/* ============================================================
   SimulAI.org - Educational-Open Knowledge Hub
   Palette: Library Blue, Reading Lamp Gold, Parchment, Ink
   Typography: Playfair Display, Source Serif 4, Source Sans 3
   ============================================================ */

:root {
    --library-blue: #1B3A5C;
    --reading-lamp-gold: #C8A24E;
    --parchment: #F7F3EC;
    --ink: #2C2418;
    --margin-gray: #5A7B99;
    --shelf-wood: #8B6F4E;
    --page-edge: #E8DFD0;
    --highlight: #FFF4D6;
    --paper-light: #F0EDE8;
    --accent-blue: #3B6EA5;

    --font-display: 'Playfair Display', Georgia, serif;
    --font-body: 'Source Serif 4', Georgia, serif;
    --font-annotation: 'Source Sans 3', 'Helvetica Neue', sans-serif;
    --font-nav: 'Source Sans 3', 'Helvetica Neue', sans-serif;
    --font-code: 'Source Code Pro', 'Courier New', monospace;

    --max-width: 1200px;
    --reading-column: 680px;
    --annotation-width: 280px;
    --toc-width: 220px;
    --gutter: 24px;
}

/* Reset */
*, *::before, *::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    font-size: 16px;
}

body {
    font-family: var(--font-body);
    font-weight: 400;
    font-size: clamp(1rem, 1.1vw, 1.15rem);
    line-height: 1.72;
    color: var(--ink);
    background-color: var(--parchment);
    overflow-x: hidden;
}

/* ============================================================
   Bookmark Progress Ribbon
   ============================================================ */

.bookmark-ribbon {
    position: fixed;
    top: 0;
    left: 0;
    width: 3px;
    height: 0%;
    background-color: var(--reading-lamp-gold);
    z-index: 1000;
    transition: height 50ms linear;
}

/* ============================================================
   Table of Contents (Sidebar)
   ============================================================ */

.toc {
    position: fixed;
    left: 0;
    top: 50%;
    transform: translateY(-50%);
    width: var(--toc-width);
    padding: 24px 20px;
    background-color: var(--page-edge);
    border-right: 1px solid var(--page-edge);
    z-index: 900;
    opacity: 0;
    pointer-events: none;
    transition: opacity 400ms ease-out;
}

.toc.visible {
    opacity: 1;
    pointer-events: auto;
}

.toc-title {
    font-family: var(--font-nav);
    font-weight: 600;
    font-size: 0.8rem;
    letter-spacing: 0.06em;
    text-transform: uppercase;
    color: var(--margin-gray);
    margin-bottom: 16px;
    padding-bottom: 8px;
    border-bottom: 2px solid var(--page-edge);
}

.toc-list {
    list-style: none;
}

.toc-item {
    display: flex;
    align-items: baseline;
    gap: 6px;
    padding: 8px 0;
    cursor: pointer;
    transition: color 300ms ease-out;
}

.toc-item:hover {
    color: var(--reading-lamp-gold);
}

.toc-item.active {
    color: var(--library-blue);
}

.toc-item.active .toc-number {
    color: var(--reading-lamp-gold);
}

.toc-number {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: 0.85rem;
    color: var(--margin-gray);
    min-width: 24px;
    transition: color 300ms ease-out;
}

.toc-leader {
    flex: 1;
    border-bottom: 1px dotted var(--margin-gray);
    margin-bottom: 3px;
    opacity: 0.4;
}

.toc-label {
    font-family: var(--font-nav);
    font-weight: 600;
    font-size: 0.75rem;
    letter-spacing: 0.04em;
    text-transform: uppercase;
    max-width: 100px;
    text-align: right;
}

/* ============================================================
   Section 1: Opening Foyer
   ============================================================ */

.opening-foyer {
    position: relative;
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: var(--library-blue);
    overflow: hidden;
}

.paper-texture {
    position: absolute;
    inset: 0;
    background-image: url("data:image/svg+xml,%3Csvg viewBox='0 0 256 256' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='noise'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.9' numOctaves='4' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23noise)' opacity='0.04'/%3E%3C/svg%3E");
    background-size: 256px 256px;
    opacity: 0.3;
    pointer-events: none;
}

.foyer-content {
    text-align: center;
    position: relative;
    z-index: 1;
    padding: 40px;
}

.ribbon-drop {
    width: 4px;
    height: 0;
    background-color: var(--reading-lamp-gold);
    margin: 0 auto 32px;
    animation: ribbonDrop 600ms ease-out 200ms forwards;
}

@keyframes ribbonDrop {
    from { height: 0; }
    to { height: 80px; }
}

.site-title {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: clamp(3rem, 8vw, 6rem);
    color: var(--parchment);
    letter-spacing: -0.01em;
    line-height: 1.15;
    margin-bottom: 24px;
}

.welcome-text {
    font-family: var(--font-body);
    font-weight: 400;
    font-size: clamp(1.05rem, 1.5vw, 1.25rem);
    color: var(--page-edge);
    max-width: 560px;
    margin: 0 auto;
    line-height: 1.72;
    opacity: 0;
    animation: fadeIn 500ms ease-out 800ms forwards;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(8px); }
    to { opacity: 1; transform: translateY(0); }
}

.scroll-indicator {
    margin-top: 48px;
    animation: gentlePulse 2s ease-in-out infinite;
}

.page-turn-arrow {
    color: var(--reading-lamp-gold);
    opacity: 0.7;
}

@keyframes gentlePulse {
    0%, 100% { transform: translateY(0); opacity: 0.7; }
    50% { transform: translateY(6px); opacity: 1; }
}

/* ============================================================
   Section 2: Chapter Navigation (Book Spine Cards)
   ============================================================ */

.chapter-nav {
    position: relative;
    padding: 120px var(--gutter) 100px;
    background: linear-gradient(to bottom, var(--library-blue) 0%, var(--parchment) 40%);
    min-height: 80vh;
    display: flex;
    align-items: center;
    justify-content: center;
}

.chapter-nav-inner {
    max-width: var(--max-width);
    width: 100%;
    text-align: center;
}

.section-label {
    font-family: var(--font-nav);
    font-weight: 600;
    font-size: 0.8rem;
    letter-spacing: 0.06em;
    text-transform: uppercase;
    color: var(--margin-gray);
    margin-bottom: 48px;
}

.bookshelf {
    display: grid;
    grid-auto-flow: column;
    grid-auto-columns: 140px;
    gap: 12px;
    justify-content: center;
    padding: 32px 0;
    perspective: 800px;
}

.book-spine {
    height: 420px;
    background: linear-gradient(135deg, var(--shelf-wood) 0%, #7A6143 50%, var(--shelf-wood) 100%);
    border-radius: 4px 8px 8px 4px;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 24px 12px;
    cursor: pointer;
    position: relative;
    transition: transform 400ms ease-out, box-shadow 400ms ease-out;
    box-shadow: 2px 2px 8px rgba(0, 0, 0, 0.15), inset -2px 0 4px rgba(0, 0, 0, 0.1);
    opacity: 0;
    transform: translateY(40px);
}

.book-spine.revealed {
    opacity: 1;
    transform: translateY(0);
}

.book-spine:hover {
    transform: rotateY(-3deg) translateY(-4px);
    box-shadow: 4px 4px 16px rgba(0, 0, 0, 0.25), inset -2px 0 4px rgba(0, 0, 0, 0.1);
}

.book-spine::before {
    content: '';
    position: absolute;
    left: 0;
    top: 10%;
    bottom: 10%;
    width: 3px;
    background-color: var(--reading-lamp-gold);
    border-radius: 0 2px 2px 0;
}

.spine-number {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: 1.8rem;
    color: var(--parchment);
    margin-bottom: 24px;
}

.spine-title {
    font-family: var(--font-nav);
    font-weight: 600;
    font-size: 0.7rem;
    letter-spacing: 0.08em;
    text-transform: uppercase;
    color: var(--page-edge);
    writing-mode: vertical-rl;
    text-orientation: mixed;
    flex: 1;
    display: flex;
    align-items: center;
}

.spine-ornament {
    width: 20px;
    height: 2px;
    background-color: var(--reading-lamp-gold);
    margin-top: 24px;
    opacity: 0.6;
}

/* ============================================================
   Chapter Sections
   ============================================================ */

.chapter {
    position: relative;
    max-width: var(--max-width);
    margin: 0 auto;
    padding: 80px var(--gutter) 100px;
    padding-left: calc(var(--toc-width) + var(--gutter) + 24px);
}

.bookmark-tab {
    position: absolute;
    left: calc(var(--toc-width) + var(--gutter) + 24px - 40px);
    top: 60px;
    width: calc(100% - var(--toc-width) - var(--gutter) + 16px);
    height: 48px;
    background-color: var(--library-blue);
    display: flex;
    align-items: center;
    padding-left: 20px;
    border-radius: 0;
}

.tab-numeral {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: 1rem;
    color: var(--reading-lamp-gold);
    letter-spacing: 0.05em;
}

.watermark-numeral {
    position: absolute;
    top: 40px;
    right: 40px;
    font-family: var(--font-display);
    font-weight: 700;
    font-size: 15rem;
    line-height: 1;
    color: var(--ink);
    opacity: 0.04;
    pointer-events: none;
    user-select: none;
}

.chapter-layout {
    display: grid;
    grid-template-columns: var(--reading-column) var(--annotation-width);
    grid-template-areas: "main aside";
    gap: var(--gutter);
    margin-top: 48px;
    position: relative;
}

.reading-column {
    grid-area: main;
    position: relative;
    padding-left: 20px;
    border-left: 4px solid transparent;
}

.chapter.active .reading-column {
    border-left-color: var(--reading-lamp-gold);
    transition: border-color 400ms ease-out;
}

.chapter-heading {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: clamp(2rem, 4.5vw, 3.2rem);
    color: var(--ink);
    letter-spacing: -0.01em;
    line-height: 1.15;
    margin-bottom: 32px;
}

.reading-column p {
    margin-bottom: 1.5em;
    opacity: 0;
    transform: translateY(12px);
    transition: opacity 500ms ease-out, transform 500ms ease-out;
}

.reading-column p.revealed {
    opacity: 1;
    transform: translateY(0);
}

.reading-column p + p {
    text-indent: 1.2em;
    margin-top: 0;
}

.drop-cap-paragraph::first-letter {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: 3.5em;
    float: left;
    line-height: 0.8;
    margin: 0.05em 0.12em 0 0;
    color: var(--reading-lamp-gold);
}

/* Annotation Margin */
.annotation-margin {
    grid-area: aside;
    position: relative;
    padding-top: 60px;
}

.annotation {
    margin-bottom: 32px;
    opacity: 0;
    transform: translateX(12px);
    transition: opacity 500ms ease-out 200ms, transform 500ms ease-out 200ms;
}

.annotation.revealed {
    opacity: 1;
    transform: translateX(0);
}

.annotation-line {
    width: 24px;
    height: 1px;
    background-color: var(--page-edge);
    margin-bottom: 8px;
    transform: rotate(-15deg);
    transform-origin: left center;
}

.annotation-text {
    font-family: var(--font-annotation);
    font-weight: 400;
    font-size: 0.85rem;
    line-height: 1.55;
    color: var(--margin-gray);
}

.annotation-text em {
    font-style: italic;
}

/* Horizontal rules (double-line notebook style) */
.chapter + .chapter::before {
    content: '';
    display: block;
    position: absolute;
    top: 0;
    left: calc(var(--toc-width) + var(--gutter) + 24px);
    right: var(--gutter);
    height: 2px;
    background-color: var(--page-edge);
    box-shadow: 0 6px 0 0 var(--page-edge);
}

/* ============================================================
   Section: Reading Room (Closing)
   ============================================================ */

.reading-room {
    position: relative;
    min-height: 80vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: var(--library-blue);
    padding: 80px var(--gutter);
    margin-top: 80px;
}

.reading-room-content {
    text-align: center;
    max-width: 600px;
}

.closing-heading {
    font-family: var(--font-display);
    font-weight: 700;
    font-size: clamp(2rem, 4vw, 3rem);
    color: var(--parchment);
    letter-spacing: -0.01em;
    line-height: 1.15;
    margin-bottom: 24px;
}

.closing-text {
    font-family: var(--font-body);
    font-weight: 400;
    font-size: clamp(1rem, 1.2vw, 1.15rem);
    color: var(--page-edge);
    line-height: 1.72;
    margin-bottom: 48px;
}

/* Open Book Spread */
.book-spread {
    display: flex;
    align-items: stretch;
    justify-content: center;
    margin: 48px auto;
    height: 200px;
    max-width: 400px;
}

.book-page {
    width: 45%;
    background-color: var(--parchment);
    border-radius: 2px;
    position: relative;
}

.book-page-left {
    transform: perspective(600px) rotateY(8deg);
    box-shadow: -4px 2px 12px rgba(0, 0, 0, 0.2);
    border-right: 1px solid var(--page-edge);
}

.book-page-left::after {
    content: '';
    position: absolute;
    inset: 16px;
    background: repeating-linear-gradient(
        transparent 0px,
        transparent 22px,
        var(--page-edge) 22px,
        var(--page-edge) 23px
    );
    opacity: 0.5;
}

.book-page-right {
    transform: perspective(600px) rotateY(-8deg);
    box-shadow: 4px 2px 12px rgba(0, 0, 0, 0.2);
    border-left: 1px solid var(--page-edge);
}

.book-page-right::after {
    content: '';
    position: absolute;
    inset: 16px;
    background: repeating-linear-gradient(
        transparent 0px,
        transparent 22px,
        var(--page-edge) 22px,
        var(--page-edge) 23px
    );
    opacity: 0.5;
}

.book-spine-center {
    width: 10%;
    background: linear-gradient(90deg, rgba(0,0,0,0.1), rgba(0,0,0,0.02) 30%, rgba(0,0,0,0.02) 70%, rgba(0,0,0,0.1));
    min-width: 12px;
}

.closing-invitation {
    font-family: var(--font-annotation);
    font-weight: 400;
    font-size: 0.9rem;
    color: var(--margin-gray);
    letter-spacing: 0.04em;
    margin-top: 32px;
}

.ribbon-curl {
    width: 4px;
    height: 40px;
    background-color: var(--reading-lamp-gold);
    margin: 32px auto 0;
    border-radius: 0 0 4px 4px;
    position: relative;
}

.ribbon-curl::after {
    content: '';
    position: absolute;
    bottom: -6px;
    left: -3px;
    width: 10px;
    height: 10px;
    border-radius: 50%;
    border: 2px solid var(--reading-lamp-gold);
    border-top: none;
    border-left: none;
    transform: rotate(45deg);
}

/* ============================================================
   Page Curl on Cards
   ============================================================ */

.book-spine::after {
    content: '';
    position: absolute;
    bottom: 0;
    right: 0;
    width: 16px;
    height: 16px;
    background: linear-gradient(135deg, transparent 50%, rgba(0,0,0,0.08) 50%);
    clip-path: polygon(100% 0, 100% 100%, 0 100%);
}

/* ============================================================
   Responsive Design
   ============================================================ */

@media (max-width: 1200px) {
    .chapter {
        padding-left: calc(var(--toc-width) + 16px);
    }

    .chapter-layout {
        grid-template-columns: 1fr var(--annotation-width);
    }

    .bookmark-tab {
        left: calc(var(--toc-width) + 16px - 40px);
    }
}

@media (max-width: 1024px) {
    .toc {
        display: none;
    }

    .chapter {
        padding-left: var(--gutter);
        padding-right: var(--gutter);
    }

    .bookmark-tab {
        left: 0;
        width: 100%;
    }

    .chapter-layout {
        grid-template-columns: 1fr;
        grid-template-areas:
            "main"
            "aside";
    }

    .annotation-margin {
        padding-top: 24px;
        padding-left: 16px;
        border-left: 2px solid var(--page-edge);
    }

    .annotation {
        transition-delay: 0ms;
    }

    .watermark-numeral {
        font-size: 8rem;
        right: 16px;
    }

    .chapter + .chapter::before {
        left: var(--gutter);
        right: var(--gutter);
    }
}

@media (max-width: 768px) {
    .bookshelf {
        grid-auto-flow: row;
        grid-auto-columns: unset;
        grid-template-columns: repeat(2, 140px);
        justify-content: center;
    }

    .book-spine {
        height: 320px;
    }

    .site-title {
        font-size: clamp(2.5rem, 10vw, 4rem);
    }

    .chapter-heading {
        font-size: clamp(1.6rem, 5vw, 2.4rem);
    }

    .book-spread {
        height: 140px;
        max-width: 280px;
    }
}

@media (max-width: 480px) {
    .bookshelf {
        grid-template-columns: repeat(2, 120px);
    }

    .book-spine {
        height: 260px;
    }

    .chapter {
        padding: 60px 16px 80px;
    }
}

/* ============================================================
   Reduced Motion
   ============================================================ */

@media (prefers-reduced-motion: reduce) {
    .ribbon-drop {
        animation: none;
        height: 80px;
    }

    .welcome-text {
        animation: none;
        opacity: 1;
    }

    .scroll-indicator {
        animation: none;
    }

    .reading-column p {
        opacity: 1;
        transform: none;
        transition: none;
    }

    .annotation {
        opacity: 1;
        transform: none;
        transition: none;
    }

    .book-spine {
        opacity: 1;
        transform: none;
        transition: none;
    }

    .book-spine:hover {
        transform: none;
    }

    .toc {
        transition: none;
    }

    .bookmark-ribbon {
        transition: none;
    }
}

/* ============================================================
   Code blocks (if needed)
   ============================================================ */

code, pre {
    font-family: var(--font-code);
    font-weight: 400;
    font-size: 0.9rem;
    background: var(--paper-light);
    border-left: 3px solid #3B6EA5;
    padding: 12px 16px;
    display: block;
    overflow-x: auto;
}

/* ============================================================
   Selection styling
   ============================================================ */

::selection {
    background-color: var(--highlight);
    color: var(--ink);
}