<?php
require_once __DIR__ . '/db.php';

function h(?string $s): string
{
    return htmlspecialchars($s ?? '', ENT_QUOTES, 'UTF-8');
}

/* ---------------------------------------------------------------------------
 * Video duration probing (used by the scanner so listings can show durations
 * before anyone plays the video). Pure PHP — no external tools involved; a
 * file the mp4 parser can't read simply has no duration until the player
 * reports one on first play.
 * ------------------------------------------------------------------------- */
function probeDuration(string $file): ?float
{
    $d = mp4Duration($file);
    return ($d !== null && $d > 0) ? $d : null;
}

/** Read the duration from an mp4's moov/mvhd box. Returns seconds or null. */
function mp4Duration(string $path): ?float
{
    $fp = @fopen($path, 'rb');
    if (!$fp) return null;
    $fileSize = filesize($path) ?: 0;
    $duration = null;

    $readBox = function (int $pos) use ($fp): ?array {
        fseek($fp, $pos);
        $hdr = fread($fp, 8);
        if (strlen($hdr) < 8) return null;
        $size   = unpack('N', substr($hdr, 0, 4))[1];
        $type   = substr($hdr, 4, 4);
        $hdrLen = 8;
        if ($size === 1) {                    // 64-bit box size
            $ext = fread($fp, 8);
            if (strlen($ext) < 8) return null;
            $size   = unpack('J', $ext)[1];
            $hdrLen = 16;
        }
        return ($size >= $hdrLen) ? [$size, $type, $hdrLen] : null;
    };

    // Walk top-level boxes to find moov, then its mvhd child.
    for ($pos = 0; $pos < $fileSize;) {
        $box = $readBox($pos);
        if (!$box) break;
        [$size, $type, $hdrLen] = $box;
        if ($size === 0) $size = $fileSize - $pos;   // box extends to EOF

        if ($type === 'moov') {
            for ($p = $pos + $hdrLen, $end = $pos + $size; $p < $end;) {
                $child = $readBox($p);
                if (!$child) break;
                [$cSize, $cType, $cHdrLen] = $child;
                if ($cType === 'mvhd') {
                    fseek($fp, $p + $cHdrLen);
                    $body = fread($fp, 32);
                    if (strlen($body) >= 20) {
                        if (ord($body[0]) === 1 && strlen($body) >= 32) {   // version 1
                            $timescale = unpack('N', substr($body, 20, 4))[1];
                            $dur       = unpack('J', substr($body, 24, 8))[1];
                        } else {                                            // version 0
                            $timescale = unpack('N', substr($body, 12, 4))[1];
                            $dur       = unpack('N', substr($body, 16, 4))[1];
                        }
                        if ($timescale > 0) $duration = $dur / $timescale;
                    }
                    break 2;
                }
                $p += $cSize;
            }
            break;
        }
        $pos += $size;
    }
    fclose($fp);
    return $duration;
}

/* ---------------------------------------------------------------------------
 * Quiz file parser
 * Format ("*_quiz_solved.txt"):
 *   [&] Question text (Select two)
 *   - wrong option
 *   - right option -- Correct
 * Returns [['text', 'options' => [...], 'correct' => [optIndex,...], 'multi'], ...]
 * ------------------------------------------------------------------------- */
function parseQuizFile(string $absPath): array
{
    $raw = @file_get_contents($absPath);
    if ($raw === false) return [];

    $questions = [];
    $cur = null;
    foreach (preg_split('/\r\n|\r|\n/', $raw) as $line) {
        $line = trim($line);
        if ($line === '') continue;

        if (str_starts_with($line, '[&]')) {
            if ($cur && $cur['options']) $questions[] = $cur;
            $cur = ['text' => trim(substr($line, 3)), 'options' => [], 'correct' => []];
        } elseif (str_starts_with($line, '-') && $cur !== null) {
            $opt = trim(substr($line, 1));
            if (preg_match('/\s*--\s*correct\s*$/i', $opt)) {
                $opt = trim(preg_replace('/\s*--\s*correct\s*$/i', '', $opt));
                $cur['correct'][] = count($cur['options']);
            }
            $cur['options'][] = $opt;
        } elseif ($cur !== null && !$cur['options']) {
            $cur['text'] .= ' ' . $line;   // wrapped question text
        }
    }
    if ($cur && $cur['options']) $questions[] = $cur;

    foreach ($questions as &$q) {
        $q['multi'] = count($q['correct']) > 1;
    }
    return $questions;
}

/* ---------------------------------------------------------------------------
 * Library scanner
 * Walks VIDEOS_ROOT recursively, upserts every directory into `folders`,
 * every *.mp4 into `videos`, and every *_quiz_solved.txt into `quizzes`
 * (keyed by relative path so IDs stay stable across rescans), then flags
 * rows whose files disappeared as is_missing.
 * ------------------------------------------------------------------------- */
function scanLibrary(): array
{
    $root = rtrim(VIDEOS_ROOT, '/');
    if (!is_dir($root)) {
        throw new RuntimeException("VIDEOS_ROOT does not exist: $root");
    }

    $db = db();
    $stats = ['folders' => 0, 'videos' => 0, 'quizzes' => 0,
              'missing_folders' => 0, 'missing_videos' => 0, 'missing_quizzes' => 0];
    $seenFolderIds = [];
    $seenVideoIds  = [];
    $seenQuizIds   = [];

    $upsertFolder = $db->prepare(
        'INSERT INTO folders (parent_id, name, path, depth, sort_order, is_missing)
         VALUES (?, ?, ?, ?, ?, 0)
         ON DUPLICATE KEY UPDATE parent_id = VALUES(parent_id), name = VALUES(name),
             depth = VALUES(depth), sort_order = VALUES(sort_order), is_missing = 0,
             id = LAST_INSERT_ID(id)'
    );
    $upsertVideo = $db->prepare(
        'INSERT INTO videos (folder_id, name, filename, path, size_bytes, sort_order, is_missing)
         VALUES (?, ?, ?, ?, ?, ?, 0)
         ON DUPLICATE KEY UPDATE folder_id = VALUES(folder_id), name = VALUES(name),
             filename = VALUES(filename), size_bytes = VALUES(size_bytes),
             sort_order = VALUES(sort_order), is_missing = 0,
             id = LAST_INSERT_ID(id)'
    );
    $upsertQuiz = $db->prepare(
        'INSERT INTO quizzes (folder_id, name, filename, path, num_questions, sort_order, is_missing)
         VALUES (?, ?, ?, ?, ?, ?, 0)
         ON DUPLICATE KEY UPDATE folder_id = VALUES(folder_id), name = VALUES(name),
             filename = VALUES(filename), num_questions = VALUES(num_questions),
             sort_order = VALUES(sort_order), is_missing = 0,
             id = LAST_INSERT_ID(id)'
    );

    $walk = function (string $dir, ?int $parentId, string $relBase, int $depth)
        use (&$walk, $db, $upsertFolder, $upsertVideo, $upsertQuiz,
            &$stats, &$seenFolderIds, &$seenVideoIds, &$seenQuizIds): void {

        $entries = scandir($dir) ?: [];
        $entries = array_values(array_filter($entries, fn($e) => $e[0] !== '.'));
        usort($entries, 'strnatcasecmp'); // "Video2" before "Video10"

        $folderSort = 0;
        $videoSort  = 0;
        $quizSort   = 0;
        foreach ($entries as $entry) {
            $full = $dir . '/' . $entry;
            $rel  = $relBase === '' ? $entry : $relBase . '/' . $entry;

            if (is_dir($full)) {
                $upsertFolder->execute([$parentId, $entry, $rel, $depth, $folderSort++]);
                $folderId = (int)$db->lastInsertId();
                $seenFolderIds[] = $folderId;
                $stats['folders']++;
                $walk($full, $folderId, $rel, $depth + 1);
            } elseif (is_file($full) && strtolower(pathinfo($entry, PATHINFO_EXTENSION)) === 'mp4') {
                if ($parentId === null) {
                    continue; // mp4 directly in the root is skipped; use course folders
                }
                $name = pathinfo($entry, PATHINFO_FILENAME);
                $upsertVideo->execute([$parentId, $name, $entry, $rel, filesize($full) ?: 0, $videoSort++]);
                $seenVideoIds[] = (int)$db->lastInsertId();
                $stats['videos']++;
            } elseif (is_file($full) && preg_match('/_quiz_solved\.txt$/i', $entry)) {
                if ($parentId === null) {
                    continue;
                }
                $numQuestions = count(parseQuizFile($full));
                if ($numQuestions === 0) {
                    continue; // unparseable file — don't index an empty quiz
                }
                $name = trim(preg_replace('/_quiz_solved\.txt$/i', '', $entry));
                $upsertQuiz->execute([$parentId, $name, $entry, $rel, $numQuestions, $quizSort++]);
                $seenQuizIds[] = (int)$db->lastInsertId();
                $stats['quizzes']++;
            }
        }
    };

    $db->beginTransaction();
    try {
        $walk($root, null, '', 0);

        // Anything not seen this pass no longer exists on disk.
        $flagMissing = function (string $table, array $seenIds) use ($db): int {
            if ($seenIds) {
                $in = implode(',', array_map('intval', $seenIds));
                $sql = "UPDATE $table SET is_missing = 1 WHERE is_missing = 0 AND id NOT IN ($in)";
            } else {
                $sql = "UPDATE $table SET is_missing = 1 WHERE is_missing = 0";
            }
            return $db->exec($sql);
        };
        $stats['missing_folders'] = $flagMissing('folders', $seenFolderIds);
        $stats['missing_videos']  = $flagMissing('videos', $seenVideoIds);
        $stats['missing_quizzes'] = $flagMissing('quizzes', $seenQuizIds);

        $db->commit();
    } catch (Throwable $e) {
        $db->rollBack();
        throw $e;
    }

    // Fill in durations for videos that don't have one yet (new files, or
    // indexed before duration probing existed). Only missing ones are probed,
    // so rescans stay fast.
    $stats['durations_probed'] = 0;
    $rows = $db->query(
        'SELECT id, path FROM videos WHERE duration_seconds IS NULL AND is_missing = 0'
    )->fetchAll();
    $upd = $db->prepare('UPDATE videos SET duration_seconds = ? WHERE id = ?');
    foreach ($rows as $r) {
        $d = probeDuration($root . '/' . $r['path']);
        if ($d !== null && $d > 0) {
            $upd->execute([$d, (int)$r['id']]);
            $stats['durations_probed']++;
        }
    }

    appMetaSet('last_scan_at', gmdate('Y-m-d H:i:s'));

    return $stats;
}

/* ---------------------------------------------------------------------------
 * Progress helpers
 * ------------------------------------------------------------------------- */

/**
 * One pass over a folder's whole subtree (videos + quizzes), matched by path
 * prefix: how much is in there, how big, how long, and how far the user got.
 * Course cards need all of it, so it is gathered in two queries rather than
 * one per fact.
 */
function folderRollup(int $userId, string $folderPath): array
{
    $args = [$userId, $folderPath, likePrefix($folderPath) . '/%'];

    $stmt = db()->prepare(
        "SELECT COUNT(*) AS total,
                COALESCE(SUM(COALESCE(v.duration_seconds, vp.duration_seconds)), 0) AS duration,
                COALESCE(SUM(v.size_bytes), 0) AS bytes,
                COALESCE(SUM(vp.completed), 0) AS completed,
                COALESCE(SUM(
                    CASE
                        WHEN vp.completed = 1 THEN 1
                        WHEN vp.duration_seconds > 0
                             THEN LEAST(vp.position_seconds / vp.duration_seconds, 1)
                        ELSE 0
                    END
                ), 0) AS credit
         FROM videos v
         JOIN folders f ON f.id = v.folder_id
         LEFT JOIN video_progress vp ON vp.video_id = v.id AND vp.user_id = ?
         WHERE v.is_missing = 0 AND f.is_missing = 0
           AND (f.path = ? OR f.path LIKE ?)"
    );
    $stmt->execute($args);
    $videos = $stmt->fetch();

    $stmt = db()->prepare(
        "SELECT COUNT(*) AS total,
                COALESCE(SUM(qp.completed), 0) AS completed,
                COALESCE(SUM(
                    CASE
                        WHEN qp.completed = 1 THEN 1
                        WHEN q.num_questions > 0
                             THEN LEAST(COALESCE(qp.best_score, 0) / q.num_questions, 1)
                        ELSE 0
                    END
                ), 0) AS credit
         FROM quizzes q
         JOIN folders f ON f.id = q.folder_id
         LEFT JOIN quiz_progress qp ON qp.quiz_id = q.id AND qp.user_id = ?
         WHERE q.is_missing = 0 AND f.is_missing = 0
           AND (f.path = ? OR f.path LIKE ?)"
    );
    $stmt->execute($args);
    $quizzes = $stmt->fetch();

    $total  = (int)$videos['total'] + (int)$quizzes['total'];
    $credit = $videos['credit'] + $quizzes['credit'];

    return [
        'videos'    => (int)$videos['total'],
        'quizzes'   => (int)$quizzes['total'],
        'duration'  => (float)$videos['duration'],
        'bytes'     => (int)$videos['bytes'],
        'total'     => $total,
        'completed' => (int)$videos['completed'] + (int)$quizzes['completed'],
        'percent'   => $total > 0 ? (int)round(100 * $credit / $total) : 0,
    ];
}

/**
 * Progress for one folder's whole subtree.
 * Returns ['total' => n, 'completed' => n, 'percent' => 0-100].
 * Percent gives partial credit for half-watched videos and partial scores.
 */
function folderProgress(int $userId, string $folderPath): array
{
    $r = folderRollup($userId, $folderPath);
    return ['total' => $r['total'], 'completed' => $r['completed'], 'percent' => $r['percent']];
}

/** A course (or section) as the library screens show it: contents + progress. */
function courseSummary(int $userId, array $folder): array
{
    $r = folderRollup($userId, $folder['path']);
    $percent = $r['percent'];

    return [
        'id'       => (int)$folder['id'],
        'name'     => $folder['name'],
        'path'     => $folder['path'],
        'videos'   => $r['videos'],
        'quizzes'  => $r['quizzes'],
        'duration' => $r['duration'],
        'bytes'    => $r['bytes'],
        'added_at' => $folder['created_at'] ?? null,
        'progress' => ['total' => $r['total'], 'completed' => $r['completed'], 'percent' => $percent],
        // The shelves the library groups by — the app has no course categories,
        // so progress is what separates one shelf from another.
        'state'    => $percent >= 100 ? 'finished' : ($percent > 0 ? 'progress' : 'fresh'),
    ];
}

/** Every top-level folder as a course card. */
function courseCards(int $userId): array
{
    return array_map(fn($f) => courseSummary($userId, $f), childFolders(null));
}

/** Headline numbers for the home screen, computed from the course cards. */
function userStats(int $userId, array $courses): array
{
    $inProgress = 0;
    $finished   = 0;
    foreach ($courses as $c) {
        if ($c['state'] === 'finished')      $finished++;
        elseif ($c['state'] === 'progress')  $inProgress++;
    }

    // Watched time: full duration for completed videos, position for the rest.
    $stmt = db()->prepare(
        'SELECT COALESCE(SUM(
                    CASE WHEN vp.completed = 1
                         THEN COALESCE(v.duration_seconds, vp.duration_seconds, vp.position_seconds)
                         ELSE vp.position_seconds END
                ), 0) AS watched
         FROM video_progress vp
         JOIN videos v ON v.id = vp.video_id AND v.is_missing = 0
         WHERE vp.user_id = ?'
    );
    $stmt->execute([$userId]);
    $watched = (float)$stmt->fetchColumn();

    $stmt = db()->prepare(
        'SELECT COUNT(*) AS taken,
                COALESCE(AVG(qp.best_score / NULLIF(q.num_questions, 0)), 0) AS avg_score
         FROM quiz_progress qp
         JOIN quizzes q ON q.id = qp.quiz_id AND q.is_missing = 0
         WHERE qp.user_id = ? AND qp.attempts > 0'
    );
    $stmt->execute([$userId]);
    $quiz = $stmt->fetch();

    return [
        'in_progress'     => $inProgress,
        'finished'        => $finished,
        'courses'         => count($courses),
        'watched_seconds' => $watched,
        'quizzes_taken'   => (int)$quiz['taken'],
        'quiz_average'    => (int)round(100 * (float)$quiz['avg_score']),
    ];
}

/** Courses the scanner indexed most recently — the "found in the last scan" strip. */
function recentCourses(int $userId, int $limit = 3): array
{
    $stmt = db()->prepare(
        'SELECT * FROM folders
         WHERE parent_id IS NULL AND is_missing = 0
         ORDER BY created_at DESC, id DESC
         LIMIT ' . (int)$limit
    );
    $stmt->execute();

    $week = time() - 7 * 86400;
    return array_map(function ($f) use ($userId, $week) {
        $c = courseSummary($userId, $f);
        $c['is_new'] = $f['created_at'] !== null && strtotime($f['created_at']) >= $week;
        return $c;
    }, $stmt->fetchAll());
}

/** Library-wide counts for the sidebar and header. */
function libraryTotals(): array
{
    $row = db()->query(
        'SELECT (SELECT COUNT(*) FROM folders WHERE parent_id IS NULL AND is_missing = 0) AS courses,
                (SELECT COUNT(*) FROM videos WHERE is_missing = 0) AS videos,
                (SELECT COUNT(*) FROM quizzes WHERE is_missing = 0) AS quizzes,
                (SELECT COALESCE(SUM(size_bytes), 0) FROM videos WHERE is_missing = 0) AS bytes,
                (SELECT COALESCE(SUM(duration_seconds), 0) FROM videos WHERE is_missing = 0) AS duration'
    )->fetch();

    return [
        'courses'  => (int)$row['courses'],
        'videos'   => (int)$row['videos'],
        'quizzes'  => (int)$row['quizzes'],
        'bytes'    => (int)$row['bytes'],
        'duration' => (float)$row['duration'],
    ];
}

/* ---------------------------------------------------------------------------
 * app_meta — small key/value store (currently just the last scan time).
 * Reads tolerate the table not existing yet, so an install created before this
 * table was added keeps working until its next scan creates it.
 * ------------------------------------------------------------------------- */
function appMetaGet(string $key): ?string
{
    try {
        $stmt = db()->prepare('SELECT v FROM app_meta WHERE k = ?');
        $stmt->execute([$key]);
        $value = $stmt->fetchColumn();
        return $value === false ? null : (string)$value;
    } catch (Throwable $e) {
        return null;
    }
}

function appMetaSet(string $key, string $value): void
{
    $db = db();
    $db->exec(
        'CREATE TABLE IF NOT EXISTS app_meta (
            k          VARCHAR(64) PRIMARY KEY,
            v          VARCHAR(255) NOT NULL,
            updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
         ) ENGINE=InnoDB'
    );
    $stmt = $db->prepare('INSERT INTO app_meta (k, v) VALUES (?, ?) ON DUPLICATE KEY UPDATE v = VALUES(v)');
    $stmt->execute([$key, $value]);
}

/** Escape LIKE wildcards in a path used as a prefix. */
function likePrefix(string $path): string
{
    return str_replace(['\\', '%', '_'], ['\\\\', '\\%', '\\_'], $path);
}

/** Escaped "contains" pattern for LIKE. */
function likeContains(string $term): string
{
    return '%' . likePrefix($term) . '%';
}

/**
 * Wipe the user's progress for a scope:
 *   all    — everything the user has watched/answered
 *   folder — the folder's whole subtree (id = folder id)
 *   video  — one video (id = video id)
 *   quiz   — one quiz (id = quiz id)
 * Returns ['videos' => n, 'quizzes' => n] rows removed.
 * Rows for files that went missing are cleared too, so a reset is complete.
 */
function resetProgress(int $userId, string $scope, int $id = 0): array
{
    $db = db();

    switch ($scope) {
        case 'all':
            $v = $db->prepare('DELETE FROM video_progress WHERE user_id = ?');
            $v->execute([$userId]);
            $q = $db->prepare('DELETE FROM quiz_progress WHERE user_id = ?');
            $q->execute([$userId]);
            break;

        case 'folder':
            $folder = getFolder($id);
            if (!$folder) {
                throw new InvalidArgumentException('Folder not found');
            }
            $args = [$userId, $folder['path'], likePrefix($folder['path']) . '/%'];
            $v = $db->prepare(
                'DELETE vp FROM video_progress vp
                 JOIN videos v  ON v.id = vp.video_id
                 JOIN folders f ON f.id = v.folder_id
                 WHERE vp.user_id = ? AND (f.path = ? OR f.path LIKE ?)'
            );
            $v->execute($args);
            $q = $db->prepare(
                'DELETE qp FROM quiz_progress qp
                 JOIN quizzes q ON q.id = qp.quiz_id
                 JOIN folders f ON f.id = q.folder_id
                 WHERE qp.user_id = ? AND (f.path = ? OR f.path LIKE ?)'
            );
            $q->execute($args);
            break;

        case 'video':
            if ($id <= 0) {
                throw new InvalidArgumentException('Video not found');
            }
            $v = $db->prepare('DELETE FROM video_progress WHERE user_id = ? AND video_id = ?');
            $v->execute([$userId, $id]);
            $q = null;
            break;

        case 'quiz':
            if ($id <= 0) {
                throw new InvalidArgumentException('Quiz not found');
            }
            $q = $db->prepare('DELETE FROM quiz_progress WHERE user_id = ? AND quiz_id = ?');
            $q->execute([$userId, $id]);
            $v = null;
            break;

        default:
            throw new InvalidArgumentException('Unknown reset scope');
    }

    return [
        'videos'  => $v ? $v->rowCount() : 0,
        'quizzes' => $q ? $q->rowCount() : 0,
    ];
}

/* ---------------------------------------------------------------------------
 * Search — substring match on names (and on the relative path, so
 * "Course 1/Intro" finds everything under it).
 * ------------------------------------------------------------------------- */

function searchFolders(string $term, int $limit = 25): array
{
    $like = likeContains($term);
    $stmt = db()->prepare(
        'SELECT * FROM folders
         WHERE is_missing = 0 AND (name LIKE ? OR path LIKE ?)
         ORDER BY depth, path
         LIMIT ' . (int)$limit
    );
    $stmt->execute([$like, $like]);
    return $stmt->fetchAll();
}

function searchVideos(int $userId, string $term, int $limit = 50): array
{
    $like = likeContains($term);
    $stmt = db()->prepare(
        'SELECT v.*, f.path AS folder_path,
                vp.position_seconds, vp.duration_seconds AS watched_duration, vp.completed
         FROM videos v
         JOIN folders f ON f.id = v.folder_id
         LEFT JOIN video_progress vp ON vp.video_id = v.id AND vp.user_id = ?
         WHERE v.is_missing = 0 AND f.is_missing = 0 AND (v.name LIKE ? OR v.path LIKE ?)
         ORDER BY f.path, v.sort_order, v.name
         LIMIT ' . (int)$limit
    );
    $stmt->execute([$userId, $like, $like]);
    return $stmt->fetchAll();
}

function searchQuizzes(int $userId, string $term, int $limit = 25): array
{
    $like = likeContains($term);
    $stmt = db()->prepare(
        'SELECT q.*, f.path AS folder_path,
                qp.best_score, qp.last_score, qp.attempts, qp.completed
         FROM quizzes q
         JOIN folders f ON f.id = q.folder_id
         LEFT JOIN quiz_progress qp ON qp.quiz_id = q.id AND qp.user_id = ?
         WHERE q.is_missing = 0 AND f.is_missing = 0 AND (q.name LIKE ? OR q.path LIKE ?)
         ORDER BY f.path, q.sort_order, q.name
         LIMIT ' . (int)$limit
    );
    $stmt->execute([$userId, $like, $like]);
    return $stmt->fetchAll();
}

function getFolder(int $id): ?array
{
    $stmt = db()->prepare('SELECT * FROM folders WHERE id = ? AND is_missing = 0');
    $stmt->execute([$id]);
    return $stmt->fetch() ?: null;
}

function getVideo(int $id): ?array
{
    $stmt = db()->prepare(
        'SELECT v.*, f.path AS folder_path, f.name AS folder_name
         FROM videos v JOIN folders f ON f.id = v.folder_id
         WHERE v.id = ? AND v.is_missing = 0'
    );
    $stmt->execute([$id]);
    return $stmt->fetch() ?: null;
}

function childFolders(?int $parentId): array
{
    $sql = 'SELECT * FROM folders WHERE is_missing = 0 AND parent_id '
         . ($parentId === null ? 'IS NULL' : '= ?')
         . ' ORDER BY sort_order, name';
    $stmt = db()->prepare($sql);
    $stmt->execute($parentId === null ? [] : [$parentId]);
    return $stmt->fetchAll();
}

/** Videos in a folder, joined with the user's progress. */
function folderVideos(int $folderId, int $userId): array
{
    $stmt = db()->prepare(
        'SELECT v.*, vp.position_seconds, vp.duration_seconds AS watched_duration, vp.completed
         FROM videos v
         LEFT JOIN video_progress vp ON vp.video_id = v.id AND vp.user_id = ?
         WHERE v.folder_id = ? AND v.is_missing = 0
         ORDER BY v.sort_order, v.name'
    );
    $stmt->execute([$userId, $folderId]);
    return $stmt->fetchAll();
}

function getQuiz(int $id): ?array
{
    $stmt = db()->prepare(
        'SELECT q.*, f.path AS folder_path, f.name AS folder_name
         FROM quizzes q JOIN folders f ON f.id = q.folder_id
         WHERE q.id = ? AND q.is_missing = 0'
    );
    $stmt->execute([$id]);
    return $stmt->fetch() ?: null;
}

/** Quizzes in a folder, joined with the user's results. */
function folderQuizzes(int $folderId, int $userId): array
{
    $stmt = db()->prepare(
        'SELECT q.*, qp.best_score, qp.last_score, qp.attempts, qp.completed
         FROM quizzes q
         LEFT JOIN quiz_progress qp ON qp.quiz_id = q.id AND qp.user_id = ?
         WHERE q.folder_id = ? AND q.is_missing = 0
         ORDER BY q.sort_order, q.name'
    );
    $stmt->execute([$userId, $folderId]);
    return $stmt->fetchAll();
}

/** Absolute, containment-checked path of a quiz file (null if gone). */
function quizFilePath(array $quiz): ?string
{
    $file = realpath(rtrim(VIDEOS_ROOT, '/') . '/' . $quiz['path']);
    $root = realpath(VIDEOS_ROOT);
    if ($file === false || $root === false
        || !str_starts_with($file, $root . DIRECTORY_SEPARATOR) || !is_file($file)) {
        return null;
    }
    return $file;
}

/**
 * Absolute, containment-checked path of a video's sibling subtitle file
 * ("Lesson.mp4" -> "Lesson.srt"), or null if there isn't one.
 */
function srtFilePath(array $video): ?string
{
    $rel = preg_replace('/\.mp4$/i', '.srt', $video['path']);
    if ($rel === null || $rel === $video['path']) {
        return null;
    }
    $file = realpath(rtrim(VIDEOS_ROOT, '/') . '/' . $rel);
    $root = realpath(VIDEOS_ROOT);
    if ($file === false || $root === false
        || !str_starts_with($file, $root . DIRECTORY_SEPARATOR) || !is_file($file)) {
        return null;
    }
    return $file;
}

/**
 * SubRip -> WebVTT (the only caption format browsers take in <track>).
 * The formats differ by a header line and the decimal separator in
 * timestamps; cue numbers are legal VTT cue identifiers and pass through.
 */
function srtToVtt(string $srt): string
{
    if (str_starts_with($srt, "\xEF\xBB\xBF")) {
        $srt = substr($srt, 3);
    }
    if (!preg_match('//u', $srt)) {   // not valid UTF-8 — legacy single-byte file
        $recoded = function_exists('iconv')
            ? @iconv('Windows-1252', 'UTF-8//TRANSLIT', $srt)
            : (function_exists('mb_convert_encoding')
                ? mb_convert_encoding($srt, 'UTF-8', 'Windows-1252') : false);
        if (is_string($recoded)) {
            $srt = $recoded;
        }
    }
    $srt = str_replace(["\r\n", "\r"], "\n", $srt);

    // Normalise timestamps to HH:MM:SS.mmm. SRT uses a comma; generated files
    // in the wild also carry single-digit hours and six-digit fractions, and
    // the WebVTT parsing algorithm rejects any fraction that isn't exactly
    // three digits — such cues would silently drop.
    $srt = preg_replace_callback(
        '/(\d+):(\d{2}):(\d{2})[.,](\d+)/',
        fn($m) => sprintf('%02d:%s:%s.%s', $m[1], $m[2], $m[3],
                          substr(str_pad($m[4], 3, '0', STR_PAD_RIGHT), 0, 3)),
        $srt
    );

    // Some ".srt" files are already WebVTT — don't stack a second header.
    $srt = trim($srt);
    return (str_starts_with($srt, 'WEBVTT') ? $srt : "WEBVTT\n\n" . $srt) . "\n";
}

/** Breadcrumb chain root -> ... -> folder. */
function folderBreadcrumbs(array $folder): array
{
    $crumbs = [$folder];
    while ($folder['parent_id'] !== null) {
        $folder = getFolder((int)$folder['parent_id']);
        if (!$folder) break;
        array_unshift($crumbs, $folder);
    }
    return $crumbs;
}

/** Most recently watched, unfinished videos for the "Continue watching" strip. */
function continueWatching(int $userId, int $limit = 6): array
{
    $stmt = db()->prepare(
        'SELECT v.id, v.name, v.folder_id, f.path AS folder_path,
                vp.position_seconds, vp.duration_seconds, vp.updated_at
         FROM video_progress vp
         JOIN videos v  ON v.id = vp.video_id AND v.is_missing = 0
         JOIN folders f ON f.id = v.folder_id
         WHERE vp.user_id = ? AND vp.completed = 0 AND vp.position_seconds > 5
         ORDER BY vp.updated_at DESC
         LIMIT ' . (int)$limit
    );
    $stmt->execute([$userId]);
    return $stmt->fetchAll();
}

/** The top-level course a folder belongs to (itself, if it's already top-level). */
function courseIdOf(array $folder): int
{
    $crumbs = folderBreadcrumbs($folder);
    return (int)$crumbs[0]['id'];
}

function formatDuration(?float $seconds): string
{
    if ($seconds === null || $seconds <= 0) return '';
    $s = (int)round($seconds);
    return $s >= 3600
        ? sprintf('%d:%02d:%02d', intdiv($s, 3600), intdiv($s % 3600, 60), $s % 60)
        : sprintf('%d:%02d', intdiv($s, 60), $s % 60);
}