  flex: 0 0 auto; font-family: var(--font-mono); font-size: 11.5px; border: 1px solid var(--line);
  border-radius: 6px; padding: 3px 7px; background: var(--surface); color: var(--muted);
  font-variant-numeric: tabular-nums; white-space: nowrap;
}
.qchip b { color: var(--ink); font-weight: 500; }
.qchip.next { border-color: var(--accent); background: var(--accent-soft); color: var(--accent); }
.qchip.next b { color: var(--accent); }
.qchip.done { opacity: .55; }

/* ---- lower grid ---- */
.grid2 { display: grid; grid-template-columns: minmax(0,340px) minmax(0,1fr); gap: 14px; align-items: start; }
@media (max-width: 760px) { .grid2 { grid-template-columns: minmax(0,1fr); } }

.tablewrap { max-height: 340px; overflow-y: auto; }
table.ct { width: 100%; border-collapse: collapse; font-family: var(--font-mono); font-size: 12px; }
table.ct thead th {
  position: sticky; top: 0; background: var(--surface); z-index: 1;
  font-size: 9.5px; letter-spacing: .06em; text-transform: uppercase; color: var(--muted);
  font-weight: 500; text-align: right; padding: 6px 8px; border-bottom: 1px solid var(--line);
}
table.ct thead th:first-child, table.ct thead th.l { text-align: left; }
table.ct td { padding: 3px 8px; border-bottom: 1px solid var(--line); text-align: right; font-variant-numeric: tabular-nums; vertical-align: middle; }
table.ct td.l { text-align: left; }
table.ct tr:last-child td { border-bottom: 0; }
table.ct tbody tr { cursor: pointer; }
table.ct tbody tr:hover, table.ct tbody tr.hot { background: var(--accent-soft); }
table.ct tbody tr.hot td { color: var(--accent); }
.glyphbtn {
  font-family: var(--font-mono); font-size: 13px; min-height: 24px; padding: 1px 7px;
  border: 1px solid var(--line); border-radius: 6px; background: var(--bg); color: var(--ink); min-width: 30px;
}
tr.hot .glyphbtn { border-color: var(--accent); color: var(--accent); background: var(--surface); }
.wslab { font-size: 9.5px; color: var(--muted); letter-spacing: .04em; margin-left: 5px; }
.bar { display: block; height: 8px; border-radius: 3px; background: var(--muted); opacity: .35; min-width: 2px; }
tr.hot .bar { background: var(--accent); opacity: 1; }
.code { color: var(--ink); letter-spacing: .06em; }
tr.hot .code { color: var(--accent); font-weight: 600; }

/* ---- bitstring ---- */
.bits {
  font-family: var(--font-mono); font-size: 12.5px; line-height: 2.05; letter-spacing: .04em;
  padding: 10px 12px; max-height: 240px; overflow-y: auto; word-break: break-all;
}
.bits:focus-visible { outline: 2px solid var(--accent); outline-offset: -2px; border-radius: 8px; }
.sym { border-radius: 4px; padding: 2px 0; }
.sym.alt { background: var(--tint); }
.sym.hot { background: var(--accent-soft); color: var(--accent); box-shadow: 0 0 0 1px var(--accent); }
.sym.cur { box-shadow: 0 0 0 2px var(--accent); background: var(--accent-soft); color: var(--accent); }
.readout {
  border-top: 1px solid var(--line); padding: 8px 12px; background: var(--bg);
  font-family: var(--font-mono); font-size: 12px; color: var(--muted); min-height: 36px;
  display: flex; align-items: center; gap: 8px; flex-wrap: wrap;
}
.readout b { color: var(--ink); font-weight: 600; }
.readout .k { color: var(--accent); }
.trunc { padding: 0 12px 10px; font-family: var(--font-mono); font-size: 11px; color: var(--muted); }

.empty { padding: 26px 14px; text-align: center; color: var(--muted); font-family: var(--font-mono); font-size: 12.5px; }
.hint { margin: 12px 2px 0; font-size: 12.5px; color: var(--muted); }
.hint b { font-family: var(--font-mono); font-weight: 600; color: var(--ink); }
.hint kbd { font-family: var(--font-mono); font-size: 11px; border: 1px solid var(--line); border-bottom-width: 2px; border-radius: 5px; padding: 1px 5px; background: var(--surface); color: var(--ink); }
footer { margin-top: 18px; font-family: var(--font-mono); font-size: 11px; color: var(--muted); letter-spacing: .04em; }
.sr { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }

@media (max-width: 620px) {
  .wrap { padding: 16px 10px 24px; }
  header h1 { font-size: 19px; }
  .stats { grid-template-columns: repeat(3, 1fr); }
  .stat .val { font-size: 15px; }
  .meter { grid-template-columns: 60px 1fr auto; gap: 7px; }
  .bits { font-size: 12px; line-height: 1.95; }
  .tablewrap { max-height: 300px; }
}
@media (max-width: 380px) {
  .stats { grid-template-columns: repeat(2, 1fr); }
  button { font-size: 13px; padding: 6px 9px; }
}
@media (prefers-reduced-motion: reduce) {
  .fade, .meter .fill { transition: none; }
  * { scroll-behavior: auto !important; }
}
</style>
</head>
<body>
<div class="wrap">

  <header>
    <div class="chip">Huffman coding · built live in your browser</div>
    <h1>Compression Tree</h1>
    <p class="lede">Normal text spends 8 bits on every character, even the ones you use constantly. Huffman coding builds a tree that gives common characters short codes and rare ones long codes — then the same sentence fits in far fewer bits. Type anything and watch the tree get built, merge by merge.</p>
  </header>

  <!-- INPUT -->
  <section class="panel">
    <div class="phead">
      <h2>Text</h2>
      <span class="sub" id="presetLab">try</span>
      <button type="button" class="tiny" data-preset="0">Tongue twister</button>
      <button type="button" class="tiny" data-preset="1">Mississippi</button>
      <button type="button" class="tiny" data-preset="2">All different</button>
      <button type="button" class="tiny" data-preset="3">One letter</button>
    </div>
    <div class="inbox">
      <label class="sr" for="txt">Text to compress</label>
      <textarea id="txt" rows="2" maxlength="400" spellcheck="false" autocomplete="off"></textarea>
      <div class="inmeta">
        <span class="lab">edit the text — everything below updates</span>
        <span class="count" id="count">0 / 400</span>
      </div>
    </div>
  </section>

  <!-- SCORE -->
  <section class="panel" aria-label="Compression result">
    <div class="stats">
      <div class="stat"><div class="lab">characters</div><div class="val" id="sChars">0</div></div>
      <div class="stat"><div class="lab">distinct</div><div class="val" id="sSyms">0</div></div>
      <div class="stat"><div class="lab">original</div><div class="val" id="sOrig">0 <small>bits</small></div></div>
      <div class="stat"><div class="lab">encoded</div><div class="val" id="sEnc">0 <small>bits</small></div></div>
      <div class="stat"><div class="lab">avg / char</div><div class="val" id="sAvg">0 <small>bits</small></div></div>
      <div class="stat"><div class="lab">longest code</div><div class="val" id="sDepth">0 <small>bits</small></div></div>
    </div>
    <div class="verdict" id="verdict">
      <div class="vline">
        <span class="vbadge" id="vbadge">idle</span>
        <span class="vnum hero" id="vhero">—</span>
        <span class="vnum" id="vdetail" style="color:var(--muted)"></span>
      </div>
      <div class="meters">
        <div class="meter"><span class="mlab">original</span><span class="track"><span class="fill" id="mOrig"></span></span><span class="mval" id="mOrigV">0</span></div>
        <div class="meter enc"><span class="mlab">huffman</span><span class="track"><span class="fill" id="mEnc"></span></span><span class="mval" id="mEncV">0</span></div>
      </div>
      <div class="tblrow">
        <label><input type="checkbox" id="cbTable"> also count the code table you must ship with the data</label>
        <code id="tblCost">+0 bits</code>
      </div>
    </div>
  </section>

  <!-- TREE -->
  <section class="panel" aria-label="Tree construction">
    <div class="controls">
      <button type="button" class="primary" id="btnPlay">▶ Build</button>
      <button type="button" id="btnStep">Step ▸</button>
      <button type="button" id="btnEnd">Skip to end</button>
      <button type="button" id="btnReset">↺ Restart</button>
      <span class="spacer"></span>
      <div class="field">
        <label for="speed">Speed</label>
        <select id="speed">
          <option value="1100">Slow</option>
          <option value="700" selected>Normal</option>
          <option value="380">Fast</option>
        </select>
      </div>
    </div>

    <p class="note" id="note"><span class="dot"></span><span id="noteText">ready</span></p>

    <div class="canvas-shell" id="shell">
      <div class="canvas-wrap" id="cwrap">
        <svg class="tree" id="svg" width="400" height="220" role="img" aria-labelledby="svgT" aria-describedby="svgD">
          <title id="svgT">Huffman tree</title>
          <desc id="svgD">The tree being built.</desc>
          <g id="edgeL"></g>
          <g id="labelL"></g>
          <g id="nodeL"></g>
        </svg>
      </div>
      <div class="fade fade-l" aria-hidden="true"></div>
      <div class="fade fade-r" aria-hidden="true"></div>
    </div>

    <div class="queue" id="queue" aria-hidden="true"></div>
  </section>

  <div class="grid2">
    <!-- CODE TABLE -->
    <section class="panel" style="margin-top:14px" aria-label="Frequency and code table">
      <div class="phead">
        <h2>Frequency &amp; codes</h2>
        <span class="spacer" style="flex:1"></span>
        <button type="button" class="tiny" id="sortBtn" title="Change sort order">sort: frequency</button>
      </div>
      <div class="tablewrap" id="tablewrap">
        <table class="ct" id="ct">
          <thead><tr>
            <th class="l">char</th><th>freq</th><th class="l">share</th><th class="l">code</th><th>bits</th>
          </tr></thead>
          <tbody id="ctb"></tbody>
        </table>
      </div>
    </section>

    <!-- BITSTRING -->
    <section class="panel" style="margin-top:14px" aria-label="Encoded bitstring">
      <div class="phead">
        <h2>Encoded bits</h2>
        <span class="sub" id="bitsSub">—</span>
      </div>
      <div class="bits" id="bits" tabindex="0" role="group"
           aria-label="Encoded bitstring. Use left and right arrow keys to step through characters."></div>
      <p class="trunc" id="trunc" hidden></p>
      <div class="readout" id="readout"><span>hover any run of bits — or any row in the table — to see which character it is</span></div>
    </section>
  </div>

  <p class="hint">The tree is read <b>0 = left</b>, <b>1 = right</b>; a character's code is the path from the root down to its box. Most-used characters end up nearest the root, so they get the shortest codes. Try <b>One letter</b> for the extreme case, or <b>All different</b> to see what happens when nothing repeats.</p>
  <p class="sr" id="sr" aria-live="polite" role="status"></p>
  <footer>demo · sloprun.dev</footer>
</div>

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  var PRESETS = [
    "she sells sea shells by the sea shore",
    "mississippi riverbank",
    "abcdefghijklmnop",
    "aaaaaaaaaaaaaaaaaaaa"
  ];

  /* header cost model: canonical Huffman ships one byte per symbol plus a 5-bit
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  var SORTS = [
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    { key: "len", lab: "code length" },
    { key: "char", lab: "character" }
  ];

  var M = null;                 /* current model */
  var step = 0;                 /* merges applied */
  var playing = false;
  var timer = null;
  var hotChar = null;           /* highlighted character */
  var cursor = -1;              /* bitstring roving cursor (char index) */
  var rowIdx = 0;               /* table roving tabindex */
  var sortIdx = 0;

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     glyphs
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  function glyph(ch) {
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    if (ch === "\t") return "⇥";
    var c = ch.charCodeAt(0);
    if (c < 32 || c === 127) return "·";
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  function wsLabel(ch) {
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    if (ch === "\n") return "newline";
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  function nameOf(ch) {
    var w = wsLabel(ch);
    return w ? w : "“" + ch + "”";
  }
  function fmt(n) { return n.toLocaleString("en-US"); }

  /* =======================================================================
     model
     ======================================================================= */
  function analyze(text) {
    var chars = Array.from(text);
    var counts = new Map();
    for (var i = 0; i < chars.length; i++) {
      counts.set(chars[i], (counts.get(chars[i]) || 0) + 1);
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    if (counts.size === 0) return null;

    var seq = 0, id = 0, nodes = [], live = [];
    counts.forEach(function (w, ch) {
      var n = { id: id++, ch: ch, w: w, leaf: true, left: null, right: null, parent: null,
                bit: "", seq: seq++, createdAt: -1, absorbedAt: Infinity, slot: 0, minLeaf: 0 };
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    });
    /* stable, reproducible starting order: rarest first, ties by code point */
    live.sort(function (a, b) { return a.w - b.w || (a.ch < b.ch ? -1 : a.ch > b.ch ? 1 : 0); });
    live.forEach(function (n, i) { n.seq = i; });
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    var steps = [];
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    while (live.length > 1) {
      live.sort(function (a, b) { return a.w - b.w || a.seq - b.seq; });
      var a = live.shift(), b = live.shift();
      var p = { id: id++, ch: null, w: a.w + b.w, leaf: false, left: a, right: b, parent: null,
                bit: "", seq: seq++, createdAt: k, absorbedAt: Infinity, slot: 0, minLeaf: 0 };
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      a.absorbedAt = k; b.absorbedAt = k;
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      steps.push({ a: a, b: b, p: p });
      k++;
    }
    var root = live[0];

    /* codes + in-order leaf slots */
    var leaves = [];
    var maxDepth = 0;
    (function walk(n, code, d) {
      if (n.leaf) {
        n.code = code === "" ? "0" : code;   /* lone symbol still needs one bit */
        n.depth = n.code.length;
        if (n.depth > maxDepth) maxDepth = n.depth;
        n.slot = leaves.length; n.minLeaf = n.slot;
        leaves.push(n);
        return;
      }
      walk(n.left, code + "0", d + 1);
      walk(n.right, code + "1", d + 1);
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    })(root, "", 0);

    var codeOf = new Map();
    leaves.forEach(function (n) { codeOf.set(n.ch, n); });

    var payload = 0;
    leaves.forEach(function (n) { payload += n.w * n.depth; });

    return {
      text: text, chars: chars, len: chars.length,
      nodes: nodes, root: root, leaves: leaves, steps: steps,
      codeOf: codeOf, maxDepth: maxDepth,
      origBits: chars.length * 8, payload: payload,
      header: headerBits(leaves.length)
    };
  }

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     geometry
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    el.svg.setAttribute("height", String(Math.round(h)));
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  /* =======================================================================
     tree rendering (persistent elements + tween)
     ======================================================================= */
  var els = Object.create(null);   /* node id -> {g, cur, tgt, from} */
  var edges = Object.create(null); /* child id -> {path, lab} */
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  function mk(tag, attrs) {
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      }
      rec.from = { x: rec.cur.x, y: rec.cur.y };
      rec.tgt = { x: p.x, y: p.y };

      if (n.absorbedAt < k && !edges[n.id]) edges[n.id] = makeEdgeEl(n.id);
    });

    /* per-step accents */
    Object.keys(els).forEach(function (i) { els[i].g.classList.remove("born", "pick"); });
    Object.keys(edges).forEach(function (i) { edges[i].path.classList.remove("fresh"); });
    if (k > 0 && k <= M.steps.length) {
      var s = M.steps[k - 1];
      if (els[s.p.id]) els[s.p.id].g.classList.add("born");
      if (edges[s.a.id]) edges[s.a.id].path.classList.add("fresh");
      if (edges[s.b.id]) edges[s.b.id].path.classList.add("fresh");
    }
    if (k < M.steps.length) {
      var nx = M.steps[k];
      if (els[nx.a.id]) els[nx.a.id].g.classList.add("pick");
      if (els[nx.b.id]) els[nx.b.id].g.classList.add("pick");
    }

    var dur = (animate && !RM.matches) ? Math.min(420, Math.max(180, +el.speed.value * 0.55)) : 0;
    startTween(dur);
    applyHot();
  }

  function startTween(dur) {
    if (raf) { cancelAnimationFrame(raf); raf = 0; }
    tweenDur = dur; tweenStart = 0;
    if (dur <= 0) { paint(1); return; }
    raf = requestAnimationFrame(function loop(ts) {
      if (!tweenStart) tweenStart = ts;
      var t = Math.min(1, (ts - tweenStart) / tweenDur);
      paint(t < 1 ? 1 - Math.pow(1 - t, 3) : 1);
      if (t < 1) raf = requestAnimationFrame(loop); else raf = 0;
    });
  }

  function paint(t) {
    var id;
    for (id in els) {
      var r = els[id];
      r.cur.x = r.from.x + (r.tgt.x - r.from.x) * t;
      r.cur.y = r.from.y + (r.tgt.y - r.from.y) * t;
      r.g.setAttribute("transform", "translate(" + r.cur.x.toFixed(2) + "," + r.cur.y.toFixed(2) + ")");
      if (r.fresh) {
        r.g.setAttribute("opacity", t >= 1 ? "1" : (0.15 + 0.85 * t).toFixed(3));
        if (t >= 1) r.fresh = false;
      }
    }
    for (id in edges) {
      var child = els[id]; if (!child) continue;
      var par = els[child.node.parent ? child.node.parent.id : -1];
      if (!par) continue;
      var x1 = par.cur.x, y1 = par.cur.y + GEO.r - 1;
      var x2 = child.cur.x, y2 = child.cur.y - (child.node.leaf ? GEO.leafH / 2 : GEO.r) + 1;
      edges[id].path.setAttribute("d", "M" + x1.toFixed(2) + " " + y1.toFixed(2) + "L" + x2.toFixed(2) + " " + y2.toFixed(2));
      var lx = x1 + (x2 - x1) * 0.42, ly = y1 + (y2 - y1) * 0.42;
      var lab = edges[id].lab;
      lab.setAttribute("x", (lx + (x2 < x1 ? -7 : 7)).toFixed(2));
      lab.setAttribute("y", ly.toFixed(2));
      lab.firstChild.nodeValue = child.node.bit;
    }
  }

  /* =======================================================================
     queue strip
     ======================================================================= */
  function renderQueue(k) {
    el.queue.textContent = "";
    var lab = document.createElement("span");
    lab.className = "qlab";
    lab.textContent = k < M.steps.length ? "waiting to merge → lightest two first" : "one tree left";
    el.queue.appendChild(lab);

    var roots = M.nodes.filter(function (n) { return n.createdAt < k && n.absorbedAt >= k; });
    roots.sort(function (a, b) { return a.w - b.w || a.seq - b.seq; });
    roots.forEach(function (n, i) {
      var c = document.createElement("span");
      c.className = "qchip" + (k < M.steps.length && i < 2 ? " next" : "");
      var b = document.createElement("b");
      b.textContent = n.leaf ? glyph(n.ch) : "●";
      c.appendChild(b);
      c.appendChild(document.createTextNode(" " + n.w));
      el.queue.appendChild(c);
    });
  }

  /* =======================================================================
     stats / verdict
     ======================================================================= */
  function renderStats() {
    var withTbl = el.cbTable.checked;
    var enc = M.payload + (withTbl ? M.header : 0);
    var orig = M.origBits;

    $("sChars").innerHTML = fmt(M.len);
    $("sSyms").innerHTML = fmt(M.leaves.length);
    $("sOrig").innerHTML = fmt(orig) + ' <small>bits</small>';
    $("sEnc").innerHTML = fmt(enc) + ' <small>bits</small>';
    $("sAvg").innerHTML = (M.payload / M.len).toFixed(2) + ' <small>bits</small>';
    $("sDepth").innerHTML = fmt(M.maxDepth) + ' <small>bits</small>';

    el.tblCost.textContent = "+" + fmt(M.header) + " bits (" + M.leaves.length + " × 13 + 8)";

    var smaller = enc < orig;
    var same = enc === orig;
    el.verdict.classList.toggle("good", smaller);
    el.verdict.classList.toggle("bad", !smaller && !same);
    el.vbadge.textContent = smaller ? "smaller" : same ? "no change" : "bigger";
    var pct = orig ? Math.abs(orig - enc) / orig * 100 : 0;
    el.vhero.textContent = smaller
      ? pct.toFixed(1) + "% smaller"
      : same ? "same size" : pct.toFixed(1) + "% bigger";
    el.vdetail.textContent = fmt(enc) + " bits vs " + fmt(orig) + " bits"
      + (withTbl ? "  (" + fmt(M.payload) + " data + " + fmt(M.header) + " table)" : "")
      + "  ·  " + (enc / orig).toFixed(3) + "×";

    var mx = Math.max(orig, enc, 1);
    el.mOrig.style.width = (orig / mx * 100).toFixed(2) + "%";
    el.mEnc.style.width = (enc / mx * 100).toFixed(2) + "%";
    el.mOrigV.textContent = fmt(orig) + " b";
    el.mEncV.textContent = fmt(enc) + " b";
  }

  /* =======================================================================
     code table
     ======================================================================= */
  function sortedLeaves() {
    var a = M.leaves.slice();
    var key = SORTS[sortIdx].key;
    if (key === "freq") a.sort(function (x, y) { return y.w - x.w || x.depth - y.depth || (x.ch < y.ch ? -1 : 1); });
    else if (key === "len") a.sort(function (x, y) { return x.depth - y.depth || y.w - x.w || (x.ch < y.ch ? -1 : 1); });
    else a.sort(function (x, y) { return x.ch < y.ch ? -1 : x.ch > y.ch ? 1 : 0; });
    return a;
  }

  var rowByChar = new Map();

  function renderTable() {
    el.ctb.textContent = "";
    rowByChar = new Map();
    var rows = sortedLeaves();
    var maxW = rows.reduce(function (m, n) { return Math.max(m, n.w); }, 1);
    if (rowIdx >= rows.length) rowIdx = 0;

    rows.forEach(function (n, i) {
      var tr = document.createElement("tr");
      tr.dataset.ch = n.ch;

      var td1 = document.createElement("td"); td1.className = "l";
      var btn = document.createElement("button");
      btn.type = "button"; btn.className = "glyphbtn";
      btn.textContent = glyph(n.ch);
      btn.tabIndex = i === rowIdx ? 0 : -1;
      btn.setAttribute("aria-label",
        nameOf(n.ch) + ", appears " + n.w + " time" + (n.w === 1 ? "" : "s") +
        ", code " + n.code.split("").join(" ") + ", " + n.depth + " bits");
      btn.addEventListener("focus", function () { rowIdx = i; retab(); setHot(n.ch, -1); });
      btn.addEventListener("click", function () { setHot(hotChar === n.ch ? null : n.ch, -1); });
      btn.addEventListener("keydown", function (e) { tableKey(e, i, rows); });
      td1.appendChild(btn);
      var ws = wsLabel(n.ch);
      if (ws) { var s = document.createElement("span"); s.className = "wslab"; s.textContent = ws; td1.appendChild(s); }
      tr.appendChild(td1);

      var td2 = document.createElement("td"); td2.textContent = String(n.w); tr.appendChild(td2);

      var td3 = document.createElement("td"); td3.className = "l";
      var bar = document.createElement("span");
      bar.className = "bar";
      bar.style.width = Math.max(3, Math.round(n.w / maxW * 62)) + "px";
      td3.appendChild(bar);
      tr.appendChild(td3);

      var td4 = document.createElement("td"); td4.className = "l code"; td4.textContent = n.code; tr.appendChild(td4);
      var td5 = document.createElement("td"); td5.textContent = String(n.depth); tr.appendChild(td5);

      tr.addEventListener("mouseenter", function () { setHot(n.ch, -1); });
      tr.addEventListener("mouseleave", function () { setHot(null, -1); });
      el.ctb.appendChild(tr);
      rowByChar.set(n.ch, tr);
    });
  }

  function retab() {
    var btns = el.ctb.querySelectorAll(".glyphbtn");
    for (var i = 0; i < btns.length; i++) btns[i].tabIndex = i === rowIdx ? 0 : -1;
  }

  function tableKey(e, i, rows) {
    var next = -1;
    if (e.key === "ArrowDown") next = Math.min(rows.length - 1, i + 1);
    else if (e.key === "ArrowUp") next = Math.max(0, i - 1);
    else if (e.key === "Home") next = 0;
    else if (e.key === "End") next = rows.length - 1;
    else return;
    e.preventDefault();
    rowIdx = next; retab();
    var btns = el.ctb.querySelectorAll(".glyphbtn");
    if (btns[next]) btns[next].focus();
  }

  /* =======================================================================
     bitstring
     ======================================================================= */
  var symSpans = [];            /* index -> span (rendered chars only) */

  function renderBits() {
    el.bits.textContent = "";
    symSpans = [];
    var shown = Math.min(M.len, MAX_RENDER_CHARS);
    var frag = document.createDocumentFragment();
    for (var i = 0; i < shown; i++) {
      var ch = M.chars[i];
      var leaf = M.codeOf.get(ch);
      var sp = document.createElement("span");
      sp.className = "sym" + (i % 2 ? " alt" : "");
      sp.textContent = leaf.code;
      sp.dataset.i = String(i);
      sp.dataset.ch = ch;
      (function (c, idx, node) {
        node.addEventListener("mouseenter", function () { setHot(c, idx); });
        node.addEventListener("mouseleave", function () { setHot(null, -1); });
      })(ch, i, sp);
      frag.appendChild(sp);
      symSpans.push(sp);
    }
    el.bits.appendChild(frag);

    var shownBits = 0;
    for (var j = 0; j < shown; j++) shownBits += M.codeOf.get(M.chars[j]).depth;
    el.bitsSub.textContent = fmt(M.payload) + (M.payload === 1 ? " bit total" : " bits total");
    if (shown < M.len) {
      el.trunc.hidden = false;
      el.trunc.textContent = "showing the first " + shown + " of " + fmt(M.len) + " characters (" + fmt(shownBits) + " of " + fmt(M.payload) + " bits)";
    } else {
      el.trunc.hidden = true; el.trunc.textContent = "";
    }
    cursor = -1;
  }

  /* =======================================================================
     highlighting
     ======================================================================= */
  function setHot(ch, idx) {
    hotChar = ch; cursor = idx;
    applyHot();
    renderReadout();
  }

  function applyHot() {
    /* table */
    rowByChar.forEach(function (tr, ch) { tr.classList.toggle("hot", ch === hotChar); });
    /* bits */
    for (var i = 0; i < symSpans.length; i++) {
      var sp = symSpans[i];
      sp.classList.toggle("hot", hotChar !== null && sp.dataset.ch === hotChar);
      sp.classList.toggle("cur", i === cursor);
    }
    /* tree */
    var id;
    for (id in els) els[id].g.classList.remove("hot");
    for (id in edges) { edges[id].path.classList.remove("hot"); edges[id].lab.classList.remove("hot"); }
    if (hotChar !== null && M) {
      var n = M.codeOf.get(hotChar);
      while (n) {
        if (els[n.id]) els[n.id].g.classList.add("hot");
        if (edges[n.id]) { edges[n.id].path.classList.add("hot"); edges[n.id].lab.classList.add("hot"); }
        n = n.parent;
      }
    }
  }

  function renderReadout() {
    el.readout.textContent = "";
    if (hotChar === null) {
      var s = document.createElement("span");
      s.textContent = "hover any run of bits — or any row in the table — to see which character it is";
      el.readout.appendChild(s);
      return;
    }
    var n = M.codeOf.get(hotChar);
    var parts = [];
    parts.push(["b", nameOf(hotChar)]);
    if (cursor >= 0) parts.push(["s", "position " + (cursor + 1)]);
    parts.push(["s", "×" + n.w + " in the text"]);
    parts.push(["k", n.code]);
    parts.push(["s", n.depth + (n.depth === 1 ? " bit" : " bits") + " instead of 8"]);
    parts.push(["s", "→ " + fmt(n.w * n.depth) + " bits here vs " + fmt(n.w * 8) + " uncompressed"]);
    parts.forEach(function (p, i) {
      var e2 = document.createElement(p[0] === "b" ? "b" : "span");
      if (p[0] === "k") e2.className = "k";
      e2.textContent = (i ? "· " : "") + p[1];
      el.readout.appendChild(e2);
      el.readout.appendChild(document.createTextNode(" "));
    });
  }

  /* =======================================================================
     playback
     ======================================================================= */
  function setNote(kind, text) {
    el.note.className = "note" + (kind ? " " + kind : "");
    el.noteText.textContent = text;
  }

  function describe(k) {
    if (!M.steps.length) {
      setNote("ok", "one distinct character — no tree to build, and one bit says it all");
      return;
    }
    if (k === 0) {
      setNote("", M.leaves.length + " characters to encode · " + M.steps.length +
        " merges ahead · each step joins the two lightest piles");
      return;
    }
    var s = M.steps[k - 1];
    var an = s.a.leaf ? glyph(s.a.ch) : "node";
    var bn = s.b.leaf ? glyph(s.b.ch) : "node";
    var txt = "merge " + k + "/" + M.steps.length + " · " + an + "(" + s.a.w + ") + " +
              bn + "(" + s.b.w + ") → node(" + s.p.w + ")";
    if (k === M.steps.length) {
      setNote("ok", txt + " · tree complete — root weight " + s.p.w + " = every character");
    } else {
      setNote("info", txt);
    }
  }

  function setStep(k, animate) {
    step = Math.max(0, Math.min(M.steps.length, k));
    renderTree(step, animate);
    renderQueue(step);
    describe(step);
    updateButtons();
    updateSvgDesc();
    updateFades();
  }

  function updateSvgDesc() {
    el.svgD.textContent = step >= M.steps.length
      ? "Completed Huffman tree with " + M.leaves.length + " character leaves, deepest code " + M.maxDepth + " bits."
      : "Huffman forest after " + step + " of " + M.steps.length + " merges.";
  }

  function updateButtons() {
    if (!M) return;
    var atEnd = step >= M.steps.length;
    el.btnStep.disabled = atEnd;
    el.btnEnd.disabled = atEnd;
    el.btnReset.disabled = step === 0 && !playing;
    el.btnPlay.disabled = M.steps.length === 0;
    el.btnPlay.textContent = playing ? "⏸ Pause" : (atEnd ? "↺ Replay build" : "▶ Build");
  }

  function tick() {
    if (!playing || !M) return;
    if (step >= M.steps.length) { stop(); return; }
    setStep(step + 1, true);
    if (step >= M.steps.length) {
      stop();
      el.sr.textContent = "Tree complete. " + M.leaves.length + " characters, longest code " + M.maxDepth + " bits.";
      return;
    }
    timer = setTimeout(tick, +el.speed.value);
  }

  function play() {
    if (!M || !M.steps.length) return;
    if (step >= M.steps.length) setStep(0, false);
    playing = true; updateButtons();
    timer = setTimeout(tick, RM.matches ? 0 : 260);
  }
  function stop() {
    playing = false;
    if (timer) { clearTimeout(timer); timer = null; }
    updateButtons();
  }

  /* =======================================================================
     empty / rebuild
     ======================================================================= */
  function showEmpty() {
    stop();
    clearTree();
    el.svg.setAttribute("width", "320"); el.svg.setAttribute("height", "120");
    el.svg.setAttribute("viewBox", "0 0 320 120");
    var t = document.createElementNS(SVG_NS, "text");
    t.setAttribute("x", "160"); t.setAttribute("y", "66");
    t.setAttribute("class", "rootcap");
    t.appendChild(document.createTextNode("type something above"));
    el.nodeL.appendChild(t);
    el.svgD.textContent = "No tree yet — the text box is empty.";
    el.queue.textContent = "";
    el.ctb.textContent = "";
    el.bits.textContent = "";
    el.trunc.hidden = true;
    el.bitsSub.textContent = "—";
    rowByChar = new Map(); symSpans = []; hotChar = null; cursor = -1;
    setNote("", "no text yet — nothing to compress");
    ["sChars", "sSyms"].forEach(function (i) { $(i).textContent = "0"; });
    $("sOrig").innerHTML = '0 <small>bits</small>';
    $("sEnc").innerHTML = '0 <small>bits</small>';
    $("sAvg").innerHTML = '0 <small>bits</small>';
    $("sDepth").innerHTML = '0 <small>bits</small>';
    el.verdict.className = "verdict";
    el.vbadge.textContent = "idle"; el.vhero.textContent = "—"; el.vdetail.textContent = "";
    el.mOrig.style.width = "0%"; el.mEnc.style.width = "0%";
    el.mOrigV.textContent = "0 b"; el.mEncV.textContent = "0 b";
    el.tblCost.textContent = "+0 bits";
    el.readout.textContent = ""; renderReadout();
    /* updateButtons() bails out while M is null, so reset the play control by hand */
    el.btnPlay.textContent = "▶ Build";
    el.btnPlay.disabled = true; el.btnStep.disabled = true; el.btnEnd.disabled = true; el.btnReset.disabled = true;
  }

  function rebuild(autoplay) {
    var text = el.txt.value;
    el.count.textContent = Array.from(text).length + " / 400";
    el.count.classList.toggle("warn", Array.from(text).length >= 400);

    var m = analyze(text);
    if (!m) { M = null; showEmpty(); return; }
    M = m;
    stop();
    hotChar = null; cursor = -1; rowIdx = 0;
    clearTree();
    fitGeometry();
    renderStats();
    renderTable();
    renderBits();
    renderReadout();
    if (autoplay && !RM.matches && M.steps.length) {
      setStep(0, false);
      play();
    } else {
      setStep(M.steps.length, false);
    }
  }

  /* =======================================================================
     scroll fades
     ======================================================================= */
  function updateFades() {
    var w = el.cwrap;
    var can = w.scrollWidth - w.clientWidth > 2;
    el.shell.classList.toggle("can-l", can && w.scrollLeft > 2);
    el.shell.classList.toggle("can-r", can && w.scrollLeft < w.scrollWidth - w.clientWidth - 2);
  }

  /* =======================================================================
     wiring
     ======================================================================= */
  var debounce = null;
  el.txt.addEventListener("input", function () {
    if (debounce) clearTimeout(debounce);
    debounce = setTimeout(function () { rebuild(false); }, 130);
  });

  document.querySelectorAll("[data-preset]").forEach(function (b) {
    b.addEventListener("click", function () {
      el.txt.value = PRESETS[+b.dataset.preset];
      rebuild(true);
      el.sr.textContent = "Loaded example. " + (M && M.steps.length
        ? "Building the tree, " + M.steps.length + " merges."
        : "Only one distinct character, so there is no tree to build.");
    });
  });

  el.btnPlay.addEventListener("click", function () { if (playing) stop(); else play(); });
  el.btnStep.addEventListener("click", function () { stop(); setStep(step + 1, true); });
  el.btnEnd.addEventListener("click", function () { stop(); setStep(M.steps.length, false); });
  el.btnReset.addEventListener("click", function () { stop(); setStep(0, false); });
  el.speed.addEventListener("change", function () {
    if (playing) { if (timer) clearTimeout(timer); timer = setTimeout(tick, +el.speed.value); }
  });
  el.cbTable.addEventListener("change", function () {
    if (M) { renderStats(); el.sr.textContent = el.cbTable.checked ? "Code table included in the total." : "Code table excluded."; }
  });
  el.sortBtn.addEventListener("click", function () {
    sortIdx = (sortIdx + 1) % SORTS.length;
    el.sortBtn.textContent = "sort: " + SORTS[sortIdx].lab;
    rowIdx = 0;
    if (M) { renderTable(); applyHot(); }
  });

  el.cwrap.addEventListener("scroll", updateFades, { passive: true });
  window.addEventListener("resize", updateFades);

  /* bitstring roving cursor */
  el.bits.addEventListener("keydown", function (e) {
    if (!symSpans.length) return;
    var n = symSpans.length, next = cursor;
    if (e.key === "ArrowRight") next = cursor < 0 ? 0 : Math.min(n - 1, cursor + 1);
    else if (e.key === "ArrowLeft") next = cursor < 0 ? n - 1 : Math.max(0, cursor - 1);
    else if (e.key === "Home") next = 0;
    else if (e.key === "End") next = n - 1;
    else if (e.key === "Escape") { setHot(null, -1); return; }
    else return;
    e.preventDefault();
    setHot(symSpans[next].dataset.ch, next);
    var sp = symSpans[next];
    if (sp.scrollIntoView) sp.scrollIntoView({ block: "nearest", inline: "nearest" });
    el.sr.textContent = nameOf(symSpans[next].dataset.ch) + ", code " + sp.textContent;
  });
  el.bits.addEventListener("focus", function () {
    if (cursor < 0 && symSpans.length) setHot(symSpans[0].dataset.ch, 0);
  });
  el.bits.addEventListener("blur", function () { setHot(null, -1); });

  /* ---------- boot ---------- */
  el.txt.value = PRESETS[0];
  rebuild(true);
  updateFades();
})();
</script>
</body>
</html>