'use client';

import { useEffect, useState, useMemo } from 'react';
import Link from 'next/link';
import { useParams, useRouter } from 'next/navigation';
import {
  ArrowLeft,
  Edit,
  Trash2,
  ChevronLeft,
  ChevronRight,
  CheckCircle2,
  Circle,
  Lock,
  BookOpen,
  Clock,
  GraduationCap,
  ChevronDown,
  ChevronUp,
  Play,
} from 'lucide-react';
import { Button } from '@/components/ui/button';
import { Progress } from '@/components/ui/progress';
import {
  AlertDialog,
  AlertDialogAction,
  AlertDialogCancel,
  AlertDialogContent,
  AlertDialogDescription,
  AlertDialogFooter,
  AlertDialogHeader,
  AlertDialogTitle,
  AlertDialogTrigger,
} from '@/components/ui/alert-dialog';
import { api } from '@/lib/api';
import { useAuth } from '@/contexts/auth-context';
import { LessonContentViewer } from '@/components/editor/lesson-content-viewer';
import { LessonQuiz } from '@/components/dashboard/lesson-quiz';
import { cn } from '@/lib/utils';

type Lesson = { id: string; title: string; durationMinutes?: number | null; order: number };
type Chapter = { id: string; title: string; description?: string | null; order: number; lessons: Lesson[] };
type CourseData = {
  id: string;
  title: string;
  description?: string | null;
  status: string;
  authorId: string;
  chapters: Chapter[];
};

export default function CoursePage() {
  const params = useParams();
  const router = useRouter();
  const { loading: authLoading, backendUser } = useAuth();
  const id = params?.id as string;
  const [course, setCourse] = useState<CourseData | null>(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState<string | null>(null);
  const [deleting, setDeleting] = useState(false);
  const [selectedLessonId, setSelectedLessonId] = useState<string | null>(null);
  const [lessonContent, setLessonContent] = useState<Record<string, unknown> | null>(null);
  const [lessonContentLoading, setLessonContentLoading] = useState(false);
  const [completedLessons, setCompletedLessons] = useState<Set<string>>(new Set());
  const [expandedChapters, setExpandedChapters] = useState<string[]>([]);
  const [sidebarOpen, setSidebarOpen] = useState(true);
  const [enrollmentProgress, setEnrollmentProgress] = useState<any>(null);
  const [showQuiz, setShowQuiz] = useState(false);
  const [quizQuestions, setQuizQuestions] = useState<any[]>([]);
  const [generatingCertificate, setGeneratingCertificate] = useState(false);

  // Flat list of all lessons
  const flatLessons = useMemo(() => {
    if (!course) return [];
    return course.chapters
      .sort((a, b) => a.order - b.order)
      .flatMap((ch) =>
        ch.lessons.sort((a, b) => a.order - b.order).map((l) => ({ ...l, chapterId: ch.id, chapterTitle: ch.title }))
      );
  }, [course]);

  const currentLessonIndex = flatLessons.findIndex((l) => l.id === selectedLessonId);
  const totalLessons = flatLessons.length;
  const completedCount = completedLessons.size;
  const progressPercent = totalLessons > 0 ? Math.round((completedCount / totalLessons) * 100) : 0;

  // Load course and progress
  useEffect(() => {
    if (!id || authLoading) return;
    let cancelled = false;
    (async () => {
      setLoading(true);
      setError(null);
      try {
        const [courseData, progressData] = await Promise.all([
          api.getCourse(id),
          api.getEnrollmentProgress(id).catch(() => null),
        ]);
        if (!cancelled) {
          setCourse(courseData);
          setEnrollmentProgress(progressData);
          if (progressData?.lessons) {
            const completed: Set<string> = new Set(
              progressData.lessons
                .filter((l: any) => l.completedAt)
                .map((l: any) => String(l.lessonId))
            );
            setCompletedLessons(completed);
          }
          const first = courseData.chapters?.[0]?.lessons?.[0];
          if (first) setSelectedLessonId(first.id);
          setExpandedChapters(courseData.chapters.map((ch: Chapter) => ch.id));
        }
      } catch (e: any) {
        if (!cancelled) setError(e?.message || 'Не удалось загрузить курс');
      } finally {
        if (!cancelled) setLoading(false);
      }
    })();
    return () => { cancelled = true; };
  }, [id, authLoading]);

  // Load lesson content
  useEffect(() => {
    if (!id || !selectedLessonId) {
      setLessonContent(null);
      return;
    }
    let cancelled = false;
    setLessonContentLoading(true);
    (async () => {
      try {
        const data = await api.getLesson(id, selectedLessonId);
        const content = data?.content;
        if (!cancelled)
          setLessonContent(
            typeof content === 'object' && content !== null ? (content as Record<string, unknown>) : null
          );
      } catch {
        if (!cancelled) setLessonContent(null);
      } finally {
        if (!cancelled) setLessonContentLoading(false);
      }
    })();
    return () => { cancelled = true; };
  }, [id, selectedLessonId]);

  const handleDelete = async () => {
    if (!course?.id || deleting) return;
    setDeleting(true);
    try {
      await api.deleteCourse(course.id);
      router.push('/dashboard');
      router.refresh();
    } catch (e: any) {
      setError(e?.message || 'Не удалось удалить курс');
    } finally {
      setDeleting(false);
    }
  };

  const markComplete = async () => {
    if (!selectedLessonId || !id) return;
    try {
      await api.completeLesson(id, selectedLessonId);
      setCompletedLessons((prev) => new Set(prev).add(selectedLessonId));
    } catch {
      setCompletedLessons((prev) => new Set(prev).add(selectedLessonId));
    }
  };

  const handleStartQuiz = async () => {
    if (!selectedLessonId || !id) return;
    try {
      const quiz = await api.getLessonQuiz(id, selectedLessonId);
      setQuizQuestions(quiz.questions || []);
      setShowQuiz(true);
    } catch {
      setQuizQuestions([]);
    }
  };

  const handleQuizComplete = async (score: number) => {
    if (!selectedLessonId || !id) return;
    try {
      await api.submitQuizScore(id, selectedLessonId, score);
      markComplete();
    } catch {
      markComplete();
    }
  };

  const handleGetCertificate = async () => {
    if (!id || generatingCertificate) return;
    setGeneratingCertificate(true);
    try {
      const { certificateUrl } = await api.getCertificate(id);
      window.open(certificateUrl, '_blank');
    } catch {
      // silent
    } finally {
      setGeneratingCertificate(false);
    }
  };

  const goToNextLesson = () => {
    if (currentLessonIndex < flatLessons.length - 1) {
      markComplete();
      setSelectedLessonId(flatLessons[currentLessonIndex + 1].id);
    }
  };

  const goToPrevLesson = () => {
    if (currentLessonIndex > 0) {
      setSelectedLessonId(flatLessons[currentLessonIndex - 1].id);
    }
  };

  const toggleChapter = (chapterId: string) => {
    setExpandedChapters((prev) =>
      prev.includes(chapterId)
        ? prev.filter((id) => id !== chapterId)
        : [...prev, chapterId]
    );
  };

  if (authLoading || loading) {
    return (
      <div className="flex h-[calc(100vh-4rem)] items-center justify-center">
        <div className="flex flex-col items-center gap-3">
          <div className="h-8 w-8 animate-spin rounded-full border-4 border-primary border-t-transparent" />
          <p className="text-muted-foreground">Загрузка курса...</p>
        </div>
      </div>
    );
  }

  if (error || !course) {
    return (
      <div className="flex flex-col gap-4 p-6">
        <Button variant="ghost" asChild>
          <Link href="/dashboard"><ArrowLeft className="mr-2 h-4 w-4" />Назад к курсам</Link>
        </Button>
        <p className="text-destructive">{error || 'Курс не найден'}</p>
      </div>
    );
  }

  const activeLessonMeta = selectedLessonId
    ? flatLessons.find((l) => l.id === selectedLessonId)
    : null;
  
  const isAuthor = course && backendUser && course.authorId === backendUser.id;
  const courseCompleted = completedCount >= totalLessons && totalLessons > 0;

  return (
    <div className="flex h-[calc(100vh-4rem)] -m-6 flex-col">
      {/* Top bar */}
      <div className="flex shrink-0 items-center justify-between border-b bg-background px-4 py-2.5 shadow-sm">
        <div className="flex items-center gap-3">
          <Button variant="ghost" size="sm" asChild>
            <Link href="/dashboard">
              <ArrowLeft className="mr-1.5 h-4 w-4" />
              Мои курсы
            </Link>
          </Button>
          <div className="h-5 w-px bg-border" />
          <div className="flex items-center gap-2">
            <GraduationCap className="h-4 w-4 text-primary" />
            <span className="font-medium truncate max-w-[300px]">{course.title}</span>
          </div>
        </div>
        <div className="flex items-center gap-2">
          {/* Progress badge */}
          <div className="hidden sm:flex items-center gap-2 mr-2 px-3 py-1.5 bg-muted rounded-full">
            <div className="h-2 w-2 rounded-full bg-primary" />
            <span className="text-xs font-medium">{progressPercent}% пройдено</span>
          </div>
          
          {/* Certificate button - show when course completed */}
          {courseCompleted && (
            <Button size="sm" variant="default" onClick={handleGetCertificate} disabled={generatingCertificate}>
              <GraduationCap className="mr-1.5 h-3.5 w-3.5" />
              {generatingCertificate ? 'Генерация...' : 'Получить сертификат'}
            </Button>
          )}
          
          {/* Edit/Delete - only for author */}
          {isAuthor && (
            <>
              <Button size="sm" variant="outline" asChild>
                <Link href={`/dashboard/courses/${course.id}/edit`}>
                  <Edit className="mr-1.5 h-3.5 w-3.5" />
                  Редактировать
                </Link>
              </Button>
              <AlertDialog>
                <AlertDialogTrigger asChild>
                  <Button variant="ghost" size="icon" className="h-8 w-8 text-muted-foreground hover:text-destructive">
                    <Trash2 className="h-4 w-4" />
                  </Button>
                </AlertDialogTrigger>
                <AlertDialogContent>
                  <AlertDialogHeader>
                    <AlertDialogTitle>Удалить курс?</AlertDialogTitle>
                    <AlertDialogDescription>
                      Курс «{course.title}» будет удалён безвозвратно.
                    </AlertDialogDescription>
                  </AlertDialogHeader>
                  <AlertDialogFooter>
                    <AlertDialogCancel>Отмена</AlertDialogCancel>
                    <AlertDialogAction
                      onClick={(e) => { e.preventDefault(); handleDelete(); }}
                      className="bg-destructive text-destructive-foreground hover:bg-destructive/90"
                      disabled={deleting}
                    >
                      {deleting ? 'Удаление...' : 'Удалить'}
                    </AlertDialogAction>
                  </AlertDialogFooter>
                </AlertDialogContent>
              </AlertDialog>
            </>
          )}
        </div>
      </div>

      <div className="relative flex flex-1 min-h-0">
        {/* ─── Left sidebar: course navigation ─── */}
        <div
          className={cn(
            'border-r bg-muted/10 flex flex-col transition-all duration-300 ease-in-out',
            sidebarOpen ? 'w-80 shrink-0' : 'w-0 overflow-hidden'
          )}
        >
          {sidebarOpen && (
            <div className="flex flex-col h-full">
              {/* Course progress */}
              <div className="p-4 border-b bg-background">
                <div className="flex items-center justify-between mb-2">
                  <span className="text-xs font-medium text-muted-foreground uppercase tracking-wide">Прогресс курса</span>
                  <span className="text-xs font-bold text-primary">{completedCount}/{totalLessons}</span>
                </div>
                <Progress value={progressPercent} className="h-2" />
              </div>

              {/* Chapters & lessons */}
              <div className="flex-1 overflow-auto py-2">
                {course.chapters
                  .sort((a, b) => a.order - b.order)
                  .map((chapter, chapterIdx) => {
                    const isExpanded = expandedChapters.includes(chapter.id);
                    const chapterLessons = chapter.lessons.sort((a, b) => a.order - b.order);
                    const chapterComplete = chapterLessons.every((l) => completedLessons.has(l.id));
                    const chapterStarted = chapterLessons.some((l) => completedLessons.has(l.id));

                    return (
                      <div key={chapter.id} className="mb-1">
                        <button
                          className="flex items-center gap-2 w-full px-4 py-2.5 text-sm hover:bg-muted/50 transition-colors"
                          onClick={() => toggleChapter(chapter.id)}
                        >
                          <div className={cn(
                            'flex h-6 w-6 shrink-0 items-center justify-center rounded-full text-xs font-bold',
                            chapterComplete
                              ? 'bg-green-100 text-green-700 dark:bg-green-900 dark:text-green-300'
                              : chapterStarted
                              ? 'bg-primary/10 text-primary'
                              : 'bg-muted text-muted-foreground'
                          )}>
                            {chapterComplete ? <CheckCircle2 className="h-4 w-4" /> : chapterIdx + 1}
                          </div>
                          <span className="flex-1 text-left font-medium truncate">{chapter.title}</span>
                          {isExpanded ? <ChevronUp className="h-4 w-4 text-muted-foreground" /> : <ChevronDown className="h-4 w-4 text-muted-foreground" />}
                        </button>

                        {isExpanded && (
                          <div className="ml-4 border-l-2 border-muted pl-2 mb-2">
                            {chapterLessons.map((lesson, lessonIdx) => {
                              const isActive = selectedLessonId === lesson.id;
                              const isCompleted = completedLessons.has(lesson.id);
                              const globalIdx = flatLessons.findIndex((l) => l.id === lesson.id);
                              // Lesson is locked if sequential mode: all previous must be complete
                              // For now, don't lock (allow free navigation)
                              const isLocked = false;

                              return (
                                <button
                                  key={lesson.id}
                                  disabled={isLocked}
                                  className={cn(
                                    'flex items-center gap-2.5 w-full rounded-lg px-3 py-2 text-sm transition-all',
                                    isActive
                                      ? 'bg-primary text-primary-foreground shadow-sm'
                                      : isLocked
                                      ? 'opacity-40 cursor-not-allowed'
                                      : 'hover:bg-muted/80 text-foreground/80'
                                  )}
                                  onClick={() => !isLocked && setSelectedLessonId(lesson.id)}
                                >
                                  {isCompleted ? (
                                    <CheckCircle2 className={cn('h-4 w-4 shrink-0', isActive ? 'text-primary-foreground' : 'text-green-500')} />
                                  ) : isLocked ? (
                                    <Lock className="h-4 w-4 shrink-0" />
                                  ) : isActive ? (
                                    <Play className="h-4 w-4 shrink-0 fill-current" />
                                  ) : (
                                    <Circle className="h-4 w-4 shrink-0" />
                                  )}
                                  <span className="truncate text-left">{lesson.title}</span>
                                  {lesson.durationMinutes && (
                                    <span className={cn('ml-auto text-xs shrink-0', isActive ? 'text-primary-foreground/70' : 'text-muted-foreground')}>
                                      {lesson.durationMinutes} мин
                                    </span>
                                  )}
                                </button>
                              );
                            })}
                          </div>
                        )}
                      </div>
                    );
                  })}
              </div>
            </div>
          )}
        </div>

        {/* Sidebar toggle */}
        <button
          type="button"
          className={cn(
            'absolute top-4 z-10 flex h-8 w-6 items-center justify-center rounded-r-md border border-l-0 bg-background shadow-sm hover:bg-muted transition-all',
          )}
          style={{ left: sidebarOpen ? '19.9rem' : 0 }}
          onClick={() => setSidebarOpen(!sidebarOpen)}
        >
          {sidebarOpen ? <ChevronLeft className="h-4 w-4" /> : <ChevronRight className="h-4 w-4" />}
        </button>

        {/* ─── Main content area ─── */}
        <main className="flex-1 flex flex-col min-h-0">
          {/* Lesson content */}
          <div className="flex-1 overflow-auto">
            <div className="max-w-3xl mx-auto w-full px-8 py-10">
              {/* Chapter & lesson header */}
              {activeLessonMeta && (
                <div className="mb-8">
                  <p className="text-sm text-primary font-medium mb-1">
                    {activeLessonMeta.chapterTitle}
                  </p>
                  <h1 className="text-3xl font-bold text-foreground">
                    {activeLessonMeta.title}
                  </h1>
                  <div className="flex items-center gap-4 mt-3 text-sm text-muted-foreground">
                    <span className="flex items-center gap-1">
                      <BookOpen className="h-4 w-4" />
                      Урок {currentLessonIndex + 1} из {totalLessons}
                    </span>
                    {activeLessonMeta.durationMinutes && (
                      <span className="flex items-center gap-1">
                        <Clock className="h-4 w-4" />
                        {activeLessonMeta.durationMinutes} мин
                      </span>
                    )}
                  </div>
                  <div className="mt-4 h-px bg-border" />
                </div>
              )}

              {/* Content */}
              {lessonContentLoading ? (
                <div className="flex items-center justify-center py-20">
                  <div className="h-6 w-6 animate-spin rounded-full border-2 border-primary border-t-transparent" />
                </div>
              ) : selectedLessonId ? (
                <>
                  <LessonContentViewer
                    content={lessonContent}
                    className="min-h-[400px]"
                  />
                  {!showQuiz && !completedLessons.has(selectedLessonId) && (
                    <div className="mt-8 p-6 border rounded-xl bg-muted/20 text-center">
                      <h3 className="font-semibold mb-2">Проверьте свои знания</h3>
                      <p className="text-sm text-muted-foreground mb-4">
                        Пройдите тест, чтобы закрепить материал и получить сертификат
                      </p>
                      <Button onClick={handleStartQuiz}>Начать тест</Button>
                    </div>
                  )}
                  {showQuiz && (
                    <LessonQuiz
                      courseId={id}
                      lessonId={selectedLessonId}
                      questions={quizQuestions}
                      onComplete={handleQuizComplete}
                    />
                  )}
                </>
              ) : (
                <div className="flex flex-col items-center justify-center py-20 text-center">
                  <BookOpen className="h-12 w-12 text-muted-foreground/30 mb-4" />
                  <p className="text-muted-foreground">Выберите урок для начала обучения</p>
                </div>
              )}
            </div>
          </div>

          {/* Bottom navigation */}
          <div className="shrink-0 border-t bg-background px-6 py-3">
            <div className="max-w-3xl mx-auto flex items-center justify-between">
              <Button
                variant="outline"
                size="sm"
                onClick={goToPrevLesson}
                disabled={currentLessonIndex <= 0}
              >
                <ChevronLeft className="mr-1.5 h-4 w-4" />
                Предыдущий
              </Button>

              <Button
                variant="ghost"
                size="sm"
                onClick={markComplete}
                disabled={!selectedLessonId || completedLessons.has(selectedLessonId)}
                className={cn(
                  completedLessons.has(selectedLessonId ?? '')
                    ? 'text-green-600'
                    : 'text-muted-foreground hover:text-primary'
                )}
              >
                <CheckCircle2 className="mr-1.5 h-4 w-4" />
                {completedLessons.has(selectedLessonId ?? '') ? 'Пройден' : 'Отметить пройденным'}
              </Button>

              {currentLessonIndex < flatLessons.length - 1 ? (
                <Button size="sm" onClick={goToNextLesson}>
                  Следующий урок
                  <ChevronRight className="ml-1.5 h-4 w-4" />
                </Button>
              ) : (
                <div className="text-sm text-muted-foreground">
                  {courseCompleted ? 'Курс пройден!' : 'Последний урок'}
                </div>
              )}
            </div>
          </div>
        </main>
      </div>
    </div>
  );
}