import * as THREE from 'https://esm.sh/three@0.165.0';
import { OrbitControls } from 'https://esm.sh/three@0.165.0/examples/jsm/controls/OrbitControls.js';

const canvas = document.getElementById('scene');
const progressInput = document.getElementById('progress');
const progressValue = document.getElementById('progress_value');
const pageCountInput = document.getElementById('page_count');
const pageCountValue = document.getElementById('page_count_value');
const urlParams = new URLSearchParams(window.location.search);

const renderer = new THREE.WebGLRenderer({ canvas, antialias: true });
renderer.setPixelRatio(Math.min(window.devicePixelRatio || 1, 2));
renderer.setClearColor(0x202124, 1);

const scene = new THREE.Scene();
const camera = new THREE.PerspectiveCamera(34, 1, 0.1, 30);
if (urlParams.get('view') === 'profile') {
    camera.position.set(0, 0.82, 5.8);
} else if (urlParams.get('view') === 'top') {
    camera.position.set(0, 5.8, 0.001);
} else {
    camera.position.set(0, 3.25, 5.4);
}

const controls = new OrbitControls(camera, canvas);
controls.target.set(0, urlParams.get('view') === 'profile' ? 0.13 : 0.18, 0);
controls.enableDamping = true;
controls.minDistance = 2.2;
controls.maxDistance = 8.0;
controls.update();

const book = new THREE.Group();
scene.add(book);

const guide = new THREE.GridHelper(5.6, 16, 0x4c4c4c, 0x343434);
guide.position.y = -0.12;
scene.add(guide);

const materials = {
    cover: new THREE.MeshBasicMaterial({ color: 0x2c1810, side: THREE.DoubleSide }),
    spine: new THREE.MeshBasicMaterial({ color: 0x9c1f1f, side: THREE.DoubleSide }),
    pagesLeft: new THREE.MeshBasicMaterial({ color: 0xd8c7a4, side: THREE.DoubleSide }),
    pagesRight: new THREE.MeshBasicMaterial({ color: 0xe7d6b4, side: THREE.DoubleSide }),
    topPage: new THREE.MeshBasicMaterial({ color: 0xf1dfba, side: THREE.DoubleSide }),
    edge: new THREE.MeshBasicMaterial({ color: 0xb99a68, side: THREE.DoubleSide }),
    hinge: new THREE.MeshBasicMaterial({ color: 0x2b0808 })
};

const BOOK_PROFILE = {
    tableY: 0,
    coverThickness: 0.03,
    raisedHingeY: 0.056,
    paperContactOffset: 0.0012,
    bundleSpacing: 0.014
};

let readingProgress = readInitialProgress();
let pageCount = readInitialPageCount();
let lastLengthError = 0;
let lastSpacingError = 0;
progressInput.value = readingProgress.toFixed(3);
progressValue.value = readingProgress.toFixed(2);
pageCountInput.value = String(pageCount);
pageCountValue.value = String(pageCount);
rebuildBook();
resize();
animate();

progressInput.addEventListener('input', () => {
    setReadingProgress(progressInput.value);
});

pageCountInput.addEventListener('input', () => {
    setPageCount(pageCountInput.value);
});

window.addEventListener('resize', resize);

window.BookShapeLab = {
    get progress() {
        return readingProgress;
    },
    get pageCount() {
        return pageCount;
    },
    get lastLengthError() {
        return lastLengthError;
    },
    get lastSpacingError() {
        return lastSpacingError;
    },
    setReadingProgress(value) {
        setReadingProgress(value);
        return readingProgress;
    },
    setPageCount(value) {
        setPageCount(value);
        return pageCount;
    }
};

function readInitialProgress() {
    const parsed = Number.parseFloat(urlParams.get('progress') ?? '0.25');
    return Number.isFinite(parsed) ? THREE.MathUtils.clamp(parsed, 0, 1) : 0.25;
}

function readInitialPageCount() {
    const parsed = Number.parseInt(urlParams.get('pages') ?? '240', 10);
    if (!Number.isFinite(parsed)) return 240;
    return snapPageCount(parsed);
}

function snapPageCount(value) {
    return THREE.MathUtils.clamp(Math.round(value / 10) * 10, 40, 600);
}

function setReadingProgress(value) {
    const next = THREE.MathUtils.clamp(Number.parseFloat(value), 0, 1);
    if (!Number.isFinite(next)) return;
    readingProgress = next;
    progressInput.value = readingProgress.toFixed(3);
    progressValue.value = readingProgress.toFixed(2);
    rebuildBook();
}

function setPageCount(value) {
    const next = snapPageCount(Number.parseFloat(value));
    if (!Number.isFinite(next)) return;
    pageCount = next;
    pageCountInput.value = String(pageCount);
    pageCountValue.value = String(pageCount);
    rebuildBook();
}

function rebuildBook() {
    clearGroup(book);

    const coverDepth = 2.30;
    const coverThickness = BOOK_PROFILE.coverThickness;
    const pageWidth = 1.62;
    const pageDepth = 2.24;
    const bundleCount = Math.max(4, Math.round(pageCount / 10));
    const spineWidth = Math.max(0.16, bundleCount * BOOK_PROFILE.bundleSpacing);
    const lines = simulatePageLines(bundleCount, pageWidth, spineWidth);
    lastLengthError = measureLineLengthError(lines, pageWidth);
    lastSpacingError = measureStackSpacingError(lines);

    addCoverAssembly(pageWidth, coverDepth, coverThickness, spineWidth);
    addClothSpine(pageDepth, spineWidth);
    addSimulatedStackBodies(lines, pageDepth);
    addSimulatedPageLines(lines, pageDepth);
}

function clearGroup(group) {
    while (group.children.length) {
        const child = group.children.pop();
        child.geometry?.dispose();
        if (Array.isArray(child.material)) {
            child.material.forEach((material) => material.dispose?.());
        }
    }
}

function addCoverAssembly(pageWidth, depth, thickness, spineWidth) {
    const cover = new THREE.Mesh(createCoverAssemblyGeometry(pageWidth, depth, thickness, spineWidth), materials.cover);
    book.add(cover);
}

function createCoverAssemblyGeometry(pageWidth, depth, thickness, spineWidth) {
    const overhang = 0.13;
    const spineHalf = spineWidth * 0.5;
    const hingeInset = 0.07;
    const outerX = pageWidth + overhang;
    const hingeX = spineHalf + hingeInset;
    const outerTopY = BOOK_PROFILE.tableY + thickness;
    const connectionTopY = BOOK_PROFILE.raisedHingeY;
    const spineTopY = BOOK_PROFILE.tableY + thickness;
    const section = [
        { x: -outerX, y: outerTopY },
        { x: -hingeX, y: connectionTopY },
        { x: -spineHalf, y: spineTopY },
        { x: spineHalf, y: spineTopY },
        { x: hingeX, y: connectionTopY },
        { x: outerX, y: outerTopY }
    ];
    const positions = [];
    const uvs = [];
    const indices = [];
    const frontTop = [];
    const backTop = [];
    const frontBottom = [];
    const backBottom = [];

    const push = (x, y, z, u, v) => {
        const index = positions.length / 3;
        positions.push(x, y, z);
        uvs.push(u, v);
        return index;
    };

    section.forEach((point, index) => {
        const u = index / (section.length - 1);
        frontTop[index] = push(point.x, point.y, depth * 0.5, u, 1);
        backTop[index] = push(point.x, point.y, -depth * 0.5, u, 0);
        frontBottom[index] = push(point.x, point.y - thickness, depth * 0.5, u, 1);
        backBottom[index] = push(point.x, point.y - thickness, -depth * 0.5, u, 0);
    });

    for (let i = 0; i < section.length - 1; i += 1) {
        indices.push(frontTop[i], backTop[i], frontTop[i + 1], frontTop[i + 1], backTop[i], backTop[i + 1]);
        indices.push(frontBottom[i], frontBottom[i + 1], backBottom[i], frontBottom[i + 1], backBottom[i + 1], backBottom[i]);
        indices.push(frontTop[i], frontTop[i + 1], frontBottom[i], frontTop[i + 1], frontBottom[i + 1], frontBottom[i]);
        indices.push(backTop[i], backBottom[i], backTop[i + 1], backTop[i + 1], backBottom[i], backBottom[i + 1]);
    }

    const last = section.length - 1;
    indices.push(frontTop[0], frontBottom[0], backTop[0], backTop[0], frontBottom[0], backBottom[0]);
    indices.push(frontTop[last], backTop[last], frontBottom[last], backTop[last], backBottom[last], frontBottom[last]);

    const geometry = new THREE.BufferGeometry();
    geometry.setIndex(indices);
    geometry.setAttribute('position', new THREE.Float32BufferAttribute(positions, 3));
    geometry.setAttribute('uv', new THREE.Float32BufferAttribute(uvs, 2));
    geometry.computeVertexNormals();
    return geometry;
}

function addClothSpine(depth, spineWidth) {
    const spine = new THREE.Mesh(createClothSpineGeometry(depth, spineWidth), materials.spine);
    book.add(spine);
}

function createClothSpineGeometry(depth, spineWidth) {
    const profile = [];
    for (let i = 0; i <= 32; i += 1) {
        const u = i / 32;
        profile.push(spineCurvePoint(u, spineWidth));
    }
    const positions = [];
    const indices = [];
    const front = [];
    const back = [];
    const push = (point, z) => {
        const index = positions.length / 3;
        positions.push(point.x, point.y, z);
        return index;
    };

    profile.forEach((point) => {
        front.push(push(point, depth * 0.5 + 0.024));
        back.push(push(point, -depth * 0.5 - 0.024));
    });
    for (let i = 0; i < profile.length - 1; i += 1) {
        indices.push(front[i], back[i], front[i + 1]);
        indices.push(front[i + 1], back[i], back[i + 1]);
    }

    const geometry = new THREE.BufferGeometry();
    geometry.setIndex(indices);
    geometry.setAttribute('position', new THREE.Float32BufferAttribute(positions, 3));
    geometry.computeVertexNormals();
    return geometry;
}

function spineCurvePoint(t, spineWidth) {
    const radiusX = spineWidth * 0.42;
    const radiusY = 0.018;
    const baseY = BOOK_PROFILE.tableY + BOOK_PROFILE.coverThickness + 0.002;
    const theta = Math.PI * (1 - THREE.MathUtils.clamp(t, 0, 1));
    return {
        t: THREE.MathUtils.clamp(t, 0, 1),
        x: Math.cos(theta) * radiusX,
        y: baseY + Math.sin(theta) * radiusY
    };
}

function simulatePageLines(bundleCount, pageWidth, spineWidth) {
    const lines = [];
    const segments = 24;
    const stepLength = pageWidth / segments;
    const entries = [];
    const spineSamples = sampleSpineByArc(bundleCount, spineWidth);
    const leftLimit = Math.min(bundleCount - 2, Math.floor((bundleCount - 1) * readingProgress));
    for (let index = 0; index < bundleCount; index += 1) {
        const t = spineSamples[index].t;
        const side = index <= leftLimit ? -1 : 1;
        entries.push({ index, t, side });
    }
    [-1, 1].forEach((side) => {
        const sideEntries = entries.filter((entry) => entry.side === side);
        sideEntries.forEach((entry, rank) => {
            entry.rank = rank;
            entry.sideCount = sideEntries.length;
        });
    });
    [-1, 1].forEach((side) => {
        const sideEntries = entries
            .filter((entry) => entry.side === side)
            .sort((a, b) => side < 0 ? a.t - b.t : b.t - a.t);
        if (!sideEntries.length) return;

        let lowerLine = null;
        sideEntries.forEach((entry, rank) => {
            const anchor = spineCurvePoint(entry.t, spineWidth);
            const target = restingTarget(side, pageWidth, rank, sideEntries.length);
            const points = buildSupportSolvedLine(anchor, target, lowerLine, side, segments, stepLength, bundleCount);
            const line = { index: entry.index, t: entry.t, side, anchor, points, endpoint: points[points.length - 1] };
            lines.push(line);
            lowerLine = line;
        });
    });
    return lines;
}

function measureLineLengthError(lines, pageWidth) {
    return lines.reduce((maxError, line) => {
        let length = 0;
        for (let i = 0; i < line.points.length - 1; i += 1) {
            length += Math.hypot(line.points[i + 1].x - line.points[i].x, line.points[i + 1].y - line.points[i].y);
        }
        return Math.max(maxError, Math.abs(length - pageWidth));
    }, 0);
}

function measureStackSpacingError(lines) {
    let maxViolation = 0;
    [-1, 1].forEach((side) => {
        const sideLines = lines
            .filter((line) => line.side === side)
            .sort((a, b) => side < 0 ? a.t - b.t : b.t - a.t);
        for (let row = 1; row < sideLines.length; row += 1) {
            const lower = sideLines[row - 1];
            const upper = sideLines[row];
            for (let col = 1; col < upper.points.length; col += 1) {
                const closest = closestPointOnPolyline(upper.points[col], lower.points);
                const distance = Math.hypot(upper.points[col].x - closest.x, upper.points[col].y - closest.y);
                maxViolation = Math.max(maxViolation, Math.max(0, BOOK_PROFILE.bundleSpacing - distance));
            }
        }
    });
    return maxViolation;
}

function sampleSpineByArc(count, spineWidth) {
    const samples = [];
    const steps = 240;
    let length = 0;
    let previous = spineCurvePoint(0, spineWidth);
    samples.push({ point: previous, length });
    for (let i = 1; i <= steps; i += 1) {
        const t = i / steps;
        const point = spineCurvePoint(t, spineWidth);
        length += Math.hypot(point.x - previous.x, point.y - previous.y);
        samples.push({ point, length });
        previous = point;
    }
    const points = [];
    for (let i = 0; i < count; i += 1) {
        const target = count === 1 ? length * 0.5 : length * (i / (count - 1));
        const found = samples.findIndex((sample) => sample.length >= target);
        if (found <= 0) {
            points.push(samples[0].point);
            continue;
        }
        const before = samples[found - 1];
        const after = samples[found];
        const span = after.length - before.length || 1;
        const t = THREE.MathUtils.lerp(before.point.t, after.point.t, (target - before.length) / span);
        points.push(spineCurvePoint(t, spineWidth));
    }
    return points;
}

function initialPageLine(anchor, target, segments) {
    const points = [];
    for (let i = 0; i <= segments; i += 1) {
        const u = i / segments;
        const sag = 0.04 * Math.sin(Math.PI * u);
        points.push({
            x: THREE.MathUtils.lerp(anchor.x, target.x, u),
            y: THREE.MathUtils.lerp(anchor.y, target.y, u) - sag * u
        });
    }
    return points;
}

function restingTarget(side, pageWidth, rank, sideCount) {
    const local = sideCount <= 1 ? 0 : rank / (sideCount - 1);
    const foreCurve = 0.11 * Math.sin(Math.PI * local);
    const x = side * (pageWidth - foreCurve);
    const y = BOOK_PROFILE.coverThickness + BOOK_PROFILE.paperContactOffset + rank * BOOK_PROFILE.bundleSpacing + 0.002 * Math.sin(Math.PI * local);
    return { x, y };
}

function buildSupportSolvedLine(anchor, target, lowerLine, side, segments, stepLength, bundleCount) {
    const points = [{ x: anchor.x, y: anchor.y }];
    let tangent = coverTangentAtX(anchor.x, side);
    for (let index = 1; index <= segments; index += 1) {
        const u = index / segments;
        const supportTangent = lowerLine ? lineTangentAt(lowerLine.points, index) : coverTangentAtX(points[index - 1].x, side);
        const point = chooseClosestSupportedPoint(points[index - 1], tangent, supportTangent, target, lowerLine, index, side, stepLength, bundleCount, u);
        points.push(point);
        tangent = normalizedVector(point.x - points[index - 1].x, point.y - points[index - 1].y);
    }
    return points;
}

function chooseClosestSupportedPoint(previous, tangent, supportTangent, target, lowerLine, index, side, stepLength, bundleCount, u) {
    const blendTangent = normalizedVector(tangent.x + supportTangent.x * 2, tangent.y + supportTangent.y * 2);
    const angleHint = Math.atan2(blendTangent.y, blendTangent.x);
    let best = null;
    let fallback = null;
    for (let sample = 0; sample < 720; sample += 1) {
        const angle = sample / 720 * Math.PI * 2;
        const candidate = {
            x: previous.x + Math.cos(angle) * stepLength,
            y: previous.y + Math.sin(angle) * stepLength
        };
        const score = scoreSupportedPoint(candidate, previous, tangent, supportTangent, angle, angleHint, target, lowerLine, index, side, bundleCount, u);
        if (best === null || score < best.score) best = { point: candidate, score };
        const fallbackScore = scoreSupportedPoint(candidate, previous, tangent, supportTangent, angle, angleHint, target, lowerLine, index, side, bundleCount, u, true);
        if (fallback === null || fallbackScore < fallback.score) fallback = { point: candidate, score: fallbackScore };
    }
    return Number.isFinite(best?.score) ? best.point : fallback.point;
}

function scoreSupportedPoint(candidate, previous, tangent, supportTangent, angle, angleHint, target, lowerLine, index, side, bundleCount, u, allowViolation = false) {
    const backward = Math.max(0, side * (previous.x - candidate.x));
    if (!allowViolation && backward > 0.00001) return Number.POSITIVE_INFINITY;

    let supportError;
    let supportViolation = 0;
    if (lowerLine) {
        const closest = closestPointOnPolyline(candidate, lowerLine.points);
        const closestDistance = Math.hypot(candidate.x - closest.x, candidate.y - closest.y);
        supportViolation = Math.max(0, BOOK_PROFILE.bundleSpacing - closestDistance) + Math.max(0, closest.y - candidate.y);
        if (!allowViolation && supportViolation > 0.00001) return Number.POSITIVE_INFINITY;
        supportError = closestDistance - BOOK_PROFILE.bundleSpacing;
    } else {
        const floor = coverTopYAtX(candidate.x) + coverClearance(bundleCount);
        supportViolation = Math.max(0, floor - candidate.y);
        if (!allowViolation && supportViolation > 0.00001) return Number.POSITIVE_INFINITY;
        supportError = candidate.y - floor;
    }

    const candidateTangent = normalizedVector(candidate.x - previous.x, candidate.y - previous.y);
    const bend = 1 - Math.max(-1, Math.min(1, candidateTangent.x * tangent.x + candidateTangent.y * tangent.y));
    const supportAlignment = 1 - Math.max(-1, Math.min(1, candidateTangent.x * supportTangent.x + candidateTangent.y * supportTangent.y));
    const angleDelta = Math.abs(Math.atan2(Math.sin(angle - angleHint), Math.cos(angle - angleHint)));
    const outwardTarget = Math.max(0, side * (target.x - candidate.x));
    const targetHeight = Math.abs(candidate.y - target.y);
    return Math.abs(supportError) * 1200 + supportViolation * 100000 + backward * 100000 + supportAlignment * 0.85 + bend * 0.22 + angleDelta * 0.04 + outwardTarget * 0.01 + targetHeight * 0.006;
}

function closestPointOnPolyline(point, polyline) {
    let best = polyline[0];
    let bestDistance = Number.POSITIVE_INFINITY;
    for (let i = 0; i < polyline.length - 1; i += 1) {
        const candidate = closestPointOnSegment(point, polyline[i], polyline[i + 1]);
        const distance = Math.hypot(point.x - candidate.x, point.y - candidate.y);
        if (distance < bestDistance) {
            best = candidate;
            bestDistance = distance;
        }
    }
    return best;
}

function closestPointOnSegment(point, a, b) {
    const dx = b.x - a.x;
    const dy = b.y - a.y;
    const lengthSquared = dx * dx + dy * dy || 0.0001;
    const t = THREE.MathUtils.clamp(((point.x - a.x) * dx + (point.y - a.y) * dy) / lengthSquared, 0, 1);
    return {
        x: a.x + dx * t,
        y: a.y + dy * t
    };
}

function coverTangentAtX(x, side) {
    const delta = 0.002;
    const y0 = coverTopYAtX(x - delta);
    const y1 = coverTopYAtX(x + delta);
    return normalizedVector(side * delta * 2, y1 - y0);
}

function lineTangentAt(points, index) {
    const previous = points[Math.max(0, index - 1)];
    const next = points[Math.min(points.length - 1, index + 1)];
    return normalizedVector(next.x - previous.x, next.y - previous.y);
}

function normalizedVector(x, y) {
    const length = Math.hypot(x, y) || 0.0001;
    return { x: x / length, y: y / length };
}

function relaxPageLine(points, anchor, stepLength, side, local, bundleCount) {
    const gravity = 0.00072;
    const stackPressure = 0.0011 * (1 - local);
    const bendStrength = 0.52;
    const iterations = 72;
    for (let iteration = 0; iteration < iterations; iteration += 1) {
        points[0].x = anchor.x;
        points[0].y = anchor.y;
        for (let i = 1; i < points.length; i += 1) {
            const u = i / (points.length - 1);
            points[i].y -= gravity * u + stackPressure * u * u;
        }
        applyBendingResistance(points, bendStrength);
        for (let pass = 0; pass < 3; pass += 1) {
            points[0].x = anchor.x;
            points[0].y = anchor.y;
            enforceLineLength(points, anchor, stepLength, 3);
            keepPageAboveCover(points, side, bundleCount);
        }
    }
}

function applyBendingResistance(points, strength) {
    const updates = points.map((point) => ({ x: point.x, y: point.y }));
    for (let i = 1; i < points.length - 1; i += 1) {
        const previous = points[i - 1];
        const current = points[i];
        const next = points[i + 1];
        updates[i].x += (previous.x + next.x - current.x * 2) * strength;
        updates[i].y += (previous.y + next.y - current.y * 2) * strength;
    }
    for (let i = 1; i < points.length - 1; i += 1) {
        points[i].x = updates[i].x;
        points[i].y = updates[i].y;
    }
}

function enforceLineLength(points, anchor, stepLength, passes) {
    for (let pass = 0; pass < passes; pass += 1) {
        points[0].x = anchor.x;
        points[0].y = anchor.y;
        for (let i = 0; i < points.length - 1; i += 1) {
            constrainSegment(points[i], points[i + 1], stepLength, i === 0);
        }
        for (let i = points.length - 2; i >= 0; i -= 1) {
            constrainSegment(points[i], points[i + 1], stepLength, i === 0);
        }
    }
}

function constrainSegment(a, b, length, anchorA) {
    const dx = b.x - a.x;
    const dy = b.y - a.y;
    const distance = Math.hypot(dx, dy) || 0.0001;
    const correction = (distance - length) / distance;
    if (anchorA) {
        b.x -= dx * correction;
        b.y -= dy * correction;
        return;
    }
    a.x += dx * correction * 0.5;
    a.y += dy * correction * 0.5;
    b.x -= dx * correction * 0.5;
    b.y -= dy * correction * 0.5;
}

function enforceForwardLineLength(points, anchor, stepLength) {
    points[0].x = anchor.x;
    points[0].y = anchor.y;
    for (let i = 1; i < points.length; i += 1) {
        const previous = points[i - 1];
        const current = points[i];
        const dx = current.x - previous.x;
        const dy = current.y - previous.y;
        const distance = Math.hypot(dx, dy) || 0.0001;
        current.x = previous.x + dx / distance * stepLength;
        current.y = previous.y + dy / distance * stepLength;
    }
}

function keepPageAboveCover(points, side, bundleCount) {
    for (let i = 1; i < points.length; i += 1) {
        points[i].y = Math.max(points[i].y, coverTopYAtX(points[i].x) + coverClearance(bundleCount));
        points[i].x = side < 0 ? Math.min(points[i].x, -0.01) : Math.max(points[i].x, 0.01);
    }
}

function coverClearance(bundleCount) {
    return BOOK_PROFILE.paperContactOffset + 0.0002 * bundleCount;
}

function enforceStackConstraints(lines, stepLength, bundleCount) {
    const iterations = 44;
    [-1, 1].forEach((side) => {
        const sideLines = lines
            .filter((line) => line.side === side)
            .sort((a, b) => side < 0 ? a.t - b.t : b.t - a.t);
        for (let iteration = 0; iteration < iterations; iteration += 1) {
            sideLines.forEach((line) => {
                line.points[0].x = line.anchor.x;
                line.points[0].y = line.anchor.y;
                applyBendingResistance(line.points, 0.22);
                enforceLineLength(line.points, line.anchor, stepLength, 3);
                keepPageAboveCover(line.points, side, bundleCount);
            });

            for (let row = 1; row < sideLines.length; row += 1) {
                const lower = sideLines[row - 1];
                const upper = sideLines[row];
                for (let col = 1; col < upper.points.length; col += 1) {
                    const normal = upwardNormalAt(lower.points, col);
                    const targetX = lower.points[col].x + normal.x * BOOK_PROFILE.bundleSpacing;
                    const targetY = lower.points[col].y + normal.y * BOOK_PROFILE.bundleSpacing;
                    upper.points[col].x = THREE.MathUtils.lerp(upper.points[col].x, targetX, 0.28);
                    upper.points[col].y = Math.max(upper.points[col].y, THREE.MathUtils.lerp(upper.points[col].y, targetY, 0.42));
                }
                upper.points[0].x = upper.anchor.x;
                upper.points[0].y = upper.anchor.y;
                applyBendingResistance(upper.points, 0.2);
                enforceLineLength(upper.points, upper.anchor, stepLength, 3);
                keepPageAboveCover(upper.points, side, bundleCount);
            }
        }
        sideLines.forEach((line) => {
            applyBendingResistance(line.points, 0.32);
            enforceLineLength(line.points, line.anchor, stepLength, 10);
            keepPageAboveCover(line.points, side, bundleCount);
            enforceLineLength(line.points, line.anchor, stepLength, 6);
        });
        sideLines.forEach((line) => {
            line.endpoint = line.points[line.points.length - 1];
        });
    });
}

function offsetPageLine(basePoints, anchor, distance) {
    return basePoints.map((point, index) => {
        if (index === 0) return { x: anchor.x, y: anchor.y };
        const normal = upwardNormalAt(basePoints, index);
        return {
            x: point.x + normal.x * distance,
            y: point.y + normal.y * distance
        };
    });
}

function upwardNormalAt(points, index) {
    const previous = points[Math.max(0, index - 1)];
    const next = points[Math.min(points.length - 1, index + 1)];
    const dx = next.x - previous.x;
    const dy = next.y - previous.y;
    const length = Math.hypot(dx, dy) || 0.0001;
    let nx = -dy / length;
    let ny = dx / length;
    if (ny < 0) {
        nx = -nx;
        ny = -ny;
    }
    return { x: nx, y: ny };
}

function coverTopYAtX(x) {
    const ax = Math.abs(x);
    const spineHalf = currentSpineHalf();
    const hingeX = spineHalf + 0.07;
    const outerX = 1.62 + 0.055;
    if (ax <= spineHalf) return BOOK_PROFILE.coverThickness;
    if (ax <= hingeX) {
        const t = (ax - spineHalf) / (hingeX - spineHalf);
        return THREE.MathUtils.lerp(BOOK_PROFILE.coverThickness, BOOK_PROFILE.raisedHingeY, t);
    }
    const t = THREE.MathUtils.clamp((ax - hingeX) / (outerX - hingeX), 0, 1);
    return THREE.MathUtils.lerp(BOOK_PROFILE.raisedHingeY, BOOK_PROFILE.coverThickness, t);
}

function currentSpineHalf() {
    return Math.max(0.16, Math.round(pageCount / 10) * BOOK_PROFILE.bundleSpacing) * 0.5;
}

function addSimulatedPageLines(lines, depth) {
    const leftMaterial = new THREE.LineBasicMaterial({ color: 0x8f7750, transparent: true, opacity: 0.72 });
    const rightMaterial = new THREE.LineBasicMaterial({ color: 0x9a8058, transparent: true, opacity: 0.72 });
    const z = depth * 0.5 + 0.006;
    lines.forEach((line) => {
        const points = line.points.map((point) => new THREE.Vector3(point.x, point.y, z));
        book.add(new THREE.Line(new THREE.BufferGeometry().setFromPoints(points), line.side < 0 ? leftMaterial : rightMaterial));
    });
}

function addSimulatedStackBodies(lines, depth) {
    [-1, 1].forEach((side) => {
        const sideLines = lines.filter((line) => line.side === side);
        if (!sideLines.length) return;
        const material = side < 0 ? materials.pagesLeft : materials.pagesRight;
        const bodyLines = sideLines.length === 1 ? createSinglePageBodyLines(sideLines[0]) : sideLines;
        book.add(new THREE.Mesh(createLoftedLineBody(bodyLines, depth), material));
        book.add(new THREE.Line(createEndpointPolyline(bodyLines, depth), new THREE.LineBasicMaterial({ color: 0xb99a68, transparent: true, opacity: 0.62 })));
    });
}

function createSinglePageBodyLines(line) {
    const bundleCount = Math.max(4, Math.round(pageCount / 10));
    const supportPoints = line.points.map((point) => ({
        x: point.x,
        y: Math.max(coverTopYAtX(point.x) + coverClearance(bundleCount), point.y - BOOK_PROFILE.bundleSpacing)
    }));
    return [
        { ...line, points: supportPoints, endpoint: supportPoints[supportPoints.length - 1] },
        line
    ];
}

function createLoftedLineBody(lines, depth) {
    const positions = [];
    const indices = [];
    const smoothLines = lines.map((line) => line.points);
    const push = (point, z) => {
        const index = positions.length / 3;
        positions.push(point.x, point.y, z);
        return index;
    };
    const front = smoothLines.map((points) => points.map((point) => push(point, depth * 0.5)));
    const back = smoothLines.map((points) => points.map((point) => push(point, -depth * 0.5)));
    for (let row = 0; row < smoothLines.length - 1; row += 1) {
        for (let col = 0; col < smoothLines[row].length - 1; col += 1) {
            indices.push(front[row][col], front[row + 1][col], front[row][col + 1]);
            indices.push(front[row][col + 1], front[row + 1][col], front[row + 1][col + 1]);
            indices.push(back[row][col], back[row][col + 1], back[row + 1][col]);
            indices.push(back[row][col + 1], back[row + 1][col + 1], back[row + 1][col]);
        }
    }
    for (let row = 0; row < smoothLines.length - 1; row += 1) {
        const last = smoothLines[row].length - 1;
        indices.push(front[row][last], front[row + 1][last], back[row][last]);
        indices.push(front[row + 1][last], back[row + 1][last], back[row][last]);
    }
    for (let col = 0; col < smoothLines[0].length - 1; col += 1) {
        const bottomRow = 0;
        const topRow = smoothLines.length - 1;
        indices.push(front[bottomRow][col], front[bottomRow][col + 1], back[bottomRow][col]);
        indices.push(front[bottomRow][col + 1], back[bottomRow][col + 1], back[bottomRow][col]);
        indices.push(front[topRow][col], back[topRow][col], front[topRow][col + 1]);
        indices.push(front[topRow][col + 1], back[topRow][col], back[topRow][col + 1]);
    }
    for (let row = 0; row < smoothLines.length - 1; row += 1) {
        indices.push(front[row][0], back[row][0], front[row + 1][0]);
        indices.push(front[row + 1][0], back[row][0], back[row + 1][0]);
    }
    const geometry = new THREE.BufferGeometry();
    geometry.setIndex(indices);
    geometry.setAttribute('position', new THREE.Float32BufferAttribute(positions, 3));
    geometry.computeVertexNormals();
    return geometry;
}

function createEndpointPolyline(lines, depth) {
    const points = lines.map((line) => new THREE.Vector3(line.endpoint.x, line.endpoint.y, depth * 0.5 + 0.008));
    return new THREE.BufferGeometry().setFromPoints(points);
}

function resize() {
    const width = window.innerWidth;
    const height = window.innerHeight;
    renderer.setSize(width, height, false);
    camera.aspect = width / height;
    camera.updateProjectionMatrix();
}

function animate() {
    requestAnimationFrame(animate);
    if (urlParams.get('animate') === '1') {
        const t = performance.now() * 0.00035;
        setReadingProgress(0.5 + Math.sin(t) * 0.48);
    }
    controls.update();
    renderer.render(scene, camera);
}