:root{
    --bg0:#f4f7fb; --bg1:#eaf0f7; --panel:#ffffff;
    --ink:#1f2937; --muted:#5f6f82; --line:#d5e0ea;
    --c1:#64b5f6; --c2:#81c784; --c3:#ffb74d; --c4:#e57373; --c5:#ba68c8;
    --shadow:0 18px 42px rgba(80,100,130,.12);
    --code-bg:#1e1e2e; --code-fg:#cdd6f4;
  }
  *{box-sizing:border-box;margin:0;padding:0}
  html{scroll-behavior:smooth}
  body{
    font-family:"Hiragino Sans","Noto Sans JP","Yu Gothic",system-ui,-apple-system,sans-serif;
    background:radial-gradient(900px 360px at 10% 0%,rgba(100,181,246,.15),transparent 60%),
      radial-gradient(760px 320px at 100% 2%,rgba(129,199,132,.12),transparent 58%),
      linear-gradient(180deg,var(--bg0),var(--bg1) 55%,#f2f6fa);
    color:var(--ink);line-height:1.72;min-height:100vh;
  }
  header.hero{position:relative;padding:48px 28px 28px;text-align:center;overflow:hidden;border-bottom:1px solid rgba(148,163,184,.16)}
  header.hero::after{content:"";position:absolute;left:50%;bottom:0;transform:translateX(-50%);width:min(1020px,92vw);height:1px;background:linear-gradient(90deg,transparent,#cfdae7 18%,#cfdae7 82%,transparent)}
  header.hero h1{font-size:34px;font-weight:800;letter-spacing:.02em;color:#1a3050}
  header.hero p{color:var(--muted);font-size:14px;margin:8px 0 0}
  .topic-picker{margin-top:18px;display:flex;gap:10px;justify-content:center;flex-wrap:wrap;position:relative;z-index:2}
  .topic-btn{background:rgba(255,255,255,.82);color:#4a5a70;border:1px solid #c9d7e5;padding:11px 22px;border-radius:14px;font-size:13.5px;font-weight:800;cursor:pointer;transition:.2s;box-shadow:0 6px 16px rgba(119,150,183,.12);font-family:inherit}
  .topic-btn:hover{color:#24384b;background:#f8fbff;border-color:#a9c5e6;transform:translateY(-1px)}
  .topic-btn.active{color:#fff;background:linear-gradient(135deg,#4a8cc7,#64b5f6);border-color:#4a8cc7;box-shadow:0 10px 24px rgba(74,140,199,.28)}
  .tabs{position:sticky;top:0;z-index:20;backdrop-filter:blur(10px);background:rgba(248,251,255,.88);border-bottom:1px solid var(--line);display:flex;gap:8px;overflow-x:auto;padding:11px 16px;scrollbar-width:none}
  .tabs::-webkit-scrollbar{display:none}
  .tab{flex:0 0 auto;background:#ffffff;color:#516577;border:1px solid #d6e0ea;padding:8px 14px;border-radius:999px;font-size:13px;cursor:pointer;font-weight:700;transition:.15s;box-shadow:0 4px 10px rgba(148,163,184,.08);font-family:inherit}
  .tab:hover{color:#24384b;background:#f8fbff;border-color:#bdd0e3}
  .tab.active{color:#1f3552;background:linear-gradient(180deg,#ffffff,#eef6ff);border-color:#a9c5e6;box-shadow:0 8px 20px rgba(96,134,176,.18)}
  .panel{display:none;max-width:1180px;margin:24px auto;padding:0 20px}
  .panel.active{display:block;background:linear-gradient(180deg,rgba(255,255,255,.96),rgba(248,251,255,.94));border:1px solid var(--line);border-radius:22px;padding:28px 28px 24px;box-shadow:var(--shadow);position:relative;overflow:hidden}
  .panel.active::before{content:"";position:absolute;inset:0 0 auto 0;height:5px;pointer-events:none;background:linear-gradient(90deg,#64b5f6 0%,#81c784 35%,#ffb74d 68%,#e57373 100%)}
  .panel h2{margin:6px 0 10px;font-size:22px;font-weight:800;color:#1f2f44;border-left:5px solid transparent;border-image:linear-gradient(180deg,#64b5f6,#81c784) 1;padding-left:14px}
  .desc{background:linear-gradient(180deg,#fff,#f8fbfe);border:1px solid #dce6ef;color:#33475d;padding:14px 18px;border-radius:14px;margin:8px 0 18px;line-height:1.75;font-size:14px;box-shadow:0 6px 14px rgba(148,163,184,.08)}
  .desc b{color:#26415e}
  .desc code,.icode{background:#eef3f8;padding:2px 6px;border-radius:4px;font-size:13px;font-family:"Consolas","Monaco","Courier New",monospace;color:#2d5986}
  .tip{margin:12px 0;background:rgba(100,181,246,.1);border-left:4px solid #64b5f6;padding:12px 16px;border-radius:0 12px 12px 0;font-size:13.5px}
  .warn{margin:12px 0;background:rgba(255,183,77,.12);border-left:4px solid #ffb74d;padding:12px 16px;border-radius:0 12px 12px 0;font-size:13.5px}
  .err-box{margin:12px 0;background:rgba(229,115,115,.1);border-left:4px solid #e57373;padding:12px 16px;border-radius:0 12px 12px 0;font-size:13.5px}
  .controls{display:flex;gap:10px;flex-wrap:wrap;align-items:center;margin:8px 0 16px}
  button.btn{background:linear-gradient(180deg,#fff,#edf6ff);color:#24415c;border:1px solid #b8d0e6;padding:9px 18px;border-radius:12px;font-weight:800;cursor:pointer;font-size:13.5px;box-shadow:0 6px 16px rgba(119,150,183,.14);transition:.15s;font-family:inherit}
  button.btn:hover{transform:translateY(-1px);box-shadow:0 10px 22px rgba(119,150,183,.18)}
  .btn-run{background:linear-gradient(180deg,#e8f5e9,#c8e6c9)!important;color:#2e7d32!important;border-color:#a5d6a7!important}
  .btn-step{background:linear-gradient(180deg,#fff3e0,#ffe0b2)!important;color:#e65100!important;border-color:#ffcc80!important}
  .btn-reset{background:linear-gradient(180deg,#fce4ec,#f8bbd0)!important;color:#c62828!important;border-color:#f48fb1!important}
  .btn-check{background:linear-gradient(180deg,#e8eaf6,#c5cae9)!important;color:#283593!important;border-color:#9fa8da!important}
  .code-area{display:grid;grid-template-columns:1fr 1fr;gap:16px;margin:12px 0}
  @media(max-width:800px){.code-area{grid-template-columns:1fr}}
  .code-editor{background:var(--code-bg);border-radius:14px;overflow:hidden;border:1px solid #313244;box-shadow:0 12px 26px rgba(30,30,46,.3)}
  .code-editor-header{background:#313244;padding:8px 14px;font-size:12px;color:#a6adc8;display:flex;align-items:center;gap:8px}
  .dot{width:10px;height:10px;border-radius:50%;display:inline-block}.dot.r{background:#f38ba8}.dot.y{background:#f9e2af}.dot.g{background:#a6e3a1}
  .code-editor textarea{width:100%;min-height:240px;background:transparent;color:var(--code-fg);border:none;outline:none;resize:vertical;padding:14px;font-size:14px;line-height:1.6;font-family:"Consolas","Monaco","Courier New",monospace;tab-size:4}
  .code-editor textarea::selection{background:#45475a}
  .output-area{background:#0f0f1a;border-radius:14px;overflow:hidden;border:1px solid #313244;box-shadow:0 12px 26px rgba(15,15,26,.3);display:flex;flex-direction:column}
  .output-header{background:#1a1a2e;padding:8px 14px;font-size:12px;color:#a6adc8;display:flex;align-items:center;justify-content:space-between}
  .output-content{flex:1;padding:14px;color:#a6e3a1;font-family:"Consolas","Monaco","Courier New",monospace;font-size:13px;line-height:1.6;overflow-y:auto;white-space:pre-wrap;min-height:240px}
  .output-content .err{color:#f38ba8}.output-content .info{color:#89b4fa}.output-content .warn-t{color:#f9e2af}
  .step-viz{margin:16px 0;display:grid;grid-template-columns:1fr 1fr;gap:16px}
  @media(max-width:800px){.step-viz{grid-template-columns:1fr}}
  .step-code{background:var(--code-bg);border-radius:14px;overflow:hidden;border:1px solid #313244}
  .step-code-header{background:#313244;padding:8px 14px;font-size:12px;color:#a6adc8}
  .step-code-lines{padding:10px 0;font-family:"Consolas","Monaco","Courier New",monospace;font-size:13px;line-height:1.7}
  .step-line{padding:2px 14px;color:#cdd6f4;transition:background .2s}
  .step-line.active{background:rgba(249,226,175,.15);border-left:3px solid #f9e2af}
  .step-line .ln{color:#585b70;display:inline-block;width:28px;text-align:right;margin-right:12px;user-select:none}
  .step-state{background:var(--panel);border:1px solid var(--line);border-radius:14px;padding:16px;box-shadow:0 8px 18px rgba(148,163,184,.08)}
  .step-state h3{font-size:15px;font-weight:800;color:#1f3552;margin:10px 0 8px}
  .step-state h3:first-child{margin-top:0}
  .var-table{width:100%;border-collapse:collapse;font-size:13px;margin-bottom:10px}
  .var-table th{text-align:left;padding:6px 10px;background:#eef3f8;color:#3a5a7c;border-bottom:2px solid #d5e0ea;font-weight:700}
  .var-table td{padding:6px 10px;border-bottom:1px solid #e8eef4;font-family:"Consolas",monospace}
  .stdout-box{background:#f5f5f5;border:1px solid #e0e0e0;border-radius:8px;padding:8px 12px;font-family:monospace;font-size:13px;min-height:36px;color:#212121;margin-top:4px}
  .stats{display:grid;grid-template-columns:repeat(auto-fit,minmax(180px,1fr));gap:12px;margin-top:14px}
  .stat-card{background:linear-gradient(180deg,#fff,#f8fbfe);border:1px solid #d9e4ee;border-radius:14px;padding:12px 16px;box-shadow:0 8px 18px rgba(148,163,184,.08);border-left:4px solid #64b5f6}
  .stat-label{font-size:11px;color:var(--muted);text-transform:uppercase;letter-spacing:.06em}
  .stat-value{font-size:22px;font-weight:800;color:#27415d;margin-top:2px}
  .flow-container{display:flex;flex-direction:column;align-items:center;gap:0;margin:16px 0}
  .flow-box{padding:10px 20px;border-radius:10px;font-size:13px;font-weight:700;text-align:center;min-width:160px;transition:all .3s}
  .flow-box.start{background:#e8f5e9;border:2px solid #66bb6a;color:#2e7d32;border-radius:24px}
  .flow-box.process{background:#e3f2fd;border:2px solid #64b5f6;color:#1565c0}
  .flow-box.decision{background:#fff3e0;border:2px solid #ffb74d;color:#e65100}
  .flow-box.end{background:#fce4ec;border:2px solid #ef5350;color:#c62828;border-radius:24px}
  .flow-box.active{box-shadow:0 0 0 4px rgba(100,181,246,.4);transform:scale(1.05)}
  .flow-arrow{width:2px;height:20px;background:#90a4ae;position:relative}
  .flow-arrow::after{content:"▼";position:absolute;bottom:-8px;left:50%;transform:translateX(-50%);color:#90a4ae;font-size:10px}
  pre.c-code{background:var(--code-bg);color:var(--code-fg);padding:16px;border-radius:12px;font-size:13.5px;line-height:1.7;overflow-x:auto;font-family:"Consolas","Monaco","Courier New",monospace;border:1px solid #313244;margin:8px 0}
  .kw{color:#cba6f7}.fn{color:#89b4fa}.str{color:#a6e3a1}.cm{color:#6c7086;font-style:italic}.num{color:#fab387}.type{color:#89dceb}.pp{color:#f38ba8}
  /* Memory visualization */
  .mem-cell-box{text-align:center;cursor:pointer;transition:transform .2s}
  .mem-cell-box:hover{transform:scale(1.05)}
  .mem-cell-inner{width:64px;height:64px;border:2px solid #64b5f6;border-radius:10px;display:flex;align-items:center;justify-content:center;font-size:18px;font-weight:800;color:#1565c0;background:#e3f2fd;transition:all .3s}
  .mem-cell-inner.active-cell{background:#fff3e0;border-color:#ffb74d;color:#e65100;transform:scale(1.1)}
  /* Quiz */
  .quiz-card{background:#fff;border:2px solid #e0e7ef;border-radius:16px;padding:20px;margin:16px 0;box-shadow:0 8px 20px rgba(148,163,184,.1)}
  .quiz-card h3{font-size:18px;font-weight:800;color:#1f3552;margin-bottom:10px}
  .quiz-option{display:block;padding:10px 16px;margin:6px 0;border:2px solid #dce6ef;border-radius:10px;cursor:pointer;font-size:14px;transition:all .2s;background:#f8fbff}
  .quiz-option:hover{border-color:#90caf9;background:#e3f2fd}
  .quiz-option.correct{border-color:#66bb6a;background:#e8f5e9;color:#2e7d32;font-weight:700}
  .quiz-option.wrong{border-color:#ef5350;background:#fce4ec;color:#c62828}
  .quiz-option.disabled{pointer-events:none;opacity:.7}
  .quiz-explain{display:none;margin-top:12px;padding:14px;background:#f0f4fa;border-radius:10px;font-size:13.5px;line-height:1.7}
  .quiz-explain.show{display:block}
  /* Scanf input sim */
  .scanf-sim{display:flex;gap:8px;align-items:center;margin:8px 0}
  .scanf-input{padding:8px 12px;border:2px solid #64b5f6;border-radius:8px;font-family:monospace;font-size:14px;width:120px;background:#e3f2fd}
  .scanf-input:focus{outline:none;border-color:#1565c0;box-shadow:0 0 0 3px rgba(21,101,192,.15)}
  /* ===== Ad slots ===== */
  .ad-slot{max-width:1180px;margin:0 auto;padding:8px 20px;text-align:center}
  .ad-slot-inner{background:#f8f9fa;border:1px dashed #d0d7de;border-radius:8px;padding:12px;color:#aaa;font-size:12px}
  .ad-slot-mid{margin:24px 0}
  /* ===== Affiliate / Book recommendation ===== */
  .affiliate-section{max-width:1180px;margin:32px auto;padding:0 20px}
  .affiliate-box{background:linear-gradient(180deg,#fff,#f8fbfe);border:1px solid #dce6ef;border-radius:18px;padding:28px;box-shadow:0 8px 24px rgba(148,163,184,.1)}
  .affiliate-box h2{font-size:20px;font-weight:800;color:#1f3552;margin-bottom:6px}
  .affiliate-box .aff-sub{font-size:13px;color:#5f6f82;margin-bottom:20px}
  .affiliate-grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(280px,1fr));gap:16px}
  .aff-card{display:flex;gap:14px;background:#fff;border:1px solid #e2eaf2;border-radius:14px;padding:16px;transition:.2s}
  .aff-card:hover{border-color:#90caf9;box-shadow:0 6px 18px rgba(148,163,184,.14);transform:translateY(-2px)}
  .aff-img{flex:0 0 80px;height:110px;background:#eef3f8;border-radius:8px;display:flex;align-items:center;justify-content:center;font-size:32px;color:#90a4ae;overflow:hidden}
  .aff-img img{width:100%;height:100%;object-fit:cover;border-radius:8px}
  .aff-info{flex:1;display:flex;flex-direction:column;justify-content:center}
  .aff-title{font-size:14px;font-weight:800;color:#1f3552;line-height:1.4}
  .aff-author{font-size:12px;color:#90a4ae;margin-top:2px}
  .aff-desc{font-size:12px;color:#5f6f82;line-height:1.6;margin-top:6px}
  .aff-link{display:inline-block;margin-top:8px;padding:6px 14px;background:linear-gradient(135deg,#ff9900,#ffad33);color:#fff;border-radius:8px;font-size:12px;font-weight:700;text-decoration:none;transition:.2s}
  .aff-link:hover{transform:translateY(-1px);box-shadow:0 4px 12px rgba(255,153,0,.3)}
  .aff-disclosure{font-size:11px;color:#b0b8c4;margin-top:14px;line-height:1.6}
  /* ===== Common page footer ===== */
  .page-footer{max-width:1180px;margin:0 auto;padding:24px 20px;color:#90a4ae;font-size:12px;border-top:1px solid #e2eaf2;display:flex;gap:16px;justify-content:space-between;align-items:center;flex-wrap:wrap}
  .page-footer nav{display:flex;gap:16px;flex-wrap:wrap}
  .page-footer a{color:#6f86a0;text-decoration:none}