import { NextResponse } from 'next/server'
import { auth } from '@/auth'
import prisma from '@/lib/prisma'
import { isCardMastered } from '@/lib/flashcards/sm2'

export async function GET() {
  try {
    const session = await auth()
    if (!session?.user?.id) {
      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
    }

    const userId = session.user.id
    const now = new Date()

    const [reviews, allCards] = await Promise.all([
      prisma.flashcardReview.findMany({
        where: { card: { deck: { userId } } },
        select: { reviewedAt: true, grade: true },
        orderBy: { reviewedAt: 'desc' },
        take: 500,
      }),
      prisma.flashcard.findMany({
        where: { deck: { userId } },
        select: { id: true, interval: true, easinessFactor: true, front: true, deck: { select: { name: true } } },
      }),
    ])

    const heatmapMap = new Map<string, number>()
    for (const r of reviews) {
      const day = r.reviewedAt.toISOString().slice(0, 10)
      heatmapMap.set(day, (heatmapMap.get(day) || 0) + 1)
    }
    const heatmap = Array.from(heatmapMap.entries())
      .map(([date, count]) => ({ date, count }))
      .sort((a, b) => a.date.localeCompare(b.date))
      .slice(-90)

    const totalCards = allCards.length
    const masteredCount = allCards.filter((c) => isCardMastered(c.interval)).length
    const retentionRate = totalCards > 0 ? Math.round((masteredCount / totalCards) * 100) : 0

    const weekMs = 7 * 24 * 60 * 60 * 1000
    const retentionByWeek: { week: string; rate: number }[] = []
    for (let i = 7; i >= 0; i--) {
      const weekEnd = new Date(now.getTime() - i * weekMs)
      const weekStart = new Date(weekEnd.getTime() - weekMs)
      const cardsAtWeek = allCards.filter((c) => c.interval >= 7 * (8 - i))
      const masteredAtWeek = cardsAtWeek.filter((c) => isCardMastered(c.interval)).length
      const rate = cardsAtWeek.length > 0
        ? Math.round((masteredAtWeek / cardsAtWeek.length) * 100)
        : retentionRate
      retentionByWeek.push({
        week: weekStart.toISOString().slice(0, 10),
        rate,
      })
    }

    const difficultCards = [...allCards]
      .sort((a, b) => a.easinessFactor - b.easinessFactor)
      .slice(0, 5)
      .map((c) => ({
        id: c.id,
        front: c.front.slice(0, 120),
        easinessFactor: c.easinessFactor,
        deckName: c.deck.name,
      }))

    return NextResponse.json({
      heatmap,
      retentionRate,
      retentionByWeek,
      difficultCards,
      totalReviews: reviews.length,
    })
  } catch (error) {
    console.error('[flashcards/stats]', error)
    return NextResponse.json({ error: 'Failed to load stats' }, { status: 500 })
  }
}